Plastics Extrusion: Material Behavior, Equipment and Process Control

In maximizing performance, or at least meeting performance requirements of extruded plastics as well as minimizing cost to extrude products it is important to understand the processing behavior of the different plastics. In producing the different extruded products certain plastics can be used. An understanding of factors such as their rheological to decomposition behaviors as well as problems that can develop provide information that will make it easier to extrude products.

Downstream of the die, the extrudate (melt) is calibrated, cooled and packaged by an array of ancillary auxiliary devices including vacuum calibrators, water tanks, cooling rolls, haul-offs, cutters, and winders. Upstream of the die, a melt pump may be interposed between the extruder and the die to produce a more uniform extrudate. The exact selection and arrangement of these component parts of an extrusion system will depend on the end product and tolerance requirements that have to be met.

On leaving the extruder, the extrudate (melt) is drawn by a pulling action or other device, at which stage it is subject to cooling, usually by water and/or blown air device. This is an important aspect of downstream control if tight dimensional requirements exist or conservation of plastics is desired. The processor's target is to determine the tolerance required for the pull rate and to see that the downstream equipment meet the total line requirements. Even if tight dimensional requirements are not required, the probability is that better control of the pull speed will permit tighter tolerances so that a reduction in the material's output will occur resulting in lower product cost.

By far practically all extruders use screws in their plasticators to melt the plastics. However ram, over a century popular melting system, is used to process plastics that cannot be melted by a screw. The ram devices are essentially batch devices, and although it is possible to achieve a constant output by sequentially operating two or more rams, the method is of virtually no importance for practically all TPs. The ram is used for processing plastics that come close to not being meltable. Another possibility is the rotary extruder, a device in which rotating discs or rotors are used to generate shear. However, TPS extrusion depends almost entirely on the rotating screw as a melt delivery device.

TPS are characterized by low thermal conductivity, high specific heat, and high melt viscosity. Preparation of a uniform homogeneous melt and its delivery at adequate pressure and a constant rate could pose considerable problems if not properly processed. The principal extruder variants are the single-screw and the twin-screw types. Of these, the single-screw extruder is by far the most versatile and popular in use.