polymer processing - Calendering process.pptx

Polymer Processing

1. Polymer Processing PE-4104

2. CALENDERING Calendering is a highly developed art and theory, rather recently elucidated by combining the complexity of melt behavior with the mechanics of screw rotating machines. 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. ( 3) To handle thicker sheets more effectively .

3. Calendering 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 .

4. CALENDERING OPERATION The calendering process prepares plastic materials or compounds. It melts the material and then passes the paste like melt through the nips of two or more precision heated, counter rotating, speed controlled rolls into webs of specific thickness and width.

5. Calenders may consist of two to at least seven rolls . Examples of the layout of the rolls are the true L, conventional inverted L, reverse fed inverted L, I , Z, and so on . Examples of a few of the configurations 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 inverted- 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.

6. ARRANGEMENTS OF PARALLEL ROLLERS

7. 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 .

8. Surface finishing Calenders provide styling on films and sheets . A sharper, deeper, and more precise matte finish can be applied to film or sheet by the calender . To make two-sided matte products, such as two-sided credit card stock, 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 .

9. Credit cards Card manufacturers are not plastic processors, but rather plastic printing and lamination specialists. They apply the features, including the security aspects, to sheets of plastic core stock that is usually rigid PVC.

10. 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, printing, paper, textile , and many more.

11. PLASTIC MATERIALS A wide variety of TPs can be used with the majority being PVC, ABS, PE, PP , and PS . 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.

12. Fluxing and feeding Fluxing is the preparation of a plastic composition to improve melt flow. Fluxing units used in calendering lines for preparing TP materials include batch-type Banbury mixers, Farrel continuous mixers (FCMs ), Buss Ko -Kneaders (BKKs), and planetary gear extruders (PCEs).

13. 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 . A micron size piece of metal will destroy the rolls, etc . Replacing these very expensive rolls is expensive and time consuming. Magnetic sensors/detectors are positioned practically anywhere the plastic material moves. This includes locating detectors under the conveyor belts that directs the web or rope up to the rolls.

14. APPLICATIONS Applications are many, including: Packaging , Building and construction (tiles, window shade, etc.), Fabric coating, Swimming pool liners, Electrical tape, etc.

15. TROUBLESHOOTING As an example, calendering problems that will affect product costs include: (1) bank marks and shiny patches on the surface; Bank marks are minimized by optimizing formulations, calendaring speeds , and roller temperatures so as to obtain the most orderly behavior of the roll banks of stock at the calender's nip entrances. ( 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 un-plasticized plastic particles; ( 5) W atermarking usually due to lubrication contamination.