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CALENDERING PROCESS

CALENDERING PROCESS. CALENDERING PROCESS. BASIC PRINCIPLE Calendaring is the process of squeezing a plastic melt between two or more counter rotating cylinders or rolls to form a continuous film and sheet .

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CALENDERING PROCESS

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  1. CALENDERING PROCESS CORPORATE TRAINING AND PLANNING

  2. CALENDERING PROCESS BASIC PRINCIPLE • Calendaring is the process of squeezing a plastic melt between two or more counter rotating cylinders or rolls to form a continuous film and sheet. • The art of forming a plastic sheet has been adopted from the technique of making paper, metal and rubber in industries CORPORATE TRAINING AND PLANNING

  3. Introduction • Both rigid and plasticized compounds of PVC resin can be used in calendaring process. • The sheet is formed in the range of 0.1 to 1.0 mm and above thickness. The sheet can be transparent, colored, embossed, printed or laminated form. CORPORATE TRAINING AND PLANNING

  4. Plastic material and its Compound • The PVC resin is generally processed in calendaring plant, and it needs blending, mixing and compounding by addition of suitable chemicals called as additives and fillers to make compound before processing in the calendering plant. • Other then PVC resin ABS, PE, PP and Styrene are also used for calendering process. CORPORATE TRAINING AND PLANNING

  5. Compound formulation • PVC resin for rigid compounds made either by the suspension or bulk polymerization process is best in the 1.7 to2.0 relative viscosity range. • Homopolymer grades are used alone and in combination with 2 to 10 percent acetate copolymer. • Heat stabilizers are needed, when heated PVC naturally tends to degrade, first by yellowing then by turning dark brown and losing its physical properties. • Many types of stabilizers are used, Metallic salts, mixed metal salts, organotins and tin mercaptides are the major categories. CORPORATE TRAINING AND PLANNING

  6. Compound formulation • Stabilizers added to a formula between 0.1 and 5.0 percent to retard degradation by tying up hydrochloric acid (HCL) generated by the heat of processing. • Plasticizers are used to make PVC soft and flexible but thereby lower melt viscosity, modulus and transition temperature. • Two common plasticizer groups are in use such as Dioctyl phthalate(DOP) and Epoxidized soyabean oil. CORPORATE TRAINING AND PLANNING

  7. Compound formulation • Impact modifiers are used to absorb shock to improve impact resistance of PVC compound. The selection of ABS, MBS, Chlorinated polyethylene( CPE) or acrylic polymer chosen on the basis of their impact efficiency, clarity, weather ability and stress whitening, as well as their processing characteristics • Process aids assist stabilization and increase the melt strength of the sheet during calendering and post processes such as thermoforming. CORPORATE TRAINING AND PLANNING

  8. Compound formulation • Lubricants are added to reduce the tendency of PVC to stick to the hot metal rolls of Calender • Lubricants are either external or internal depending on where they are most effective. Stearic acid or zinc and inorganic stearates and soaps are classified as internal lubricant. Paraffin and polyethylene waxes are external. CORPORATE TRAINING AND PLANNING

  9. Coloring materials and fillers • The pigments or dyes which can withstand temperatures of processing at about 180°C should be considered. Pigments which are suitable for coloring plastic materials are metallic salts, lead, Cadmium based and also organic colors are used. CORPORATE TRAINING AND PLANNING

  10. Coloring materials and fillers • Inorganic materials in the form of fine powder can be added in the Plastic compound which helps to reduce cost and improve properties. The popular fillers are china clay (a hydrated aluminum silicate) and the various grades of chalks (Calcium Carbonates). Also red mud (hydrated aluminum silicate) is also used as fillers. The fillers improve the modulus and the low temperature crack resistance is reduced CORPORATE TRAINING AND PLANNING

  11. Formulation for calendering • General purpose flexible PVC fiim formulation • Details parts • PVC resin 100 • Dop Plasticizer 30 • Epoxy thalate 5 • Phosphate 15 • CaCo3 10 • Ba – cd liquid 2 • Ba –cd soap 1 • Stearic acid 0.3 • Silica 0.5 CORPORATE TRAINING AND PLANNING

  12. Compound preparation (Blending & Fluxing) • The resin is premixed and blended with stabilizers and other additives by using Ribbon blender or High speed mixture. The premixing is done up to 80°C for better absorption and soaking of the resin. • After blending of the two compounds, it is fed to a fluxing machine. Fluxing is done under temperature at about 150°C. • Fluxing can be done in banbury mixture or a continuous extruder • The residence time of the plastic flux at high temperature must be limited. CORPORATE TRAINING AND PLANNING

  13. Sectional diagram of ribbon blender CORPORATE TRAINING AND PLANNING

  14. sectional diagram of a high speed mixture Fig No. 2 CORPORATE TRAINING AND PLANNING

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  17. Calendering process with plant details • The rolls are heated either by steam or hot oil and roll temperature ranges from 150° to 180°C. • Fluxed material delivered to the first calendar nip is regulated to form a rolling bank. The sheet passing the first nip forms another bank between the second and third rolls, at the final nip, the desired thickness is obtained from the smallest bank possible to minimize stress in the sheet. CORPORATE TRAINING AND PLANNING

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  19. Sheet Take off roll CORPORATE TRAINING AND PLANNING

  20. Embossing and laminating • To create leather like grain or other surface structure, the hot sheet coming from the stripper rolls is passed between a steel roll nipped by a rubber roll that forces the PVC into the grain. • To laminate another sheet or a fabric to the calendared sheet, it may be nipped against the sheet on the last calendar roll or a laminating station may be installed in place of the embossing rolls. CORPORATE TRAINING AND PLANNING

  21. Calendering process with plant details • After fluxing and gelation, either a two roll mill or an extruder is used to form strands or any acceptable forms. • Melt is maintained and delivered to the calender at around 140-160°C. • The calender roll squeezes a plastic melt into a flat sheet. CORPORATE TRAINING AND PLANNING

  22. Cooling and winding of the sheet • The sheet is cooled by passing it over cooling rolls then through a thickness gauge (e.g. betaray device) prior to being wound-up. • If the sheet is wound up too hot, it has a tendency to shrink during the cooling process. • The sheet is then trimmed to required width and wound on a tube or cut to length and stacked as sheets. • The trimmed edges of the sheet can be recycled. CORPORATE TRAINING AND PLANNING

  23. Precautions to be taken during processing • Preparation of stock for calendering, conditions on the calender, the take off, thickness measurements, and control to wind up. • Other considerations include whether the plastic is laminated to a fabric on the calender. Whether the web is embossed or slit in line; what finish is required (glossy, matt or semi-matt). CORPORATE TRAINING AND PLANNING

  24. Types of calenders • Superimposed calenders (I –Type) • Offset calenders (L or Inverted L Type) • Z type of Calenders • The ‘ I ‘ type calender rolls has the advantage of taking up a minimum of floor space and offers the designer a more simple problem for the strength of the side frames and gives good visibility. CORPORATE TRAINING AND PLANNING

  25. Types of calenders • Z’ type calender has the advantage that bending of the rolls caused by pressure of material in the nip has no effect on the succeeding nip and therefore tends to improve thickness accuracy. • The inverted ‘L’ offers a good compromise, since the first pair of horizontal rolls hold the input material easily succeeding two nips can be arranged to give good control of gauge CORPORATE TRAINING AND PLANNING

  26. - Type CORPORATE TRAINING AND PLANNING

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  28. Calender rolls and its Heating arrangement • Calendar rolls are normally made of chilled cast iron. Calendars are also made with steel rolls which are more resistant and which exhibits less deflection under the pressure of the material in the nip. • Heating system • High pressure steam is used on calendaring plants. Working pressure range – 680-1380 KN/m and a temperature range 120-185°C. CORPORATE TRAINING AND PLANNING

  29. Typical drilled calendar roll CORPORATE TRAINING AND PLANNING

  30. High pressure hot water (H.P.H.W.),This is effective method to heat and cool the calender rolls by circulating H.P.W.H. through each roll. • The hot oil can be circulated through the rolls to achieve the high operating temperature. Processing Parameters The processing parameter needs to be set and control are temperature, speed, pressure and the nip gap. CORPORATE TRAINING AND PLANNING

  31. Examples: Roll No. Roll Temp.(°C) Speed( m/min.) 1st 165 80 2nd 168 82 3rd 171 85 4th 174 88 Nip gap between rollers: Roll No. Nip Gap 1st - 2nd High 2nd -3rd Medium 3rd -4th Low CORPORATE TRAINING AND PLANNING

  32. TABLE 1 • Roll No. Roll Temp. (°C) Speed ( m/min.) • 1st 145-150 75-80 • 2nd 150-155 80-82 • 3rd 155-160 83-85 • 4th 160-165 85-90 • Nip gap between rolls • Nip gap high Roll 1 Roll 2 Roll 3 • Nip gap medium Roll No Nip Gap 1-2 High 2-3 Medium 3-4 Low Roll 4 Nip gap low CORPORATE TRAINING AND PLANNING

  33. Mechanism against Roll bending • The calender lines during operation generate very high forces exerted on the rolls to squeeze the plastic melt into a thin web or sheet. High forces can bend the rolls. • Crowned rolls , which have a greater diameter in the middle than at the edges • Crossing the rolls slightly, thus increasing the nip of opening at either end of the rolls. • Roll bending, where the bending movement 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. CORPORATE TRAINING AND PLANNING

  34. Crown CORPORATE TRAINING AND PLANNING

  35. Troubleshooting in calendaring process CORPORATE TRAINING AND PLANNING

  36. Advantages of Calendering Process • Delivery/ output of a calender is high • Versatile in product design and pattern and permits lamination & embossing without additional equipment. • Increase in maximum width of film / sheet. • Better control of film / sheet thickness. • The properties are more uniform across the width of the product. • Better optical properties due to cooling chilled rolls. • PVC being heat sensitive material, it is safer by calendering process than extrusion process. CORPORATE TRAINING AND PLANNING

  37. Applications • The applications include sheets in hospitals, Printer blankets, rain coats, table cloths, draperies, water tank liners, wall covers, thermoform sheets, packaging applications, medical applications, stationery items, files, folders, luggage bag applications, counter tops, flooring, upholstery, automobiles applications, and Toys. • They may be heat-sealed, heat shrunk for blister packaging or used plain as wrapping materials, mulches, swimming pool liners, reservoir films, vapor barriers. CORPORATE TRAINING AND PLANNING

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