Plastic Shrinkage Calculator: Rates by Material (Full Table)

A plastic shrinkage calculator is only as good as the input data feeding it. Use wrong shrinkage values and your cavity dimensions will be off, your parts will reject, and you will spend $8,000 to $25,000 on rework or steel removal. This reference covers 30-plus resins, the math behind cavity sizing, and how to distinguish shrinkage from warp before you cut steel.

What the Standard Actually Says

ASTM D955 is the governing test method for measuring mold shrinkage of thermoplastics. It defines shrinkage as the difference between the mold cavity dimension and the molded part dimension at 24 hours post-ejection, divided by the mold cavity dimension, expressed as a percentage. ISO 294-4 covers the same measurement for the international market and uses millimeters per millimeter (mm/mm) rather than percent, though the math is identical.

Published shrinkage values from resin datasheets are measured on flat test plaques, typically 60 mm x 60 mm x 2 mm, under controlled injection conditions. Real-world part shrinkage can deviate 15% to 40% from those datasheet values depending on wall thickness, gate location, flow direction, pack pressure, and cooling rate. Treat published values as a starting point, not a guarantee.

Two directions matter on every part: flow direction (parallel to melt flow) and transverse direction (perpendicular to flow). Semi-crystalline resins like polypropylene and nylon shrink significantly more in the transverse direction. Amorphous resins like ABS and polycarbonate shrink nearly isotropically, meaning flow and transverse values are close to each other.

How to Calculate Plastic Shrinkage: The Core Formula

The cavity sizing formula is straightforward. You work backward from your finished part dimension to the steel dimension you need to cut.

The formula is:

Cavity Dimension = Part Dimension / (1, Shrinkage Rate)

Shrinkage rate is expressed as a decimal. A 1.8% shrinkage rate enters the formula as 0.018.

Worked Example: Polypropylene Housing

Your part drawing calls for a 150.00 mm outside diameter on a polypropylene (PP) housing. Your resin datasheet shows a polypropylene shrinkage rate of 1.5% to 2.0% in the flow direction. You have a center gate, so flow is radial and relatively uniform. You pick 1.8% as your nominal value.

Cavity Dimension = 150.00 / (1, 0.018) = 150.00 / 0.982 = 152.75 mm

You cut the cavity to 152.75 mm. After first shots, you measure the part. If it comes in at 150.30 mm, your actual shrinkage was 1.6%, not 1.8%. You steel-safe on the cavity side and add 0.10 mm of steel rather than removing it. This is why we always bias cavity dimensions toward the steel-safe condition when shrinkage uncertainty spans a range.

Shrinkage vs. Warpage: Do Not Confuse Them

Shrinkage is volumetric reduction. Every part shrinks. Warpage is differential shrinkage across a part geometry that causes distortion, bow, or twist. A flat panel that measures the correct overall length but is bowed 2 mm in the middle has a warp problem, not a shrinkage problem. Fixing it requires cooling circuit changes, gate relocation, or wall thickness balancing, not a cavity dimension change. Our project managers flag this distinction in every first-shot review because mixing them up wastes sampling cycles.

Mold Shrinkage Chart: 30+ Resins

Values below are compiled from resin supplier datasheets and ASTM D955 test data. Flow and transverse values are given where the difference is meaningful. Use these as your starting point and confirm against your specific grade’s datasheet. Filled grades (glass, mineral, carbon fiber) shrink less than unfilled grades and are noted separately.

Resin	Abbreviation	Shrinkage Flow (%)	Shrinkage Transverse (%)	Type	Notes
Acrylonitrile Butadiene Styrene	ABS	0.4 to 0.7	0.4 to 0.7	Amorphous	ABS shrinkage rate is nearly isotropic
ABS + Polycarbonate Blend	ABS/PC	0.4 to 0.7	0.4 to 0.7	Amorphous	Widely used in enclosures
Polypropylene, Unfilled	PP	1.5 to 2.0	1.8 to 2.5	Semi-crystalline	High transverse vs. flow delta
Polypropylene, 20% GF	PP-GF20	0.4 to 0.8	1.0 to 1.5	Semi-crystalline	Glass reduces flow shrinkage more than transverse
Polypropylene, 40% Talc	PP-T40	0.8 to 1.2	1.0 to 1.6	Semi-crystalline	Common in automotive interior
Polyethylene, High Density	HDPE	1.5 to 3.0	1.5 to 3.0	Semi-crystalline	Wide range; wall thickness sensitive
Polyethylene, Low Density	LDPE	1.5 to 4.0	1.5 to 4.0	Semi-crystalline	Flexible applications
Polycarbonate	PC	0.5 to 0.7	0.5 to 0.7	Amorphous	Tight tolerance parts; low shrinkage variance
Nylon 6	PA6	0.6 to 1.4	1.0 to 2.0	Semi-crystalline	Moisture content at mold time affects values
Nylon 6/6	PA66	0.8 to 1.5	1.5 to 2.5	Semi-crystalline	Higher transverse shrinkage than PA6
Nylon 6/6, 30% GF	PA66-GF30	0.3 to 0.6	0.8 to 1.5	Semi-crystalline	Structural hardware, brackets
Polyoxymethylene (Acetal)	POM	1.8 to 2.5	1.8 to 2.5	Semi-crystalline	High shrinkage; gear and bearing housings
Polyethylene Terephthalate	PET	0.2 to 0.8	0.8 to 1.5	Semi-crystalline	Amorphous when quench-cooled
Polybutylene Terephthalate	PBT	1.5 to 2.2	1.8 to 2.5	Semi-crystalline	Electrical connectors; flame retardant grades common
PBT, 30% GF	PBT-GF30	0.2 to 0.5	0.8 to 1.3	Semi-crystalline	Connector housings
Polystyrene, General Purpose	GPPS	0.3 to 0.6	0.3 to 0.6	Amorphous	Brittle; optical clarity grades available
High Impact Polystyrene	HIPS	0.4 to 0.7	0.4 to 0.7	Amorphous	Slightly higher than GPPS due to rubber phase
Styrene Acrylonitrile	SAN	0.3 to 0.7	0.3 to 0.7	Amorphous	Chemical resistance over GPPS
Polymethyl Methacrylate	PMMA	0.2 to 0.6	0.2 to 0.6	Amorphous	Acrylic; optical parts
Polyvinyl Chloride, Rigid	PVC-R	0.2 to 0.6	0.2 to 0.6	Amorphous	Corrosive offgas; requires stainless tooling (420SS)
Polyvinyl Chloride, Flexible	PVC-F	1.0 to 2.5	1.0 to 2.5	Amorphous	Plasticizer content drives wide variance
Thermoplastic Polyurethane	TPU	0.5 to 2.0	0.5 to 2.0	Semi-crystalline	Hardness (Shore A/D) affects shrinkage
Thermoplastic Elastomer	TPE	0.5 to 2.5	0.5 to 2.5	Varies by grade	Confirm with specific compound supplier
Polyphenylene Oxide Blend	PPO/PPE	0.5 to 0.8	0.5 to 0.8	Amorphous	Noryl-type blends
Polyphenylene Sulfide	PPS	0.2 to 0.6	0.4 to 1.0	Semi-crystalline	High-temperature; H13 tooling recommended
PPS, 40% GF	PPS-GF40	0.1 to 0.3	0.3 to 0.6	Semi-crystalline	Automotive under-hood; mold temp 135 to 150C
Polyetheretherketone	PEEK	0.9 to 1.4	1.1 to 1.7	Semi-crystalline	Mold temp 160 to 200C; H13 or S7 tooling
PEEK, 30% CF	PEEK-CF30	0.1 to 0.4	0.5 to 1.0	Semi-crystalline	Abrasive; requires hardened steel, min 52 HRC
Liquid Crystal Polymer	LCP	0.0 to 0.2	0.5 to 1.5	Semi-crystalline	Extreme anisotropy; gate location is critical
Polyetherimide	PEI	0.5 to 0.7	0.5 to 0.7	Amorphous	Ultem; high mold temp 150 to 175C
Polysulfone	PSU	0.5 to 0.7	0.5 to 0.7	Amorphous	Medical and fluid handling
Cyclic Olefin Copolymer	COC	0.5 to 0.7	0.5 to 0.7	Amorphous	Optical clarity; low moisture absorption

Shrinkage rate by material varies most dramatically between amorphous and semi-crystalline families. Amorphous resins solidify without an ordered crystal structure and shrink 0.2% to 0.9% in most cases. Semi-crystalline resins form ordered crystal lattices during cooling and release that volume as they solidify, producing shrinkage rates from 1.5% to 4.0% for unfilled grades.

How Processing Conditions Shift Your Shrinkage Values

The mold shrinkage chart above gives you the range. Your process determines where inside that range you actually land. Four variables move the needle most.

• Pack pressure: Higher pack pressure forces more material into the cavity and reduces shrinkage. A 10% increase in pack pressure on an unfilled PP part typically reduces shrinkage by 0.1% to 0.2%.
• Mold temperature: Higher mold temperature gives semi-crystalline resins more time to crystallize fully. POM run at 90C mold temperature will shrink more than POM run at 60C because crystallinity is higher.
• Wall thickness: Thicker walls cool more slowly, allowing more crystallization and higher shrinkage. A 4 mm PP wall will shrink more than a 2 mm PP wall of the same part, often by 0.3% to 0.5%.
• Gate size and location: Undersized gates freeze off before pack is complete, cutting pack efficiency and increasing shrinkage. Gate freeze-off time should be confirmed by gate seal study during process development.

In our shops in China, we document nominal shrinkage, confirmed shrinkage after T1 sampling, and process window boundaries in a single qualification record. That record travels with the mold and prevents the next process engineer from recalculating from scratch when the program moves to a new press or facility.

Applying Shrinkage to Cavity Sizing: Steel-Safe Strategy

When shrinkage spans a range, you need a bias direction before you cut. The rule is simple: bias toward the condition where you can add steel rather than remove it. Adding steel means welding, which weakens P20 tool steel and is essentially not possible on hardened H13 without full re-heat treatment. Removing steel is clean machining.

For an external dimension (an outside diameter or a width across a housing), the part grows if the cavity is larger. You want the first shots to run slightly large on the part, which means you cut the cavity slightly small. You then open the cavity (remove steel) to grow the part if needed. This is the steel-safe direction for external dimensions.

For an internal dimension (a bore, a slot, a pocket), the part grows if the core is smaller. Cut the core slightly large