CALENDERING Introduction This process is used to convert thermoplastic materials into continuous sheets, films, and for applying plastic coatings to textiles, paper, or other supporting material. When coating the calendering line is also called a coating machine. Calendering is an alternative to extrusion with the usual film at three or more mils (75 microns) thick (Chapter 5). For the production of sheet or film plastic melt is compounded and pressed as it passes through the nips of a series of three or more heated highly polished steel rolls. A plastic bank is formed into a web in the nip between the first pair of rolls. Passing through the second and third nips further reduces the thickness. Final thickness of the sheet is determined by the gap between the last pair of rolls called the gauging rolls. Finally, a take-off roll pulls the hot sheet around a chilled roll to cool the sheet or film web (Figure 9.1). In this industry bank is identified as the quantity of plastic present in the nip formed between two rolls (Figure 9.1). [Bank marks are surface roughness on sheet caused by incorrect temperature or sizes of banks. They can be minimized by optimizing formulations, calendering speeds, and roll temperatures so as to obtain the most orderly behavior of the rolling banks of stock at the calender-nip entrances.] Calendering converts plastic into a melt and then passes the pastclike melt through roll nips of a series of heated and corotating speed- controlled rolls into wcbs of specific thickness and width. The web may be polished or embossed, either rigid or flcxiblc. Proper calendering rcquircs precise control of the complete roll tcmpcraturcs, pressures, and specd of rotations. An cmbosscd design can be produced on the surface by using an engraved roll, calendering a mixture of granular 370 Plastic Product Material and Process Selection Handbook Figure 9.I Example of the sheet or film passing through nip rolls to decrease thickness plastic chips of varying color may produce unusual decorative effects such as marblization, and so forth. Calendering often processes vinyl plastics. The complete equipment usually consists of a mixer such as a Banbury mixer followed by the heated rolls, chilled rolls, and finally a windup roll. 3 The windup roll controls the tension on the film or sheeting as it moves through the calender rolls. Calenders arc generally designed to meet the specific needs of the customer. Once installed and operating continuously, the cost per pound of film or sheet is lower than by any other process such as extrusion. The capital cost for a calendering line will average at least $10 million. A line, probably the largest in the world processing PVC sheet was build by Kleinewefers Kunststoffanlagen GmbH, Munich, Germany. Cost for this 5 roll L-type configuration was $33 million (1999). It has 3,500 mm roll-face widths and 770 mm diameters with an output rate at 4,000 kg/h. Plastics that melt to a rather low viscosity are not suitable for calendering. Additives can have a major influence on processability. With this understanding comes the ability to make calenders more productive by increasing their speed. They also produce films and 9 9 Calendering 371 sheets with tighter thiclmess tolerances and improved uniformity and can handle thicker sheets more effectively. Equipment The purpose for the calender is to provide sufficient energy to convert a mass of plastic into film or sheet form without supplying so much heat as to cause degradation. This is a very important consideration parti- cularly when processing rigid PVC. Variations in these multi-million dollar calender lincs are dictated by the very high forces exerted on the rolls to compress the plastic melt into thin film or sheet web constructions. Important is the complete removal of any metal or hard surface material. This includes microscopic particles. As an example a micron size piece of metal or slight scratch will destroy the rolls, etc. Replacing these very expensive very heavy rolls is expensive. This type of equipment may not be in the storeroom. 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 environment in the plant to be a relatively clean room. Calenders vary in respect to the number of rolls and their arrange- ments. Examples of the layout of the rolls are the true L, conventional inverted L, revcrsc fed inverted L, 1, Z, and so on. These large diameter heated rolls have the function to convert the high viscosity plastic melt into film or sheet. Figures 9.2 and 9.3 provide examples of lines. Figure 9,2 Calender line starting with mixer In the early days of calendering plastics three-roll vertical rubber machines were used. Problems developed in processing plastics. They 372 Plastic Product Material and Process Selection Handbook Figure 9.3 Examples of the arrangements of rolls in a calender line included difficulty in feeding horizontal nip, gauge variations, temperature variations due to using cored rolls, no capability for cross- axis or roll bending adjustments, and roll floating due to pressure variations in the feed nips. As time passed these problems were continually reduced or eliminated particularly on the smaller calenders. The offset rolls were designed to eliminate the major difficulty of the horizontal feed nip. Because the material drops by gravity into the vertical pass, the offset feed nip provides important savings in manpower and yield. Mso, the pressure fluctuations of the feed to the other nips are minimized because roll No. 2 will tend to float horizontally rather than vertically in relation to roll No. 3 (Figure 9.3). To reduce gauge variation in this setup fitting roller bearings can stabilize roll No. 3 floating roll. Cross-axis and/or roll bending may be fitted to roll No. 3 or roll No. 4. With this compact setup it is still easily accessible for starting up and operating the machine. The Z-type roll arrangements followed developments in offset rolls. This design eliminated the floating No. 3 roll on a calender fitted with bearings. Each roll can be preloaded on to its bearings at a point that is 9 9 Calendering 373 the resultant of the material pressures and the roll weight. This approach had other advantages that included reduction of the height required for the installation of rolls. In turn plant space requirement was reduced along with reduced building cost. Its disadvantage is limiting the ease of access to roll No. 2 or No. 3 in the case of the inverted Z. With the inclined Z it is more difficult to feed than a standard type Z because the nip does not hold as much material. Calenders with at least four to six rolls are used to fabricate thin rigid sheet where the extra nips greatly improve the surface finish of the sheet. The more popular are the four-roll inverted L calender and Z calender. The Z calender has 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 compensation for roll bending. This compensation occurs because there arc no more than two rolls in any vertical direction as opposed to three rolls in a four roll inverted L type calender. The speed of the calendering rolls usually differs. They operate at different speeds to provide the best performance of the melt, particularly the required shearing action (Chapter 1). High pressures of at least up to 6,000 psi (40 MPa) can bend or deflect the rolls. This calender bowl deflection is the distortion suffered by calender rolls resulting from the pressure of the plastic running between them. If not corrected, the deflection produces film or sheets thicker in the middle than at the edges. The amount of thrust exerted by the material depends on processing factors such as method of feeding stock into the calender, plastic temperature, melt flow behavior (Chapter 1), required thickness and width, and speed of the calendering line. Unfortunately the rolls do not bend like a simple beam that is freely supported at each end and uniformly loaded along its length. Each calender roll varies in thickness between the face and its journal. Because it rotates the pressure distribution across the roll is not exactly equal. Thus it does not deflect on conformation with the classical engineering equation 1 but in such a manner simulating a profile of a U-shaped frame forming a collar about an ox's neck resembling an oxbow. In order to compensate for this thicl~ess variation requires the surface of the roll to fit a certain profile (crown). The amount of crown, that is the difference in roll section radius between ends and center, will vary depending on the rhcological properties of the plastic being processed (Chapter 1). Rolls arc crowned resulting in having a greater diameter in the middle. The equipment also provides for different types of adjust- mcnts and controls (crossing of rolls and roll bending) to correct 374 Plastic Product Material and Process Selection Handbook distortion. Example is crossing the rolls slightly rather than having them truly parallel; results in increasing the nip opening at both ends of the roll. Less deflection at high operating conditions can be achieved by the use of stiffer rolls, based on higher modulus of elasticity steels or dual-steel construction. Another approach is to bend the roll so that the bending moment is applied to the end of each roll by having a second bearing on each roll neck. In turn a hydraulic cylinder loads it. Calenders require high temperatures with little variations or fluctuations across the rolls during the application of the high pressures on the stock. Flow of stock relates to the friction between the stock and the roll faces, stock viscoelasticity, and pressure applied on the plastic. The first matching rolls provide initial control feeding plastic into the calender system. The final matching rolls provide the final roll thickness control of the sheet or film. Those matching rolls in between provide a gradual thickness metering action. Adjusting roll temperatures and speeds controls the final product dimensions. Roll loads run 1000 to 2000 Ib/linear in. of roll face for soft sheeting, and occasionally approach 5000 lb/linear in. for thin, rigid material processed cool at 330F (166C) on larger rolls. Total connected horsepower can run from 2 yd./min, on 24 in. calenders, to as much as 8 to 10 for a large 36 by 96 in. machine on tough plastics. Any unevenness in the temperature and pressure along the roll's length, that could include uneven temperature across the melt, is reflected as variations in the product thickness. Other causes of thickness changes across the web include nonhomogeneous rheology of the stock (Chapter 1 ), problems with material's lubricity, malfunctioning pressure and temperature sensors, equipment line control malfunctioning, use of damaged calender rolls, and so on. Also critical is the cooling of film or sheet that use multiple water-cooled rolls in the calender line with roll temperatures gradually reduced as the plastic travels downstream. 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. At least two to ten to possibly 20 cooling rolls are used depending on the thickness of web and the speed of production line. With more cooling rolls the line permits slower cooling to room temperature eliminating a shock cooling situation for certain plastics that reduces physical and mechanical properties such as rigid PVC. If embossing is to be applied, the embossing roll precedes these cooling rolls. After leaving the last large diameter calendering heated rolls, the film can be literally dropped vertically into an embosser, usually with three rolls- that is the embossing roll itself, a cooling rubber roll, and a contact cooling to the 9 9 Calendering 375 rubber roll. Temperature accuracy is usually controlled within +IC (e2F). Since the heated plastic clings to the calender rolls the web does not drop off the last roll. It has to be pulled off evenly across the width of the roll. This is accomplished by the stripper roll which is normally positioned 3 to 6 in. (75 to 150 mm) from the last roll, and at a height that gives the sheet approximately 270 ~ lap round the roll. Overall after the heated plastic passes through the rolls it can go through operations of stripping, embossing, cooling, trimming, and wind-up. Because here the hot plastic is in contact with a comparatively cold roller, for PVC there may be a problem of plasticizer and moisture condensing on the metal surface of the stripper roll. This condensate will mark or, in the case of condensed plasticizer, attack the sheet surface. To overcome this damaging action the stripper roll is covered with a highly absorbent material such as cloth. The thinner the sheet the greater the degree of roll cling, Thus the speed of the stripper roll must be varied with respect to the calender speed. Once the desired speed differential is set it is maintained. As the calender speed is altered, the stripper roll speed maintains a constant ratio with the calender speed. Different types of controls arc available to meet specific operating conditions (Chapter 3). Propcr use of all controls is required to meet product performances and minimize costs. The controls can call for adjustments on different line equipment, such as the nip openings, roll bending, neckdown, and so on. As an example proper use of ncckdown roll permits windups to bc run faster than the final calender roll on many thin, unsupported film products. Calenders and rake-offs arc 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 arc generally obtained in a median thiclmess range. Trimming can be performed either on the calender or later when the sheet is cold just prior to winding. It is economically sound to trim at the calender stage where the material, owing to its existing temperature, can be readily conveyed back to the calender feed nip, to a set of rolls, an extruder feeder for recycling, or a granulator and blended with virgin plastic. Following cooling the plastic can bc trimmed at the edges and wound. Trim material can account for up to 5% of the width depending on the line's operating efficiency. The target is to have as little trim as possible. This operation is to cool the sheet to ambient temperatures. If 376 Plastic Product Material and Process Selection Handbook warm or hot sheet is wound up, high internal strains may be caused and blocking and de-embossing problems may be introduced. Ideally, sheet should be wound up at approximately I OC (5OF). Wind-up occurs at the end of the line. The two usual methods of winding into rolls are center-core winding and surface batching. Not all calendered sheets are wound up into rolls. They are also cut into panels by rotary cutters or automatic guillotines that may be installed instead of wind-up equipment. With center-core winding one end of the mandrel is fitted into a socket which is power driven. It requires that uniform sheet or film tension is used or the product will not be uniform in thickness, etc. As the roll increases in size the moment of inertia builds up and the take-up force per revolution increases. Unless the drive can compensate for this force increase, the winding tension varies throughout the roll. By appropriately adjusting the tensions, winding can be applied to rigid or flexible plastics. Methods used to overcome this tension situation include a slipping clutch between the mandrel and the drive, or more usually, having the drive to the mandrel transmitted by a motor drive. This action controls the sheet tension at a predeter- mined value regardless of the increasing diameter as the roll winds up. To facilitate roll changing the winding station is usually duplicated, thus allowing one roll to wind while the other is being removed. Other auxiliary equipment can be included in the line such as orienting by stretching in the machine direction and/or transverse direction using the cooling rolls or setup bioriented stretching (Chapters 5 and 18), annealing, decorating, slitting, heat sealing, festooning, and so on. Corn pou nd i ng/B lending Different plastics, each with variations in type and quantity of additives, fillers and/or reinforcements, result in providing different processing conditions and end product performances. Important is the proper preparation of the plastic compounded stock to be processed based on weight as well as order of mixing. Stock prepared effects factors such as how the calender is to be operated, take-off thickness measurements, windup system requirements, and line speed controls. Other factors that influence the preparation of a stock is related to the finish (glossy, semi-matte, matte, etc.), product requiring coating or laminated to a substraight (fabric, plastic film or sheet, aluminum foil, etc.), embossed, etc.), or include if web is slit in line. With the finished product special properties may be required such as optical clarity and mono or biaxial orientation (Chapter 5). 9 9 Calendering 377 Blending or compounding of the plastic with different additives and fillers is a critical part of the process, particularly of PVCs. The PVC compounds require heat stabilizers in order to be properly processed. Heat stabilizer system imparts during processing primarily heat stability, as well as adequate lubricating characteristics to reduce or control frictional heat. Stabilizers 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 that in turn interferes with obtaining an acceptable surface finish of the film or sheet. This deposit may start out as a soft, waxy material barely visible on the metallic contact surfaces of the processing equipment. When plate-out occurs the line has to be shut down and the contamination removed. Processing 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 nip of the rolls. The residence time of the plastic flux at high heat must be controlled and limited. PVC is especially sensitivity to heat and time at heat. What is required is proper setting of the machine controls and operation within set limits. The processing variables of a PVC plastic (such as flow, heat stability and softening point) are strongly influenced by polymerization technique, MWD, and the extent of any polymerization (Chapter 1). Due to the plastic's viscosity, a melt shear effect is developed throughout the process. This shear is of prime importance between the calender rolls. The calender forms the web as a continuous extrusion between the rolls (Chapter 1). 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. TO improve PVC melt flow the stock is subjected to fluxing or fusion. It is the heating of the vinyl compound to produce a homogeneous mixture. Fluxing units used in calendering lines include batch-type Banbury mixers, Farrel continuous mixers (FCMs), Buss Ko-Kneaders (BKKs), and planetary gear extruders (PGEs). The dry blend is fed into the mixer/extruder. Proper mixing within a short dwell time and heat transfer control contributes to an improved product. During fluxing, each particle receives the same gentle treatment, generating less heat 378 Plastic Product Material and Process Selection Handbook 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. Rigid PVC manufacturers usual prefer the L-type with four to seven rolls being fed from the floor level. Since there is no disturbing vapors 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 4-roll inverted L- or an F-type. 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. 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. The stock delivered to the first calender nip needs to be well fused, homogeneous in composition, and relatively uniform in temperature. The optimum average temperature for good fusion depends on the formulation. A rigid PVC formulation based on medium molecular weight plastic (intrinsic viscosity of 0.90 to 1.15)211 has a typical optimum temperature of 180 to 190C (355 to 375F) at the first calender nip. For best calendering, there should be no cold volume elements below 180C (356F) and no hot spots above 200C (392F). Required is close control of temperature to ensure proper fusion and mixing conditions. This interaction depends on stock temperature and in turn on the performance of PVC melts. Flexible PVC is normally calendered at temperatures of 10 to 20C (50 to 68F) lower than rigid PVC. In flexible PVC production, a short single screw extruder acting as a strainer filters out contaminants from stock before reaching the calender. This important method is not applicable to rigid PVC because it drastically increases the head pressure and the consequent overheating would cause the stock to decompose. Market Products from calenders go into many different markets such as credit cards, upholstery, luggage, water reservoir, rainwear, loose-leaf book, and footwear. Different plastics are used such as ABS and ABS/PVC alloys go into margarine pack, luggage, panels, and chlorinated PE go into roofing, and pond liners. There are unsupported and supported as well as rigid products and coated substrates. Unsupported flexible PVC [...]... as paper, plastic processing, and wood coating where 392 Plastic Product Material and Process Selection Handbook long term economic gains made the changes viable The National Coil Coaters Association, Chicago, II1., organized in 1962, has developing industry standards, exchange of technical information, preparing technical manuals and keeping records of sales growth Property Plastic coating materials... coating system (as well as other plastics such as acrylics and polyesters), it can satisfactorily withstand one of the most complex bends or back-to-back bend cycles Spread Coating This technique involves that the material to be coated passes over a roller and under a long blade or knife The plastic coating compound is 390 Plastic Product Material and Process Selection Handbook ... provides for short production runs Calendered sheet is usually less glossy than extruded material Calendering may be preferable for certain applications requiring its higher tensile properties, product uniformity, and unusually close gauge 379 380 Plastic Product Material and Process Selection Handbook Figure 9.4 Exampleof roll covering Table 9,1 Exampleof comparing calendering and extrusion processes iiiiiii... used 3 396 Plastic Product Material and Process Selection Handbook Process Different casting processes are used They tend to overlap and could be identified by other processing methods An example is liquid injection molding that can be identified as injection molding (Chapter 4) or reaction injection molding (Chapter 12) Many decades ago the reaction injection molding process was... Automatic systems may havc their applicators mounted on fixed stands, on rcciprocaring or rotating machines, on robots, and so on Hame Spray Coating Flame spray coating involves blowing a plastic powder through a flame that partially melts the powder and fuses it as it contacts the substrate The 388 Plastic Product Material and Process Selection Handbook Dip Air Knife Kiss Gravure Reversegravure Offsetgravure... 6 Plastic Product Material and Process Selection Handbook When a dry film is prepared, the forces that stabilize the dispersion of TP particles must be overcome and the particles must coalesce into a continuous film The rate of coalescence is controlled by the free volume available, that in turn depends mainly on Tg (Chapter 1) TSs not properly stored can lose their stability before use With TS plastics... cost 397 398 Plastic Product Material and Process Selection Handbook and meet regulations, solvent recovery systems are used that have explosive-proof hazard safety capabilities There are also systems that use water-based solvent solutions such as polyvinyl alcohol plastic Spin casting can use plastic molds, such as silicone, to produce close tolerance, highly cost effective, limited production in... most desirable method of all are maximum rates and speeds, accuracy of gauge, speed of gauge adjustment, processing and product range versatility, lower raw plastic costs, high on-stream time factors, fast on-line time, and case of accommodation of automatic gauging and control 381 COATI N G Overview Coated products using thermoplastics (TP) and thcrmoset plastics (TS) are literally all around us worldwide... made in a batch process within a mold or cell or continuously between stainless steel belts Basically the processing cells consist of two pieces of polished (or tempered) plate glass slightly larger in area than the finished sheet is to 402 Plastic Product Material and Process Selection Handbook be The cell is hcld together by spring clips that rcspond to thc contraction of the acrylic material during... starting material is usually in liquid form rather than the usual solid plastic used in other processes There is also the difference that the liquid could bc a monomer rather than the plastic used in most other processes and in turn the monomer is converted to a polymer /plastic (Chapter 1) 11 9 Casting 3 9 5 This process has the advantages of low cost equipment, but is a relatively slow process and labor . evaporation. 386 Plastic Product Material and Process Selection Handbook When a dry film is prepared, the forces that stabilize the dispersion of TP particles must be overcome and the particles must. for different types of adjust- mcnts and controls (crossing of rolls and roll bending) to correct 374 Plastic Product Material and Process Selection Handbook distortion. Example is crossing. ambient temperatures. If 376 Plastic Product Material and Process Selection Handbook warm or hot sheet is wound up, high internal strains may be caused and blocking and de-embossing problems may