product that is {{0}}.25mm or greater in thickness. Some industries use 0.l0mm as the dividing line between film and sheet. Widths can be up to at least 3m (loft).
Sheet extrusion technology produces single layer products as well as more complex coextruded or multi-layer engineered products.
heated or cooled) polishing three roll stack (stand), take away equipment[such as gauge monitors and controls edge trimmers, antistatic devices, air coolers, cooling tunnels, conveyors, slitters, pull or nip rolls, tension rolls, and/or cross cutting devices], and either a (turret)wind-up or stacking equipment. Throughput capacities for these lines can be up to at least 400-1100kg/h.
Temperature throughout the line must be kept above the plastic's melting point. A reverse heat profile is used with PET. The highest from the extruder's feed throat to the barrel end could be 316-260 degree (600-500 degree F) .that are run on the first two heating zones to obtain rapid melting. Die exit ranges from 300-260 degree (570-500 degree F). If the first two zones are not sufficiently hot,the result will be surging. Too hot a melt temperature results in IV degradation.
Poly(vinylchloride) PVC has advantages over conventional materials due to its toughness and flexibility. PVC is also an easily processable and low-cost material. During processing, PVC requires comparatively cost material. During processing, PVC requires comparatively less energy compared with the manufacture of, for example, paper and metal. Its physical and resistance properties make PVC a substitute for many applications. For some PVC products, mechanical properties such as strength and toughness are inadequate .
The thermal stability of PVC is purely dependent upon temperature and residence time in an extruder. PVC compounds usually require a melt temperature of 190–195 degree . The melt temperature is needed to develop excellent physical properties in PVC products. Streamlined equipment provides a long run without interrupting the process. A small increase in temperature will provide better output and melt uniformity. Higher processing temperature without appropriate stability and lubricants can lead to earlier degradation.
Maintaining the uniformity of the thickness of the sheet is very important. Sheet thickness is controlled by adjusting The flow from the die or adjusting the gap between the rolls.The temperature of the rolls must be balanced to have the best surface and fast cooling.the rolls must be balanced to have the best surface and fast cooling.residual stress. The roll temperatures used are 50–90 degree .
Different and important components of the sheet extrusion lines follow the dies. Just as improper material handling equipment in the beginning up-stream) of the line can restrict or damage the line's operation, the same is true with the down-stream equipment. Different equipment com-ponents are required that are properly aligned and interfaced with each other.
The hot melt from a slit die is directed to usually a three roll stack that are equipped with quick-acting safety releases where the gap between the rolls open in emergencies.As the extrudate exits the die, it must be supported as it is hot and in a semimolten state. Support is provided almost immediately by the roll stand. Different diameter polishing rolls such as 20-76 cm (8-30in)] are used; the larger rolls provide processing advantages with higher output rates.
Rolls can be embossed to impart a surface finish on the sheet. They can provide specific patterns or textures, such as prismatics for the lighting industry, grained surfaces for luggage, paneling, and decorative overlays,etc. Embossing can be done continuously during sheet (also profiles, etc.)extrusion with little or no decrease in output. Depending on pattern and plastic used, colder than the usual roll temperatures are used, such as 50 degree (125 degree F) on the embossing roll, to prevent recovery of the surface impressions.The quality of the sheet is directly related to the surface quality of the stacked rolls. Therefore, chrome plated
rolls which have been ground to a surface of 76-15Onm (3-6μin) are Usually desired for good gloss.
The volume of water and oil through the double-shell rolls is a major consideration for cooling and temperature control.
Stock temperature, sheet thickness, and linear travel rate are important for optimizing roll diameter. As an example, when processing ABS at 365kg/h (8001b/h) using a 11.4cm (4.5in) screw extruder sheet line, it would be sufficient to use 30cm (12in) diameter rolls. At rates over 910kg/h (20001b/h), rolls could range from 60 to 70cm (24 to 30in) in diameter. Usually having rolls too large is not a problem since the roll temperatures can be elevated .
coextruded or laminated material using many different lay-ups can be extruded In turn, the coextruded melt can be directed into the three roll stack to produce sheets.
Laminated sheet products also can be made using the three roll stacks.In this case, a film or any web material is unwound from a roll (pay-off station) supported above the stack (Figs. 9.19 and 9.20). The film is fed into the nip together with the molten web that exited the extruder die. The film bonds to the sheet and transfers its special surface properties
Up-stream of the pull rolls, trim knives are used. They provide the means to edge trim both sides of the sheet and also when required slit the sheet into multiple widths using specialty designed razor slitters.
At the end of line, thick sheets can be cut to length using equipment on casters to fit the take-off tract. There are different types of cutters to handle the variety of different plastic sheets that range from being brittle to nonbrittle They are designed to cut the sheet uniformly and prevent chatter marks in the sheet. Thickness up to 8mm (0.30in)are usually sheared but it depends on the plastic's cutting characteristics.
As an example, when processing PS, the melt could be run through dies which have either constant diameter or tapered manifolds. With ABS, the streamlined manifold dies, such as the coathangers with flexible lips and minimum dwell time, are used. Since ABS is more viscous than PS, the ABS die should be constructed to operate at pressures of {{0}}MPa (1500-32OOpsi) for 120mm (4.5in) extruder. Lower pressures are used with PS.With the higher viscosity plastics, such as PC, the internal pressures require the dies to be stronger so no die deflection occurs during processing. The dies for PC has to withstand pressures of 28-42MPa (4000-6OOOpsi). Even higher pressures can be generated when thin sheets are processed. The dies require very narrow die gap settings of possibly less than 0.8 mm (0.030 in).
The different constructions of these sheets provide different performance advantages for the different products. Performances include: (1)high gloss expensive layer over tough nongloss less expensive substrate;
(2)thin weatherable, usually expensive, layer over a tough, less expensive,nonweatherable substrate;
(3) low gloss layer over a tough, lower cost,substrate;
(4)thin expensive decorative (wood grain, marble, etc.) layer over a tough, less expensive substrate;
(5) vapor barrier over a tough and strong substrate;
(6) chemical resistive layer over a less expensive substrate;
(7) soft touch layer over a cost effective tough substrate;
(8) correctly color matched cap eover a noncolored less expensive substrate ;
(9) sheet with a core of recycled plastic with different overlays;and
(10) others.
