CHAPTER 1 INTRODUCTION 1.1 Introduction Injection Molding Is The

CHAPTER 1

INTRODUCTION

1.1 Introduction

Injection molding is the most commonly used manufacturing process for the fabrication of plastic parts. Injection moulding can be performed with a host of materials, including metals, glasses, elastomers, confections, and most commonly thermoplastic and thermosetting polymers. A wide variety of products are manufactured using injection molding, which vary greatly in their size, complexity, and application. The injection molding process requires the use of an injection molding machine, raw plastic material, and a mold. The plastic is melted in the injection molding machine and then injected into the mold, where it cools and solidifies into the final part. There are three parts in the injection molding machine. The parts are clamping unit and injection unit.

Figure 1.1

1 The injection molding process are shown below,

1. Granules of plastic powder are poured or fed into a hopper which stores it until it is needed. 2. A heater heats up the tube and when it reaches a high temperature, a screw thread starts turning. 3. A motor turns a thread which pushes the granules along the heater section which melts and turns into a liquid. The liquid is forced into a mould where it cools into the shape. 4. The mould then opens and the unit is removed.

1.2 Objective

The objective of the experiment is for the students to learn processing techniques associated with injection moulding.

2 CHAPTER 2

LITERATURE REVIEW

The materials that is used in this experiment are Polypropylene and Acrylonitrile- Butadiene-Styrene (ABS).

Polypropylene is a type of thermoplastic polymer resin. It is a part of both the average household and is in commercial and industrial applications. The chemical designation is C3H6. One of the benefits of using this type of plastic is that it can be useful in numerous applications including as a structural plastic or as a fiber-type plastic. There are a lot of benefits of Polypropylene. Polypropylene has a high melting point compared to similarly weighted plastics. Next, it is easy to customize. It has the ability to add dye to it. It can be colored in various ways without degrading the quality of the plastic. This is also one of the reasons it is commonly used to make up the fibers in carpeting. It also adds strength and durability. Then, It does not absorb water like other plastics. It does not mold or otherwise deteriorate in the presence of bacteria, mold or other elements. The newer versions contain an elastic element to them. This gives them a rubber-like composition and opens the door for new uses. In addition, it is unlikely to shatter and will take significant damage prior to breaking, though it is not as sturdy as other plastics such as polyethylene. Lastly, it is lightweight and very flexible.

ABS (Acrylonitrile Butadiene Styrene) is a thermoplastic resin commonly used for injection molding applications.

ABS Plastic is a copolymer of Acrylonitrile, Butadiene, and Styrene, and generally possess medium strength and performance at medium cost. ABS is a common thermoplastic resin and can often meet the property requirements at a reasonable price, falling between standard resins (PVC, polyethylene, polystyrene, etc.) and engineering resins (acrylic, nylon, acetal, etc.). ABS is considered the best of the styrenic family. It is tough, hard and rigid and has good chemical resistance and dimensional stability.

3 ABS is derived from acrylonitrile, butadiene, and styrene. Acrylonitrile is a synthetic monomer produced from propylene and ammonia; butadiene is a petroleum hydrocarbon obtained from butane; and styrene monomers, derived from coal, are commercially obtained from benzene and ethylene from coal. The advantage of ABS is that this material combines the strength and rigidity of the acrylonitrile and styrene polymers with the toughness of the polybutadiene rubber. The most amazing mechanical properties of ABS are resistance and toughness. A variety of modifications can be made to improve impact resistance, toughness, and heat resistance. The impact resistance can be amplified by increasing the proportions of polybutadiene in relation to styrene and acrylonitrile although this causes changes in other properties. Impact resistance does not fall off rapidly at lower temperatures. Stability under load is excellent with limited loads.

Even though ABS plastics are used largely for mechanical purposes, they also have good electrical properties that are fairly constant over a wide range of frequencies. These properties are little affected by temperature and atmospheric humidity in the acceptable operating range of temperatures. The final properties will be influenced to some extent by the conditions under which the material is processed to the final product; for example, molding at a high temperature improves the gloss and heat resistance of the product whereas the highest impact resistance and strength are obtained by molding at low temperature.

ABS is an opaque thermoplastic that possesses a diverse combination of properties. ABS is characterized by resistance to chemicals, heat and impact. Typical applications for ABS include appliance housings, luggage, camera bodies, power tool housings, battery cases, tool boxes, packing crates, radio cases and various furniture components.

ABS is a low cost engineering plastic that is easy to machine and fabricate. ABS is an ideal material for structural applications when impact resistance, strength, and stiffness are required. It is widely used for machining pre-production prototypes since it has excellent dimensional stability and is easy to paint and glue. Natural (beige) ABS and black ABS are FDA compliant for use in food processing applications.

4 CHAPTER 3

METHODOLOGY

2.1 Materials

Table 2.1 : Materials Name Abbreviation

Acrylonitrile-Butadiene-Styrene ABS

Nylon PA

Polycarbonate PC

Polypropylene PP

Polystyrene GPPS

2.2 Procedure

WARNING : In this laboratory experiment we MUST wear safety glasses and heat-resistant gloves.

 Making the part 1. The safety glassses and heat-resistant gloves is wore. 2. The main parts of the injection molding machine is observed before starting the lab. 3. The main power is switched on followed by starting the cooling system and hydraulic system. 4. Turn the temperatures at different zones in the barrel, nozzle and mold as listed below:  Rear 160°C  Middle 180°C  Front 200°C  Nozzle 200°C  Mold 32°C

The back pressure and screw speed is set as 0.6 MPa and 50 rpm respectively.

*The setting might change depend on material used.

WARNING : We now must use EXTREME caution around the barrel and the nozzle as they will become hot enough to leave severe burns.

5. Do not proceed until the nozzle and the barrel have reached the set temperatures.

5 6. The polymer is feed into the hopper. 7. The process is started with manual operation mode. 8. The mold release is applied to both sides of the mold. 9. The different steps in manual operation is followed to inject the polymer into the mold.  The barrel is filled with the resin  It is melted through the barrel  The mold is closed and the polymer melt is injected into the mold  The mold is opened and the injected sample is ejected out. 10. The injection molding chamber door is closed while the melt is injected into the mold. The machine has a safety switch that will not allow it to be run unless the door is closed. 11. The machine in semi-automatic and automatic modes is operated by changing the mode of operation from the main control panel. 12. The polymer weight for injection is changed to see its influence on the quality of the product.

 Purging the Machine 1. The purging polymer pellets is fed into the hopper. 2. The temperature limits is set at different zones on the battle from the main control unit. 3. The temperatures is waited at different zones to stabilize. 4. The nozzle is moved back from the mold by switching to manual mode of operation. 5. The desired amount of the purging polymer will be pushed out of the barrel to clean the barrel and nozzle. 6. The polymer chips struck to the injection mold and gates is cleared. 7. The barrel temperature is turned to zero. 8. The nozzle temperature is turned to zero. 9. The cooling system and hydraulic systems is turned off. 10. The key is turned to the off position (counter clockwise 90°).

CHAPTER 4

RESULTS & DISCUSSION

3.1 Results

6 Figure 3.1

3.2 Discussion

There are several injection molding defects which occur in the product. The defects are short molded, flow marks, silver streaking, poor weld line, voids, sink marks and warpage.

The product will be short molded when the mold cavity is not filled completely. This is due to poor material flowability, poor gas venting and insufficient injection molding machine performance (shot capacity and plasticizing capacity). Next, the flow marks is the ring shaped miniature bands appear on the molded product surface around gates or narrow sections. This is because of the molten polymer cools rapidly within the mold and becomes highly viscous. Once that molten polymer starts to coagulate, it gets pushed by the molten polymer injected afterward and forms miniature bands. Then, silver streaking is streaks of a silvery white color appear in the material flow direction. The causes of silver streaking are the material is absorbing water, the material is contaminated with another type of material, aeration is occuring during the molding and too much additive is present.

Poor weld line is the material splits off and flows in two or more directions. When the divergent flow fronts converge again, hair-like lines appear where they meet. This is due to polymer temperature is too low, the polymer is not fully melted and the difficulties in venting gas and air. There is also voids which are holes inside the molded products. This will happened when there is change in density as the material changes from a molten to a coagulated state. The surface of the molded product loses heat rapidly through the mold wall and the polymer hardens. As a result, the polymer is pulled outward, causing all shrinkage to

7 concentrate in the thickest parts of the product leaving holes within the product. In molded products with a thickness greater than 6 mm, voids are very difficult to avoid. Air gets trapped within the molten polymer, leaving bubbles within the product is also the cause of voids.

Sink marks is the surface of the molded product collapses somewhat-something that is particularly common in thicker products. The causes of sink marks is same as the causes of voids, but sink marks arise when the surface cools too slowly. The warpage defect is the molded product warps immediately following or at some time following molding where the mold cavity should be straight. This is due to residual stress from the molding process is released, causing deformation.

There are four parameters that must be established and controlled in order to achieve proper injection molding of a plastic part which are pressure, temperature, distance and time. the pressure and temperature areas are the ones most commonly considered during the troubleshooting process. Based on the requirements of any particular plastic material, the pressure must be sufficient to inject the plastic material and to hold the mold closed. In addition, the temperature of the injected plastic and mold must be correctly maintained. There are four types of pressure which are backpressure, injection pressure, holding pressure and clamp pressure.

The backpressure is pressure that is created during the return action of the screw after injecting material. This pressure is used for better mixing of the plastic (especially if colors are added at the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Next is the holding pressure. Once the majority of the plastic (95%) has been injected using standard injection pressure, the machine should drop into hold pressure. This pressure is about half of the injection pressure and is used to finish filling the mold by packing the molecules together in an orderly fashion. Hold pressure is required until the gate freezes off, normally in 3 to 4 seconds. At the other end of the machine, we have clamp pressure. The only reason to have clamp pressure is to keep the mold closed against injection pressure. Therefore, the amount of clamp pressure required is based on the material being molded. The easier flow materials require less injection pressure, thus they require less clamp pressure. Conversely, the stiffer flow materials will require more injection pressure, thus more clamp pressure.

8 Heat is used to soften the plastic to the point of being able to inject it, but heat is also found in the mold and in the heat exchanger of the machine. There are four zones of heat that must be controlled in the injection unit. They are the rear, center, front, and nozzle zones. Each is controlled independently of the others (refer Figure 3.2).

Figure 3.2 : Injection Unit Heat Zones

CHAPTER 5

CONCLUSION

9 4.1 Conclusion

As conclusion, the students are able to understand the process of injection molding. The students also are able to know the function of different parts in injection molding machine.

REFERENCES

1. Kauffer, P. (2011). Injection molding process, design, and applications. New York: Nova Science.

10 2. Injection Molding. (n.d.). SpringerReference. 3. Really Fast Custom Parts. (n.d.). Retrieved November 7, 2015. 4. InjectionMolding.com - The latest injection molding news and information on startups. (n.d.). Retrieved November 7, 2015. 5. Tobin, W. (1989). Basic injection molding (2nd (rev.) ed.). Los Angeles, CA: T/C Press, Division of Technology Conferences. 6. InjectionMolding.com - The latest injection molding news and information on startups. (n.d.). Retrieved April 26, 2016, from http://www.injectionmolding.com/