A Guide to Polyolefin Injection Molding A Guide to Polyolefin Injection Molding

Table of Contents Introduction ...... 2 Polyolefins are derived from petrochemicals ...... 2 Molecular structure and composition affect properties and processability ...... 2 Chain branching ...... 3 Density ...... 3 Molecular weight ...... 4 Molecular weight distribution ...... 4 Copolymers ...... 5 Modifiers and additives ...... 5 Working closely with molders ...... 5 How polyolefins are made ...... 5 Low density polyethylene (LDPE) ...... 6 High density polyethylene (HDPE) ...... 6 Linear low density polyethylene (LLDPE) ...... 7 Polypropylene ...... 7 Shipping and handling polyolefin resins ...... 7 Material handling ...... 7 How to solve material handling problems ...... 9 Other material handling practices ...... 10 The injection molding process ...... 11 Injection units ...... 11 Plasticator specifications ...... 13 Screw designs ...... 13 Nozzles ...... 14 Clamp mechanisms ...... 15 Clamp specifications ...... 16 Injection molds ...... 17 Types of mold ...... 17 Sprues and runners ...... 17 Mold venting ...... 19 Gating ...... 20 Mold cooling ...... 20 Ejection devices ...... 21 Spiral flow measurement ...... 21 General injection molding operating procedures ...... 22 General safety ...... 22 Heat ...... 23 Electricity ...... 23 Machinery motion ...... 23 The injection molding process and its effect on part performance ...... 23 The molding cycle ...... 23 Shrinkage ...... 29 Warpage ...... 29 Color dispersion and air entrapment ...... 30 Part ejection and mold release ...... 31 Clarity ...... 32 Gloss ...... 32 Polypropylene integral hinges ...... 32 Appendices 1. Injection Molding Terms ...... 33 2. Metric Conversion Guide ...... 38 3. Abbreviations ...... 40 4. ASTM test methods applicable to polyolefins ...... 41 5. Injection molding problems, causes and solutions ...... 42 6. ASTM and ISO sample preparation and test procedures ...... 46 7. Compression and injection molded sample preparation for HDPE ...... 47

1 A Guide To Polyolefin Injection Molding

Introduction „ Furniture compounded with colorants, flame „ Housewares retardants, blowing agents, fillers, Polyolefins are the most widely „ Industrial containers reinforce-ments, and other used plastics for injection „ Materials handling equipment functional addi-tives such as molding. This manual, A Guide to „ Packaging antistatic agents and lubricants. Polyolefin Injection Molding, „ Sporting goods contains general information „ Toys and novelties Molecular structure and concerning materials, methods and equipment for producing high This manual contains extensive composition affect quality, injection molded, information on the injection mold- polyolefin products at optimum ing of polyolefins. However, it properties and production rates. makes no specific processability recommendations for the Polyolefins that can be processing of LyondellBasell resins Four basic molecular properties injection molded include: for specific applications. For more affect most of the resin „ Low density polyethylene detailed information please contact characteris-tics essential to (LDPE) your LyondellBasell polyolefins injection molding high quality „ Linear low density polyethylene sales or technical service polyolefin parts. These molecular (LLDPE) representative. properties are: „ High density polyethylene „ Chain branching (HDPE) Polyolefins are „ Crystallinity or density „ Ethylene copolymers, such as „ Average molecular weight ethylene vinyl acetate (EVA) derived from „ Molecular weight distribution „ Polypropylene and propylene copolymers (PP) petrochemials The materials and processes used „ Thermoplastic olefins (TPO) to produce the polyolefins determine these molecular In general, the advantages of Polyolefins are plastic resins properties. injection molded polyolefins com- polymerized from petroleum-based pared with other plastics are: gases. The two principal gases are The basic building blocks for the ethylene and propylene. Ethylene gases from which polyolefins are „ Lightweight is the principal raw material for derived are hydrogen and carbon „ Outstanding chemical mak-ing polyethylene (PE and atoms. For polyethylene, these resistance ethylene copolymer resins; atoms are combined to form the „ Good toughness at lower propylene is the main ingredient for ethylene monomer, C2H4. temperatures making polypropylene (PP) and „ Excellent dielectric properties propylene copolymer resins. „ Non-hygroscopic Polyolefin resins are classified as thermoplastics, which means that The basic properties of polyolefins they can be melted, solidified and can be modified with a broad melted again. This contrasts with In the polymerization process, the range of fillers, reinforcements thermoset resins, such as double bond connecting the carbon and chemical modifiers. phenolics, which, once solidified, atoms is broken. Under the right Furthermore, polyolefins are can not be reprocessed. conditions, these bonds reform considered to be relatively easy to with other ethylene molecules to Most polyolefin resins for injection injection mold. form long molecular chains. molding are used in pellet form. Major application areas for poly- The pellets are about 1/8 inch long olefin injection molding are: and 1/8 inch in diameter and usual- ly somewhat translucent to white in „ Appliances color. Many polyolefin resins con- „ Automotive products tain additives, such as thermal The resulting product is polyethyl- „ Consumer products stabi-lizers. They also can be ene resin.

2 3 For polypropylene, the hydrogen or degradation may cause cross- range of 0.895 to 0.905 g/cm , and carbon atoms are combined linking in polyethylenes and chain which is the lowest for a to form the propylene monomer, scission in polypropylenes. commodity thermo-plastic and CH CH:CH . 3 2 does not vary appreciably from manufacturer to manufacturer.

The third carbon atom forms a side branch which causes the backbone chain to take on a spiral shape. Polypropylene, on the other hand, can be described as being linear (no branching) or very highly branched. Although the suspended carbon forms a short branch on every repeat unit, it is also responsible for the unique spiral and linear configuration of the Ethylene copolymers, such as polypropylene molecule. ethylene vinyl acetate (EVA), are made by the polymerization of ethylene units with randomly Density distributed vinyl acetate (VA) Polyolefins are semi-crystalline comonomer groups. For polyethylene, the density and polymers which means they are crystallinity are directly related, the composed of molecules which are Chain branching higher the degree of crystallinity, arranged in a very orderly the higher the resin density. Higher Polymer chains may be fairly (crystalline) structure and density, in turn, influences linear, as in high density molecules which are randomly numerous properties. As density polyethylene, or highly branched oriented (amorphous). This mixture increases, heat softening point, as in low density polyethylene. For of crystalline and amorphous resistance to gas and moisture every 100-ethylene units in the regions (Figure 2) is essential in vapor permeation and stiffness polyethylene molecular chain, providing the desired properties to increase. However, increased there can be one to ten short or injection molded parts. A totally density generally results in a long branches that radiate three- amorphous polyolefin would be reduction of stress cracking dimensionally (Figure 1). The grease-like and have poor physical resistance and low temperature degree and type of branching are properties. A totally crystalline poly- toughness. controlled by the process (reactor), olefin would be very hard and catalyst, and/or any comonomers brittle. „ LDPE resins have densities used. HDPE resins have linear molecular ranging from 0.910 to 0.930 grams per cubic centimeter Chain branching affects many of chains with comparatively few side (g/cm3) the properties of polyethylenes chain branches. Therefore, the „ LLDPE resins range from including density, hardness, chains are packed more closely 0.915 to 0.940 g/cm3 flexibility and transparency, to together (Figure 3). The result is „ HDPE resins range from name a few. Chain branches also crystallinity up to 95 percent. LDPE >0.940 to >0.960 g/cm3 become points in the molecular resins generally have crystallinity structure where oxidation may from 60 percent to 75 percent. As can be seen, all natural occur. If excessively high LLDPE resins have crystallinity polyolefin resins, i.e, those without temperatures are reached during from 60 percent to 85 percent. PP any fillers or reinforcements, have processing, oxidation can occur resins are highly crystalline, but densities less than 1.00 g/cm3. This which may adversely affect the they are not very dense. PP resins light weight is one of the key polymer’s properties. This oxidation have a nominal specific gravity

3 advantages for parts injection molded from polyolefins. A general Table 1. General guide to the effects of polyethylene physical properties on guide to the effects of density on prperties and processing the properties for various types of polyethylene resins is shown in Table 1.

Molecular weight Atoms of different elements, such as carbon, hydrogen, etc., have different atomic weights. For carbon, the atomic weight is 12 and for hydrogen it is one. Thus, the molecular weight of the ethylene unit is the sum of the weight of its six atoms (two carbon atoms x 12 + four hydrogen x 1) or 28.

Unlike simple compounds, like ethylene or propylene, every polyolefin resin consists of a mixture of large and small chains, i.e., chains of high and low molecular weights. The molecular weight of the polymer chain generally is in the thousands and minutes (g/10 min). Melt flow rate weight distribution, to measure the may go up to over one million. The is inversely related to the resin’s flow and other properties of resins. average of these is called, quite average molecular weight: as the Generally, injection molding resins appropriately, the average average molecular weight are characterized as having molecular weight. increases, MFR decreases and medium, high or very high flow. vice versa. As average molecular weight For injection molding grades, the increases, resin toughness Melt viscosity, or the resistance of MFR (MI) values for polyethylenes increases. The same holds true for a resin to flow, is an extremely are generally determined at 190°C tensile strength and environmental important property since it affects (374°F) using a static load of 2,160 stress crack resistance (ESCR) – the flow of the molten polymer g. MFR values for polypropylenes cracking brought on when molded filling a mold cavity. Polyolefins are determined at the same load parts are subjected to stresses in with higher melt flow rates require but at a higher temperature 230°C the pres-ence of materials such as lower injection molding processing (446°F). The MFR of other solvents, oils, detergents, etc. pressures, temperatures and thermoplastics may be determined However, high-er molecular weight shorter molding cycles (less time using different combinations of results in an increase in melt needed for part cooling prior to temperatures and static load. For viscosity and greater resistance to ejection from the mold). Resins this reason, the accurate prediction flow making injection molding more with high viscosities and, therefore, of the relative processability of difficult as the average molecular lower melt indices, require the different materials using MFR data weight increases. opposite conditions for injection is not possible. molding. Melt flow rate (MFR) is a simple Molecular weight measure of a polymer’s melt It should be remembered that viscosity under standard conditions pressure influences flow distribution of temperature and static load properties. Two resins may have During polymerization, a mixture of (pressure). For polyethylenes, it is the same melt index, but different molecular chains of widely varying often referred to as melt index (MI). high-pressure flow properties. lengths is produced. Some may be MFR is the weight in grams of a Therefore, MFR or MI must be short; others may be extremely melted resin that flows through a used in conjunction with other long containing several thousand standard-sized orifice in 10 characteristics, such as molecular monomer units.

4 The relative distribution of large, copolymer ethylene ethyl acrylate „ Nucleators medium and small molecular (EEA) and vinyl acetate to „ Clarifiers chains in the polyolefin resin is produce ethylene vinyl acetate „ Lubricants important to its properties. When (EVA). the distribu-tion is made up of Ethylene is used as a comonomer chains close to the average length, with propylene to produce Working closely the resin is said to have a “narrow polypropylene random molecular weight distribution.” with molders copolymers. Polypropylene can be Polyolefins with “broad molecular made more impact resistant by weight distribution” are resins with LyondellBasell offers a wide range producing a high ethylene- a wider variety of chain lengths. In of polyolefin resins for injection propylene copolymer in a second general, resins with narrow mold-ing, including Alathon® and reactor forming a finely dispersed molecular weight distributions have Petrothene® HDPE,Petrothene® secondary phase of ethylene- ® good low-temperature impact LDPE, LLDPE, and PP, Ultrathene propylene rubber. Products made strength and low warpage. Resins EVA copolymers and Flexathene® in this manner are commonly with broad molecular weight TPOs. These resins are tailored to referred to as impact copolymers. distributions generally have greater meet the requirements of many stress cracking resistance and areas of application. greater ease of processing (Figure Polyolefin resins with distinctly dif- 4). ferent properties can be made by The type of catalyst and the controlling the four basic molecular polymerization process used to properties during resin production produce a polyolefin determines its and by the use of modifiers and molecular weight distribution. The additives. Injection molders can molecular weight distribution work closely with their (MWD) of PP resins can also be LyondellBasell polyolefins sales or altered during production by con- technical service representative to trolled rheology additives that determine the resin that best meets selec-tively fracture long PP their needs. molecular chains. This results in a LyondellBasell polyolefins technical narrower molecular weight service representatives are also distribution and a higher melt flow available to assist injection molders rate. and end-users by providing guidance for tool and part design and the development of specialty Copolymers products to fulfill the requirements of new, demanding applications. Polyolefins made with one basic type of monomer are called Modifiers and homopolymers. There are, How polyolefins however, many polyolefins, called additives copolymers, that are made of two are made Numerous chemical modifiers and or more monomers. Many injection additives may be compounded with High-purity ethylene and propylene molding grades of LLDPE, LDPE, polyolefin injection molding resins. gases are the basic feedstocks for HDPE and PP are made with In some grades, the chemical making polyolefins (Figure 5). comonomers that are used to modifiers are added during resin These gases can be petroleum provide specific property manufacture. Some of these refinery by-products or they can be improvements. additives include: extracted from an ethane/propane The comonomers most often used „ Antioxidants liquified gas mix coming through with LLDPE and HDPE are called pipelines from a gas field. High „ Acid scavengers alpha olefins. They include efficiency in the ethane/propane butene, hexene and octene. Other „ Process stabilizers cracking and purification results in comonomers used with ethylene „ Anti-static agents very pure ethylene and propylene, to make injection molding grades „ Mold release additives which are critical in the production are ethyl acrylate to make the „ Ultraviolet (UV) light stabilizers of high quality polyolefins.

5 LyondellBasell can produce polyolefins by more polymerization technologies and with a greater range of catalysts than any other supplier can. Two of LyondellBasell’s plants are pictured in Figure 6.

Low density polyethylene (LDPE) To make LDPE resins, LyondellBasell uses high pressure, high temperature tubular and autoclave polymerization reactors (Figures 7 and 8). Ethylene is pumped into the reactors and combined with a catalyst or initiator to make LDPE. The LDPE melt formed flows to a separator where unused gas is removed, recovered, and recycled back into the process. The LDPE is then fed to an extruder for pelletization. Additives, if required for specific applications, are incorporated at this point. High density polyethylene (HDPE) There are a number of basic processes used by LyondellBasell for mak-ing HDPE for injection molding applications —including the solution process and the slurry Figure 7. LDPE high temperature tubular process diagram process. In the multi-reactor slurry process used by LyondellBasell (Figure 9), ethylene and a comonomer (if used), together with an inert hydrocarbon carrier, are pumped into reactors where they are combined with a catalyst. However, in contrast to LDPE production, relatively low pressures and temperatures are used to produce HDPE. The granular polymer leaves the reactor system in a liquid slurry and is separated and dried. It is then conveyed to an extruder where additives are incorporated prior to pelletizing.

6 Linear low density Figure 8. High temperature autoclave process diagram polyethylene (LLDPE) LyondellBasell uses a gas phase process for making LLDPE (Figure 11). This process is quite different from the LDPE process, but somewhat similar to the HDPE process. The major differences from the LDPE process are that relatively low pressure and low temperature polymerization reactors are used. Another difference is that the ethylene is copolymerized with butene or hexene comonomers in the reactor. Unlike HDPE, the polymer exits the reactor in a dry granular form, which is subsequently compounded with additives in an extruder. With changes in catalysts and operating conditions, HDPE resins also can be produced in some of these LLDPE reactors. Figure 9. HDPE parallel reactors — slurry process Polypropylene To make PP, LyondellBasell uses a vertical, stirred, fluidized-bed, gas- phase process (Figure 12). LyondellBasell was the first polypropylene supplier in the United States to use gas-phase technology to produce PP. Impact copolymers are produced using two, fluidized bed, gas phase reactors operating in series. LyondellBasell’s polyolefin production facilities are described in Shipping and handling of polyolefin resins

LyondellBasell also utilizes a multi- Because both of these processes It is of utmost importance to keep reactor solution process for the utilize multiple reactors, polyolefin resins clean. production of HDPE (Figure 10). In has the capability LyondellBasell LyondellBasell ships polyolefin this process, the HDPE formed is of tailoring and optimizing the resins to molders in hopper cars, dissolved in the solvent carrier and molecular weight distribution of hopper trucks, corrugated boxes, then precipitated in a downstream the various product grades to and 50-pound plastic bags. Strict process. An additional adsorption provide a unique range of quality control throughout resin step results in a very clean product processability and physical manufacture and subsequent with virtually no catalyst residues. properties. handling, right through delivery to the molder, ensures the cleanliness of the products. 7 Table 2. LYB polyolefin Figure 10. HDPE solution process production facilities

BAYPORT, TX

Figure 11. LLDPE fluidized bed process virgin resin. In all cases, the proportion of regrind used should be carefully controlled to assure consistency of processing and part performance.

Material handling LyondellBasell utilizes material handling systems and inspection procedures that are designed to prevent external contamination and product cross-contamination during production, storage, loading and shipment. Since polyolefin resins are non- hygroscopic (do not absorb water) they do not require drying prior to being molded. However, under certain conditions, condensation may form on the pellet surfaces.

When cartons of resin are When bulk containers are Reground resin, whether used as moved from a cold warehouse delivered, the molder must use a blend or as is, should also be environment to a warm appropriate procedures for stringently protected to keep it molding area or when transferring unloading the resin. Maintenance free of contamination. Whenever cold pellets from a silo to an indoor of the in-plant material handling possible, the regrind material storage system is also essential. When should be used as it is generated. system, the temperature of the bags and boxes are used, special When this is not possible, the material should be allowed to care is necessary in opening the scrap should be collected in a equilibrate, for up to eight containers, as well as covering closed system and recycled with hours to drive off any condensation` them, as they are unloaded. the same precautions taken for before molding.

8 the pellets. Proper design of the Figure 12. PP dual reactors – gas-phase process transfer lines is also critical in terms of utilizing the optimum bend radii, blind tees, and proper angles. Consult your LyondellBasell technical service engineer for guidance in this area. How to solve material handling problems Since smooth piping is a leading contributor to angel hair and streamers, one solution is to roughen the interior wall of the piping. This causes the pellets to tumble instead of sliding along the pipe, minimizing streamer formation. However, as the rapidly moving polyolefin pellets contact an extremely rough surface, small particles may be broken off the pellets creating fines or dust. Two pipe finishes, in particular, have proven to be effective in conveyed through a transfer line The best way to improve resin minimizing buildup and giving the travels at a very high velocity. As utilization is to eliminate longest life in transfer systems. the pellet contacts the smooth pipe contaminants from transfer One is a sand-blasted finish of 600 wall, it slides and friction is systems. If bulk handling systems to 700 RMS roughness. This finish generated. The friction, in turn, are not dedicated to one material is probably the easiest to obtain. creates sufficient heat to raise the or are not adequately purged, However, due to its sharp edges, it temperature of the pellet surface to there is always the possibility of will initially create dust and fines the resin’s softening point. As this contamination resulting from until the edges become rounded. remnants of materials previously happens, a small amount of molten transferred. polyolefin is deposited on the pipe The other finish is achieved with wall and freezes almost instantly. shot blasting using a #55 shot with Occasionally, clumps of “angel Over time, this results in deposits 55-60 Rockwell hardness to hair” or “streamers” may described as angel hair or produce a 900 RMS roughness. accumulate in a silo and plug the streamers. Variations of this finish are commonly known as “hammer- exit port. Contaminants of this type As the pellets meet the pipe wall, finished” surfaces. The shot can also cause plugging of transfer along the interior surface of a long blasting allows deeper penetration system filters and/or problems that radius bend, the deposits become and increases hardness, which in affect the molding machine. All of almost continuous and streamers turn leads to longer surface life. these problems can result in are formed. Eventually, the angel molding machine downtime, hair and streamers are dislodged The rounded edges obtained excessive scrap and the time and from the pipe wall and find their minimize the initial problems costs of cleaning silos, transfer way into the molding process, the encountered with dust and lines and filters. Polyolefin dust, storage silo or the transfer filters. fines. They also reduce metal fines, streamers and angel hair The amount of streamers formed contamination possibly associated contamination may be generated increases with increased transfer with the during the transfer of polymer air temperature and velocity. sandblasted finish. through smoothbore piping. These transfer systems also may contain Other good practices of material Whenever a new transfer long radius bends to convey the handling include control (cooling) system is installed or when a resin from a hopper car to the silo of the transfer air temperature to portion of an existing system is or holding bin. A polyolefin pellet minimize softening and melting of replaced, the interior surfaces should be treated by either

9 sand or shot blasting. The Regardless of the type of Other material initial cost of having this done equipment used or the materials handling practices is far outweighed by the transferred, a transfer system prevention of future problems. should be maintained and kept Beside-the-press vacuum loaders clean in the same manner as any are used to feed many injection Elimination of long-radius other piece of production molding machines. These units bends where possible is also equipment. Periodic washing and draw resin pellets from drums or important as they are probably drying of silos and holding bins cartons placed beside the the leading contributor to reduces the problem of fines and machine. In some set-ups, the streamer formation. When this dust build-up due to static vacuum loaders draw from multiple type of bend is used, it is charges. sources and directly feed the critical that the interior surface hopper with resin, regrind, Other steps to eliminate should be either sand- or shot- colorants and other concentrate contamination include: blasted. additives. Good housekeeping The use of self-cleaning, „ Inspect the entire transfer procedures are particularly impor- stainless steel “tees” in place system on a regular basis tant when working with beside-the- of long bends prevents the „ Clean all filters in the transfer press loaders since contaminants formation of streamers along system periodically can easily get into the material the curvature of the bend, „ Ensure that the suction line is containers. causing the resin to tumble not lying on the ground during Blending with colorants, additives instead of slide (Figure 13). storage or when the system is and other materials is done using However, there is a loss of started to prevent debris from on-the-machine blending units efficiency within the transfer entering the system consisting of multiple hoppers system when this method is „ Place air filters over hopper car feeding different resin compound used. Precautions should be hatches and bottom valves ingredients. Colorants, additives, taken to ensure that sufficient during unloading to prevent regrind and base resin are blower capacity is available to debris or moisture from combined using either volumetric prevent clogging of the transfer contaminating the material or, the more accurate, weight-loss lines and maintain the required „ Purge the lines with air and feeding (gravimetric) techniques. transfer rate. then with a small amount of Microprocessor controls monitor product prior to filling storage and control the amount of material Figure 13. Eliminate long-radius silos or bins fed into a mixing chamber below bends where possible. The use the hoppers. Recipe data can be „ Allow blowers to run for of stainless steel "tees" stored in the control unit for instant several minutes after prevents the formation of retrieval. streamers along the curvature unloading to clear the lines and of the bend. reduce the chance of cross- Central blending units can also be contamination of product. used especially when much higher overall volumes are required. A central vacuum loading system Information regarding transfer transfers the finished blend to the systems and types of interior individual molding machines. finishes available can be obtained from most suppliers of materials handling equipment or by The injection consulting your LyondellBasell technical service engineer. molding process To extend the life of the transfer Complete systems can be piping, it should be rotated 90° at supplied which, when properly The injection molding process periodic intervals. Resin pellets maintained, efficiently convey begins with the gravity feeding of tend to wear grooves in the contamination-free product. polyolefin pellets from a hopper bottom of the piping as they are into the plasticating/injection unit of transferred which not only the molding machine. Heat and contributes to fines and streamer pressure are applied to the formation but also accelerated polyolefin resin, causing it to melt wear due to non-uniform abrasion. and flow. The melt is injected under high pressure into the mold.

10 Pressure is maintained on the The reciprocating screw injection heater bands. The friction is material in the cavity until it cools molder is the most common caused by the pellets sliding and solidifies. When the part molding machine in use today for against themselves and the temperatures have been reduced mold-ing polyolefins. The injection inner wall of the barrel and the sufficiently below the material's unit (Figure 15) mixes, plasticates screw surface. The heat from distortion temperature, the mold and injects a thermoplastic melt the friction and conduction opens and the part is ejected. into a closed mold. The cause the pellets to melt. The reciprocating screw accomplishes majority of the melting occurs The complete process is called a this in the following manner: in the transition section of the molding cycle. The period between screw, where compression of the start of the injection of the melt 1. The injection cycle starts with the polymer is taking place as into the mold cavity and the thescrew in the forward the root diameter of the screw opening of the mold is called the position. is increased. clamp close time. The total 2. Initially, the screw begins to 5. Next, the melted polymer is injection cycle time consists of the rotate in the heated barrel. further mixed and clamp close time plus the time Resin pellets are forced by this homogenized in the metering required to open the mold, eject action to move forward through section of the screw. In the the part, and close the mold again. the channels of the screw. metering section of the screw, There are four basic components 3. As the pellets move forward, the root diameter has reached to an injection molding machine: they are tumbled, mixed and its maximum, and no further gradually compressed together compression takes place. 1. Injection unit/plasticator as the screw channels become 6. The polymer melt flows in front 2. Clamp unit shallower. The section of the of the screw tip and the 3. Injection mold screw nearest the hopper is pressure produced by the 4. Control system called the feed section, in build-up of polymer in front of which no compression takes the screw causes the screw to Injection units place. be pushed backward in the 4. As the pellets travel down the barrel as it continues to rotate. Plunger injection units (Figure 14) barrel, they are heated by 7. The screw stops turning when were the first types used for friction and the heat conducted the volume of melt produced injection molding, but their use from the external electric ahead of the screw tip is today is quite limited.

Figure 14. Schematic cross-section of a typical plunger (or ram or piston) injection molding system

11 Figure 15. Schematic cross-section of a typical screw injection molding machine, showing the screw in the retracted (A) and forward (B) position

Figure 16. In this 2-stage injection molding machine, the screw-type preplasticizer is atop and parallel to the horizontal plunger injection cylinder and chamber

12 sufficient to completely fill the (Figure 16) in which the this value to polyolefins. Injection mold cavity and runner system plasticating unit feeds a separate rate is the maximum rate at which (the channels leading to the injection cylinder called an the plasticized material can be mold cavity). This amount of accumulator. Melt is injected into injected through the nozzle in cubic material is called the shot size the mold using a ram in the inches/minute at a stated pressure. and the period during which accumulator. Machines equipped Injection pressure is generally the screw rotates is called the with accumulators can be used for expressed as the hydraulic screw recovery time. molding parts requiring very large pressure in psi (pounds/square 8. The screw is then forced shot sizes, for the high-speed inch) applied to the screw during forward, injecting the melt into injection needed to fill long and injection. The maximum injection the mold. This is called the narrow mold cavities, and for pressure available varies and the injection stage. molding parts requiring better actual pressure required depends control of shot size and injection on the resin, melt temperature, In order to compensate for material pressure. mold cooling, part design and mold shrinkage in the cavity due to cool- design. Most plasticating units ing, an excess amount of material Plasticator have a chart which relates the is generally held in front of the specifications hydraulic pressure to the pressure screw at the end of the injection actually applied to the polymer. stroke. This extra material is called Injection capacity is defined as the the cushion and, during the maximum shot size in ounces (oz.) packing phase, some of the of general-purpose polystyrene Screw designs cushion material continues to be (PS). In equating this to Numerous plasticating screw slowly injected into the cavity to polyolefins, use approximately designs are available for injection compensate for the volume lost 90% – 95% of the capacity stated molding polyolefins (Figure 17). due to the shrinkage of the for PS. The plasti-cating rate is However, since it is impossible to material in the mold and the usually given in pounds/hour or have a screw designed for every compressibility of the plastic. ounces/second for PS. Because of molding job, general-purpose Backpressure is the amount of differences in melting character- screws are most commonly used. hydraulic pressure applied to istics and different sensitivities to The shallower the screw channels, the back of the screw as it screw design variables, it is not the smaller the resin volume rotates. Varying the amount of possible to easily convert or apply conveyed to the tip of the screw. backpressure alters the pressure exerted on the polymer in front of the screw. Increasing backpressure also changes the amount of internal energy transmitted to the melt Figure 17. Screw type configurations used in injection molding machines by the shearing action of the rotating screw. An increase in backpressure raises the melt temperature without requiring an increase in heating cylinder temperatures and improves mixing and plasticating. Unfortunately, increasing backpressure also reduces screw recovery rates and can add unnecessary shear (heat) to the polymer which may lead to polymer degradation. Typically, backpressure is set at a minimum unless additional mixing is required.

Two-stage systems, also called screw preplasticators, are available

13 On the other hand, while deep screw channels accommodate Table 3. Functions of the three sections of an extrusion screw larger shot sizes more quickly, they do not heat and plasticate the melt as efficiently as a screw with shallower channels. The three basic screw sections are described in Table 3. There are a number of barrier screw designs available which offer some benefits not provided by general-purpose screws. Barrier screws provide more efficient mixing without increased backpressure and, in some cases, recovery times may be decreased. Figure 18. Typical sliding check rink showing injection stage These advantages are offset by the (top) and retraction stage (bottom) increased risk of black speck formation. The deep flights in a barrier screw may have stagnant areas in which there is a reduction in the flow of the material. The molten plastic tends to stay in these areas and degrade, ultimately causing black specks in the parts as the degraded material flakes off the screw. When purchasing barrier screws, it is recommended that the molder work closely with the screw designer to ensure that stagnant areas are avoided and that the screw is properly designed for the material being used.

Plasticating screws for thermoplastics generally have interchangeable tips. The two most commonly used tips in the injection molding of polyolefins are sliding check ring and ball-check non- The typical length-to-diameter (L/D) return valves. In the molding cycle, The nozzle extends into the fixed ratio for polyolefin reciprocating as the screw moves forward to platen and mates to an indentation screws is about 20-30:1, with a inject material into the mold, the in the front of the mold called the compression ratio of 2-3:1. Longer non-return valves close to prevent sprue bushing. screw lengths are generally material from flowing back over the preferred as they provide better The nozzle may have a positive flights of the screw. Typical sliding homogeneity of temperature and shut-off device or it may be open check ring and ball check valves and rely on the freezing-off of the melt quality. are shown in Figures 18 and 19. melt in the gate areas of the mold to keep the resin from flowing back Because of their tendency to wear, Nozzles it is critical to periodically inspect into the injection unit. Some the condition of sliding ring shut-off The injection-unit nozzle is nozzles may be connected to a tips. Excessive wear will result in connected to the barrel and directs temperature control device to inconsistencies in shot size and the flow of the melt into the mold. control the melt temperature. melt temperature.

14 Clamp mechanisms

Figure 19. Typical ball check assembly showing injection There are three basic types of stage (top) and front discharge-retraction injection moldingmachine clamps: stage (bottom) mechanical, also called toggle units, hydraulic and a combination of these called hydromechanical clamps. Toggle clamps, which are less expensive to build, are most widely used on small tonnage machines (typically, less than 500 tons). The toggle action can best be under- stood by looking at your arm when it is bent at the elbow and then when it is fully extended. In the toggle clamp, a hydraulic cylinder moves the unit’s crosshead forward, extending the toggle links and pushing the platen forward. The mechanical advantage is low as the clamp opens or closes, which permits rapid clamp movement. This action slows and the mechanical advantage increases as the platen reaches the mold-close position. The slow Figure 20. Toggle clamping system speed is important for mold protection.

Full clamp pressure is reached when the linkage is fully extended. To adjust the toggle clamp to different mold heights, the entire toggle mechanism and moving platen assembly are moved along tie rods. The position of the toggle mechanism depends on where the mold closes when the toggle is at full extension. The toggle opens when hydraulic pressure is applied to the opposite side of the clamp cylinder. See Figure 20. Figure 21. Hydraulic clamping system Hydraulic clamps generally are used on injection molding machines in the 150 ton to 1,000+ ton clamp tonnage range. In this type of clamp, hydraulic oil is used to move the platen through the full closing and opening strokes. The fluid is metered into a reservoir behind the main ram. At first quite rapid, the oil flow is slowed as the ram reaches the mold-close position in order to protect the

15 mold. An oil fill valve closes when Figure 22 . Hydromechanical clamping system. Top view shows the mold is closed. The area clamp open position of piston and ram. Bottom view behind the ram is then pressurized shows clamp closed, toggled and ram pressurized. to build full clamp tonnage. To open the mold, the oil valve is first partially opened to smoothly open the mold. Once the mold halves are separated, the clamp accelerates to a fast open speed (Figure 21). Mold set-up is much easier with a hydraulic clamp than with a toggle clamp since hydraulic clamp tonnage can be reached anywhere along the clamp stroke. Mold set-up is accomplished by setting the clamp position from the machine’s control center. Hydromechanical clamps are commonly used on very large injection molding machines, i.e., over 1000 tons. In the hydromechanical clamp, a hydraulically actuated toggle mechanism pushes the moving platen at high speed to a point where the mold halves are nearly Generally, clamp specifications horizontal direction but, generally, closed. A mechanical locking plate also state the mini-mum mold should not extend outside of the or links prevent rearward thickness for which the clamp can platens. movement during final build-up to develop its full tonnage. full clamp tonnage. Short-stroke Maximum daylight opening is the Clearance between tie-bars is hydraulic cylinders are used to given for the distance (inches) move the platen the final short distance (inches) between the two platens when the clamp is com- between the top tie-bars closing distance and develop full (horizontal) and sidebars clamp tonnage. See Figure 22. pletely open. This measurement is a major factor in determining the (vertical). Since the tie-bars are effective maximum mold thickness fixed on most injection molding Clamp specifications which takes into account the mold clamps, the distance between opening required for part ejection them dictates the maximum size Key clamp specifications to or removal. Complicated molds of a mold that can be placed in consider in choosing an injection may require more opening space the clamp. molding machine are: and rigid mounting surfaces, since Clamp tonnage is the maximum „ Clamp stroke high platen deflection under load force which the clamp can „ Minimum mold thickness could damage the mold. An allowable deflection of 0.001 in/ft develop. A clamping pressure of „ Clearance between tie bars of span with full clamp load on the five-tons-per-square-inch of the „ Maximum daylight opening center of the platen is, generally, projected area of the molding „ Platen size considered acceptable. (including the runner system) is „ Clamp tonnage more than adequate for Platen size is given in horizontal polyolefins. However, where Clamp stroke is the maximum dis- and vertical measurements packing is not a major factor, this tance (inches) the moving platen (inches) for the full platen. Since pressure may be as low as 2 tons/in2. can travel. Clamp stroke is a there are tie-bars running through An industry rule-of-thumb is that a major factor in determining the the corners of the platens, mold- clamp force of 2 to 3 tons per in2 minimum mold thickness that can size limits are less than full platen of the projected area of the part(s) be used with the machine. size. A mold can extend between and cold runner system is the tie-bars in either the vertical or adequate for reciprocating-screw-

16 type, injection-molding machines. Figure 23. Two-plate mold Some thin-wall stack molds may require 5 tons/in2 for optimum per- formance.

Injection molds There are many types of injection molds and tooling in use today, such as two-plate, three-plate and stack molds. Two and three-plate molds are more commonly used for heavy wall and non-packaging products. Both cold and hot-runner systems are used for two and three-plate molds. All stack molds use a hot manifold to convey the melt to the cavities. Each mold component must be machined and finished to exact dimensions with Figure 24. Three-plate mold very tight tolerances and must be heat-treated to be able to withstand very high injection and clamp pressures. Injection molds are the most expensive molds used in plastics processing with very long lead times required for their design and fabrication. Every mold must be tested and debugged to prove- out the ejection system, cooling and/or heating system and operating components before it is placed in production.

Types of molds A two-plate mold (Figure 23) has only one parting line. If a runner is used, it is connected to the molded product and requires manual removal and separation after the fed directly to a recycling system. is much more expensive and takes part is ejected. This type of mold is This type of mold is more longer to build. Three level stack the least expensive to make and is expensive than the two-plate molds are very new and four level commonly used for parts with mold. stack molds have been around for less than five years. The dairy relatively simple geometries. Stack molds (Figures 25 and 26) container and lid industries can be two, three or four levels. commonly use stack molds. The Three-plate molds (Figure 24) The advantage of the stack mold is four-level is common for lids, and have two parting lines, one for the that it can, generally, produce a the two-level is common for runner system and one for the larger number of products versus a containers. molded product. When the mold two or three-plate mold utilizing the opens, the runner is automatically same machine clamp tonnage. The separated from the product to disadvantage is that it requires a Sprues and runners allow separate handling. This molding press with much greater eliminates the need for manual The sprue and runner system is daylight opening to accommodate the conduit that connects the separation and removal and the the mold height. This type of mold sprue and runner system may be machine nozzle to the cavities.

17 During the injection phase of the Figure 25. 2 x 1 wash basin stack mold Photo courtesy of Tradesco Mold, Ltd. molding cycle, the molten material flows through the sprue and runner to the cavities. The sprue connects the machine nozzle to the runner and may be either a cold or a hot sprue. In the hot sprue design, the sprue has heating elements that maintain the plastic in a molten state eliminating the need for separation and recla- mation. Ideally, the sprue should be as short as possible to minimize the pressure loss during injection. A cold sprue is tapered for easy release from the mold.

There are three basic runner types in use: „ Cold Runner

Figure 26. 4 x 24 stack mold Photo courtesy of Tradesco Mold, Ltd. „ Insulated Runner „ Hot Runner

Cold runners are commonly used in two and three-plate molds, but not in stack molds which require the use of a hot runner. The most com-monly used runner designs are full-round, half-round and trapezoidal (Figure 27. The full- round is gener-ally preferred for ease of machining and lower pressure loss. For fast cycles a full-round is not recommended since the greater mass of material takes longer to cool and may control the cycle time. The runner should be polished for ease of mold filling and part ejection. The insulated runner (Figure 29) is the precursor to the hot runner. Figure 27. Schematic showing typical runner designs found in The runner diameter is very large injection molds and a thick skin is formed on the outside of the runner. The molten plastic flows in the center and, due to external insulation and the low thermal conductivity of the polymer, remains molten during the cycle. This design eliminates the need for removing and/or recycling the runner. The problem with this design is that when the machine is down for any extended period of time the runner solidifies and has to be physically removed before

18 Figure 28. Insulated runner system Vents should be located at the extremities of the part and at locations where melt flow fronts come together. Venting is also easily achieved around ejector pins and core slides provided that there is sufficient clearance between the pin/slide and the mold. Typical mold vents are channels cut from the cavity or runner straight to the edge of the mold. Closest to the part, they are typically 0.0005- 0.001 in. (0.013-0.025 mm) in depth and 0.063-0.5 in. (1.6-12.7 mm) in width. The initial vent thickness should be maintained for about 0.5 in. (12.7 mm) and then the depth can be increased to Figure 29. Hot-runner system about 0.003 in. (0.076 mm) to the edge of the mold. The vents should be polished towards the edge of the mold to make them ‘self- cleaning.’ Build-up in the vents will eventually affect mold filling resulting in non-uniform fill and unbalanced cavities. For this reason, it is important that vents be inspected between production runs to ensure that they are clean and within specification. In some cases, reduction of the injection rate prior to final filling of the cavity will prevent burning and also prevent the mold from opening.

Continuous parting-line venting may be necessary in high speed molding operations. Even though beginning the next molding cycle. according to the mold maker’s burning is not evident, the lack of As molders have become more specifications to prevent damage burn marks does not ensure that comfortable with hot runner and material leakage in the the molds are properly vented. technology, insulated runners are manifold. Increasing vent areas may help rapidly becoming obsolete and not reduce cycle time. Proper venting many molds are built today utilizing Mold venting will also aid mold filling by this technology. decreasing the resistance due to When molten plastic is injected air pressure on the flow front. The externally heated hot-runner into the mold, the air in the cavity system (Figure 29) also maintains has to be displaced. To accomplish Mild sand blasting or vapor the plastic in a fluid state while the this, vents are machined into the blasting of the mold cavity assists mold is running with the pressure parting line to evacuate the air and in venting and part release. at each gate approximately the are extremely important to the However, for high-gloss finishes, same. Maintaining a uniform consis-tent production of high this blasting is not advisable. Vapor temperature in the sprue bar and quality products. In many cases, honing may help alleviate a the hot-runner manifold is very this is an area in mold design and venting problem area but care critical to process and product construction that is often must be taken that honing is not consistency. Start-up procedures overlooked. too deep or wide to be noticeable must be carefully followed on the finished part.

19 Gating runners, and gates) be balanced. This depends on the size and Figure 30. Gating systems The gate is the bridge between location of the gates and is often the runner and the cavity. determined by experience. Depending on the specific Advances in mold filling simulation material and part design (wall software have provided an thickness, geometry, etc.) there additional tool for analysis prior to are many different types of gates the manufacture of the tool. Fine- which can be used (Figure 30). tuning may be required and is generally done by utilizing a series The type and size of gate are very of short shots, observing the fill critical since they can affect many pattern, and making minor factors including mold-fill time, adjustments, as required. For overall cycle, orientation, multi-cavity tools utilizing single shrinkage, warpage, and part gates and a hot runner system, appearance. adjustment of the temperatures of the individual gates may be used Because it acts as a restriction to to balance the overall fill pattern. the polymer flow, a high shear rate is created at the gate often In high-speed, thin-wall molding, it resulting in a temperature is common to provide cooling increase. There is also a high around the gate to remove the heat pressure drop across the gate produced by the high shear rates. which needs to be overcome by This may be supplemented by the increased injection pressure or use of inserts fabricated of high higher temperatures. The pressure conductivity alloys, such as drop can be reduced, to a certain beryllium-copper, in these critical degree, by using shorter gate land areas. lengths. Mold cooling A large gate provides easy filling with relatively low shear rates and Although mold cooling is pressure drops. However, if it is too extremely critical to cycle time, large, it will require an excessive warpage, molded-in stresses, amount of time to cool, lengthen- mold-filling, etc., the sizing and ing the cycle. It is also possible layout of the cooling pattern are that insufficient packing and often over-looked and neglected subsequent sinks or voids will aspects in the initial stages of tool occur if either sections of the part design. or the sprue and runner system The cooling layout should be freeze off before the gate. considered in relationship to the thickness profile of the part and the A gate which is too small will general filling pattern in order to require higher pressures to inject provide adequate cooling in critical the material and may cause sections and not overcool others problems in part filling. If the gate which may cause part warpage. In freezes off before the part cools, it areas where coolant flow may be will not be possible to develop restricted due to part geometry i.e., adequate packing, resulting in bosses, the use of inserts voids or sinks. With extremely fabricated from high thermal small gates, jetting or melt fracture conductivity alloys, such as of the polymer flow will cause beryllium-copper, should be surface appearance defects, considered. including delamination. In all cases, cooling channels To ensure uniform fill, it is critical should be sized in relation to the that the feed system (sprue, available coolant flow to ensure

20 turbulent flow which is much more maintaining the molten resin at Figure 31. Broad MWD (left) and effective for heat removal than 190°C (374°F) and 43.5 psi narrow MWD (right) lowering the temperature of the for Polyethylenes or at 230°C spiral flow coolant. Routine inspection and (446°F) for Polypropylenes. acid-cleaning of cooling channels Melt index is a good measure- are recommended to maintain the ment of a resin’s relative flow coolant flow velocity and minimize properties at low shear rates, but pressure drops. Ideally, the only for resins of the same temperature differential between molecular weight distribution coolant inlets and outlets should be (MWD). Under actual injection about 2°F. Jumpers between molding conditions, differences in cooling circuits should be avoided MWD will affect the resin’s melt in order to reduce temperature viscosity (flow characteristics) at differentials in the coolant. Spiral flow measures the flow high shear rates. Temperatures, length when molten resin is The utilization of low pressure-drop pressures and shear rates of injection molded into a long, manifolds, valves, fittings, etc. and actual molding do not conform to 0.0625" radius, half-round spiral in-line flowmeters and temperature those of the MI or MFR test channel (Figure 31). The higher indicators are also good practices methods. the spiral flow number (SFN), the to provide information regarding has a number of easier the resin is to process. The the efficiency and condition of the LyondellBasell unique manufacturing processes melt temperature is monitored and cooling system. available which allow the control maintained at 440°F (227°C) and not only of the melt index and injection molding is conducted Ejection devices density, but also MWD. This using a constant pressure of 1,000 psi (7,000 kPa). Spiral flow is a The ejection of injection molded capability results in a better overall more realistic measurement than parts is most commonly accom- balance in resin properties and melt index because it is run at a plished by air, vacuum, pins or processability. Because melt index much higher shear rate allowing strip-per plates. Depending on part and MWD play a key role in resins of similar MIs and different design, combinations of these performance in actual end-use MWD to be compared at realistic systems are used for rapid positive applications, LyondellBasell has conditions. The broader MWDs ejection. Care should be taken in utilized "Spiral Flow" (SF) as a resins exhibit lower melt viscosity selecting ejection surfaces more practical method of (higher SFN) at higher shear rates because of aesthetic and measuring and comparing a resin's than narrower MWD resins with moldability requirements. performance using realistic similar melt indices (Figure 32). Wherever possible, the part should processing conditions. be ejected off the core. For small, thin-walled moldings that may shrink onto the core, air ejection through the core is usually Figure 32. Effect of MWD on spiral flow of HDPE (all materials have MFR - 5 gms/10 min) adequate for part removal. On some products with threaded or undercut features, collapsible, retractable or unscrewing cores are used.

Spiral flow measurement The relative processability of an injection molding resin is often determined by its Melt Index (MI) or Melt Flow Rate (MFR). This involves measuring the relative flow of the molten resin through a specified capillary in a calibrated laboratory instrument, while

21 Since it does not take into account Always refer to the manufacturer’s the safe operation of any piece of the effects of MWD, relying only on operating manual for any specific processing equipment. In some the melt index can be misleading. start-up and shutdown cases, these interlocks may need ® For example, ALATHON ETP H procedures. to be bypassed while performing 5057, a broad MWD, 57 melt index set-up and maintenance functions. resin for thin-wall HDPE Refer to the LyondellBasell Under no circumstance should this applications, exhibits flow suggested resin startup conditions be done by non-qualified properties similar to many narrow (Table 4) for general guidelines to personnel. In order to assure MWD resins in the 75 to 80 melt use in starting up an injection utmost safety during normal index range. molder on polyolefins. operation, interlocks should never be bypassed. LyondellBasell has established the use of spiral flow as a specification General safety Keep all molding equipment and for all high-flow (30 melt index and As with any process involving the surrounding work areas above) HDPE resins and has energy and mechanical motion, clean and well maintained. begun reporting the spiral flow injection molding can be a number for each lot on the hazardous operation if appropriate Hydraulic leaks should be repaired Certificate of Analysis (COA). This safety procedures aren’t well immediately to eliminate safety allows the molder to compare the documented and followed. (Refer hazards. Hydraulic lines, valves, spiral flow of an incoming lot of to the Manufacturer’s operating fittings and hoses should be resin with the SFN of the lot on- manual.) checked periodically per the hand and readily estimate how the Mechanical, electrical, and manufacturer’s recommendations. new lot will process relative to hydraulic interlocks are critical to current production. For example, if the current lot being run has a SFN of 20 in. and the new lot has a Table 4. Suggested start-up conditions (based on general purpose melt reported SFN of 22 in., the new lot index/flow rate products) can be processed at either lower temperatures and/or at a faster production rate. Only minor adjustments in either melt temperature and/or injection pressure may be required to compensate for SFN variability from lot to lot.

General injection molding operating procedures

Prior to starting up the injection- molding machine, be sure to have the following available: „ Safety glasses for all personnel assisting in the start-up. „ Loose fitting, heavy-duty insulat-ed work gloves. „ A large metal container or cardboard for collecting melt produced during the start-up procedure. „ Soft beryllium-copper, bronze, or aluminum tools for use in removing any plastic from the nozzle area.

22 Good housekeeping is essential. injury. Do not reach around, under, 5. COOLING: Coinciding with the Loose pellets, tools, oil, etc. on and through, or over guards while the start of plastication, the cooling around the molding machine can equipment is operating. cycle begins (Note: Since cause accidents, damage to the coolant continuously circulates Be sure all people working near equipment, or contamination of the through the mold, cooling the injection molding machine parts. technically starts as soon as know where the Emergency Shut the melt contacts the cavity Off but-on is located. Never during injection) Heat disengage any of the safety 6. MOLD OPEN: Mold opens, High temperatures are necessary mechanisms or inter-locks on the slides retract, ejector pins in the injection molding process. injection molding machine. Always use heat-resistant gloves, activate, part(s) are ejected. safety glasses and protective cloth- Some machines store energy ing. Modern injection molding (hydraulic, pneumatic, electrical, or The length of each of these steps machines have warning signs gravitational) which can be present will depend on the complexity of identifying specific hot areas on even when the machine is turned the mold, the size of the machine, molding machines; do not ignore off. Consult the manufacturer’s and the geometry and end-use these signs. Keep the splashguard operating guide for methods to requirements of the part. A typical in place during purging and when properly de-energize the cycle for a four cavity, 16 oz. the machine is operating. equipment. As with any piece of stadi-um cup can be found in potentially hazardous equipment, a (Figure 33) while one for a single Polymer left in the barrel or hot suitable lock-out/tag-out procedure cavity, 12 lb. bumper fascia can runner system may often be should be implemented and be found in (Figure 34). pressurized. Care should be taken enforced. whenever resin flow is interrupted Regardless of part size, weight, or due to blockage or mechanical mold complexity, nearly half of problems. The injection every injection cycle is spent If the molding machine will be shut molding process cooling the part(s) to a temperature down for an extended period of suffi-cient to allow ejection without time (30 minutes or longer), lower and its effect on post-mold distortion. Factors that the heats, purge the machine or part performance affect the cooling rate of the part(s) cycle it until the lower temperature will be covered later. is reached before shutting it down, leaving the screw full of resin and The molding cycle In almost all cases, part quality is in the retracted position. the result of steps 2-5. The highest As detailed in the section on the quality parts begin with a homoge- Injection Molding Process, there neous plastic melt in terms of tem- Electricity are several steps in the perature and composition. production of injection molded Therefore, the quality of the next Molding machines utilize high elec- parts. In most cases, the injection part to be produced is the result of trical voltage and have warning molding cycle begins with the the development of the shot during signs pointing out electrical mold open, ejector pins/slides the current cycle. Shot size should hazards; do not ignore these signs. retracted, and the screw/ ram be sufficient to produce a cushion Keep water away from these ready with the next shot of of material at the end of the step 3 areas. A periodic inspection of all material. The cycle then proceeds of 0.1-0.5 in. (2-13 mm). This electrical devices and connections as follows: cushion will keep the screw from for wear, looseness, etc. is very ‘bottoming out’ and help maintain important. 1. MOLD CLOSE: Mold closes plastic pressure within the cavity. and clamp develops full closing Machinery motion pres-sure Achieving a homogeneous melt is 2. INJECTION: Material is controlled by many factors includ- There is considerable mechanical injected into the mold cavity ing: screw design, screw speed, motion during the injection molding 3. PACKING: Material is packed screw and barrel wear, back process. Neckties and loose fitting into the mold to fill out the part pressure, shot to barrel capacity clothing should not be worn around 4. PLASTICATION: Screw begins ratio, soak time, and heater band molding machines since these can to rotate (or ram retracts) to settings. be caught by the equipment devel-op the next shot of movement and lead to physical material

23 provided for melting the resin Figure 33. Injection molding cycle for a 16.0z. stadium cup (4 pellets comes from shear heating cavity, HDPE, 31g each, 7.8 sec. cycle) due to friction between the pellets, screw and barrel. As the back pres-sure is increased, the screw works the material more in order to convey it forward, thereby raising the temperature of the melt more quickly. The increased work by the screw also increases the mixing of the molten plastic resulting in better temperature and homogeneity of the melt. However, too much back pressure can result in degradation of the plastic, an increase in the screw recovery time, increased energy costs, and more wear on the screw and barrel. Shear heating is also dependent on the viscosity of the plastic, screw design, screw Figure 34. Injection molding cycle for automotive facia (12 lb. shot, 0.125 in. speed, and back pressure. The thickness, 107 sec. cycle) latter two can be varied to some extent by the processor to control the shear heating and melt temperature.

The shot-to-barrel capacity ratio (SBCR) can also have an effect on melt, and therefore part quality. The ideal range for the shot to barrel capacity ratio is 30-60%. If the SBCR is less than 30%, too many shots of material reside in the barrel under the influence of heat from the heater bands and shear from the screw. This may lead to over-heating and degradation of the resin. If the SBCR is greater than 60%, less than two shots of material are in the barrel, which typically does not allow the melt temperature to Screw design for polyolefin should be set so that the screw equilibrate. A high SBCR will also processing was covered in a consistently recovers 1-2 seconds mean that the screw may recover previous section. A worn barrel before the mold opens. (develop the next shot) just before and/or screw creates an the mold opens which can lead to Backpressure is typically set at the increased gap between the two cycle alarms due to inadequate minimum level that delivers a resulting in resin moving shot size. A SBCR range of 30- homogeneous melt (no unmelted backward or staying in place 60% will provide adequate time for pellets leaving the nozzle). However, instead of being conveyed the melt temperature to equilibrate. backpressure may need to be forward by the screw. A worn increased to improve temperature screw and/or barrel can lead to In order to achieve proper melt consistency in the melt and to mini- poor melt consistency, homogeneity, all of the pellets mize or eliminate streaks due to poor degradation of the resin, and should be melted by the time they dispersion of colorant. During shot inconsistency. Screw speed reach the middle of the transition plastication, most of the energy zone on the screw. Figure 35

24 depicts the amount of energy needed to process a polypropylene Figure 35. Energy needed to bring PP to melt temperature impact copolymer. In order for the pellets to be fully melted halfway through the transition zone, 71% of the energy to reach the desired melt temperature (in this case, 450°F) must be transferred to the polymer. The heater bands on the barrel provide only a small amount of the energy needed to melt the plastic. Most of the energy from the heater bands maintains the barrel temperature during processing and raises the temperature of the solid pellets in the feed zone. There are four typical temperature setting patterns for injection molding on a barrel with five heater zones (Figure 36): 1. Increasing: This pattern has the lowest temperature setting at the feed throat and the Figure 36. Various barrel temperature profiles highest at the front of the screw with a steady increase of the temperature settings in between. The nozzle is typically set at the same temperature as the front zone. This pattern is the one most commonly used and is particularly recommended for lower melting point materials (such as EVA or EMA) to prevent bridging at the feed throat of the extruder. It is also recommended when the SBCR is low, typically <30%.

2. Decreasing: This pattern has the desired melt temperature at the front of the screw and the highest temperature at the back of the screw. The nozzle temperature set point should than desired melt temperature) is usually set at the same be used at the feed throat. In in the middle of the screw to temperature as the front zone. addition, this profile can correspond with the transition This temperature profile is increase the chance of air section where the majority of recommended when the SBCR being entrapped in the melt the melting takes place. is >50% and screw recovery instead of venting back Settings near the feed throat and residence times (time from through the hopper. are typically at the softening resin entering the extruder to point of the resin while the leaving the nozzle) are short. 3. Hump: This pattern has the settings at the front of the Sufficient feed throat cooling highest temperature settings barrel and the nozzle should must be provided to prevent (typically 20+ degrees higher be at the desired melt bridging. Otherwise, a low

25 temperature. This profile is If a heater band is on all the time, making it easier to fill the mold. recommended when SBCR is either the set point is too high to be Increasing the pressure or injection 25-50% and overall resi-dence reached or there is a problem with rate increases the shear rate, time is 2 to 4 minutes. the thermocouple or heater band (it which decreases the viscosity is working but not reading the making it easier to fill the mold. 4. Flat: This profile uses the actual temperature). If the Therefore, given a resin, machine desired melt temperature as thermocouple is fine and the and mold, there are three variables the settings for all of the barrel desired setting for the zone is not that can be used to fill out the zones except for the feed out of line, the processor can mold: throat, which should be set at increase the temperature settings „ Injection/packing pressures or below the softening point of upstream (closer to the feed throat) „ Injection rate the resin. This profile is typical of the zone in question. Should this „ Melt temperature of processes where the SBCR not reduce the time that the heater is 20 to 40%. band is on, the temperature setting on the zone should be lowered The curves in Figure 37 indicate In actual practice, the specific until the band cycles regularly. This the relative temperature-pressure screw design also plays an will prolong the life of the heater relationships for PE resins of important part in obtaining the band and reduce the energy varying melt indices. The higher desired melt. It is possible that usage. the MFR or MI, the lower the different screw designs may injection pressure and/or the If a heater band does not draw require different profiles to achieve temperature required to fill a mold. current, or does so infrequently, similar melt characteristics even if Assuming the same mold filling there are two possible problems. they have the same or an characteristics (fill speed and fill Either most of the heat going into equivalent SBCR. time), cycle time and injection the plastic at this barrel zone is via temperature, a high flow resin: By varying the screw speed and uncontrolled shear heating or the back pressure, shear heating is, for thermocouple is broken, both of 1. Will allow pressures to be the most part, an easily controlled which should be corrected. A reduced about 25% when the source of heat to the material. The broken thermocouple will typically resin MI or MFR is doubled. best parts are typically produced read out the maximum permissible 2. Will allow a decrease in melt when there is a balance of shear temperature. If all of the cavities temperature of about 70°F heating and heat from the heater are filling with acceptable cycle (40°C) when the resin MI or bands. Once a temperature profile times, the heater band set points in MFR is doubled. is chosen, it is recommended that the zones upstream of the zone in The effect of a higher flow PE resin the processor monitor current flow question should be reduced. If this on temperature and pressure can into the heater bands. The proper fails to get the heater band cycling, be seen in Figure 38. Note that as balance of shear heating and reset the upstream zone(s) to their the MI or MFR of the resin thermal heat is achieved when the original temperature(s) and increases the possible reduction in current cycles regularly (typically increase the temperature set point. temperature and/or pressure will several times a minute). This is of It may appear that the procedures become less. particular importance in the above are only serving to increase transition zone of the barrel; if the However, the switch to a polyolefin the overall melt temperature. barrel is divided into quarters along with higher flow characteristics While this is true to a small extent, the length, the transition zone is usually results in a loss of other the benefit is that they aid in typically the middle two quarters of properties such as resistance to providing a more homogeneous the barrel. Typically the heater stress cracking and impact and controllable melt temperature bands in the feed zone will cycle strength, especially at lower that will improve the molded parts. regularly or be on nearly all the temperatures. time. The heater bands in the Now that we have a homogeneous Injection and/or packing pressures metering zone and nozzle should melt stream in the barrel or accu- are typically the first settings cycle regularly but less frequently mulator, we need to examine the adjusted by the processor because than the feed zones. Because introduction of the plastic into the they have a quick response on accurate temperature control is so mold. The viscosity, or resistance mold fill. Increasing the pressures important, it is always a good to flow, of the resin is affected by will help fill out the mold correcting practice to routinely check the tem-perature and shear rate. for short shots and reducing or calibration of the controllers. Increasing the melt temperature eliminating surface defects such as reduces the viscosity of the resin sink marks and ripples near the

26 gate. The downside of increasing pressure is the chance of trapping Figure 37. Temperature-pressure relationshps for polyethylene resins of several melt indices air in the cavity resulting in burn marks or of increasing the flash on the parting line due to the mold opening. Increasing injection pressures also pack resin more tightly into the cavity, which may reduce shrinkage, increase the gate temperature(s), and increase molded-in stress. The reduced shrinkage can lead to a part sticking in the cavity and also post- mold dimensional differences. Increasing the injection rate(s) reduces the viscosity of the resin, which may reduce the amount of molded-in stress in the part. In addition, an increased injection rate may also yield a more uniform part temperature (due to faster intro-duction of material into the mold) which can reduce differential shrinkage (i.e. warpage) due to temperature variation. Increasing the injection rate(s) without a decrease in injection pressure can lead to flashing of the part. Changing the injection rate(s) also has a fast effect on part quality although it may take time to fine- tune the rate(s) for optimum quality. Excessive molded-in stress can lead to an increase in warpage Figure 38. Effect of Melt index of polyethylene resin on injection temperature and a decrease in impact strength and environmental stress cracking resistance.

Sometimes the injection rate and injection pressure are not independent variables; i.e. the machine is set with a maximum pressure and runs on injection rate settings which are set on screw position. This setup will allow the machine to vary the injection pressure based on the pressure needed to meet the rate set points. Conversely, the injection pressure can be specified based on screw position and the rate is allowed to vary. Some processors are now utilizing pressure sensors within the cavity to control the operation of the machine via cavity pressure. This is a new approach that is

27 gaining acceptance for molding through the mold. The coolant The extremities of the part tend to parts with critical tolerances. It is takes heat out of the mold and have the lowest temperature since also applicable to molds (such as therefore out of the part via the polymer melt has transferred syringes) where core shift is of conduction. Optimum cooling is heat as it has flowed through the concern. achieved when the water is in cavity. turbulent flow. In general, an The final way to control the increase in coolant flow rate will Differences in part temperature viscosity of the resin is to adjust remove more heat than a decrease can lead to differential shrinkage the melt temperature. An increase in coolant temperature. and therefore warpage. There are in temperature will decrease two ways to minimize temperature viscosity. Changing temperature As resin flows into the mold, the differentials either through settings yields a slower response material in contact with the mold differential cooling or an increase than pressure or injection rate. surface solidifies very quickly form- in injection rate. High resin temperature can lead to ing a skin layer and an inner flow degradation and require longer ‘channel’ through which material Differential cooling involves direct- cooling time while low continues to flow (Figure 39). As ing coolant towards the gate area, temperatures can lead to shot more heat is taken from the plastic which has the highest part inconsistency, higher injection by the mold, the flow channel is temperature and away from the pressures, and excessive wear/ reduced to the point that no more extremities where the part damage to the screw and barrel. material will flow. The optimum temperature is lowest. The typical process parameters should be When setting an injection rate or method is to reduce the coolant chosen to allow complete mold fill injection and packing pressure flow to the cooling channels and pack before the flow channel profile, the aim of the processor nearest the extremities and open solidifies completely. Keeping the should be to provide a smooth up the valves to the channels melt channel open allows for better delivery of material into the mold. A nearest the gate. Directing the packing of the extremities of the momentary slowing of the screw coolant flow from the hotter part. There are also differences in due to either the transfer from one portions of the tool to the part temperature depending on step to another or too large of a extremities may also be effective in proximity to the gate. step can result in a hesitation of some cases. the plastic flow front. Hesitation of Figure 39. the melt front can cause surface As covered previously, defects such as flow lines or tiger increasing the injection rate will stripes, which may lead to poor shorten the injection times weld and/or knitline strength. allowing for a more uniform part Therefore it is necessary to reduce temperature. the rates or pressures in a consistent manner to prevent Because the cooling step is usually flashing of the tool, potential core the longest time period in the injec- shift(s), and bottoming out of the tion molding cycle, a cold mold screw. temperature is generally recommended. Because of the In general, >95% of the part wide range of mold and part weight(s) should be delivered designs, it is very difficult to specify during the injection step. The final mold temperatures. A typical range part weight is achieved via the of mold surface temperatures for packing and holding step. Packing polyolefins is 70-125°F (20-50°C), pressures are typically about half Uniform cooling to the mold occurs which requires coolant the level of the injection pressure when the coolant makes one pass temperatures of 32-50°F (0-10°C). and serve to achieve final part through the mold (no looping or Materials with lower melting weight(s) and also to allow time for connecting of flow channels) and temperatures, such as EVAs, will the gates to freeze off before there is only one temperature be at the lower end of the range plastication can begin for the next controller. During even cooling, the while the higher melting materials, shot. gate area is always the hottest area of the part because such as HDPE and PP will be at During the injection and packing throughout the injection and the higher end. Cold molds, steps, coolant (typically a mixture packing steps, molten material however, tend to give a less glossy of ethylene glycol and deionized continues to flow through the gate. surface finish and will restrict the water if <45°F) is circulating flow of resin within the mold. A cold

28 mold can also lead to a higher Since the degree of shrinkage Warpage amount of molded-in stress within is partly a result of cooling, it the part. Warmer mold can be reduced by molding at Warpage results from non-uniform temperatures will increase the lower injection temperatures shrinkage of the molded part gloss level and may also improve and running a colder mold. caused by non-uniform cooling. resin flow by constricting the melt Packing the part more will When a part warps after being channel less, at the expense of also minimize shrinkage. This ejected, it is assuming its ‘natural’ increased cycle times. is done either by molding at shape by relieving the stresses The injection molder and the mold moderate temperatures and forced upon it while being cooled design typically fix the amount of high pressures or by molding in the mold. The problem, often a time needed for the mold to open, at fairly high temperatures difficult one to solve, is to minimize eject the part and then close again. and moderate pressures. the ‘locked-in’ stresses, which the Mold open time can be reduced by However, excessive part might later ‘remember’ and using only enough daylight to allow temperature or pressure can relieve during cooling to room tem- the part(s) to fall freely, reducing result in flash. perature. In cases where parts are the amount of time required for air fixtured after ejection, subsequent Another means of reducing assists and also the number of exposure to higher temperatures shrinkage is the use of higher times the ejector pins activate. may cause relaxation and pressure and longer packing warpage. Part designs incorporating time. This allows additional significant differences in cross- Shrinkage resin to flow into the mold as sectional thickness are more prone the material in the mold cools to warpage than those with a more Amorphous resins such as ABS, and shrinks, packing out the uniform thickness, due to higher Polycarbonate, and Polystyrene mold as much as possible but residual temperatures in the thicker have much lower shrinkage values may also increase cycle time sections. than the polyolefins. The higher and higher molded-in stress. shrinkage of polyolefins is due to In addition to non-uniform the fact that, in their molten state, Longer cooling time in the cooling, locked-in stresses are they take up more volume than in mold before ejection is generated in the mold by such the solid state because polyolefin especially useful whenever an operating conditions as excessive resins are semi-crystalline. When inside dimension is critical. As molding pressures, slow fill times, the resin solidifies, the chains in the molded article cools and low backpressure, or too low a the crystalline regions pack tightly contracts around the core, the melt temperature. together resulting in a reduction in core will maintain the critical volume. In general, the polyolefins inside dimension of the part. There is no single, clear-cut can be ranked for shrinkage: Generous draft, or tapering, will allow easier part ejection. remedy for warpage. Adjusting mold conditions, redesigning the HDPE > LLDPE > LDPE > PP A longer cooling time will mean an increase in cycle part or the mold, switching to a material with a narrower MWD, or Once a resin has been selected, time, therefore many molders a combination of these may reduce shrinkage can be controlled, to will increase the mold cooling the internal stresses. Generally, the some extent, through mold design to reduce shrinkage. least warpage occurs when the and processing conditions (Table Shrinkage is a time-dependent melt temperature is set at the 5). Studies on a test mold in which dependent function. In maximum, the mold temperature is the thickness and gate area of a general, a polyolefin part has high, injection pressure is a flat plaque can be varied, indicate achieved about 90% of its minimum and the injection time is the following: total shrinkage after 48 hours. short (Table 5). Shrinkage can continue for „ Shrinkage is reduced as part several more days if the parts Molding at high temperatures thickness decreases. The are packed hot and/or are allows the stresses induced during response to a thickness stored in a warm warehouse. injection to be reduced before the change is more pronounced Parts that have shrunk after part sets. Running a warm mold with HDPE than PP. packaging typically exhibit also allows the stresses to relax ‘nesting’ problems if the parts before the melt sets. Differential „ Shrinkage is reduced when the are stacked inside each other. cooling between the mold halves is gate area is reduced.

29 Table 5. Some ways to reduce shrinkage and warpage in polyolefin injection moldings

often required to produce warp- will allow lower injection pressure, Color dispersion and free parts especially those having which can shorten the molding air entrapment large, flat sections. cycle. In addition, higher flow resins typically exhibit less “elastic Injection and packing pressures memory” which can also reduce An effective means of improving should be adequate to permit easy warpage. Lower density resins (for dispersion and preventing air fill but should not be set PE) are only slightly less bubbles from getting into the mold excessively high in order to allow susceptible to warpage than higher with the melt is to use a breaker some of the molded-in stresses to density resins. plate at the end of the barrel relax before the part sets. between the screw tip and the Differences between flow and nozzle (Figure 40). Backpressure Increasing the injection rate will transflow shrinkage can result in on the melt, in most cases, decrease the injection time, which warpage. HDPE is known to have squeezes all the air out between will allow the mold to fill faster a large difference between these the melting pellets and produces before the extremities can cool too two while PP is more balanced bubble-free moldings. The breaker much. This gives the entire between the flow and transflow plate may be ¼ inch (6.5 mm) molding a chance to cool at a more shrinkage. thick and must be large enough to uniform rate, which reduces the fit into the opening of the nozzle. warpage. Because both shrinkage and The plate is drilled with 20- to-40, warpage are strongly influenced by small diameter (1/32 inch or 0.8 Some of these remedies, such as the mold cooling patterns and part mm) holes. Another option is to high melt temperature or low injec- geometry (uniformity of thicknesses increase the back pressure on the tion pressure can increase the and flow patterns), it is very impor- screw making sure that it is not set cycle time. Switching to a higher tant that these be considered in the too high so that the screw cannot MFR/MI resin can offset the early stages of part and mold recover in time for the next cycle. increase in time. Higher flow resins design. Increasing the backpressure and/ or adding a breaker plate can also

30 aid in color dispersion and melt stick to the cavity or the mold core. EVA moldings should be generous, temperature homogeneity. When Ejection pins may be used to move as they tend to be stickier at part running at high processing the molding from either the cavity ejec-tion temperatures. Normally, a temperatures, backpressure or the core first. 3- to 5-degree draft significantly should be kept to a minimum to assists in part ejection. On shallow reduce degradation of the resin. Enough draft must always be draft molds, the use of stripper provided, especially in deep-draw rings and/or air ejection may be Part ejection and mold moldings, such as long containers. necessary. Draft angles of less release Reverse draft or even parallel than one degree are not sides should be avoided wherever recommended and should be Mold release is affected by a possible. The draft allowance for avoided unless dictated by part number of factors. Some polyolefin requirements. molding resins exhibit better mold release than others do. It has been found that these resins have accompanying disadvantages, Figure 40. Dispersion aids are inserts that may be placed in the nozzle of the such as less gloss. Resins that injection molding machine to restrict the melt flow and improve develop a grainy or frosty surface mixing of the resin for a more homogenous melt temperature or release better than smooth, high- mixture of materials that may have been added, such as colorant gloss moldings, such as those made from high MI grades or clarified polypropylene. However, even polyolefins of the same MI may vary in their mold-release properties. Resins sometimes are compounded with a mold-release additive. Changing the mold design or one or more molding conditions without affecting the end properties of the molding usually can alleviate mold release problems. Mold release may be difficult if the mold is packed too tightly in an effort to reduce shrinkage. Frequently, a molded article sticks to the mold if the cycle time is too long and the molding shrinks to a core. In such cases, shortening the cooling time may improve mold release. On the other hand, the same problem can occur if the cycle is too short to allow the molding to shrink away from the cavity walls. In such cases, length-ening the cycle time may improve mold release. Draw stoning of the surfaces in the direction of mold opening may also alleviate this problem.

Mold release greatly depends on the degree of polish on the inside of the mold. Proper surface finishes inside a mold for a deep- draw item determines whether the part can be ejected easily or will

31 Clarity Polypropylene integral gates must be placed so a weld hinges line does not occur in the hinge. If High melt temperature and low multiple gates are used, it is pressure are necessary to recommended that a section, eliminate flow marks in molded Polypropylene can be molded into slightly thicker than the wall thick- parts. Clarity of molded parts also a hinged part that can be flexed ness, be placed parallel to the can be improved by lowering the many cycles before failure. hinge. This ‘flow collector’ will pro- mold temperature, especially in Lifetimes in excess of a million mote a uniform flow pattern across thin-walled sections when higher cycles have been measured for the hinge. MI resins are used. This reduces properly designed and the size of the crystals formed manufactured hinged parts. Molding conditions that lead to which, in turn, reduces the light optimum hinge properties are a diffraction. The hinged section must be thin high melt temperature (typically enough to flex properly but thick 500°F to 525°F/260°C to 275°C), Polypropylene random copolymers enough to prevent tearing. Normal fast injection speed and a warm have greater clarity than PP homo- hinge thickness is 0.008-0.015 in. mold (120°F to 150°F/50°C to polymers. The clarity of random (0.2-0.38 mm). The greater thick- 65°C). In order to develop optimum copolymer resins can be further ness is necessary if the hinge properties, the hinge should be enhanced by the addition of clarify- requires strength or load bearing flexed several times immediately ing agents. Optimum clarity for PP properties. after removal from the mold. In articles in the nominal 0.050 in. some applications such as multi- thickness range is obtained at melt A typical cross-section of an cavity, hinged closures, it is not temperatures of about 430°F integral hinge is shown in Figure possible to perform this flexing. (220°C) and mold temperatures 41. A shallow relief or clearance However, if the hinge is properly about 50 to 80°F (10 to 25°C). must be provided to prevent designed, it will still perform Generally, high injection rates also gathering and excessive stress adequately for the requirements of enhance clarity. Highly polished when the hinge is in the closed the application. All polypropylenes tools are necessary for highest position. Variations may be made can be used for living hinges but clarity. on this design to achieve specific the most optimum hinges are results. achieved with homopolymer PP Gloss followed by random copolymer and impact PP. Acceptable hinges may Surface gloss of the molding is Figure 41. Polypropylene integral be formed from PP impact affected by resin properties, the hinge specifications copolymers but there is some condition of the mold and molding potential for delamination in the conditions. The higher the MI or hinge area. MFR of the polyolefin resin the It is also possible to produce greater the gloss of the molding. hinges utilizing secondary Further, higher density polyethyl- operations. One post-forming enes give higher gloss than lower procedure involves a heated steel density resins. die at about 425°F (220°C) which is forced into the molded part at Highly polished molds are one of The gating into the part must be 50-100 psi (345-690 kPa) of the most important factors for designed to allow the polymer to pressure. A rolling, heated die can obtaining high-gloss parts. For flow through the entire hinged also be used; this process is often polyethylenes, a warm mold gives section in a uniform and referred to as ‘coining’ a hinge. If better gloss than a cold mold. continuous fashion with the flow the polymer thickness is reduced Gating also may contribute to front perpendicular to the hinge. to 0.005 to 0.015 in. (0.13-0.38 obtaining a high-gloss part. This arrangement ensures mm) a satisfactory hinge results. Restrictive gating produces higher optimum hinge strength without This technique may be used to put gloss because it keeps the delamination. It is preferable that hinges in very large or complex temperature high as the melt is all gates be on one side of the parts. injected into the mold cavity. hinge to eliminate the possibility of Highest gloss for polypropylene weld lines; however, some designs resins is obtained with a cold mold are so complicated that this and a fast injection rate. arrangement is not possible and

32 Appendix 1 Co-injection Molding: A special Daylight Opening: The maximum multimaterial injection process in distance between the stationary which a mold cavity is first and moving platens of the clamp Some terms pertaining to partially filled with one plastic and unit in the fully open position. then a second shot is injected to injection molding Delamination: When the surface encapsulate the first shot. of a finished part separates or Antioxidant: Additive used to help Cooling Channels: Channels appears to be composed of layers. protect plastics from degradation located within the body of a mold Strata or fish-scale-type through sources such as heat, age, through which a cooling medium is appearance where the layers may chemicals, stress, etc. circulated to control the mold be separated. Antistatic Agent: Additive used to surface temperature. help eliminate or lessen static elec- Diaphragm Gate: Used in tricity from the surface of the Clarifiers: Additive used in symmetrical cavity filling to reduce plastic part. polypropylene random copolymers weld-line formations and improve to improve clarity. filling rates. Aspect Ratio: Ratio of total flow length to average wall thickness. Cushion: Extra material left in Direct Gate: The sprue that feeds barrel during cycle to try and directly into the mold cavity. Back Pressure: The pressure ensure that the part is packed out Dispersion Aids: Perforated applied to the plastic during screw during the hold time. recovery. By increasing back pres- plates placed in the plasticator sure, mixing and plasticating are Cycle: The complete sequence of nozzle to aid in mixing or improved; however, screw operations in a process to dispersing colorant as it flows recovery rates are reduced. complete one set of moldings. The through the perforations (Figure cycle is taken at a point in the 40). Backing Plate: A plate used as a operation and ends when this point support for the mold cavity block, is again reached. guide pins, bushings, etc. Boss: Protuberance on a plastic Figure 42. Schematic showing a typical injection mold with some of the part designed to add strength, points identified facilitate alignment, provide fastening, etc. Breaker Plate: See Figure 40. Cavity: The space inside a mold into which material is injected. (Figure 42). Charge: The measurement or weight of material necessary to fill a mold during one cycle. Clamping Plate: A plate fitted to a mold and used to fasten the mold to a platen (Figure 42). Clamping Pressure: The pressure applied to the mold to keep it closed during a cycle, usually expressed in tons. Closed-loop Control: System for monitoring complete, injection- molding-process conditions of tem-perature, pressure and time, and automatically making any changes required to keep part production within preset tolerances.

33 Draft: The degree of taper of a Flow: A qualitative description of Jig: A tool for holding parts of an mold-cavity sidewall or the angle the fluidity of a plastic material assembly during the of clearance designed to facilitate during the process of molding. A manufacturing process. removal of parts from a mold. measure of its moldability Knit Lines: See weld lines. generally expressed as melt flow Drooling: The extrudation or rate or melt index. Knockout Pins: A rod or device leakage of molten resin from a for knocking a finished part out of plasticator nozzle or nozzle sprue- Flow Line: Marks visible on the a mold. bushing area while filling or shoot- finished items that indicate the ing the mold. direction of the flow of the melt L/D Ratio: A term used to help Dwell: A pause in the applied into the mold. define an injection screw. This is pres-sure to a mold during the Flow Marks: Wavy surface the screw length-to-diameter ratio. injection cycle just before the appear-ances on a molded part Melt Flow Rate: A measure of the mold is completely closed. This caused by improper flow of the molten viscosity of a polymer dwell allows any gases formed or melt into the mold. determined by the weight of present to escape from the polymer extruded through an molding material. Gate: An orifice through which the melt enters the mold cavity. orifice under specified conditions Ejector Pins: Pins that are Hob: A master model in hardened of pressure and temperature. pushed into a mold cavity from the steel. The hob is used to sink the Particular conditions are rear as the mold opens to force shape of a mold into a soft metal dependent upon the type of the fin-ished part out of the mold. block. polymer being tested. MFR Also called knockout pins. usually is reported in grams per Homopolymer: Plastic that 10 minutes. Melt flow rate defines Ejector Return Pins: Projections results by the polymerization of a the flow of a polypropylene resin. that push the ejector assembly single monomer. An extrusion weight of 2160 back as the mold closes. Also grams at 446°F (230°C) is used. called surface pins or return pins. Hopper Dryers: Auxiliary equip- ment that removes moisture from Melt Index: Term that defines the Ejector Rod: A bar that actuates resin pellets. melt flow rate of a polyethylene the ejector assembly when the resin. An extrusion weight of 2160 mold opens. Hopper Loader: Auxiliary equip- ment for automatically loading grams at 310°F (190°C) is used. Family Mold: A multi-cavity mold resin pellets into machine hopper. Mold Changer: An automated where each of the cavities forms device for removing one mold one of the component parts of an Hot-runner Mold: A mold in which from a machine and replacing it assembled finished part. the runners are insulated from the chilled cavities and are kept hot. with another mold. Fan Gate: A gate used to help Hot-runner molds make parts that Mold Frame: A series of steel reduce stress concentrations in have no scrap. plates which contain mold compo- the gate area by spreading the nents, including cavities, cores, opening over a wider area. Less Injection Pressure: The pressure runner system, cooling system, warping of parts can usually be on the face of the injection screw ejection system, etc. expected by the use of this type of or ram when injecting material into gate. the mold, usually expressed in Mold-temperature-control Unit: psi. Auxiliary equipment used to Fill: The packing of the cavity or control mold temperature. Some cavities of the mold as required to Insulated Runner: See hot- units can both heat and cool the give a complete part or parts that runner mold. mold. Others, called chillers, only are free of flash. Izod Impact Test: Test to deter- cool the mold. Fin: The web of material mine impact strength of a sample Moving Platen: The platen of an remaining in holes or openings in by holding a sample bar at one injection molding machine that is a molded part which must be end and broken by striking. moved by a hydraulic ram or removed for final assembly. Sample specimen can be either notched or unnotched. mechanical toggle. (Figure 20-21) Flash: Extra plastic attached to a Multi-cavity Mold: A mold having molding usually along the mold Jetting: A turbulent flow in the two or more impressions for form- parting line. melt caused by an undersized gate or where a thin section ing finished items in one machine rapidly becomes thicker. cycle.

34 Multi-material Molding: The Pinpoint Gate: A restricted gate of Restricted Gate: A very small injection of two-or-three materials, 0.030 in or less in diameter, this orifice between runner and cavity in sequence, into a single mold gate is common on hot-runner in an injection mold. When the part dur-ing a single molding cycle. The molds. is ejected, this gate readily breaks injection molding machine is free of the runner system. equipped with two-or-three plasti- Generally, the part drops through cators. (See also co-injection) one chute and the runner system through another leading to a Nest plate: A retainer plate in the granulator and scrap-reclaim mold with a depressed area for system. cavity blocks. Retainer Plate: The plate on which Non-return Valve: Screw tip that demountable pieces, such as mold allows for material to flow in one cavities, ejector pins, guide pins direction and closes to prevent and bushings are mounted during back flow and inject material into molding. the mold. Piston: See ram. Retractable Cores: Used when Nozzle: The hollow-cored, metal Plasticate: To soften by heating and molding parts in cavities not per- nose screwed into the injection end mixing. pendicular to the direction in which of a plasticator. The nozzle the part is ejected from the mold. matches the depression in the Plasticator: The complete melting The cores are automatically pulled mold. This nozzle allows transfer of and injection unit on an injection from the mold prior to the mold the melt from the plasticator to the molding machine. opening and reinserted when the runner system and cavities. mold closes again and prior to Platens: The mounting plates of a injection. Nucleating Agent: Additive used press on which the mold halves with polypropylene to increase are attached. Rib: A reinforcing member of a crystallization rate by providing Plate-out: The blooming of addi- molded part. additional sites for crystal growth. tives onto machinery during pro- cessing of plastics. Ring Gate: Used on some Orange Peel: A surface finish on a cylindrical shapes. This gate molded part that is rough and Plunger: See ram. encircles the core to permit the splotchy. Usually caused by melt to first move around the core moisture in the mold cavity. Pressure Pads: Reinforcements of before filling the cavity. hardened steel distributed around Packing: The filling of the mold the dead areas in the faces of a cavity or cavities as full as possible mold to help the land absorb the without causing undue stress on final pressure of closing without the molds or causing flash to collapsing. appear on the finished parts. Purging: The forcing one molding Part Picker: An auxiliary unit material out of the plasticator with usually mounted on fixed platen, another material prior to molding a which reaches into the open mold new material. Special purging to grab parts and remove them compounds are used. prior to next molding cycle. Also called a robot, the device is used Ram: The forward motion of the Robot: Automated devices for when you do not want to drop parts screw in the plasticator barrel that removing parts upon ejection from from mold upon ejection. forces the melt into the mold cavity. an open mold rather than letting the parts drop. Also see parts Parting Line: On a finished part, Recovery Time: The length of time picker. Robots also can perform this line shows where the two mold for the screw to rotate and create a secondary functions, such as halves met when they were closed. shot. inspection, degating, precise placement of parts on a conveyor, etc.

35 RMS Roughness: A measure of Side Bars: Loose pieces used to Sprue Gate: A passageway the surface roughness/smoothness carry one or more molding pins through which melt flows from the of a material. The root mean and operated from outside the nozzle to the mold cavity. square (RMS) average of the mold. Sprue Lock: The portion of resin "peaks and valleys” of a surface is Side-draw Pins: Projections used retained in the cold-slug well by determined using a Profilometer. to core a hole in a direction other an undercut. This lock is used to The lower the number, the than the line of closing of a mold pull the sprue out of the bushing smoother the surface: a reading of and which must be withdrawn as the mold opens. The sprue one or two would be a very before the part is ejected from the lock itself is pushed out of the polished and smooth surface. mold. See also Retractable Cores. mold by an ejector pin. Rockwell Hardness: A measure of Silver Streaks: See splay marks. Sprue: The feed opening the surface hardness of a material. provided in injection molding A value derived from the increase Single-cavity Mold: A mold having between the nozzle and cavity or in depth of an impression as the only one cavity and producing only runner system. load of a steel indenter is one finished part per cycle. increased from a fixed minimum Stack Molds: Two or more molds Sink Mark: A shallow depression value to a higher value and then of a similar type that are or dimple on the surface of a returned to the minimum value. positioned one behind the other to finished part created by shrinkage The values are quoted with a letter allow for additional parts to be or low fill of the cavity. prefix corresponding to a scale manufactured during a cycle. relating to a given combination of Slip Agent: Additive used to Stationary Platen: The large front load and indendter. provide lubrication during and plate of an injection molding press immedi-ately following processing to which the front plate of the Runner: The channel that of plastics. mold is secured. This platen does connects the sprue with the gate Slip plane: Marks evident in or on not move during normal operation. for transferring the melt to the finished parts due to poor welding cavities. Stress Cracking: There are or shrinking upon cooling. three types of stress cracking: Runnerless molding: See hot- Spiral Flow: Test performed by 1. Thermal stress cracking is runner mold. injection molding a sample into a caused by prolonged exposure spiral mold and used to compare of the part to elevated Screw Travel: The distance the the processability of different temperatures or sunlight. screw travels forward when filling resins. 2. Physical stress cracking occurs the mold cavity. Splash Marks: See splay marks. between crystalline and amorphous portions of the part Short Shot: Failure to completely Splay Marks: Marks or droplet- when the part is under an fill the mold or cavities of the mold. type imperfections on the surface internally or externally induced of the finished parts that may be strain. Shot: The complete amount of caused by the splaying of the melt 3. Chemical stress cracking melt injected during a molding through the gates and into the cool occurs when a liquid or gas cycle, including that which fills the cavity where they set up. permeates the part's surface. runner system. Split-ring Mold: A mold in which a All of these types of stress Shot Capacity: Generally based split cavity block is assembled in a cracking have the same end on polystyrene, this is the channel to permit the forming of result: the splitting or fracturing of maximum weight of plastic that can undercuts in a molded piece. the molding. be displaced or injected by a single These parts are ejected from the injection stroke. Generally mold and then separated from the Striations: Marks evident on the expressed as ounces of piece. molded-part surfaces that indicate melt flow directions or impinge- polystyrene. Sprue Bushing: A hardened-steel ment. insert in the mold that accepts the Shrinkage: The dimensional plasticator nozzle and provides an Stringing: Occurs between the fin- differences between a molded opening for transferring the melt. ished part and the sprue when the part and the actual mold mold opens and the melt in this dimensions. area has not cooled sufficiently.

36 Stripper Plate: A plate that strips a Valve Gating: A type of gate where molded piece from core pins or a pin is held in the gate or channel force plugs. The stripper plate is by spring tension. As the injection set into operation by the opening of stroke moves forward, this gate the mold. compresses the plastic in the runner. When this pressure build- Structural Foam Molding: up is sufficient to overcome the Process for making parts that have spring tension, the pin is then solid outer skin and foamed core. pushed back (pulled) and the fast Submarine Gate: A gate where the decompression of the melt fills the opening from the runner into the cavity at extremely high speed. mold cavity is located below the Vent: A shallow channel or opening parting line. Also called a tunnel cut in the cavity to allow air or gate. gases to escape as the melt fills Suck-back: When the pressure on the cavity. the sprue is not held long enough Vented Barrel: Special plasticator for the melt to cool before the unit with a vent port over the com- screw returns. Some of the melt in pression section of the screw to the cavities or runner system may permit escape of gases prior to expand back into the nozzle and injecting melt into mold. Often used cause sinks marks on the finished when molding moisture-sensitive part. resins. Tab Gate: A small removable tab Vertical Flash Ring: The about the same thickness as the clearance between the force plug molded item, but usually perpendi- and the ver-tical wall of the cavity cular to the part for easy removal. in a positive or semi-positive mold. Tie-bar Spacing: The space Also the ring of excess melt which between the horizontal tie-bars on escapes from the cavity into this an injection molding machine. clearance space. Basically, this measurement limits Warpage: Dimensional distortion in the size of molds that can be a molded object. placed between the tie-bars and into the molding machine. Weld Line: Marks visible on a finished part made by the meeting Toggle: A type of clamping mecha- of two melt-flow fronts during nism that exerts pressure by apply- molding. ing force on a knee joint. A toggle is used to close and exert pressure Wisps: Similar to stringing but on a mold in a press. smaller in size. These also may occur as slight flashing when the Tunnel Gate: See submarine gate. mold is overpacked or forced Undercut: A protuberance or open slightly. Mold-parting-line indentation that impedes wear or misalignment can also withdrawal from a two-piece rigid cause wisps. mold.

37 Appendix 2 Metric Conversion Guide

Appendix 3: Metric Conversion Guide

To Convert From To Multiply By

Area square inches square meters 645.2 square millimeters square inches 0.0016 square inches square centimeters 6.452 square centimeters square inches 0.155 square feet square meters 0.0929 square meters square feet 10.76

Density pounds/cubic inch grams/cubic centimeter 27.68 grams/cubic centimeter pounds/cubic inch 0.036 pounds/cubic toot grams/cubic centimeter 0.016 grams/cubic centimeter pounds/cubic foot 62.43

Energy foot-pounds Joules 1.356 Joules foot-pounds 0.7375 inch-pound Joules 0.113 Joules inch-pounds 8.85 foot-pounds/inch Joules/meter 53.4 Joules/meter foot-pounds/inch 0.0187 foot-pounds/inch Joules/centimeter 0.534 Joules/centimeter foot-pounds/inch 1.87 foot-pounds/square inch kilo Joules/square meter 2.103 kilo Joules/square meter foot-pounds/square inch 0.4755

Length Mil millimeter 0.0254 millimeter mil 39.4 inch millimeter 25.4 millimeter inch 0.0394

Output pounds/minute grams/second 7.56 grams/second pounds/minute 0.132 pounds/hour kilograms/hour 0.454 kilograms/hour pounds/hour 2.20

Power kilowatts horsepower(metric) 1.34 horsepower (metric) kilowatts 0.76

38 To Convert From To Multiply By

Pressure pounds/square inch (psi) kilopascals (kPa) 6.895 kilopascals (kPa) pounds/square inch (psi) 0.145 pounds/square inch (psi) bar 0.0689 bar pounds/square inch (psi) 14.5

Temperature °F °C (°F-32)/1.8) °C °F 1.8 x °C +32

Thermal Conductivity Btu-in/hr, sq. ft.,°F W/(m-°K) 0.1442 W / (m-°K) Btu-in/hr,sq ft, °F 6.933

Thermal Expansion inches/inch,°F meters/meter,°C I . 8 meters/meter, °C inches/inch, °F 0.556

Viscosity poise Pa-sec. 0.1 Pa-sec poise 10

Volume cubic inch cubic centimeter 16.39 cubic centimeter cubic inch 0.061 cubic foot cubic decimeter 23.32 cubic decimeter cubic foot 0.0353

Weight ounce gram 28.35 kilogram ounce 0.0353 pound kilogram 0.4536 kilogram pound 2.205 ton (US) ton (metric) 0.907 ton (metric) ton (US) 1.102

39 Appendix 3:

AbAbAbbrbrbreeeviaviaviationstionstions ASTM American Society for Testing and Materials Btu British thermal unit cpm Cycles per minute cps Cylinder deg Cycles per second deflec Deflection deg Degree (angle) DTUL Deflection temperature under load E Modulus of elasticity elong Elongation EMA Ethylene-vinyl acetate copolymer ESCR Environmental stress cracking resistance EVA Ethylene vinyl acetate copolymer flex Flexural FR Flame retaratant FRP Fiber reinforced plastic g Gram HDPE High density polyethylene HDT Heat deflection temperature HMW High molecular weight IMM Injection molding machine imp Impact J Joule K Kelvin kpsi 1000 pounds per square inch L/D Length to diameter ratio of screw lbf Pound-force LDPE Low density polyethylene LLDPE Linear low density polyethylene MD Machine direction MDPE Medium density polyethylene MFR Melt flow rate Ml Melt index mod Modulus mol% Mole percent MW Molecular weight N Newton PE Polyethylene PP Polypropylene pphr Parts per hundred resin ppm Parts per million psi Pounds per square inch RH Relative humidity rpm Revolutions per minute sp gr Specific gravity SPE Society of Plastics Engineers SPI The Society of the Plastics Industry ten Tensile

Tg Glass transition temperature (crystalline polymers)

Tm Melt temperature (amorphous polymers) UL Underwriter's Laboratories ult Ultimate UV Ultraviolet yid Yield

40 Appendix 4 ASTM test methods applicable to polyethylene

PROPERTY ASTM METHOD ISO METHOD

Ash Content D 5630 ISO 3451-1, 6427 Deflection Temperature D 648 ISO 75 Density D 1505 ISO 1183 Dielectric Constant D 150 or D 1531 — Dissipatpion Factor D 150 or D 1531 — Environmental Stress Crack Resistance D 1693 ISO 4599 Flexural Modulus (secant or tangent) D 790 ISO 178 Flexural Strength D 790 ISO 178 Flow Rates using extrusion rheometry D 1238 ISO 1133 Hardness, Rockwell D 785 ISO 2039-2 Hardness, Shore D 2240 — Impact Strength D 256 for notched (Izod) ISO 180 D 4812 for unnotched ISO 180 Impact Strength, Gardner D 5420 — Impact Strength, falling Dart (Tup impact) D 5628 — Impact Strength (multi-axial instrumented) D 3763 ISO 5503-2 Impact, Tensile D 1822 ISO 8256 Low Temperature Brittleness D 746 ISO 974 Melt Flow Rate D 1238 ISO 1133 Melt Index D 1238 ISO 1133 Rheological properties using capillary rheometer D 3835 ISO 11443 Shrinkage, mold D 955 ISO 294-4 Specific Gravity D 792 ISO 1183 Surface Resistivity D 257 IEC 93 Tensile Modulus D 638 ISO 527-1, 2 Tensile Strength D 638 ISO 527-1, 2 Thermal Conductivity C 177 — Thermal Expansion D 696 — Torsional Expansion D 1043 ISO 458-1 Vicat Softening Point D 1525 ISO 306 Volume Resistivity D 257 IEC 93 Water Absorption D 570 ISO 62

None: Caution should be exercised when comparing data obtained using ASTM and ISO test procedures. In many cases, the data are not equivalent due to differences in specimen preparation, geometry, and/or testing procedures.

41 Appendix 5 Injection Molding Problems, Causes and Solutions

42 Appendix 5 Injection Molding Problems, Causes and Solutions

43 Appendix 5 Injection Molding Problems, Causes and Solutions

44 Appendix 5 Injection Molding Problems, Causes and Solutions

45 Appendix 6 Comparison between ASTM and ISO Sample Preparation and Test Procedures SAMPLE PREPARATION For polyethylene, compression molding is the preferred sample preparation by ASTM. For polypropylene, injection molding is the preferred sample preparation by both ASTM and ISO.

FLEXURAL MODULUS For ASTM, the recommended test specimen is 127 mm by 12.7 mm by 3.2 mm. For ISO, the recommended test specimen is 80 mm by 10 mm by 4.0 mm.

IZOD IMPACT For ASTM, the recommended test specimen is 64 mm by 12.7 mm by 3.2 mm. For ISO, the recommended test specimen is 80 mm by 10 mm by 4.0 mm. For notched Izod Impact testing, both ASTM and ISO used a notch at an angle of 45° with a radius of curvature of 0.25 mm.

TENSILE IMPACT The test specimen geometries, as shown in the figure, are as follows:

TENSILE IMPACT For ASTM, an end-gated, single-cavity injection mold with dimensions of 127 mm by 12.7 mm by 3.2 mm is recommended. For ISO, a fan-gated, two-cavity injection mold with dimensions of 60 mm by 60 mm by 2 mm is recommended.

TENSILE STRENGTH AND ELONGATION The test specimen geometries, as shown in the figure, are as follows:

46 Appendix 7 Comparison between Compression Molded and Injection Molded Sample Preparation for HDPE*

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