Why moulded plastic parts shrink after cooling, typical shrinkage rates by material, and how to design tooling that accounts for it. Includes a shrinkage comparison chart.
Every plastic part shrinks as it cools. Not a little, and not the same amount for every material. Get the shrinkage allowance wrong at the tooling stage, and you find out the expensive way: a finished part that doesn’t fit, snap, or seal the way it did on screen.
This is the number one reason first-run tooling needs rework. Not bad design. Not bad moulding. A shrinkage rate that didn’t match the material actually used.
What Causes Shrinkage in the First Place
Plastic takes up less space solid than it does molten. As the melt cools in the mould and its molecules pack tighter, the part contracts around the core.
How much it contracts depends on the material’s structure:
- Semi-crystalline plastics (PP, PE, POM, Nylon) form ordered crystal regions as they cool. That packing is efficient, so these materials tend to shrink more.
- Amorphous plastics (ABS, PC, PMMA, PS) cool into a disordered structure with less rearrangement, so they shrink less and more predictably.
Wall thickness, gate location, packing pressure and mould temperature all shift the number further. The percentages below are a starting point, not a guarantee – they tell you which materials are naturally stable and which need tighter process control.
Shrinkage by Material
How to Use This When You’re Designing a Part
- Confirm the shrinkage rate with your moulder before you cut steel. The ranges above cover unfilled, standard grades. A glass-filled or coloured grade of the same resin can shrink differently – and differently in different directions.
- Thicker sections shrink more than thin ones. A part with uneven wall thickness will shrink unevenly, which shows up as warping, not just a size mismatch.
- Glass fibre reduces shrinkage but makes it directional. Filled nylon or filled PC shrinks less along the fibre flow direction than across it. If tight tolerances matter, this changes how you gate the tool.
- PTFE is the outlier on this chart. It isn’t injection moulded like the rest – it’s compression moulded and sintered, so its shrinkage behaves differently and shouldn’t be compared directly to the others.
Common Questions
Does a material shrink the same amount in every direction? Not always. Unfilled, amorphous materials shrink close to evenly. Semi-crystalline and glass-filled materials often shrink more along the direction of flow than across it, which is why two “identical” dimensions on the same part can come out slightly different.
Can shrinkage be controlled once the material is chosen? To a point. Mould temperature, packing pressure and cooling time all influence the final number within a material’s typical range. What they can’t do is move a material outside its natural range – PE will always shrink more than PC, regardless of process tuning.
Why did two parts from the same “PP” spec shrink differently? Different suppliers grade PP differently – homopolymer versus copolymer, filled versus unfilled, different melt flow rates. “PP” on a drawing isn’t a single number; the grade matters as much as the resin family.
Getting the shrinkage rate right is the difference between a tool that works first time and one that needs rework. For expert guidance on choosing the right plastic for your needs, contact Plastinternational today!
