Processing of Plastics Questions & Answers

Within the range of plastic polymers some are hygroscopic (take on moisture) and some are not. If using a hygroscopic material this moisture needs to be removed before processing which means the material needs drying. Commonly used materials which are hygroscopic, and therefore need drying, include ABS, Acrylic, Nylon, PEEK, Polycarbonate, PC/ABS, PBT, PET. PPS, SAN, and some TPE’s. Different materials require different drying temperatures which typically range from 80 to 120 deg C

Sink marks occur in injection moulded plastic parts during the materials cooling phase in the tool.

When a plastic part cools in a mould tool the part will shrink. During the moulding process second stage pressure is used to push more material into the tool to compensate for this shrinkage. Once the gates have frozen off, however, no more material can be injected into the tool and any shrinkage which occurs after that cannot be compensated for. In parts with thicker wall sections this shrinkage can show itself in the form of sink marks.

To avoid the potential for sink marks it is recommended thick sections should be avoided wherever possible during the component design stage. Good practice is to reduce the wall thickness of the part wherever possible by coring out the component and using ribs to maintain strength and stiffness as required.

If it is not possible to avoid thick sections in the design other methods can be used to reduce/avoid sink marks:

1. Optimising the processing conditions during the moulding process is the first step.
2. Increase the size of the runner and gate system in order that the gate freezes off later which will allow more material to be introduced during second stage pressure.
3. Using a lower shrinkage material will reduce the level of sink. This could be the same polymer but with a filler (as only the polymer shrinks, not the filler), a lower shrinkage polymer or a combination of both.
4. Introduce a blowing or foaming agent. These agents work during the moulding process causing gas to be produced. As the plastic part cools the gasses expand internally and compensate for the shrinkage which is occurring. The result is a part without sink which has a very fine “honeycomb” structure inside.
5. Gas injection or water injection. During this process gas or water in injected separately into the centre of the component which pushes the plastic to the extremities of the mould. This results in a hollow part being made which does not show sink.

It is possible to see bubbles in plastic mouldings which are transparent. The bubbles are in fact voids in the moulding which have not been filled with plastic. During the plastic moulding process, the material tends to fill the external parts in the mould cavity first and then fills internally. If the gate in the mould freezes off before the part is completely consolidated this can result in voids or bubbles in the part.

Streaks in products which are using masterbatch can be caused by several reasons.

These include:

Poor mixing or dispersion of the masterbatch in the base polymer.

Chemical incompatibility of some of the pigments in the mix with the base material.

Thermal incompatibility between the pigments and the base material.

Moisture in hygroscopic materials.

Blowing agents are generally used on thick section parts to either reduce sink marks or reduce the weight of the part.

During the moulding process chemical blowing agents decompose to produce bubbles in the polymer matrix when pressure is reduced as the resin enters the mould or exits the extrusion die.

The gas produced from this decomposition displaces the polymer forcing it to fill the cavity and forming a fine honeycomb structure inside the part. This results in a part without sink and can result in a weight reduction of up to ten per cent in components.