Plastic shrinkage is an unfortunate consequence of the injection moulding process. Shrinkage is the contraction of a plastic moulded component as it begins to cool after the injection process.
All plastics shrink as they cool from viscous liquids to solids, and every type of plastic shrinks in a slightly different way. Eliminating shrinkage is virtually impossible, although minimizing the effects of shrinkage is essential for moulding parts accurately.
Shrinkage occurs at a molecular level when plastics melt and cool. Volumetric shrinkage is caused by thermal contraction, which affects all polymers (semi-crystalline and amorphous), and influences how the material changes in volume as it transitions from a liquid to a solid. In general, plastics can shrink up 20% to 25% during the injection moulding process.
The dynamics of shrinkage depend on the type of material as it the material cools and solidifies.
Shrinkage rates are strongly determined by the polymer composition and material properties including its pressure, volume and temperature (PVT) characteristics. The processing conditions; temperature, pressure and flow rate also have an influencing factor, as does the design and geometry of the component design in terms of wall thickness, gate location and mould constraints.
Most of the component shrinkage occurs in the mould while cooling, but a small amount of shrinkage occurs after ejection, as the part continues to cool. The component may continue to shrink very slightly for several hours or even days, and any dimensional inspection should be delayed until the temperature and moisture content of the component has fully stabilised.
By changing temperatures, pressures, and packing and cooling times, it is possible to mitigate shrinkage. By applying pressure to a liquid plastic, the molecules can be compressed into a smaller volume which enables more material to be injected into the mould to compensate for shrinkage.
Managing shrinkage is a complicated task, given the many variables involved and how each one can influence the others. To help reduce and eliminate defects earlier in the product design cycle, simulation software can enable engineers to address and fine-tune plastic component designs, injection mould designs, and the control of the manufacturing process.
A leading manufacturer of product inspection machinery in the food processing industry required some simple but unique components for a new process inspection machine. The components themselves were plastic test pieces, into which a metal ball bearing was integrated to test the effectiveness of the metal detectors on a food processing line.
The test pieces themselves had to be suitably rugged and robust to ensure, that if damaged, no shards of plastic would find their way on to the production line. This specific requirement deemed that the component design would need to have an unusually large wall thickness, which then presented a challenge to mitigate the effects of shrinkage.
To ensure a high quality product was consistently produced for the client, Skar took two key steps. The first was to water-cool the parts coming out of the mould. This meant that the speed of the cooling process was increased to effectively ‘freeze’ the exterior of the moulded part very quickly, locking it into place before the slow cooling of the inner plastic could deform the outer surface…
The cooling process was combined with packing the molten plastic into the mould at particularly high pressure to further reduce the effect of the shrinkage.
These steps combined with strong quality and measurement systems enabled Skar to produce perfectly formed test pieces for the client.
For further enquiries or more detailed information please call us on 01473 828000 or email us at sales@skar.co.uk