Calendering

Calendering can achieve surprising accuracy on the thickness of a sheet. Typically the tolerance is 0.005 mm but to achieve this it is essential to have very close control over roll temperatures, speeds and proximity. In addition, the dimensions of the rolls must be very precise. The production of the rolls is akin to the manufacture of an injection moulding tool in the sense that very high machining skills are required. The particular features of a calender roll are a uniform specified surface finish, minimal eccentricity and a special barrel profile ( crown ) to compensate for roll deflection under the very high presurres developed between the rolls. 

Calendering is one of the major methods of PVC processing which cottsumes about 25% PVC. PVC is the major polymer processed by calendering ( 95% of all polymers processed). Plasticizers are used in different variations of calendering  

Calenders are usually very large, custom-designed machines 

Thickness control is very important because thickness of the produced fdm can be as small as 0.04 mm. Most or all the rolls are mounted on moveable bearings and nip is adjusted by moveable bearings. In modem calenders, movement of rolls is computer controlled with a high precision. Rolls also have individual drives and thus the required speed gradient may be selected. 

The elastic properties of materials exert pressure on calender rolls which causes their deformation. This is controlled by honing the surface of rolls, roll bending, or cross-axes. Modem calender automatically adjusts the gaps based on the data from the gauges scanning the entire width of the material. 

A modem calender is used to form a film or a sheet. Compounding operations are performed on auxiliary equipment which may be more complex than the calender itself Formulation and a repeatable compounding method frequently determine the quality of the calendered product. 

Calendering is a method of mixing in which polymer and additives are introduced into a stack of rollers, which apply work and heat and progressively reduce the thickness of the mix to that of sheet or film, with the facility also of adding an embossed surface. The equipment, which has been borrowed from the rubber industry, is usually large and costly, but laboratory-scale systems are also available, for small-scale testing of compounds and formulae. 

In the calendering process, a polymer melt is transformed into film and sheet shapes by squeezing it between co-rotating pairs of rollers. The number of rollers depends on the material processed and the final product. Rubber can be ealendeied with a two-roller system. Other thermoplastic polymers require a four-roller system to ensure good surface quality. A typical calendering system is eomposed of  

13 Plug-assisted system (from Osswald and Hernandez-Ortiz, 2006). 

In the plasticating unit, the material is melted, mixed, and continuously fed through the nip of the hrst two rollers. This step controls the feeding rate with the remaining rollers controlling product thickness. In a four-roller system the polymer passes through three nip regions  

Rollers come in different types such as I-type, F-type, L-type, inverted L-type, and Z-type configurations (Fig. 3.14). 

A variation in the gap size between the rollers (which can be caused by poor roller dimensions, setting, thermal effects or roller distortion because of high pressure in the gap region) causes a non-uniformity in the cross-machine 

Early PVC calenders were modified from those used in the rubber industry. Calendering processes and machinery for rubber and PVC products have been examined (408). 

A calender is essentially a set of four giant steel rollers mounted in a cast iron frame, usually in an inverted L-shape, in such a way that they can be rotated, and the distance between each pair of rollers adjusted. Consecutive rolls squeeze the material resulting finally in a continuous sheet of the desired thickness. PVC, ranging from rigid to semi-rigid to flexible, is produced continuously in film or sheet up to approximately 1.5 millimetres in thickness and up to and over 2.0 metres in width. 

Calendering is much more capital intensive than the extrusion process, but higher production rates and more accurate product thickness are obtained. Mini calenders or Calandrette lines (from Battenfeld) are available for the production of rigid and semi-rigid PVC films with lower investment costs. 

The passage of hot melt between the rolls creates a pressure forcing them apart and the calculation of tolerances using a statistical model has been described (417). There is a rolling bank of feed material created in the gap or nip set between the first pair and also the second pair of rolls. Passage of the material is controlled by roll temperature, surface finish and the ratio of the roll speeds at the nip. The final calender nip controls film thickness. 

Plate-out can also be a problem in this area due to the formation of coatings which adversely influence sheet quality. The chemical structure of coatings, resulting 

In a calender line, the polymer melt is transformed into films and sheets by squeezing it between pairs of co-rotating high-precision rollers. Calenders are also used to produce certain surface textures which may be required for different applications. Today, calendering lines are used to manufacture PVC sheet, floor covering, rubber sheet, and rubber tires. They are also used to texture or emboss surfaces. When producing PVC sheet and film, calender 

Controlled rheology grades of PP have been used in the production of calendered PP films [29]. Compared with conventional reactor grade material, the reduced molecular weight distribution of PP provides better flow and a slow rate of crystallisation, giving better processibility and films free from surface defects. However, the optical quality of the calendered film does not reach that of extruded film. The poor thermal oxidation resistance of PP in the presence of air is the major impediment in use of PP as a calendering material. 

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 are 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. 

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 

In the early days of calendering plastics three-roll vertical rubber machines were used. Problems developed in processing plastics. They 

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 

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 are 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 I). 

Gaskell [33] was the first to analyze the process by developing a mathematical procedure in one dimension for Newtonian fluids, derived as a specific extension for a purely viscous (nonelastic and time ind ndent) fluid. Mc-Kelvey [34] extended the analysis to indude non-Newtonian flow, assuming only that the viscosity depends on the state of shear and making the conespond-ing modification in the momentum equatkm. He also showed the qipniximate cubic dependence of maximum pressure devdoped between rolls on the leave-off distance, and specialized his non-Newtonian development for powm-law fluids. 

Vlachopoulos and Hfymak [39] suggested a hydrodynamic model for the calendering of rigid poly(vinyl diloride) (PVC), using both the isothermal as- 

For the system shown in 1. 8.5, the lubrication assunqptimi, as applied to the mass flow balance and momentum conservation (details of which are available in Ref. 42) yield 

Assuming an infinite reservoir for the PVC melt feed, the boundary condition is given as 

Note that Ci is a constant for a given polymer system and Ci for given process parameters. In the case of PVC, it follows fiom Eq. (8.26) that 

Several types of bottles are manufactured from polyvinyl chloride via blow molding. Examples include clear bottles used for cooking oils, cosmetic bottles, and clear spice bottles. 

Simply by changing the components added to polyvinyl chloride, we can create a wide range of properties. The receptiveness of the resin to different additives, the many processing methods available to producing polyvinyl chloride-based products, and its low cost combine to make it one of the most versatile polymers in commercial use today. 

Polyvinyl chloride is considered to be a self-extinguishing polymer - in other vords, it does not support combustion well. Why does it exhibit this behavior  

Why is the reaction temperature of polymerization of polyvinyl chloride so important to its manufacture  

How do primary and secondary stabilizers act to reduce degradation in polyvinyl chloride  

Polymer powders are also used in a process known as fluidized-bed coating. When a gas is passed up through a bed of particles, the bed expands and behaves much like a boiling liquid. When a heated object is dipped into a bed of fluidized polymer particles, those that contact it fuse and coat its surface. Such 100% solids coating processes are increasing in importance because they eliminate the pollution often caused by solvent evaporation when ordinary paints are used. 

Similar procedures have been in use for years with plastisols (Chapter 7). Liquid platisol is poured into a heated female mold. The plastisol in contact with the mold surface fuses and the remainder is poured out for reuse. This slush-molding process is used to produce objects such as doll s heads. Dipping a heated object into a liquid plastisol coats it with plasticized polymer. Vinyl-coated wire dishracks are familiar products of this process. 

FIGURE 17.8 Inverted L calender, illustrating process variables for the production of PVC sheet. From Modem Plastics Encyclopedia, McGraw-HiU, New York, 1%9-1970 ed. 

The present form of the hydrodynamic theory makes essentially the same assumption for the operating conditions as Gaskell [33] and Vlachopoulos and, Hrymak [39], namely, isothermal steady-state flow of an incompressible power-law liquid. The reconciliation of this assumption to the experimental data is also identical namely, it is assumed that the calendering temperature is equal to the average surface temperature plus the average temperature rise due to adiabatic viscous dissipation, as per Kiparissides and Vlachopoulos [41]. 

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In manufacture, the abrasive grain is mixed with cmde mbber, sulfur, and other ingredients for curing, then passed through calender roUs to produce a sheet of desired thickness. The wheels are stamped from this sheet and heated under pressure to vulcanise the mbber. 

Calendering. The rheological characteristics of the sheet extmsioa grades of ABS easily adapt them to caleaderiag to produce film from 0.12... 

Gut Rubber. To produce cut mbber thread, smoked mbber sheet or crepe mbber is milled with vulcanizing agents, stabilizers, and pigments. This milled stock is calendered into sheets 0.3—1.3 mm thickness, depending on the final size of the mbber thread desired. Multiple sheets are layered, heat-treated to vulcanize, then sHt into threads for textile uses (Fig. 2). Individual threads have either square or rectangular cross-sections. 

The formaldehyde approach is stiU used by Futamura Chemical (Japan). They make spun-laid viscose nonwovens where the hydroxymethylceUulose xanthate derivative formed from formaldehyde ia the spia bath allows the fibers to bond after layiag. This process was originally developed by Mitsubishi Rayon (30), who later found that the derivative was thermoplastic, and the web could be calender-bonded (120°C) prior to regeneration (31). 

Available as both film and sheet unless otherwise noted. All materials are available in PDA grades except for CTA, PVP, and PL EX = extrusion CL = calendering REG = regeneration CAST = casting BO = biaxial orientation. 

The basic methods for forming film or sheeting materials may be classified as follows melt extmsion, calendering, solution casting, and chemical regeneration. Of special note is the use of biaxial orientation as part of the critical manufacturing steps for many film and sheet products. 

Fine Powder Resins. Fine powder PTFE resins are extremely sensitive to shear. They must be handled gendy to avoid shear, which prevents processing. However, fine powder is suitable for the manufacture of tubing and wire insulation for which compression molding is not suitable. A paste-extmsion process may be appHed to the fabrication of tubes with diameters from fractions of a millimeter to about a meter, walls from thicknesses of 100—400 )J.m, thin rods with up to 50-mm diameters, and cable sheathing. Calendering unsintered extmded soHd rods produces thread-sealant tape and gaskets. 

To produce unsintered tape by paste extmsion, the fine powder is lubricated and preformed according to the procedure described above. The preform is extmded in the form of rods, which are calendered on hot roUs to the desired width and thickness (109,110). 

Overbraided hose liners are made from fine powder resins by paste extmsion, and thread-sealant tapes are produced from fine powder by calendering. Fabricated gaskets are made from granular resins and pipe liners are produced from fine powder resins. Fibers and filament forms are also available. 

R. C. Ribbans, Dnsintered Tape Manufactured Calendering Round Rods, technical release, E.I. du Pont de Nemours Co., Inc., Wilmington, Del., 1966. 

Poly(vinyl chloride). To be converted into film, poly(viayl chloride) [9002-86-22] (PVC) must be modified with heat stabilizers and plasticizers, which increase costs. Plasticized PVC film is highly transparent and soft, with a very high gas-permeation rate. Water-vapor transmission rate is relatively low. At present, PVC film is produced by blown-film extmsion, although casting and calendering are employed for heavier gauges (see Vinyl POLYAffiRS). 

Lubricants. Process aids or lubricants promote smooth and rapid extmsion and calendering, prevent sticking to extmders or calender roUs, and impart good release properties to molding compounds. In some cases use of lubricants allow slightly lower processing temperatures (see Vinyl polymers). 

AppHcatioas of dibasic lead phthalate, coated grade, iaclude 90 and 105°C rated PVC electrical iasulation, plastisols, profile extmsions, calendered sheet, and molded products. The recommended range of usage ia viayl electrical iasulatioa is 5—7 parts pet huadred resia, depending oa the particular iasulatioa classiftcatioa to be met. Ia geaeral-purpose extmded and molded PVC stocks, approximately 3—6 phr of coated dibasic lead phthalate is suggested. 

Vinyl-Goated Fabrics. Manufacturing methods for vinyl-coated fabrics now available are calendering and extmsion for thicklayer, and paste coating for thin layer. Both soHd and foam vinyl-coating layers are used. 

In the calendering method, a PVC compound which contains plasticizers (qv) (60—120 phr), pigments (qv) (0—10 phr), fillers (qv) (20—60 phr), stabilizers (10—30 phr), and other additives, is kneaded with calender roUs at 150—200°C, followed by extmsion between clearance-adjusted roUs for bonding onto the substrate. This method is employed for products with thick PVC layers, ie, of 0.05—0.75 mm thickness. The main plasticizer used is di-2-ethylhexyl phthalate (DOP). For filler to reduce cost, calcium carbonate is mainly used. A woven or knit fabric made of cotton, rayon, nylon, polyester, and their blend fiber is used as substrate. For foamed vinyl-coated fabrics, the bonded materials are heated in an oven to decompose the foam-blowing... 

For vinyl plastisol, organosol products and calendering, Day-Glo Color Corp. offers T, D, VC, and AX-Series pigments, Lawter Chemical offers the B-3500 and G-3000 Series, and Radiant Color offers P-1600 and R-203-G Series. In addition, Day-Glo offers VC Series for vinyl calendering where nonformaldehyde products are needed. 

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