10

OVERVIEW Calendering is essentially extruding a plastic material between successive pairs of corotating, parallel rolls to form a film or sheet. However, most film and sheet are prepared by extrusion techniques (Chapters 8 and 9). These different techniques can provide different: (1) properties, to meet various product performance requirements; (2) processing performances; (3) delivery schedules; and (4) costs. Thermal properties, molecular char• acteristics, and degree of crystallinity of the plastics are factors which affect the processability of plastics. Additives can have a major influence on processability (Chapter 3) As a method of film and sheet manufacture, calendering is widely used. Like extrusion, design and construction of the calendering equipment has become increasingly sophisticated and engineered. The progress made, as with extrusion, has been largely empirical and in the direction of better and faster big machines. Understanding the process and the effect of different processing variables also continues to be studied theoretically. Calendering was developed over a century ago to produce natural rubber products. With the development of TPs, multi-million dollar, ex• tremely heavy calender lines started using TPs and now process mainly TP materials. Another major product continues to be elastomeric tire• fabric coating. Calendering is a highly developed art and theory, rather recently eluci• dated by combining the complexity of melt behavior with the mechanics of screw rotating machines (Chapters 2-17). With this understanding comes the ability: (1) to make calenders more productive by increasing their speed; (2) to produce films and sheets with tighter thickness tolerances and greater uniformity; and (3) to handle thicker sheets more effectively.

D. V. Rosato, Extruding Plastics © Chapman & Hall 1998 Overview 419 The calender consists essentially of a system of three or more large diameter heated rolls whose function is to convert high viscosity plastic melt into film or sheet. The equipment can be arranged in a number of ways with different combinations available to meet different product requirements. Typical line set-ups are shown in Figs. 10.1 and 10.2. The preparation of the material or compound is important to successful calendering. It is usually done by computer controlled electronic weigh• ing scales that supply precise amounts of each ingredient to a high inten• sity mixer. It is designed to incorporate any liquids into the plastic particles and to secure uniform distribution of all powdered ingredients (Chapter 17). Blending is generally done for a specific time and to a specific tempera• ture. The still-dry, free-flowing blend is then charged to a feed hopper where it is screw fed into a continuous mixer, such as an extruder and/ or kneader [4]. Under the action of a mixer's reciprocating screw in the confined volume of the mixer chamber, the blend begins to flux or masti• cate into the required plastic state. Usually the next step is to force the plastic out of the of the mixing chamber through a die producing strands. The strands can exit as a continuous rope or be chopped into small baseball size buns. This hot plastic material may be passed through a two-roll mill and/or be directly conveyed to the top of the calender. Preparation of stock material for calendering, conditions on the calender, take-off thickness measurements, windup system, and line

."...,,"'"=~, Oven temperatures =---= A;r flows ------Damper pOSitions z=====, Lower explosive l' ----- Roll gaps ----======0;& :=== Sheet temperatures _____ Center bendmg ----:<:====::::l:!:I...... ,==J~ _ ____ Roll rati os Roll speeds ----- Air pressure

t=~~:;:::;:;;:;:;;;;::;;;;;:~~~, ViSCOSi Y =_ _ __ ==-- % Solids NiP pressure ------.....--"------Blade pressure ---

Figure 10.1 Simplified schematic of calendering. 420 Calendering

Blenders

Feed system Calender Power feeder I Stripper Kneader Winder Conveyor r;:;;:-I - • •

o

Ex~~?er o / _ " '. I / IO~· + + • ~~~ _o::-tI~W--IM'-- n~ Metal \ + '~ Banbury Mill detector '..,../ mixer Windup Thickness Cooling drums gauge

Figure 10.2 Calendering lines. control must all be adapted to the plastic compounded system being processed. Other considerations include whether: (1) only the plastic is used; (2) what finish is required, i.e., glossy, semi-matte, or matte; (3) plastic is laminated to a fabric; (4) web is embossed; (5) web is slit in line; (6) very low strain recovery is in the film or sheet; (7) the product needs special properties, such as optical clarity and mono or biaxial orientation; and (8) others. General processing considerations include temperature and pressure controls, calender line speed, dimensions of thickness and width, surface finish, and orientation.

Calendering or extrusion Film and sheet can, in principle, be made by calendering or by extrusion. Factors that govern the advantages and disadvantages of each process can interact in a complex way. Factors to be considered include: (1) type of material to be processed; (2) quantity of product to be produced; (3) thickness required on film or sheet; and (4) costs. The capital equipment Table 10.1 Guide for producing PVC film and sheet by different fabricating processes

Extruder- Blown Flex-lip Plastisol Calender calender film extruder cast Melt roll

Lines installed, USA 155 2 90 40 60 5 Relative cost lowest low higher higher highest low Machine cost ($ million) 1-6 1-2.5 0.3-0.6 0.3-0.6 0.3-0.7 0.3-1.3 Rate and range (Ibh-I ) 800-8000 500-1500 600 (411zin) 750 W,'zin) 750 100-1000 Product gauge range (in) 0.002-0.050 0.002-0.005 0.001-0.003 0.001-0.125 0.001-0.012 0.0015-0.020 Sheet accuracy (%) 3 (1-5) 3 (1-5) 10 10 7 5 (2-10) Time to heat (h) 6 5 3 3 % 3 Time for 'on stream' 2-5 min 10 min 2h 5h 10 min 2-5 min Gauge adjust time seconds seconds 5-30 min 5-30 min seconds 1 min a Autogauging capability yes yes no no no no <::l l':) Color or product change time 5-30 min 10-40 min 30-60 min 30-60 min 15min 30-60 min ""'t <::l Windup speed (ydmin- I ): 80 (150) 60 (80) 15 (20) 15 (30) 20 (40) 20 (30) R). average (max.) El Limitations High capital Lower rate, Poor accuracy, long on Fumes, Reduced rate cost, heat versatility stream time, low rate, inefficiency, and range, time problem degradation, reduced high energy soft materials versatility cost, resin only slow cost, release manual cost gauge change Applications and advantages Versatility, . Accuracy, Low investment, Grain retention Good on wall high rate, gauge multiplant capability, (pattern cast covering, thin accuracy, adjust, thin gauge (0.003 in in), soft hand material, ease and, reduced and under) and heavy and drape coated fabric, adjustment cost gauge (O.05D-O.125in) accuracy,

ease at reduced oJ::. N reprocess investment ..... 422 Calendering and replacement parts in calendering lines are expensive. The very small unsophisticated lines start about the million-dollar range compared to the much lower cost extrusion lines. Table 10.1 provides a guide comparing different fabricating processes to produce PVC film and sheet. In general, plastic materials, such as PE, PP, and PS film and sheet, are usually produced through the rather conventional extrusion lines. To produce PVC film and sheet in large quantities, calendering is almost always used since the process is less likely to cause degradation than is extrusion as well as having dimensional and cost advantages. The review in this chapter principally concerns calendering PVc. A web thickness between 0.05-O.50mm (0.002-0.020 in) is generally the kind of plasticized film and sheeting produced by calender lines. For extremely light gauges, those under 0.02mm (0.001 in), calendering could become impractical or damaging to the equipment. The reasons include factors such as, for certain materials, there exists poor strength of the thin webs and also very high forces develop on the matting heavy duty rolls. For very heavy/thick gauges, such as sheeting over 0.50mm (0.020 in), calendering may not be the optimum method of production. The reason is that there may not be enough shearing action that can be put into the rolling banks to keep the compound at uniform temperature. In addition, the separating forces on the rolls become so low that gauge variations could become prohibitive. It can be said that basically the up-stream and down-stream procedures are similar in production lines whether calenders or extruders are used (Chapters 8 and 9). For a given quantity of output, it is usually necessary to have more extruders than calenders. This situation makes the extrusion lines more flexible and more able to handle short production runs. The extrusion flexibility, when compared to calendering, includes ease of changing product thicknesses, widths, and materials. Calenders are capable of higher production speeds. Thus, there are situations where they provide a favorable situation for long runs. For these long runs, cost advantages exist. Calenders are better at producing sheets and film that have tight thickness tolerances. Calendering also provides product uniformity. Constant in-process monitoring and continuous profile adjustments is usually a significant advantage of calendering over other methods.

CALENDERING OPERATION The calendering process prepares plastic materials or compounds. It melts the material and then passes the pastelike melt through the nips of two or more precision heated, counter rotating, speed controlled rolls into webs of specific thickness and width as shown in Figs. 10.1-10.3. The (usual) 0000..

~ Reverse feed Q True "L" Conventional inverted "L" Symbols: inverted "L" ~ ;:::: Positive ~ F~ --t ------./ Cal;"-der -:~ ---0 bender Positive and s- Oq negative bender -0 Cl ~ ~ Fume suction device --- Cross axis ~ Preload ~ Pick off Driving motor ...... Let off Rubber covered roll o· I and A ;:::: and laminating : stretch roll for polished section glazing film

Quartz heater for '-'~: : extra deep emboss ~_ I __ ~ L ____ _ Polish or ______Radiant quartz heater pattern roll to glaze back of sheet Embosser unit for flexibles mounted on wheels iI3w Figure 10.3 PVC calender operations. 424 Calendering parallel rolls have extremely flat surfaces and rotate at possibly the same speed but usually at slightly different speeds depending on the plastic being processed. Although plastic forming occurs in the calender itself, down-stream precision operating equipment is needed to produce the TP film or sheet. Up-stream of the calender, a mixer blends the raw materials, usually in powdered form with the desired additives (pasticizers, stabilizers, etc.) and fillers (calcium carbonate, etc.). This pastelike melt passes through a roll mill, such as the multiple roll mill. As an example, heavy gauge products of 0.6mm (0.02 in) leave the final calendering roll. In turn, the sheet is drawn down or stretched at take-off speeds consistent with good stripping from the final heated calender roll. Lighter gauge film at 0.18 mm (0.007 in) when leaving the last calender roll is typically drawn down at a ratio of 1.2/1 and wound up at a thickness of O.ISmm (0.006in). Calenders may consist of two to at least seven rolls. They are character• ized by the number of rolls and their arrangement. Examples of the layout of the rolls are the true L, conventional inverted L, reverse fed inverted L, I, Z, and so on. Figure 10.4 provides examples of a few of the configura• tions which are: (1) one-sided coating with vertical 3-roll; (2) one-sided coating with inverted-L using 4-roll; (3) double-sided coating with in• verted-L using 4-roll; (4) double-sided coating with two 3-rolls; (5) one• sided coating with Z-configuration using 4-roll; (6) one-sided coating with S-configuration using 4-roll; and (7) one-sided coating with modified-s using 4-roll.

Feed

Feed

(d) (a) (b) (e)

(e) If) (9)

Figure 10.4 Arrangements of parallel rolls for calender coating. Calendering operation 425 The most popular are the four-roll inverted L calender and the Z calender. The Z calenders have the advantage of lower heat loss in the film or sheet because of the melts shorter travel and the machines' simpler construction. They are simpler to construct because they need less com• pensation for roll bending. This compensation occurs because there are no more than two rolls in any vertical direction as opposed to three rolls in a four roll inverted L calender and so on. The speed of the calendering rolls is rarely the same. They operate at different speeds to provide the best performance of the melt, particularly the required shearing action. A typical relationship of speed settings on an inverted L as shown in Fig. 11.4(b) can be 100% for the top in-line vertical roll, roll on its right side at 0.85%, roll just below at 1.03%, and the bottom roll at 1.06% . Rigid PVC manufacturers usual prefer the L configuration with four to seven rolls being fed from the floor level. Since there is no disturbing vapors (condensing on rolls and web) from lower calender rolls within the pickoff area, it is preferable to have the pickoff rolls on an elevated level. Flexible PVC is commonly processed using a four roll inverted L or an F calender. These systems enable the plasticizer-saturated vapors to escape via the usual suction hood located above the calender, where they are filtered before being released to the atmosphere. A universal five roll L calender is used for rigid or flexible PVC film. It provides heat stability and superior film control with good surface appearance. The major difference between this universal machine and the others is in mounting and placement of the first roll. There are configurations where the first roll is generally mounted be• side the second roll; the more recent design still has the first roll mounted horizontally but now it is next to the third roll, resembling a four roll L calender. The second roll remains in action underneath the third roll, so four gaps are still available. Rolli film wrap length, however, is reduced so dwell time is 40% less, decreasing PVC heat loss and shrinkage. Modern designs for rigid and flexible PVC lines currently show few significant differences. Both actually consist of the same roll groups, but arranged at different elevation levels. Variations in these multi-million dollar calender lines are dictated by the very high forces exerted on the rolls to squeeze the plastic melt into thin film or sheet web constructions. High forces at least up to 41 MPa (6000psi) can bend or deflect the rolls, producing gauge variations, such as a web thicker in the middle than at the edges. During calendering, particularly film, roll separating forces in the final nip may be as high as 41MPa (6000psi). This potential problem can be counteracted by different methods that include the following: (1) crowned rolls, which have a greater diameter in 426 Calendering the middle than the edged; (2) crossing the rolls slightly (rather than having them truly parallel), thus increasing the nip opening at both ends of the roll; and (3) roll bending, where a bending moment is applied to the end of each roll by having a second bearing on each roll neck, which is then loaded by a hydraulic cylinder. Controls are used to perform any roll bending and crossing of the rolls. Calenders require high temperatures, with little variations or fluctua• tions across the rolls during the application of the high forces or pressures. During calendering thin PVC film, the melt stock temperature may be 190°C (375°F), compared to heavier gauge sheeting where 177°C (350°F) stock temperatures are usually required. For the most part, this difference is due to the higher calendering speeds, resulting in generating more frictional heat, required for economical production of thin gauge films. The high forces with the temperature controls are necessary to squeeze the plastic mass into relatively thin sheets but particularly very thin film. Any unevenness in the forces along the roll, that could include uneven temperature across the melt, is reflected as variations in the product thickness. Mechanically, one cause of pressure fluctuations is too much clearance in the bearings, which can be resolved either by pulling back the rolls against one side of the bearing or by using tapered roller bearings. Although limited by a number of simplified assumptions, for many years there has been a simplified approach for calculating the calender's rolls separating forces. The equation is: F = 2vVrw(l/ho -II H) where F = total separating force, MPa; v = viscosity of stock being calendered, Pa s; V = peripheral speed of rolls, m/ s; r = radius of rolls, m; w = width of web being calendered, m; ho = separation between rolls at the nip, mm; H = height of melt stock at the nip entrance, mm. Other causes of thickness changes across the calendered web include nonhomogeneous rheology of the stock (Chapter 3), problems with ma• terial's lubricity, problems with pressure and temperature sensors, equip• ment line control malfunctioning, use of damaged calender rolls, and so on. After forming through these multiple rolls of the calender, the film or sheet is cooled. The sheet or film immediately passes through precision surfaced cooling rolls that are kept at precisely controlled temperatures and/ or a cooling tower where the web can be festooned. The temperature of the rolls is gradually reduced as the plastic travels down-stream. There may be up to 19 cooling rolls following an embosser (if used). After leaving the last large diameter calendering heated rolls, the film is 'dropped' vertically into an embosser, usually with three rolls that are the Calendering operation 427 embossing roll itself, a cooling rubber roll, and a contact cooling to the rubber roll. Diameters of rolls could be 15-25cm (6-10in). These rolls are usually driven and temperature controlled in sections between two and five rolls. Temperature accuracy can be controlled within :!:l°C (:!:2°F). In line with these rolls is usually a thickness gauge (such as a beta gauge) controlling the film or sheet thickness. Thickness gauges are usually located within this cooling section of the production line. Continuous beta scanning equipment can be used that traverses the web constantly. This provides feedback to the upstream larger rolls that makes thickness corrections automatically (Chapter 6). Gauge controls can also be used in the neckdown section, where the hot melt web leaves the last calendering roll and the web is transferred to the first of the cooling rolls. The amount of neckdown, as also reviewed in Chapter 9, provides an important control in the operation of the entire production line. Proper use of all these controls, and that include speed and stretch in the takeoff train, allows the production of extremely flat sheet with a profile tolerance of :!:5% across and down the web. The controls can call for adjustments on different line equipment, such as the nip openings, skew, roll bending, and so on. After cooling, the plastic is trimmed at the edges and wound. Trim material can account for up to 5% of the width depending on the line's operating efficiency. Of course, the target is to have as little trim as possible. The trim is usually immediately directed back through a granulator and blended with virgin material (Chapter 3). When slitting the webs, typical slit widths are made to the nearest O.8mm (O.03in) on 7.6 or 15.2cm (3 or 6in) cores with roll diameters of 36-102cm (14- 40in). We have reviewed the typical sequence for a calender line. However, there are many variations to suit the end product and reduce costs. Aux• iliary equipment can be included, such as stretching in the machine direc• tion by using the cooling rolls or setup bioriented stretching, water quench tank, annealing, decorating, slitting, heat sealing, festooning, and so on (Chapters 2 and 7-9).

Surface finishing Calenders provide styling on films and sheets. PVC surface calendered goods require depths of matte or provide an even roughness, which can be measured with a device such as a profilometer. Different markets use different terms to describe their finishes, such as satin, Sheffield, and light matte. After being processed, a one-sided matte finish may be applied 428 Calendering with just an embossing roll. A sharper, deeper, and more precise matte finish can be applied to film or sheet by the calender. The principle used in embossing PVC film or sheet is to deliver a relatively hot web to a relatively cold embossing roll. This action imparts the desired surface finish to the web and freezes it by rapidly cooling the embossment pattern. Film temperature, roll temperature, and pressure are optimized empirically for each compound formulation and pattern to be embossed. The embossing roll and the rubber pickup roll are internally cooled by a liquid medium; both of them require a very uniform surface temperature to develop the desired embossing pattern. But the rubber pickup roll tends to stick to a hot PVC web unless it is cooled externally as well as internally. External cooling in a wet trough is re• quired because most rubbers have poor heat transfer properties, in fact, so poor that a nonexternally cooled backup roll surface temperature would rise almost to the processing temperature of the PVC web. Quick roll changing techniques are used for the last two calender rolls to provide the capability of changing rolls. Changes from a high gloss to matte, or to different profiles, are usually made in about 10 h or less; in the past, it took at least 80 h. Embosser rolls can be changed in minutes instead of hours. To make two-sided matte products, such as two-sided credit , the last two calender rolls have matte surface finish. To produce these surfaces, the rolls are usually sandblasted with aluminum oxide grit of controlled particle size such as 120-180 mesh. For the heavier matte, coarser grit and more passes of the sandblasting action on the rolls are required. Proper selection of metal for the calender rolls is also necessary; certain metals are not suitable for producing these decorative, etc., surface finishes.

Credit cards This is, as we all know, big business. All kinds of data are provided through cards, and in the USA usage could be well over 40 billion cards. In USA, over 300 million are used for health cards alone, and that should at least double very quickly. Reports indicate that Germany has over 80000 health cards. China, Southwest Asia, Mexico, and parts of Europe show strong growth potential for many types of cards. About 46% of the card-making capacity is in the USA. What makes this interesting to the calendered sheet industry is that practically all of these cards are calendered PVC sheets. The sheets that are produced need to be very carefully processed and handled. Card manufacturers are not plastic processors, but rather plastic print• ing and specialists. They apply the features, including the security aspects, to sheets of plastic core stock that is usually rigid PVc. Calendering operation 429 Most are die cut from sheets 50-58 cm (20-23 in) wide by 66-76 cm (26- 30 in) long. Secure card manufacturers generally produce their cards in one of two ways: (1) by applying a 0.045-O.052mm (1.8-2.0 mil) thick PVC film laminated to both sides of a 0.66-0.67mm (26.0-26.5mil) usually homopolymer sheets; and (2) by using the split core method where two 10.34 mm (3.5 mil) thick coploymer sheets are joined through a heat pres• sure lamination process, followed with a film overlay thinner than that used in the solid core. Thickness control is important, otherwise problems will develop with card embossing equipment and the magnetic stripe reading machines. Other considerations that influence calendering sheet include ink adhe• sion, flex capabilities, cleanliness, and embossed-characteristic height retention. Secure cards tend to made from PVC copolymers, while the others with less stringent performance demands often use homopolymers. The life expectancy of cards is about three years. New plastic compounds continue to be evaluated to extend life with cost reductions. The global card market for 1994 consumption was estimated at above 26 million kg (57millionlb) of plasticS with over 90% was PVc. The USA and Canada consumed about 12 million kg (26.5millionlb). While there are over 50 fabricators of rigid calendered PVC sheet worldwide produc• ing about 1.3 billion kg (2.75billionlb) of products for all markets (cards, refrigerator door liners, etc.), fewer than ten can supply sheets meeting quality levels required in the card market. The card manufacturing business has relatively low-technical require• ments. While the industry is capital intensive, the barriers to enter into the market historically have been low. The result has been intense competi• tion. New minor developments have continued to occur since the use of cards started expanding during the early 1960s, such as unsuccessful replacement of ABS. Most recently, an important new development called 'smart cards' occurred that is targeted to replace conventional magnetic• strip cards. A variety of semiconductor computer chips can be embedded between two preprinted ABS labels by injecting ABS between the labels to form 0.8 mm (0.03in) thick cards. These cards are already popular in Europe in various applications. Numerous firms compete globally in the production of smart cards.

Roll covering . The application of natural (natural rubbers) and synthetic (plastics) elastomers to the surface of mandrels is called roll covering. Covered rolls are used in a variety of industries, such as extrusion, , pa• per, , and many more. Industrial requirements for covering rolls are forever changing. Improving quality through technology has been 430 Calendering necessary since the first roller was manufactured. Much of the technology has been directed towards improving compounding materials and pro• cesses. Many of the earliest manufacturing methods are still used today. Methods tend to be labor intense and dependent on individual skills. The diversity of the industry has slowed the evolution toward automa• tion since many roll covering companies may work with over 200 differ• ent formulations and limited productions exist. Different methods are used in this versatile industry. The traditional method is based on cutting to size calendered 1.5-3.2mm (0.06-0.125in) thick sheets. Layers of the sheet are wrapped around a mandrel with numerous splices. Strips of the calendered sheet are also used by rolling spirally. These methods gener• ally produced good quality but have many negative features, such as poor adaptability to automation, high material costs, etc. Other methods include using precured tubing (customer purchases for their own installation), mUlti-pass tape overlap (at different angles includ• ing different overlap methods), and crosshead method (similar to extru• sion profile covering mandrel). The extrusion crosshead technique has become a popular method for roll covering (Chapter 11).

PLASTIC MATERIALS A wide variety of TPs can be used with the majority being PVC, ABS, PE, PP, and PS. However over 80% are flexible and rigid PVCs. When calendering other plastics, there is a tradeoff in economy and quality. The basic limitation of the calendering process is the need to have sufficiently broad melt index to allow as wide a heat range as possible for the process. This behavior of the plastic permits the material to have a relatively high melt viscosity in the banks of the calender rolls; banks indicate where two rolls meet, or the nip of the rolls. As a result of the viscosity, a shear effect can be developed throughout the process. This shear is of prime importance between the calender rolls. Thus, the calender forms the web as a continuous 'extrusion' between the rolls. Unlike when processing just through a conventional extrusion line, the plastic mass cannot be confined when being calendered. Because of the lack of confinement, the shear effect and a broad melt band are essential aspects of calendering. The blending or compounding (Chapter 17) of the plastic with different additives and fillers is a critical part of the process. The blending must produce a uniformly colored and stabilized product in an approximate powder form. After blending, the rate of consumption during calendering dictates the temperature of the melt. The PVC compounds require stabilizer systems. There is an art (based on experience) in preparing these additive systems so they are compatible with the compound. Information on the different ingredients needed to Plastic materials 431 meet different product requirements (that do not interfere with color, etc.) is readily available from the PVC manufacturers and/or the literature. The ideal heat stabilizer system imparts during processing primarily heat stability, as well as adequate lubricating characteristics to reduce or con• trol frictional heat. These stabilizer systems are also very efficient for plate-out resistance. Plate-out is a condition where the calender rolls and/ or embossing rolls become coated with a deposit from the compound being processed. This deposit may start out as a soft, waxy material barely visible on the metallic contact surfaces of the processing equipment. During long runs of over a few 300m (1000fO, both the consistency and quality of the plate-out can become prohibitive and interfere with acceptable surface finish of the film or sheet. When plate-out occurs, the line has to be shut down and the contamina• tion removed. If the contamination is soft, generally all it needs is a wiping. When it is as hard and has an excellent adhesive bond to the steel, some type of mechanical abrasive or wire brush is used (carefully so that will not damage the roll surface). Because the plastic is processed between the reqUired heat and its critical heat of degradation, the time of heat becomes extremely critical and an important part of the complete process. For example, the processor will minimize the amount of melt in the bank (nip) of the rolls. The residence time of the plastic flux at high heat must be limited. As reviewed throughout this book, PVC is especially sensitivity to heat and time at heat. This situation is not a problem if the machine controls are properly set and operated within set conditions. The plastic mix fed to the calender may be a simple hot melt, as with PE, PP, etc. However, for PVC it is obtained by premixing the vinyl plastic, stabilizers, plasticizers, calcium carbonate, etc., in ribbon blenders. From the blenders the blend is passed usually through a Banbury mixer where the mass is gelled for a specific time and temperature period depending on the mix. A typical period for gelling is about 5-10min at 120-160°C (248-320°F). The gelled 'lumps' are made into a rough web on a two-roll mill and the web is fed to the calender rolls. Another approach is to have the gelled material go through an extruder/compounder to produce a rope when exiting the machine. This rope about 50-100mm (2-4 in) in diameter is fed to the calender rolls. It is directed over a conveyor belt to the nip of the top rolls. Rather than just being depOsited in the center of the nip, a movable 'robot' arm moves along and from side to side feeding the rope from almost one end of the roll to the other end. This method of delivering the rope provide for a more even distribution of the plastic. Ingredients for PVC film and sheet can include solid and liquid raw materials. Solids are weighed in batches and dropped into a high-speed 432 Calendering

'LINER Figure 10.5 Schematic of calendering with a fabric using a support liner. mixer. Liquids are metered into the mixer, usually by piston pumps or the volumetric metering devices. A batch mixing system is generally in-line with a continuous fluxing system that starts at a holding bin. Some opera• tions do not use batch mixers for blending. Continuous metering systems, using loss-in-weight feeders and pumps, will meter the mix directly into the continuous fluxing mixer. If fabric, paper, or other material is feed through the rolls, plastic can be pressed into the surface of the calendered web (Fig. 10.5). When operating the calendering line in this mode, it becomes a coating machine.

Fluxing and feeding Fluxing is the preparation of a plastic composition to improve melt flow; fusion is the heating of the vinyl compound to produce a homogeneous mixture. Fluxing units used in calendering lines for preparing TP ma• terials include batch-type Banbury mixers, Farrel continuous mixers (FCMs), Buss Ko-Kneaders (BKKs), and planetary gear extruders (PCEs). The dry blend is fed into the mixer / extruder. Excellent mixing within a short dwell time and heat transfer control contribute to an improved product. During fluxing, each particle receives the same 'gentle' treat• ment, generating less heat history and producing more uniform feed rate, color, gauge thickness, web surface, and so on. The feed can discharge onto a two-roll mill. Operating this way, it provides for a second fluxing action, mainly for working in scrap or for convenience as a buffer. The stock delivered to the first calender nip needs to be well fused, homogeneous in composition, and relatively uniform in temperature. The Plastic materials 433

Table 10.2 Example of flexible PVC calendering conditions

Heavy-gauge product Light-gauge product

Roll temp. Roll speed Roll temp. Roll speed Roll no. OF "C ftmin-l mmin-1 of "C ftmin-1 mmin-I

1 347 175 125 38 338 170 263 80 2 352 178 128 39 343 173 269 82 3 353 181 138 42 349 176 279 85 4 363 184 148 45 354 179 289 88 optimum average temperature for good fusion depends on the formula• tion. A rigid PVC formulation based on medium molecular weight plastic (intrinsic viscosity of 0.90-1.15) (Chapter 3) has a typical optimum tem• perature of 180-190°C (355-375°P) at the first calender nip. Por best calendering, there should be no cold volume elements below 175°C (345°P) and no hot spots above 195°C (385°P). These typical temperature readings are necessary for close control of fusion and mixing conditions; they are based on industry studies of flow effects and their relation with volume and element size. This interaction depends on stock temperature and in turn on the performance of PVC melts. Flexible PVC is normally calendered at temperatures of 10-20°C (50- 68°F) lower than rigid PVc. Typical calendering conditions for unsup• ported flexible PVC film and sheet are shown in Table 10.2. In flexible PVC production, a short single screw extruder, acting as a strainer, filters out contaminants from stock before reaching the calender. This method is not applicable to rigid PVC because it drastically increases the head pressure and the consequent overheating would cause the stock to decompose.

Heat sensitivity The target for the calender is to provide sufficient energy to convert the mass of plastic into film or sheet form without supplying so much heat as to cause degradation. This is a very important consideration particularly when processing rigid PVc. PVC is very temperature sensitive. As an example, for the manufacture of thin gauge PVC films, very high calendering speeds [over 90mm/min (300ft/min)] are encountered. Fric• tional heat build-up not only affects the compound heat stability in terms of initial color and properties, but it also affects stock transfer from one calender roll to the next. This action can be caused by loss of the lubricant additives at these high temperatures, indicating the need for a lubricant of lower volatility 434 Calendering This situation with PVC has always existed since it became commer• cially available almost a century ago. However, it is the fourth most consumed plastic material in the world using all the different processes (extrusion, injection, coating, etc.). For those that use it correctly, process• ing is conducted so that degradation is not a problem. Those that do not process it with care have problems. The energy required to turn the calender rolls is proportional to their diameter. The frictional heat developed which is transferred to the ma• terial is also dependent upon the roll diameter. To control this heating action, the calender operation and control is vital to the successful produc• tion of film or sheet.

Contamination Very vital to this operation is the 'complete' removal of any metal or hard surface material. This includes microscopic particles. From the start to the end of the calendering process, extreme care has to be taken to ensure there is no contamination of the equipment or plastic being processed. Preventative maintenance of these lines is a continuous operation that includes the operating environments. A relatively clean room has to exist. This cleanliness is required to ensure that damage to the expensive equipment does not occur, up-stream or down-stream. A micron size piece of metal will destroy the rolls, etc. Replacing these very expensive rolls is expensive and time consuming. Where this type of equipment may not be in the spare parts room, delay in refurbishing or replacing parts is very time consuming. Special treatments of multiple filtering and screening all raw materials are required. Incoming materials that include all additives and fillers are inspected and/ or filtered. Some of the equipment will use duplicate and triplicate screens. The inspection and filtering is made to eliminate both metallic and nonmetallic contaminants. Magnetic sensors/detectors are positioned practically anywhere the plastic material moves. This includes locating detectors under the con• veyor belts that directs the web or rope up to the rolls. This action is taken even though there was a very thorough inspection of raw materials on arrival at the plant and through mixers, blenders, etc., prior to reaching the conveyor belts. Thus, action is taken even though the materials had what one may consider repeated 100% foolproof inspections every step of the way.

Recycling Handling PVC scrap and cold trim from the product line can pose a very difficult problem if they are not handled correctly. The scrap and trim Processing optimization 435 could represent 1O-40wt% of a mix with virgin material in certain runs where recycled plastic can be used. The actual amount depends on the width of the calender in relation to the web width and properties obtained. The flux rate and energy required to remelt the scrap are considerably less than required to flux the virgin plastic. With this differential, there is the potential danger of material decomposition whenever scrap is pro• cessed to start up a mix. Optimum uniformity and product standardiza• tion requires the proper blend of new and old materials. Reprocessed material is best added to the blender where a standard can be established, although some plants feed it directly to the fluxing equip• ment. Careful control of the scrap percentage in the total mix is essential to obtain quality controlled products; control should be somewhat better than that required in other processes. Extreme care is required when handling and/ or transporting the plastic to be recycled to ensure that it is not contaminated.

ORIENTATION Orientation is applied to the film and sheet. The information already provided in Chapter 2 describes why orientation is used and how to process film or sheet with mono- or by-oriented properties.

PROCESS OPTIMIZATION The structure of calender rolls, supports, and other parts have been struc• turally designed to be extremely sturdy in order for the line to operate under high pressures in addition to the weight of the rolls and other parts. Individual motor drives for the rolls are used and required to provide sophisticated means of controlling line speed.

Roll Steel rolls used in the line are very heavy and of intricate design. As reviewed in Chapter 9, these heavy rolls have internal designs to ensure the proper flow of liquid and even surface heating on the roll's surface. Critical temperature control of the massive rolls is required to meet prod• uct performance requirements. The design of the rolls themselves is of great importance, particularly the ratio of length to diameter. The market requirements usually set the product width and roll length. Therefore calender sizes have generally been based on the required separating force needed between rolls. In theory, heavy gauge, high plasticized materials require extensive work in order to maintain frictional heat and uniformity of the bank (nip). To meet 436 Calendering this condition, the material should be processed through the larger diameter rolls. However, in practice the separating forces of these type materials are considerably lower and frequently are run through smaller diameter rolls. With thin gauge,low plasticized materials, the reverse is true in that the frictional heat developed in the bank (nip) is greater on large diameter rolls. Therefore, these materials should be run on calenders with the smallest possible roll diameters. However, there is a 'happy medium' that has to be met. These heavy loads develop high separating forces and considerable gauge changes can occur across the width of the film. Smaller diameter rolls are used in regions where it is desired to reduce the frictional heat of the melt. These rolls require a different method of roll adjustments. Regardless of the size of rolls, the uniformity of the product will be affected by slight changes in shape in the roll as the equipment starts its heating process. The concentricity of the rolls when operating the line at the required temperatures during processing is very important. This is not necessarily the same as ensuring that the rolls are concentric at room temperature. In fact, with the larger calender rolls and lines, it is the custom to allow the equipment to run for two to three days. This heating period is actually the beginning of the startup prior to actual production. It is required in order to have all the 'metal' parts reach equilibrium. Once the line is up to equilibrium temperature, this expensive preheat period requires a long production run. Any deviation in concentricity usually is kept below half the tolerance of the film or sheet to be produced. Some of these products require that deviations be as small as 1/20th to 1/50th of the product tolerance re• quired. As reviewed in the Chapters 8 and 9, roll surface finishes and upkeep is of considerable importance to ensure good quality products. Roll mountings are another potential source of error. This problem develops due to the possibility of alteration in nip pressure and roll alignment. Also with poor mountings, heavy oscillations can develop. The use of auxiliary rolls before the actual calender rolls has been found to improve the stability of the system. The methods used in altering the nip arrangement are usually by roll bending or roll crossing. Less power is required to alter the thickness across the web by roll crossing than by actual roll bending. Although each method finds its application in appropriate circumstances, in general it has been found that the nip between the second and third rolls of the calender assembly has the greatest influence on the quality of the product. Rolls under load suffer a tendency to separate despite their weight mass. The thinner the gauge and the lower the plasticizer content of the Applications 437

PVC material, the higher the separating force. This type of action also causes a greater amount of roll deflection. By using special grit-blasting techniques, the third and fourth calenders (last two rolls) may be custom-surfaced to generate a uniform two-sided matte product. Alternately, one or more down-stream embossing stations can be used to produce a custom surface on one or both sides of film. When processing certain plastics continuously around the clock, a satis• factory matte usually will last about five days. It will require down-time of the line to refurbish the roll(s). After separating from the last calendering roll, antistatic, slip agents, and other units may be applied to the surface(s) of the web.

CONTROL Many control features in calenders provide versatility, better quality, and higher operating rates. Complete control of friction ratios gives good tracking of stock via individual drives on all calender rolls. Close toler• ances on film profiles are obtained from axis crossing, roll straightening, and gap profiling of rolls. Better control over stock temperature is achieved by using internal high rates of circulating liquid flow lines/ compartments within the rolls to obtain the roll surface temperature control. Surface temperature can be held steady to within at least 2°C (3-4°F). In older calenders, severe cases of roll float caused by changes in the balance of forces on a roll were sometimes encountered. Preloading de• vices reduce roll floating and thus provide a better gap control between pairs of rolls, especially during startup. Web thickness can be measured with improved feedback control.

APPLICATIONS Different plasticS are formed into support and unsupported webs by calendering. These include PVC, ABS, PUR, SBR, NR (natural rubber), and others. Included are PVC and its copolymers with and without blends of ABS (Chapter 3). Applications are many, including packaging, building and construction (tiles, window shade, etc.), tire fabric coating, swimming pool liners, electrical tape, and so on. One special and important application is the coating of paper, woven and nonwoven , plastic films and sheets, credit cards, roll cover• ings, and other . A calender with three rolls is usually sufficient for one-sided coating, but four rolls are used for extremely thin coatings. Double-sided coating can be applied simultaneously on both sides of substrates, such as fabric, using a four roll calender or sequentially by two 438 Calendering three roll calenders. Specialized calendering equipment is used for certain products, such as floor tile and window curtains.

SAFETY The usual health and safety procedures in plastic operating plants have to be followed. Most of the hazards are associated with the high speeds at which webs normally travel, and the large forces involved in calendering, and/ or the high temperatures. Safety devices are located where points of danger exist. An example is locating a double-action interlocked safety at the nip (bite). As shown in View A-A of Fig. 10.6, a recommended location of the device is shown. The dimensions shown are guides for maximum allow• able for safe operation. Distance X will vary in accordance with the roll diameter, but always be great enough to ensure that no part of the opera• tor's hand can reach the bite without tripping the bar. The full calender view shows the safety bar in position at the feed bite, and also illustrates the recommended location for safety trip cables which are required in addition to the interlocked bite bar. The letters in Fig. 10.6 identify: B, safety trip cable switch; C, vertical safety trip cable; 0, adjust• able clamp for horizontal safety trip bar; E, horizontal safety trip bar; F, double action electrically interlocked bite bar; J, safety trip cable screw eye tension adjustment; K, safety trip cable guide pulley; and L, calender control switch. This device can be used but actually the safest way to feed material through the rolls is to use a strip or rope .of plastic, or the remains from the last run. The material wound around all the rolls, from start to the end, would be in the open position. Material would logically go through the line with no hands involved. Once the new material starts passing through and/or around the rolls, space between rolls is gradually reduced and adjusted to the operating gap spacings. All this involves no hands near the nip or, actually, the use of a bite safety bar.

COSTING The capital cost for a new calendering line will be about $4-5 million, not including building cost and utility services. The sum of fixed costs and the variable costs for operating a new line in USA can amount to $450- 550/h. Taking PVC as an example, these lines have a throughput of 400- 2700kg/h (90-6000Ib/h). And the average range of compounding costs for calendering PVC is about $0.49-1.12/kg ($1.08-2.47 lIb). Not including material costs, the expense of operating a large calender line may be $350-560/h, depending on degree of automation, investment, labor, power costs, and overheads. For PVC sheet 2m (7ft) wide and

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10.6 10.6 Figure Figure 440 Calendering 0.50mm (0.020 in) thick traveling at 27.4m/min (91 ft/min) and wound up at a rate of about 489kg/h (l076Ib/h), the conversion cost is about $0.21/ kg ($0.46/lb). These costs are based on PVC plastic at $0.7/kg ($1.70/lb) and plasticizer at $1.10/kg ($2.42/lb). Table 11.2 provides a general manufacturing comparison of calendering with combination extruder / calender, extrusion blown film, extruder with (flexible-lip die) flat cast film, plastisol-cast, and melt roll. To reduce the cost of products, the logical approach is to increase the line's speed. However, there are numerous factors which limit calender speed. At a given speed, more frictional heat is generated by the more highly filled and rigid stocks. Excessive speed can produce high tempera• ture melt with degradation or sticking of webs to the rolls where stock temperature is limited. Optimal speeds of the final calender roll for double polished, clear melt flexible PVC viscosities at a given temperature degrade at about the same maximum temperatures as the rigid stocks. Another target is to reduce the edge trim if possible.

TROUBLESHOOTING This chapter has reviewed all kinds of problems with information on causes and their solutions. Collectively, they provide different types of troubleshooting guides. With most problems comes unexpected expenses. As an example, calendering problems that will affect product costs in• clude: (1) bank marks and shiny patches on the surface; (2) cold marks (crow's feet) caused by the stock being to cold; (3) blistering due to high temperature or a large bank at the roll's nip; (4) pinholes caused by foreign matter or by the presence of unplasticized plastic particles; and (5) water• marking usually due to lubrication contamination. Bank marks are minimized by optimizing formulations, calendering speeds, and roll temperatures so as to obtain the most orderly behavior of the roll banks of stock at the calender's nip entrances. Proper use of drawdown permits windups to be run substantially faster than the final calender roll on many thin, unsupported film products. Calenders and take-offs are run almost synchronously on heavy gauge products. Films and sheets with a high gloss taken off a highly polished final calender roll tend to stick to the roll more than their matte counterparts. Very soft webs also tend to stick to the final calender roll. The fastest calender speeds are generally obtained in a median thickness range.