Mold shrinkage chart, melt temp and mold temp for 35 resins

Mold shrinkage, melt temperature, mold temperature and barrel residence time for 35 thermoplastic families, from manufacturer processing guides and technical data sheets. Shrinkage is shown along and across flow where they differ, because glass-filled grades can shrink several times more across flow than along it. A source note is at the bottom. Companion to our resin drying chart.

How to read this chart

These are family-level typical values from manufacturer processing guides. They are a starting point for a process and a sanity check on a data sheet, not a substitute for the sheet on the grade you are actually running. Where suppliers disagree materially, both figures appear.

Processing parameters by resin

Scroll the table sideways to see all columns

Eight things worth knowing before you use these numbers

1. Amorphous and semi-crystalline behave differently, with two exceptions

Amorphous resins (PC, ABS, SAN, PS, PMMA, PEI, sulfones, modified PPE) mostly sit in a narrow band of about 0.3–1.0% and are close to isotropic. Semi-crystalline resins (nylon, acetal, PBT, PP, PE, PPS, PEEK) shrink about 1–3% unfilled and are direction- and crystallinity-sensitive. Two exceptions catch people out: ASA is not isotropic: the commonly quoted “0.5–0.9%” is the two directions, not a grade range. And filled sulfones are not isotropic either, despite blanket statements to the contrary; one producer’s 30% glass PSU is 0.29 along flow and 0.46 across.

2. Acetal decomposition is the highest-consequence line on this page

One producer’s guide: acetal copolymer should never be heated above 446°F (230°C), nor remain above 380°F for more than 15 minutes without purging. Beyond that the resin degrades and releases formaldehyde, and if venting is inadequate, pressure builds in the equipment. A homopolymer guide: decomposition is almost entirely to gaseous products, so pressure build-up can be rapid.

3. Residence time and melt temperature are one parameter, not two

Every supplier that publishes seriously publishes a curve, not a single number. One guide puts it directly: residence time should be as short as possible when molding near the maximum suggested melt temperature. A “max residence” figure is only valid at the low end of the melt range. A polycarbonate guide adds the opposite caution: below about 4 minutes you get an insufficiently compounded melt.

4. Mold temperature sets crystallinity, and the trade runs both ways

Mold shrinkage rises with mold temperature; post-mold shrinkage falls. Cold-molded semi-crystalline parts measure fine out of the press and then move in service. Three specifics: PEEK needs a 170°C minimum tool surface or you get amorphous parts that crystallize later above Tg. PPS has a forbidden band: 190–260°F yields unpredictable crystallinity and should be avoided. And injection-grade PET needs a hot mold, the opposite of preform tooling, which is one more reason never to read bottle-resin numbers across. For acetal homopolymer, annealing will not rescue a cold-molded part. It produces uncontrolled deformation instead.

5. Measure at 24 hours minimum, 48 for polyolefins

One polyolefin producer: a polyolefin part reaches about 90% of total shrinkage after 48 hours, and shrinkage continues for days if parts are packed hot. Another producer anchors its published values to 24 hours after molding. Force-cooling parts to measure sooner (in a fridge or under water) measures thermal contraction, not shrinkage.

6. Hold pressure and gate freeze set the number as much as the resin does

One producer’s guide: shrinkage begins the moment cavity pressure reaches atmospheric, and that point moves with the holding-pressure profile, so shrinkage can be influenced directly. Another notes polystyrene shrinkage may be effectively zero in zones under high follow-up pressure. Every published figure is conditional on a specific plaque geometry and set of conditions.

7. Nylon and acrylic grow after molding

One nylon guide: in a well-molded nylon part of moderate wall thickness, expansion from water absorption dominates the shrinkage from increasing crystallinity. Acrylic does the same: one producer publishes corrections to be added: +0.1% at 40% RH, +0.2% at 65%, +0.3% at 80%. On a 0.4% nominal shrinkage that is most of the budget.

8. ISO and ASTM values are not interchangeable

ISO 294-4 and ASTM D955 use different specimens, hold conditions and timing, and reference thicknesses differ (2 mm vs 3.2 mm). One producer publishes ISO plaque shrinkage of 1.50/1.80% for unfilled PA66 but a constrained real-part value of 0.81%, roughly half. Another says it flatly: shrinkage values measured on standard ASTM or ISO test bars should be used for material comparison only, not for tool design.

Some parameters aren’t indexed to the resin at all

A family-level row will mislead you in four cases. TPU melt temperature tracks Shore hardness: 190–205°C at 70A versus 215–235°C at 74D, so a soft grade run at the family maximum will degrade. Flexible PVC melt temperature and shrinkage both track durometer. TPV flow-direction shrinkage falls from 4.1% at 49A to 1.5% at 94A while cross-flow stays flat near 0.7%. Hardness changes the anisotropy itself. And PA12 melt temperature is grade-driven: unfilled runs 230°C, 30% glass runs 270°C.

One thing that surprised us

Engineering injection grades of PET are almost always glass- or mineral-filled. Bottle-resin numbers do not apply to them. If you find an “unfilled injection PET” shrinkage figure on a chart somewhere, it was almost certainly borrowed from bottle resin, which is processed under completely different conditions.

Common questions

Qualifying a grade and want these confirmed against the producer’s current data sheet? Send us the grade. A polymer chemist reviews it.