Plastic Shrinkage Rates Table

Molded plastics shrink between about 0.4 percent for general-purpose polystyrene and 2.1 percent for acetal (POM) as they cool in the mold. This page lists the typical rate for each common injection molding resin, split into flow and cross-flow values where the producer publishes both, all taken from resin producer datasheets. The shrinkage mechanism itself, and how the molding cycle compensates for it, is covered on the injection molding hub; this table exists for dimensional planning.

How to read these shrinkage rates

Shrinkage is quoted as a percentage of the molded dimension. A 1.5 percent rate on a 100mm dimension means the part comes out about 1.5mm smaller than the cavity. Two numbers matter for every resin: the shrinkage along the melt flow direction and the shrinkage across it.

Flow and cross-flow are separate numbers

Semi-crystalline resins, and glass-filled grades especially, shrink differently along and across the flow. BASF’s 30 percent glass-filled PA66, A3EG6, lists 0.5 percent parallel to flow and 1.0 percent normal to it, so one direction of the same part shrinks twice as much as the other. That difference, not the average, is what warps parts. Unfilled acetal is the opposite case: BASF’s N2320 003 AT lists 2.10 percent parallel and 2.00 percent normal, close to isotropic, which is one reason POM still molds precise gears. Victrex PEEK 450G sits between, at 0.90 percent along flow and 1.3 percent across.

Amorphous and semi-crystalline resins behave differently

The three amorphous resins in the table, ABS, polystyrene, and polycarbonate, all land between roughly 0.4 and 0.8 percent, and their flow and cross-flow values stay close. The semi-crystalline group, PP, nylon, POM, and PEEK, starts near 0.9 percent and runs past 2 percent. Crystals pack tighter than random polymer chains, so semi-crystalline grades lose more volume as they freeze. That single fact explains most of the table.

Shrinkage rates by material

Each row states the producer’s published range or grade value. Producers cite ISO 294-4 as the mold-shrinkage test basis; the US styrenics sheets cite ASTM D955. Grade choice within a family moves the number, so treat family position as context and the grade datasheet as the value that counts.

POM and PP anchor the high end

Acetal copolymer is the high-shrinkage benchmark at 2.0 to 2.1 percent, nearly the same in every direction. Polypropylene spans 1.0 to 2.0 percent; Borealis prints that range for its RE420MO grade and states that wall thickness and molding parameters set where in the range a part lands.

Glass filler cuts shrinkage and adds direction

Glass fibers do not contract as the resin freezes, so swapping resin for fiber shrinks the whole part less. Moving from BASF’s unfilled A3K PA66, at 1.50 percent parallel and 1.80 percent normal, to the 30 percent glass A3EG6, at 0.5 percent parallel and 1.0 percent normal, cuts flow shrinkage to a third. The trade is anisotropy: the filled grade’s flow-to-cross-flow gap widens to 0.5 percentage points, and fibers aligned by flow pull dimensions toward the flow axis.

Using a rate to size a mold cavity

The first-pass rule is to divide the target dimension by 1 minus the shrinkage as a decimal, or just add the percentage to the steel, which is close enough at these rates. A 100mm PP dimension at the 1.5 percent midpoint needs a cavity near 101.5mm. Run the same sum at both ends of the range and the stakes appear: at 1.0 percent the cavity is 101.0mm, at 2.0 percent it is 102.0mm, and that 1.0mm spread is a process window a molder has to sit inside, not a rounding error.

Why the number is a range, not a constant

Borealis hangs its 1 to 2 percent PP range directly on wall thickness and molding parameters. Covestro’s Makrolon 2805 sheet separates a tested value, 0.65 percent parallel and 0.70 percent normal, from a 0.60 to 0.80 percent practical range drawn from field experience. Pack harder, run a hotter mold, or thicken a rib, and the same grade moves inside its band. Tooling decisions therefore lock the grade and the processing window together.

Moisture moves nylon after molding

Nylon is the one common resin whose dimensions keep changing in service. BASF’s A3K datasheet lists 2.5 to 3.1 percent moisture absorption at equilibrium, 23°C and 50 percent relative humidity, and 8 to 9 percent water uptake at saturation, and parts swell as they absorb. A nylon part measured dry out of the press is not the part you will measure in a humid plant a week later, so dimension nylon to its conditioned state.

Limitations

These are typical values for material families and named grades, gathered from producer datasheets (BASF editions 02/2026, Covestro, Borealis 2025, SABIC, Victrex), checked in August 2026. They support material comparison, tolerance budgeting, and first-pass cavity sizing. The number that governs a tool is the one on the datasheet of the exact grade being bought, confirmed with the molder against the part’s geometry, gating, and wall thickness. For the physics of shrink and warpage control in the molding cycle, see the injection molding hub, and for selection factors beyond shrinkage, see the materials overview.