Plastic Dies Process - A die in plastic processing is a metal restrictor or channel capable of providing a constant cross sectional profile to a stream of liquid plastic. This Plastic Dies Process allows for continuous processing of shapes such as sheets, films, pipes, rods, and other more complex profiles. This is a continuous process, allowing for constant production (assuming constant supply of plastic melt), as opposed to a sequential (non-constant) process such as injection molding.
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Die forming typically occurs immediately after polymer melt has exited an extruder. The most basic process involves guiding the stream of molten polymer under pressure through a die, which three distinct regions: manifold, approach, and lip. The 'manifold' serves to channel the polymer melt from its initial extrusion point to a near-net-shape of the final product.
The 'approach' region further guides the melt into the final shape, and begins to correct for any non-uniform flow. Finally, the 'lip' forms the melt into the final desired cross section and compensates for any remaining flow asymmetry. After exiting the lip of the die, the polymer melt will undergo die swell before curing. Die swell is an expansion of the melt as the pressure is released, and is dependent on polymer chemistry and die design. After curing, the solid, continuous part is drawn onto a take-up roller or cut into transportable lengths, depending on the type of part. This process may vary significantly depending on the type of die and extrusion process.
There are two major types of dies used in flat sheet extrusion: T-shaped and coat-hanger. A T-shaped die consists of two arms extending at right angles from the initial extrusion channel; these arms have a small slit along their length to allow the polymer melt to flow through. The melt is then further thinned by a short, flat approach before being pushed through the lips of the die. This setup can cause non-uniform flow across the width of the extruded sheet, with the melt at the center flowing faster than the melt at the edges of the die, resulting in buckling and other defects after exiting the die.
A more modern design is the coat-hanger die. This die differs from the T-shaped die in that the arms are not at right angles to the input direction; instead, the arms are at a shallower angle and are often curved. The arms also have a variable diameter, tapering down to a smaller radius further from the input channel. The approach portion of coat-hanger dies are longer than their T-shaped counterparts, further reducing any flow nonuniformity. Finally, the melt is extruded through lips as in the T-shaped die.
For products such as plastic sheets or films, cooling is achieved by pulling through a set of cooling rolls (also known as calender or chill rolls), usually 3 or 4 in number. In sheet extrusion, these rolls not only deliver the necessary cooling but also help determine sheet thickness and surface texture (in case of structured rolls; i.e. smooth, levant, haircell, etc.). A common processing defect known as nerve may occur when contact time between the rollers and extrudate is too brief, resulting in insufficient cooling time.
Coextrusion is common in sheet and film extrusion, allowing for speedy production of multi-layered parts. This is accomplished by joining multiple polymer melts in either the manifold or approach stage. Layers of different thicknesses may be formed by introducing melts at different flow rates or different manifold sizes.