OVERVIEW
Sheet extrusion transforms TP materials into roll or sheet stock through a combination of heat and pressure.The extruder plasticizes the plastic and pumps it through a die and it continues downstream in the sheet line.The terms sheet or sheeting normally describe a flat plastic
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.
An extrusion sheet line consists of a properly controlled
extruder , sheet die, temperature controlled (individually
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.
PLASTIC MATERIALS
Many different flexible to rigid thermoplastics can be extruded into transparent or colored sheets. They include ABS, AN, PA, PET, PETG, PMMA,PC, PE, PP, PS, PUR, and PVC.
Processing PET
Running a PET (polyethylene terephthalate) plastic through an extruder sheet line basically does not require a high degree of sophistication. Most PET sheet lines can be regulated by standard discrete control devices.
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°C (600-500°F) .that are run on the first two heating zones to obtain rapid melting. Die exit ranges from 300-260°C (570-500°F). If the first two zones are not sufficiently hot,the result will be surging. Too hot a melt temperature results in IV degradation.
PET is not difficult to extrude on conventional sheet extrusion equipment provided it was very carefully predried. This is because a small amount of moisture it contains reacts with the molten plastic resulting in the loss of molecular weight and in physical properties. Drying PET can be rather complex and costly.
Processing PVC
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 °C. 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 °C.
Line components
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°C (125°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.
The cooling capacity of the roll stand must be sized to the extrudate exiting rate of the extruder.
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 .
The three roll stands can be in different positions to meet different plastic material requirements and surface finishes desired.
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
Pull roll
A set of pull rolls (usually rubber covered) down the line moves the sheet off the final cooling roll and along the cooling conveyor
off the final cooling roll and along the cooling conveyor
than the pull rolls. These speed adjustments are critical during the set up. This action keeps the sheet flat and taut during the cooling process. The pull rolls keep the sheet moving and feeds it to the end of the line. They provide no cooling or shaping.
Trim and slitting
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.
Process control
A central microprocessor control for the complete line can be used.Software is available that has been preprogrammed providing precise,repeatable, central control of the sheet extrusion line. The control display would provide the information required to set and fine tune the equipment in the line. It provides an easy approach to vary the settings of each operation and interrelate them.
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.
CUTTING DIE
The obvious function of the die is to control the shape of the extrudate.The key word is control. In order to take this action, the extruder must deliver melted material to the die at a constant temperature, pressure, and output rate.
The choice of the die is influenced by the quality of the sheet required.
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 10-22MPa (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).
APPLICATIONS
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.
PP Hollow sheet extrusion line
SPC Marble sheet extrusion line