Introduction
A mould cavity is cut to one size, but the part that comes out is smaller. Get that gap wrong and you face undersized parts, poor assembly fit, sink marks, warpage and tooling rework.
A published shrinkage value is a reference range, not a constant. Resin grade, wall thickness, gate position, flow direction and processing conditions all move it. Even the supplier's own datasheet says it is for material comparison, not final design.
This article gives a grade-specific shrinkage chart, shows the calculation, compares amorphous and semi-crystalline materials, and sets out how to validate values before you finalise tooling.
Key Takeaways
- Shrinkage is the dimensional reduction from cavity to cooled part, and it continues after ejection.
- Use chart values for early comparison, then verify with grade data and mould trials.
- Semi-crystalline and reinforced materials need closer attention to directional shrinkage.
- Wall thickness, gating, packing, mould temperature and fibre orientation change final dimensions.
- Tight-tolerance parts need measured trials and documented process settings.
What Plastic Material Shrinkage Represents in Injection Moulding
Shrinkage is the dimensional difference between the cold mould cavity and the cooled, measured part. Covestro's guidance on linear shrinkage uses this definition:
Shrinkage (%) = (mould dimension − part dimension) ÷ mould dimension × 100
To convert to a ratio, divide by 100. So 1% = 0.01 mm/mm, and the same number applies in in/in.
Moulding shrinkage is not thermal expansion in service. It is set by how the part fills, packs, cools and is ejected. The cavity must be cut to allow for it, and the process must stay consistent enough to hit it every cycle.
Datasheet value, design estimate and measured value
These three are not the same thing:
- Datasheet value: measured on a standard plaque, for comparison between materials.
- Design estimate: the value you pick for your cavity, informed by the datasheet and your geometry.
- Measured value: what your actual part shows after a trial.
Shrinkage is derived from material and process together. It is not a fixed material property.
Factors That Influence Plastic Material Shrinkage in Real-World Operation
ASTM D955 states that standard specimens do not predict absolute shrinkage in production parts, because flow paths, wall thicknesses and pressure and temperature gradients differ. A test-bar number and your part can disagree.
Key influences:
- Wall thickness and cooling: thicker sections cool slowly, and more crystallisation can raise shrinkage.
- Packing pressure and time: higher pressure and longer dwell reduce shrinkage until the gate freezes.
- Melt and mould temperature: for LDPE, Celanese reports higher temperatures increase shrinkage. Treat this as material-specific.
- Gate design and flow length: gate size, position and long or asymmetric flow paths change local packing and orientation.
- Fillers and fibres: filler content and fibre orientation create directional differences.
- Recycled content: a 2014 ABS study found shrinkage reductions of up to 17% relative to virgin resin with reprocessed material.
Differential shrinkage
Uneven walls, ribs, bosses, inserts and non-uniform cooling make different areas shrink by different amounts. That shows up as sink marks from local volume contraction, and as warpage or twisting when shrinkage is unequal. Thickness differences alone are enough to drive warpage.
Plastic Material Shrinkage Value Chart and Operating Range
Use this chart to compare materials, not to fix final cavity dimensions. All values come from supplier datasheets for the named grade. P/N = parallel/normal to flow; NR = not reported. Percentages are used throughout, with ratios converted where the source gives in/in.
| Material / grade | Shrinkage | Direction | Test method |
|---|---|---|---|
| ABS, Terluran GP-22 | 0.4–0.7% | NR | ASTM D955 |
| PC, Makrolon 2405 | 0.65 / 0.70% | P/N | ISO 294-4 (2 mm) |
| PC/ABS, Bayblend T65 XF | 0.50–0.70% | P and N | ISO 294-4 (2 mm) |
| POM, Hostaform C9021 | 2.0 / 1.8% | P/N | ISO 294-4 |
| PVC, GEON M3800 | 0.2–0.5% | Flow | NR |
| GPPS, Styrolution PS 3600/3601 | 0.4–0.7% | NR | ASTM D955 |
| PP, Sasol HHR102 | 1.5 / 1.4% | As labelled by source | ISO 294-4 |
| HDPE (generic catalogue range) | 2.4–4.0% | NR | ISO 294-4 |
| PBT, Ultradur B4520 | 1.70 / 1.70% | P/N | ISO 294-4 |
| PBT-GF30, Ultradur B4300 G6 HS | 0.40 / 1.10% | P/N | ISO 294-4 |
| PA6, Ultramid B3S | 0.90 / 0.90% | P/N | ISO 294-4 |
| PA6-GF30, Ultramid B3WG6 | 0.30 / 0.75% | P/N | ISO 294-4 |
| PA66, Ultramid A3K | 1.50 / 1.80% | P/N | ISO 294-4 |
| PA66-GF30, Ultramid A3WG6 | 0.4 / 0.9% | P/N | ISO 294-4 listed |
| Mineral-filled PA6, Ultramid B3M6 | 1.30 / 1.10% | P/N | ISO 294-4 |
| PEEK, Victrex 450G | 0.90 / 1.30% | Flow/cross-flow | ISO 294-4 |
| PPS-GF40, Fortron 1140L4 | 0.3 / 0.6% | P/N | ISO 294-4 listed |
| TPE (HEXPOL generic guidance) | 1.0–2.5% | NR | NR |
Notes:
- Both Covestro entries use 60 × 60 × 2 mm specimens at 500 bar. Most other sources do not state specimen thickness.
- The TPE range depends on grade, gate and application. It is not a universal allowance.
- The HDPE row is a catalogue reference, not a single grade.
- Values change with grade, filler content, wall thickness, gate position, mould temperature, packing and cooling history.
Nominal operating range
A published range is only useful if you know its assumptions: the specimen, conditioning period, process settings, measurement timing and direction. ASTM D955 measures at 24 and 48 hours after moulding. If your inspection happens at a different age, record it.
Allowable tolerance and boundary limits
The ends of a chart range are not production tolerances. Shrinkage is what the material does. Tolerance is what the part is permitted to deviate. ISO 20457:2026 covers tolerances and acceptance conditions for moulded parts, and it is a separate matter from shrinkage.
Link your chosen value to the part's functional tolerance and the process variation you expect.
Safe operating margin
Do not rely on the midpoint of a broad range for a critical part. No source gives a universal safe percentage. MOCOM recommends warpage analysis at the design stage and checking a geometrically comparable mould under planned processing conditions.
Run a measured trial or simulation when:
- Tolerances are tight
- Geometry is complex
- The material is filled or semi-crystalline
Key Technical Properties, Specification, Measurement and Validation
Shrinkage depends on material structure, direction, time, temperature, pressure, geometry and how you measure it.
Amorphous versus semi-crystalline
Covestro gives a class benchmark: under 1% for unreinforced amorphous thermoplastics and over 1% for unreinforced semi-crystalline ones. These are broad tendencies, not guaranteed limits.
The chart shows it. Unfilled PC sits at 0.65–0.70%, while unfilled PA66 reads 1.50–1.80%. Slow cooling allows more crystallisation and more shrinkage. Rapid cooling can lower initial shrinkage but increase post-shrinkage later.
Directional shrinkage and reinforcement
Unreinforced amorphous materials are relatively isotropic. Semi-crystalline and reinforced materials are not.
PA66 shows it clearly. Unfilled A3K reads 1.50/1.80% (P/N). The 30% glass-filled A3WG6 reads 0.4/0.9%. Fibres cut the numbers, but the gap between directions remains, and in PA66-GF30 it is more than double.
Covestro's Figure 7 compares linear shrinkage ranges across thermoplastics, though it is a comparison diagram, not a grade-specific tooling chart.
Stability, moisture and post-mould change
Nylon needs particular care. Moisture uptake is a separate quantity from moulding shrinkage:
| Grade | Equilibrium uptake (23 °C/50% RH) | Saturation (water, 23 °C) |
|---|---|---|
| PA6 Ultramid B3S | 2.6–3.4% | 9–10% |
| PA66 Ultramid A3K | 2.5–3.1% | 8–9% |
These are mass percentages. BASF reports a length increase of 0.2–0.3% per 1% absorbed water. ISO 1110 covers accelerated conditioning of polyamides toward 23 °C/50% RH, so condition before final inspection.
Victrex notes that post-crystallisation and stress relaxation cause later dimensional change, and annealing can reduce it.
Specification and documentation
Record the following with any shrinkage value:
- Exact grade and formulation
- Filler percentage, colour and additive package
- Test method, direction and specimen geometry
- Processing assumptions and conditioning state
- Whether the value is rated, typical or production-tested
Measurement and verification methods
Measure the cavity reference dimension and the cooled part with calibrated equipment, at several locations and in both directions. Then apply the formula above.
ISO 294-4 compares cavity and part dimensions at room temperature, with measurement to 0.02 mm. Know which stage your data comes from:
- Laboratory screening: standard plaques, good for ranking materials
- Supplier datasheet: comparison values under stated conditions
- First-off mould trials: your tool, your gate, your process
- Production capability data: repeatability across a run
Protolabs, for example, inspects three pieces and up to five features for a partial first article, and uses at least 30 parts for a production capability report. These are supplier practices, not standards.
Log cavity dimensions, process settings, cooling time, conditioning time and measurement temperature.
Application-specific validation and scale-up
A staged workflow works best:
- Review supplier data for the exact grade
- Select an initial design value
- Run a controlled mould trial
- Measure shrinkage by direction and feature
- Adjust the tool or process
- Confirm repeatability at scale-up
For grade selection and commercial supply, Distil offers engineering plastics including PA6, PA66 and PA66-GF30 in standard, reinforced and flame-retardant grades, with material-selection support for processors and OEMs.
Implications of Operating Outside the Recommended Range and Common Misinterpretations
A wrong shrinkage assumption produces:
- Undersized or oversized parts
- Sink marks and warpage
- Poor fit, seal or clearance problems
- Rework and scrap
An Autodesk Moldflow validation shows this in simulation. For a 40 × 40 × 2 mm PA6 part with simulated shrinkage of 1.64%:
| Cavity allowance | Simulated outcome |
|---|---|
| 1.0% | Undersized; tension and inward bowing |
| 1.64% | Virtually zero deviation |
| 2.0% | Oversized; compression and outward bowing |
This is a numerical case, not a production scrap study. Estimate scrap cost from your own yield data rather than from the simulation alone.
Common misinterpretations
- Treating a nominal chart value as an absolute limit
- Ignoring flow-direction and wall-thickness effects
- Applying test-bar data directly to complex parts
- Assuming a new material will shrink like the old one
When to escalate
Bring in engineering review, mould-flow analysis, prototype tooling, extra cavities or a controlled material trial when:
- Tolerances are tight
- The part is a large flat moulding
- The material is fibre-filled
- The application is medical or electrical
Shrinkage data alone does not establish compliance for those uses.
Conclusion
Shrinkage is a governing design and process parameter. Use the chart as a starting point for comparison. Base final tooling decisions on grade-specific data, documented assumptions and measured validation.
Reliable dimensions come from collaboration between the material supplier, product designer, mould maker, moulder and technical R&D team.
Frequently Asked Questions
What is an acceptable shrinkage percentage?
It depends on the resin grade, part geometry, dimensional tolerance, process and application. Use the chart as a reference, not a universal limit.
How is shrinkage value calculated?
Subtract the cooled part dimension from the cold cavity dimension, divide by the cavity dimension, and multiply by 100. State the reference dimension and measurement timing, such as 24 or 48 hours.
What are the material properties of ABS plastic?
For Terluran GP-22, notched Charpy impact is 22 kJ/m² at 23 °C and HDT is 94 °C at 1.80 MPa. It is an easy-flow general-purpose grade with shrinkage of 0.4–0.7%.
Why does plastic shrinkage vary by material grade and moulding conditions?
Polymer structure, fillers, wall thickness, packing pressure, mould temperature, cooling rate and gate design all change it. Measurement timing matters too.
How do amorphous and semi-crystalline plastics differ in shrinkage?
Unreinforced amorphous plastics generally shrink under 1% and are relatively isotropic. Semi-crystalline ones generally exceed 1%, vary more by direction and respond more to cooling rate.
How can shrinkage be reduced in injection moulding?
Keep walls uniform, balance gating, cool evenly and pack to gate freeze. Choose a suitable material and confirm with controlled trials.
