Plastic Shrinkage Explained: Values, Causes and Dimensional Control

Every moulded plastic part is smaller than the cavity that produced it, and the toolmaker has to know by how much before cutting steel. Get the allowance wrong and you have a tool that produces dimensionally incorrect parts from the first shot, correctable only by welding and remachining.

Shrinkage is therefore one of the few plastics properties where the cost of being wrong is paid up front and in full.

The two shrinkages

Mould shrinkage is the difference between cavity dimension and part dimension, measured after the part has cooled and stabilised – conventionally 24 to 48 hours after moulding, at controlled temperature.

Post-mould shrinkage continues after that. Semi-crystalline polymers keep crystallising slowly for days or weeks, and the part keeps getting slightly smaller. Where dimensions are critical, this is why parts are sometimes annealed – to complete the shrinkage deliberately and predictably rather than have it happen in the customer’s assembly.

Measure at a consistent interval after moulding. A part measured at 1 hour and the same part measured at 48 hours are different sizes, and comparing across inconsistent intervals produces data that cannot be interpreted.

Typical mould shrinkage by polymer

Polymer	Typical mould shrinkage	Structure
PS	0.4 – 0.7%	Amorphous
ABS	0.4 – 0.9%	Amorphous
PC	0.5 – 0.7%	Amorphous
PMMA	0.3 – 0.8%	Amorphous
PVC (rigid)	0.2 – 0.6%	Amorphous
PA 6 / PA 66	0.7 – 2.0%	Semi-crystalline
PP	1.0 – 2.5%	Semi-crystalline
HDPE	1.5 – 4.0%	Semi-crystalline
LDPE	1.5 – 3.5%	Semi-crystalline
POM	1.8 – 2.5%	Semi-crystalline
PET (semi-crystalline)	1.2 – 2.0%	Semi-crystalline
Glass-filled grades	0.2 – 1.0%, strongly directional	Varies

Treat these as ranges to start a conversation, not as design values. Actual shrinkage for a specific grade in a specific part comes from the material supplier’s data sheet and, ultimately, from a moulding trial. The spread within a single polymer is wider than the difference between some polymers.

Why semi-crystalline polymers shrink so much more

An amorphous polymer solidifies into a disordered glass. Its molecules occupy roughly the same amount of space as a solid as they did as a melt, so contraction is thermal only – modest and predictable.

A semi-crystalline polymer does something extra. On cooling, sections of chain fold into ordered crystalline regions where molecules pack far more densely than in the amorphous melt. That density increase is additional volume loss on top of thermal contraction, and it is why PP shrinks three to five times more than ABS.

The important consequence: anything that changes crystallinity changes shrinkage.

– Slower cooling gives more time to crystallise, so more crystallinity and more shrinkage. A hot mould shrinks more than a cold one. – Nucleating agents give crystals more starting points, producing more, smaller crystals – and changing both the rate and the final level. – Some pigments nucleate. This is the mechanism behind a genuinely awkward production reality, covered below.

What changes shrinkage on the machine

Variable	Increase it and shrinkage…	Why
Holding pressure	decreases	More material packed in
Holding time	decreases, until gate freeze	Same, while the gate is open
Melt temperature	increases	More thermal contraction
Mould temperature	increases	More crystallinity develops
Cooling time	decreases	Part stabilises in the constraint of the tool
Wall thickness	increases	Slower cooling, more crystallinity
Injection speed	varies	Changes orientation pattern

Holding pressure is the strongest process lever, and it is also the one most often already at its practical limit.

Directional shrinkage

Shrinkage is rarely equal in all directions.

In unfilled semi-crystalline polymers, molecular orientation along the flow means shrinkage is typically somewhat greater along the flow than across it.

In glass-filled grades the effect reverses and intensifies. Fibres align with the flow and physically restrain contraction along their axis, so the part shrinks much less along the flow than across it. Differences of a factor of two or more are normal.

This is why a flat glass-filled plaque gated at one end will bow, and why the fix is gate position rather than holding pressure. Anisotropic shrinkage is also the reason a filled part’s dimensions can be excellent in one axis and out of tolerance in another with no process fault at all. See warpage in injection moulding.

Where colour comes in

This is small, real, and worth knowing because it produces a confusing production symptom.

Some pigments act as nucleating agents in semi-crystalline polymers. Certain organic pigment classes – some phthalocyanine blues and greens are the classic example – provide surfaces on which crystallisation starts. More nucleation sites means a different crystallisation rate and a different final crystallinity, and therefore a different shrinkage.

The practical consequence: the same tool, same machine, same settings and same base resin can produce a slightly different dimension in blue than in natural. The effect is small – typically a fraction of the total shrinkage – but on a tight-tolerance part it is enough to matter.

Three things follow:

Qualify dimensionally critical parts in each production colour, rather than approving in natural and assuming the rest follow.

Keep the colour formulation stable. If a supplier substitutes a pigment for supply reasons, the nucleation behaviour can change even though the shade is matched. Require change notification.

Be careful when moving a part between colours mid-programme. A part running to tolerance in black may sit differently in a new shade, and that is chemistry, not a fault.

At normal dosages the carrier resin’s contribution to shrinkage is negligible. At the high dosages used in pipe black or high-opacity white it is not, which is another reason the carrier belongs on the specification.

We keep pigment sourcing and carrier consistent lot to lot precisely so that dimensions stay where they were qualified – see why colour drifts between batches.

Getting a tool cut correctly

1. Get grade-specific shrinkage data from the resin supplier, not a generic polymer table 2. Account for direction – specify flow and cross-flow separately for filled materials 3. Consider steel-safe machining on critical dimensions, so material can be removed later but not added 4. Mould a trial tool or a prototype for genuinely critical geometry 5. Qualify in the production colour on tight-tolerance parts 6. Fix the measurement protocol – interval after moulding, temperature, humidity, datum scheme – before collecting any data

Point 3 is the one that saves programmes. Cutting a cavity slightly small on a critical dimension is cheap insurance; welding steel back into a cavity that was cut oversize is neither cheap nor reliable.

Talk to us

If you are qualifying a dimensionally critical part in multiple colours, tell us the polymer, the tolerance and the shades and we will keep pigment chemistry and carrier consistent across them, and flag where a shade change is likely to move a dimension.

Related: injection moulding defects, sink marks, automotive, engineering polymer masterbatch, household.

FAQs

What is mould shrinkage in plastics? The difference between the cavity dimension and the finished part dimension, measured after the part has cooled and stabilised – conventionally 24 to 48 hours after moulding at controlled temperature. The tool must be cut oversize by this amount.

Why do semi-crystalline plastics shrink more than amorphous ones? Because crystallisation packs molecules far more densely than the amorphous melt, adding volume loss on top of ordinary thermal contraction. PP and HDPE shrink several times more than ABS or PC for this reason.

What is the shrinkage of polypropylene? Typically in the range of 1.0 to 2.5 percent, depending on grade, filler content, wall thickness and processing conditions. Use grade-specific data from the resin supplier rather than a generic figure when cutting a tool.

What is post-mould shrinkage? Continued dimensional change after the part has apparently stabilised, as semi-crystalline polymers keep crystallising slowly over days or weeks. Annealing completes it deliberately rather than allowing it to happen later in the customer’s assembly.

Does mould temperature affect shrinkage? Yes. A hotter mould cools the part more slowly, which allows more crystallinity to develop and produces greater shrinkage. It is one of the more powerful process levers, and it interacts with surface finish and cycle time.

Why do glass-filled parts shrink differently along and across the flow? Because the fibres align with the flow and physically restrain contraction along their axis. Shrinkage across the flow can be twice that along it or more, which is what bows a flat filled plaque gated at one end.

Can masterbatch change part dimensions? Slightly, in semi-crystalline polymers. Some pigments nucleate crystallisation, changing crystallinity and therefore shrinkage, so the same tool can give a marginally different dimension in different colours. Qualify tight-tolerance parts in each production shade.

When should a part be measured after moulding? At a consistent interval, long enough for the bulk of post-mould shrinkage to have occurred – commonly 24 to 48 hours at controlled temperature. What matters most is that every measurement uses the same interval so results are comparable.