What is mould shrinkage in injection moulding?

Mould shrinkage is the percentage a moulded part contracts as it cools from melt temperature to room temperature, measured against the cavity dimensions. Typical values run from about 0.2% for glass-filled grades to 2.5% for unfilled semi-crystalline resins like POM and PP.

Which plastics shrink the most in injection moulding?

Semi-crystalline unfilled resins shrink the most: POM about 1.8-2.5%, PP about 1.0-2.5%, HDPE about 1.5-4.0%, and PA grades about 0.7-1.8%. Amorphous resins like ABS, PC and PS shrink much less, typically 0.3-0.7%.

How does glass fibre affect shrinkage?

Glass fibre cuts shrinkage in the flow direction dramatically — often from 1.5-2.0% down to 0.2-0.5% for a 30% filled grade — but much less across the flow. That anisotropy is the main driver of warpage in glass-filled parts and must be considered at DFM stage.

How does shrinkage affect tolerances?

High and variable shrinkage means wider achievable tolerances. Amorphous low-shrink materials can hold fine tolerances consistently; high-shrink semi-crystalline materials need wider tolerance bands, especially on large dimensions where a 2% shrink means 2 mm per 100 mm.

Who compensates for shrinkage — designer or toolmaker?

The toolmaker scales the cavity by the expected shrinkage factor of the specified resin. The designer's job is to keep geometry shrink-friendly (uniform walls, balanced ribs) and to fix the material before tooling — changing resin after the tool is cut changes the part dimensions.

Does wall thickness change shrinkage?

Yes. Thicker walls cool slower, crystallise more (in semi-crystalline resins) and shrink more. A part with mixed 2 mm and 5 mm sections will shrink unevenly across those sections — one reason uniform wall thickness is the first DFM rule Nordmould checks.

Injection Moulding Shrinkage: Rates for 25 Materials & How to Design for It

Mould shrinkage is the percentage a part contracts as it cools from the melt to room temperature. It is the single number that decides whether a 100.0 mm feature arrives at 99.4 mm or 98.0 mm — and it depends on the resin family, the filler, the wall thickness and the process. The toolmaker compensates by scaling the cavity; the designer's job is to specify the resin before tooling and keep the geometry shrink-friendly.

Nordmould confirms the shrinkage assumption for the specified grade during DFM review, because a wrong shrinkage factor means undersized or oversized parts no process adjustment can fully correct.

What are typical shrinkage rates by material?

Values below are typical mould-shrinkage ranges for unfilled injection grades at normal wall thickness (2-3 mm). Glass-filled values are flow-direction figures; transverse shrinkage is higher. Always design to the datasheet of the exact grade.

Material	Family	Typical shrinkage (unfilled)	With 30% glass fibre (flow)
ABS	Amorphous	0.4-0.7 %	0.1-0.3 %
ASA	Amorphous	0.4-0.7 %	0.1-0.3 %
SAN	Amorphous	0.3-0.7 %	0.1-0.3 %
Polystyrene (GPPS/HIPS)	Amorphous	0.3-0.6 %	0.05-0.3 %
PMMA (acrylic)	Amorphous	0.2-0.6 %	0.1-0.3 %
Polycarbonate (PC)	Amorphous	0.5-0.7 %	0.1-0.3 %
PC/ABS blend	Amorphous	0.4-0.7 %	0.2-0.4 %
PEI (Ultem)	Amorphous	0.5-0.7 %	0.2-0.4 %
PSU (polysulfone)	Amorphous	0.6-0.8 %	0.2-0.4 %
PVC (rigid)	Amorphous	0.2-0.6 %	0.1-0.3 %
PP (polypropylene)	Semi-crystalline	1.0-2.5 %	0.4-1.0 %
HDPE	Semi-crystalline	1.5-4.0 %	0.3-0.8 %
LDPE	Semi-crystalline	1.5-5.0 %	—
PA6 (unfilled)	Semi-crystalline	0.7-1.5 %	0.2-0.8 %
PA66 (unfilled)	Semi-crystalline	0.8-1.8 %	0.3-0.8 %
PA12	Semi-crystalline	0.8-1.6 %	0.3-0.8 %
POM (acetal)	Semi-crystalline	1.8-2.5 %	0.4-0.9 %
PBT (unfilled)	Semi-crystalline	1.5-2.2 %	0.3-1.0 %
PET	Semi-crystalline	1.5-2.0 %	0.2-0.6 %
PPS	Semi-crystalline	0.6-1.0 %	0.1-0.4 %
PEEK	Semi-crystalline	0.9-1.3 %	0.2-0.5 %
LCP	Semi-crystalline	0.0-0.3 %	0.0-0.2 %
TPE (SEBS/TPV)	Varies	1.0-2.5 %	—
TPU	Varies	0.6-1.8 %	—
PLA	Semi-crystalline	0.3-0.5 %	—

Why do semi-crystalline resins shrink more?

Amorphous resins (ABS, PC, PMMA, PS) freeze from a tangled melt with little ordering, so they contract modestly and uniformly — typically 0.3-0.8%. Semi-crystalline resins (PP, PA, POM, PBT, PEEK) pack part of their structure into dense crystals as they cool, and that ordering takes up less space — typically 1.0-2.5% total contraction.

The practical consequences: semi-crystalline parts need wider tolerance expectations, more careful cavity-scaling, and more attention to cooling, because anything that changes crystallisation — mould temperature, cooling time, wall thickness — changes the final dimension.

How does glass fibre change the picture?

Glass fibres do not shrink. Along the flow direction, fibres align and physically resist contraction, so a PA66-GF30 that shrank 1.5% unfilled shrinks only about 0.3-0.5% in flow. Across the flow, restraint is weaker and shrinkage stays higher — often 0.8-1.2%.

That directional split is anisotropic shrinkage, and it is the leading cause of warpage in glass-filled parts: flat panels dish, long housings banana. Gate location, wall design and symmetric geometry matter more with filled grades than any machine setting. Nordmould reviews fibre-direction risk on glass-filled parts as a standard DFM item.

What does shrinkage mean for tolerances and design?

Three working rules. First, scale: at 2% shrinkage a 150 mm feature moves 3 mm — big parts in high-shrink resins cannot hold tight tolerances economically. Second, material lock: the cavity is scaled for one resin's shrinkage; switching from unfilled to glass-filled PBT after tooling shrinks the parts beyond adjustment. Third, uniformity: mixed wall sections shrink by different amounts, so the first DFM rule — uniform walls — is also the first shrinkage rule.

Where a part has genuinely tight requirements, choose low-shrink amorphous resins or filled grades, specify tolerances only on functional features, and confirm the shrinkage factor used for cavity scaling before steel is cut.