Sheet extrusion die

Figure 2.24 illustrates the basic components of a sheet extrusion die. The die is built in two halves the part for easier construction and maintenance. The use of a jack bolt facilitates separation of the die halves when the die is full of plastic. The die lip can be adjusted (across the entire lip length) to enable the processor to keep the thickness of the extruded sheet within specification. 

Fig. 3-12a. Examples of dies with nomenclature. (1) Pipe or tubing die for in-line extrusion. (2) Pipe or tubing die for crosshead extrusion. (3) Cast film die. (4) Sheet extrusion die.

Figure 2, Cross-section of sheet extrusion die and three-roll stack.

FIGURE 4.7 Upper schematic diagram of a single-screw extruder. Lower a sheet extrusion die for plastics. Source Reproduced with permission from Styrolution, Styrolux, BASF. 

Auto-flex die n. Trade name for a type of sheet-extrusion die with a flexible Up in which each hp-adjusting bolt, which can either push against the hp or pull it, is paired with a nearby cartridge heater. 

Choker bar (restrictor bar) n. A bendable metal bar incorporated in a sheet-extrusion die for controlling flow distribution and lateral sheet thickness, and for reducing stagnation in the melt. The shape of a flow passage between the choker bar and the lower die body is altered by turning bolts coimected to the bar. 

Sheet Extrusion and Thermoforming. Sheet for thermoforming and analogous operations is usually formed by extmding the melt through a slot die onto a set of poHshed chill roUs. The sheet is usually approximately 150 cm wide. After rapid cooling, the web is coiled or cut into sheets. Polystyrene, PVC, polyethylene, polypropylene, and filled polypropylene are prepared in sheet form by extmsion. 

Fig. 8. Sheet extrusion A, die inlet B, die C, three-roll finisher D, support roUers E, edge-trim cutter F, puU roUs and G, saw or shear (15).

VDC copolymers in, 25 725, 734 Sheet extrusion, 10 546-547 Sheet-fed offset inks, 14 320 Sheet fluorosilicates, 12 634-636 Sheet forming, of paper, 18 118-122 Sheetmetal forming dies... 

Chicago, II., 29th Aug-2nd Sept.l993,p.403-6. 6A MULTILAYER EXTRUSION DIE GEOMETRY FOR THERMOFORMED POLYPROPYLENE FOAM SHEETS AND BIAXIALLY ORIENTED FOAM FILMS Raukola J Savolainen A Tampere,University of Technology (TAPPI)... 

Fig. 11 Foamed sheet extrusion line. 1 Primary extruder, 2 blowing agent metering and flow control, 3 screen pack/breaker plate, 4 secondary extruder, 5 annular die, 6 cooling mandrel, 7 guide rolls, 8 winders. [38]...

The advantage of calendering over direct sheet extrusion is that a complicated and expensive die is not required. Some additional mixing can be done as part of the calendering process, as well, and a surface finish can be applied by the final roller, if desired. Sheet up to a few millimeters thick and a meter wide can be made using calendering. 

Drawdown from the die to nip is typically ca 10%. Sheet extrusion requires that the resin be of high melt viscosity to prevent excessive sag of the melt between the die and the nip. The melt should reach the nip before touching any other part of the middle roll to prevent uncontrolled cooling of the resin. The appearance of the lower surface of the sheet is determined by the middle roll, ie, its polish, surface temperature, and freedom from condensed materials. The upper surface is cooled by air and has a more glossy appearance. Neither the top roll nor the bottom roll greatly affect the appearance of the top surface of the sheet. Edges of the sheet are trimmed. 

Sheeting dies. One of the most widely used extrusion dies is the coat-hanger sheeting die. A sheeting die, such as the one depicted in Fig. 3.14, is formed by the following elements ... 

Sheet extrusion requires a broad die, in which an even distribution of the flowing polymer along its width, and an equal flow resistance at each point, has to be taken care of. Sheets are being extruded up to a width of 3 or 4 m, in thickness up to 3 cm. If desired, sheets can be produced with a wave-shape, by passing the, still plastic, extrudate between two profiled rolls. Film extrusion can be done in a similar way, as a matter of fact with a narrower die gap. After extrusion, sheets and films are cooled in a water bath or on polished cooling rolls. 

In the process an extrusion die deposits a molten bead of like material continuously along the edge of one moving length of film or sheet, and a second... 

When processing plastics some type of tooling is usually required. Tools include molds, dies, mandrels, jigs, fixtures, punch dies, perforated forms, etc. The terms for tools are virtually synonymous in the sense that they have some type of female and/or negative cavity into or through which a molten plastic moves usually under heat and pressure or they are used in secondary operations such as cutting dies, stamping sheet dies, etc. These tools fabricate or shape products. In this chapter injection molds and extrusion dies are primarily reviewed because they represent over 95% of all tools made for the plastic industry. This chapter also includes information applicable to other molds and dies used in the other processes some of the other chapters too provide information applicable to their tools. 

Almost exclusively used for extrusion of PVC-U into pipe profile and sheet, extrusion equipment consists of a cylindrical barrel containing two close fitting, intermeshed, Archimedean screws. PVC dry blend is fed in via a hopper at the start of the screws and is conveyed forward, softened, sheared and finally gelled by rotation of the screws and the heat of the barrel. The resulting homogeneous viscous melt is forced through a heated die fitted to the exit end of the barrel. Thereafter the downstream equipment sizes or calibrates, cools and pulls off the product which is also cut to desired lengths. 

The extrusion of foamed products with twin screw extruders has been reviewed (375). The machinery, tools and processes for the production of foam core pipe, sheet and profile have been listed and the market reviewed (124,333). Technical details of PVC-U foam sheet extrusion lines, including dies and calibrators, have been given (258). 

Several unique products are made by extrusion and the dies needed to make these products are classified as 1) sheet dies 2) flat-film and blown-film dies 3) pipe and tubing dies 4) profile extrusion dies and 5) co-extrusion dies. Furthermore, each product type has unique hardware downstream of the die to shape and cool the extruded melt. To aid the reader, detailed illustrations of the various die designs and the complementary downstream cooling and shaping hardware are shown. 

Extrusion dies vary in shape and complexity to meet the demands of the product being manufactured. There are five basic shapes of products made with extrusion dies, as illustrated in Table Film and sheet dies... 

The most common extrusion die for sheet products is the coat hanger-type manifold die as shown schematically in Fig. lA and as a section view in Fig. IB. The key elements of Fig. lA are ... 

Another important product made with extrusion dies is the creation of multilayered materials. Multilayered sheet and film materials have two applications ... 

Fig. 9 Chill roll stack for sheet extrusion (1) sheet die with flow restrictor (2) molten sheet extrudate (3) lower chill roll (all chill rolls temperature controlled) (4) middle chill roll (imparts gloss/surface texture to sheet) (5) upper chill roll and (6) solidified sheet.

Sheet extrusion requires a broad, slit die, in which an even distribution of the fiowing polymer along its width... 

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