Plastic Material Shrinkage Rates

Plastic Material Shrinkage Rates

Each product design needs to know the plastic material shrinkage rate. So that when he designs his parts, he can present the correct scale size of the drawing to the mold manufacturing factory.
Plastic molding shrinkage is the contraction of plastic moulding as it cools after injection. Most shrinkage occurs in the mould during cooling, but some shrinkage occurs after ejection as the part cools.
After that, the part may shrink slightly for several hours or even days until the temperature and moisture content stabilise.

The polymer primarily determines the plastic shrinkage rate, but processing conditions and part design also influence the injection moulding process. The ASTM D955 standard, which provides guidelines for determining the linear shrinkage of moulded thermoplastic parts, is commonly used to measure the shrinkage compared to the injection moulding tool.

In addition to ASTM D955, there are other internationally recognised standard methods related to plastic shrinkage. ISO 294-4 is specifically for thermoplastics and outlines the procedure for measuring shrinkage during cooling and solidification. ISO 2577, on the other hand, focuses on thermosets and provides guidelines for measuring shrinkage during the curing process.

These standard methods enable manufacturers to accurately assess and quantify plastic materials’ shrinkage characteristics. This information is essential for designing injection moulds and predicting moulded parts’ final dimensions and tolerances. By understanding the shrinkage behaviour, manufacturers can adjust part designs and processing parameters to achieve the desired dimensions and minimise issues such as warpages or dimensional inaccuracies.

The specific polymer type is crucial, as different materials have varying shrinkage rates. Moreover, factors like mold temperature, cooling rate, and part geometry can also influence shrinkage behaviour. Therefore, carefully considering polymer properties, adherence to standard measurement methods, and optimising processing conditions are vital for achieving precise and consistent results in the injection moulding process.

Further Considerations for Plastic Shrinkage

In addition to the factors mentioned earlier, plastic shrinkage in the injection moulding process needs further consideration. These factors can have an impact on shrinkage behaviour and need to be taken into account for accurate predictions and effective part design. Here are some additional considerations:

1. Additives and Pigments: Certain additives and pigments used in plastic formulations can act as nucleating agents. Nucleating additives promote the formation of small crystals during the cooling and solidification process, which can influence shrinkage behavior. Depending on the specific additive and its concentration, the shrinkage of plastics may be affected, leading to variations in the final dimensions of moulded parts.
2. Fillers and Fibers in Composite Materials: When plastics are reinforced with fillers (such as glass fibers) or other composite materials, their shrinkage characteristics can differ from those of virgin polymers. Fillers and fibres can affect the material’s thermal expansion coefficients, potentially leading to differences in shrinkage rates. The direction in which the fibres align within the material can also influence shrinkage, as shrinking may occur less in one direction.
3. Mold Design and Cooling System: The design of the injection mold and the cooling system can impact shrinkage. Non-uniform cooling rates within the mold can result in uneven shrinkage, leading to warpages or dimensional inconsistencies. Proper mold design, including the distribution of cooling channels and optimized cooling times, helps to control the cooling process and minimize shrinkage-related issues.
4. Processing Conditions: Melt temperature, injection speed, and holding pressure during moulding can affect shrinkage. Optimizing these processing parameters based on the specific polymer used and desired part dimensions is crucial to achieving accurate shrinkage control.
5. Post-Molding Processes: Some post-molding processes, such as annealing or stress-relieving, can influence the final shrinkage of the plastic parts. These processes minimize residual stresses and optimize dimensional stability by subjecting the parts to controlled heating and cooling cycles.

Considering these additional factors related to plastic shrinkage allows manufacturers to refine their injection moulding processes and ensure more precise control over the dimensional accuracy and quality of the final plastic parts. By understanding and addressing these considerations, manufacturers can reduce shrinkage issues, resulting in high-quality molded components.

There are several well-defined and standardised approaches to determine the shrinkage of plastic parts. The most internationally used and recognised approaches are:

‣ ISO 294-4, ISO 294-3 (thermoplastic shrinkage).
‣ ASTM D955 (thermoplastic shrinkage).
‣ ISO 2577 (thermoset shrinkage).