Shrinkage & Expansion of plastic parts

In summary, there is a shrinkage study being conducted on a plastic part, specifically Basell Pro-fax 6331 Natural. After 5 months of observation, it was noticed that the part started to expand instead of shrink. It is possible for plastics to behave this way due to water absorption, high thermal coefficients of expansion, and residual molding stresses. It is important to control for water absorption and measure at the same temperature, and to also consider the accuracy of measurements.
  • #1
nady_sophy
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TL;DR Summary
Need to know upon shrinkage and expansion of plastic parts.
Hi,

I'm currently doing a shrinkage study upon this plastic part. I hv been observing how much can the part able to shrink over the time after molding, and now hv been on the 5th month observation. However, on the 5th month, the part starting to expand, compared with the data from the 4th month. Is this possible? Are there any plastic material that can behave this way?

By the way, this plastic part material is Basell Pro-fax 6331 Natural.
 
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  • #2
Plastics absorb water. Are you controlling for water absorption due to room humidity? Plastics have high thermal coefficients of expansion. Are you measuring at the same temperature? How accurate are you measurements? Note that accuracy is not the same as the number of significant digits on a digital measuring device. Is the part warping from residual molding stresses?
 
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1. What causes plastic parts to shrink or expand?

Plastic parts can shrink or expand due to changes in temperature, humidity, and the type of plastic material used. These factors can affect the molecular arrangement and density of the plastic, leading to dimensional changes.

2. How much can plastic parts shrink or expand?

The amount of shrinkage or expansion varies depending on the type of plastic and the specific conditions. It can range from 0.5% to 5% of the original size. Some plastics, such as polypropylene, have a higher shrinkage rate compared to others.

3. Can shrinkage or expansion be predicted?

Yes, the shrinkage or expansion of plastic parts can be predicted through computer simulations and tests. These methods take into account the material properties, processing conditions, and design geometry to determine the expected dimensional changes.

4. How does shrinkage or expansion affect the functionality of plastic parts?

Shrinkage or expansion can affect the functionality of plastic parts by causing warping, distortion, or poor fit with other components. It can also affect the mechanical properties, such as strength and stiffness, of the plastic parts.

5. How can shrinkage or expansion be minimized?

To minimize shrinkage or expansion, it is important to carefully select the plastic material and processing conditions. Using a material with lower shrinkage rate and optimizing the molding parameters can help reduce dimensional changes. Design modifications, such as adding ribs or fillets, can also help minimize the effects of shrinkage or expansion.

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