Introduction to Polymer Science and Technology

Polymer processing

screen pack

breakerplate

Figure 3.22Illustration of screen pack and breaker plate

Functions of screen pack and breaker plate also include generation of back pressure to ensure that the screw is filled and the melting and mixing is efficient. The breaker plate also turns spiral /rotational flow to linear flow. An adapter,following the breaker plate, helps to smoothly link the barrel and the die.

The die is machined to match the profile of the product or to produce a pre-form/parison for the final product as in blow moulding of bottles or tubular film production. The land length for a die (Figure 3.23) is an important parameter: longer land lengths promote flow stability and reduce die swell and drool; however as well as the greater manufacturing costs, the disadvantages of longer land lengths include higher melt pressures and higher heat and shear histories. Generally the land length should be as short as possible, without compromising the final product characteristics. Land lengths are commonly short for wire coating and long for tubing, pipe and profile applications.

land length

Figure 3.23Description of a land length for a die and mandrel or pin used for a tubular product

A gear pumpis incorporated into some extruders between the screw and the die for extrusion of some materials such as wood-flour and polymer mixtures, or to achieve accurate/uniform output from the extruder. It also reduces load on the extruder, enabling higher output and/or less wear on parts, such as thrust bearings. The use of gear pumps raises some safety concerns: the main concern being the creation of high pressures if there is a blockage ahead of the pump, since the pumping action is continued. Appropriate safety measures should be taken to avoid the risk of explosion from excessive pressure, see Strong (1996, p271) for further information. Gear pumps are expensive and are included only when it is not possible to achieve pressure/output stability and low melt temperature by screw design or other means.

Auxiliary equipmentis used to pull the material from the extruder die, where the molten polymer is shaped, at an appropriate rate. The shaped molten plastic is quickly cooled in a water-cooled tank to retain the shape. Depending on the extrudate, before the cooling tank there may be vacuum-assisted calibrators and sizing plates to ensure that the roundness of the outside shape is maintained. Calibrators are usually made of brass with peripheral holes or slots in the sleeve wall to allow the surrounding

Introduction to Polymer Science and Technology Polymer processing

vacuum to act on the extruded tube/pipe. The inside surface should be sandblasted and may also contain shallow grooves to decrease the surface drag of the extruded tube. The inside diameter of the calibrators (sizing rings) should be between 3% and 15% wider than the outside diameter of a tubular extrudate to compensate for the subsequent shrinkage of the extrudate in the cooling tank, particularly with crystalline polymers such as PEs, PP and nylons. The actual amount of oversize, for a given polymer, depends on several factors, the most important being tube diameter and line speed. An example of this is given for Hytrel (a DuPont thermoplastic polyester elastomer) in DuPont Hytrel extrusion manual.

The extrudate moves over a set of rollers and leaves the cooling tank with an air wipe to remove water droplets present on its surface. The air wipe, therefore ensures that the water is kept in the cooling/quenching tank so that no health and safety issues arise from wet and slippery floors. It also prevents extrudate slippage in the haul-off belt of a caterpillar puller. Therefore, the haul-off pulls the extrudate from the die, through the sizing/cooling to a wind-up roll or cutter. The more accurate the haul-off speed and the caterpillar belt pressure is set, the better the dimensions of the final product will be.

Twin-screw extruders(Figure 3.24) are used where more efficient material compounding and conveying is required. The mechanism of material conveying/transportation, and therefore the output rate, is different in the twin- screw extruders compared with single-screw extruders: in single screw extruders the output rate is, basically, proportional to the inner-surface area of the barrel, whereas in intermeshing twin screw extruders it is proportional to the channel volume because the work is done mainly inside the screw channel. When intermeshed, the relative motion of the flight of one screw inside the channel of the other acts as a paddle and pushes the material from screw to screw and from flight to flight. They are, therefore, more effective for achieving high outputs. They are also better suited to processing certain polymers, e.g., PVC, which is heat sensitive and therefore prone to thermal degradation during processing. There are two types of compounding/ mixing that occur in twin screw extruders: dispersive (high-shear mixing) and distributive (low-shear mixing).

Date: 2015-12-11; view: 1830