Injection Molding Reference

Plastic Resin Shrinkage Rate Chart for Injection Molding

Plastic resin shrinkage reference chart comparing common injection molding materials
Typical resin-family shrinkage values are useful for early comparison, but exact commercial grade data should take precedence before final tool compensation.

Use this reference chart to compare typical molding shrinkage ranges across common injection molding resins, including ABS, PC, PMMA, PP, PE, POM, nylon, PBT and reinforced grades. These values are intended for early material comparison and tooling discussions—not as final cavity compensation values.

Quick Engineering Answer

Plastic molding shrinkage is the dimensional reduction that occurs as a molded resin cools and stabilizes. Amorphous resins often show lower and more predictable shrinkage ranges than many semi-crystalline resins, while glass-filled grades can show different shrinkage parallel and transverse to flow.

A generic shrinkage value should therefore be treated as a starting reference rather than a final steel-sizing instruction. Before final tooling dimensions are released, confirm the exact commercial resin grade, reinforcement level, supplier-reported shrinkage data, test method and project-specific validation results.

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Reference scope: this page is intended for resin-family comparison and early shrinkage screening. Exact supplier data and grade-specific validation take precedence when setting final tooling compensation.

View Shrinkage Rate Chart Material Selection Guide
Family-Level Reference Data

Plastic Resin Shrinkage Rate Chart

Compare typical molding shrinkage ranges across common thermoplastic resin families before selecting an exact commercial grade. The values below are useful for early material comparison and initial tooling discussion, but they should not be treated as universal mold-shrinkage factors. Resin grade, reinforcement level, test method and molding conditions can change the reported result, especially for semi-crystalline and fiber-filled materials.

Data Level Family-level comparison ranges, not final cavity compensation values.
Exact-Grade Override The selected resin manufacturer's current datasheet takes precedence.
Reinforced Grades Check filler percentage and separate flow / transverse shrinkage values where published.

← Swipe horizontally to compare all table columns →

Typical family-level molding shrinkage reference ranges for common injection molding thermoplastics.
Resin Common Name Polymer Structure Grade / Filler Context Typical Reference Shrinkage (%) Direction / Exact-Grade Check
ABS Acrylonitrile Butadiene Styrene Amorphous Generic unfilled family 0.40–0.70 Confirm the exact ABS grade and supplier-reported molding shrinkage before tool compensation.
PC Polycarbonate Amorphous Generic unfilled family 0.50–0.70 Grade-specific data takes precedence; transparent, UV, FR and reinforced PC grades should not be assumed to share one value.
PC/ABS PC / ABS Blend Amorphous blend Blend ratio varies by grade 0.40–0.70 Use the selected commercial blend's own datasheet; formulation differences can change the reported range.
PMMA Acrylic Amorphous Generic unfilled family 0.30–0.70 Confirm the exact optical or general-purpose grade and the supplier test conditions.
PS Polystyrene Amorphous Generic family reference 0.40–0.70 Confirm the exact grade rather than applying a family-level number directly to steel dimensions.
PP Polypropylene Semi-crystalline Unfilled family reference 1.50–2.20 Shrinkage is strongly grade- and process-dependent. Filled PP should be checked separately from unfilled PP.
HDPE High-Density Polyethylene Semi-crystalline Generic family reference 1.50–3.00 Confirm density, grade and supplier-reported molding shrinkage under the stated test method.
LDPE Low-Density Polyethylene Semi-crystalline Generic family reference 1.50–3.50 Use the exact grade datasheet; flexible resin families can span a broad molding-shrinkage range.
POM Acetal Semi-crystalline Generic unfilled family 1.80–2.50 Confirm the exact POM grade and whether the supplier reports direction-specific or test-condition-specific data.
PA6 Nylon 6 Semi-crystalline Unfilled family reference 0.70–2.20 Exact grade and reinforcement level are essential. Keep mold shrinkage separate from later moisture-driven dimensional change.
PA66 Nylon 66 Semi-crystalline Unfilled family reference 1.00–2.00 Confirm the exact grade and keep post-mold conditioning effects separate from molding shrinkage.
PA66-GF30 30% Glass-Filled Nylon 66 Filled semi-crystalline Approx. 30% glass fiber 0.30–0.80 Do not assume one isotropic value. Check separate flow and transverse shrinkage data where the supplier publishes them.
PBT Polybutylene Terephthalate Semi-crystalline Unfilled family reference 1.20–2.00 Confirm the commercial grade because filled, FR and electrical grades may use different shrinkage data.
PBT-GF30 30% Glass-Filled PBT Filled semi-crystalline Approx. 30% glass fiber 0.30–0.90 Directional data matters. Verify filler content and flow / transverse shrinkage values for the selected grade.
PET Polyethylene Terephthalate Semi-crystalline Generic family reference 1.20–2.00 Confirm exact grade, reinforcement and supplier test conditions; crystallinity affects the reported behavior.
TPU Thermoplastic Polyurethane Elastomer Hardness / chemistry dependent 0.80–2.00 Verify the exact TPU chemistry and hardness grade; family-level values cover a broad range.
TPE Thermoplastic Elastomer Elastomer Multiple TPE families 1.00–2.50 TPE is not one formulation. Use the exact supplier grade rather than a universal TPE shrinkage factor.
PPS Polyphenylene Sulfide Semi-crystalline Often supplied in filled grades 0.60–1.50 Filler type and percentage are critical. For reinforced grades, check separate directional shrinkage data where available.
PEI Polyetherimide Amorphous High-temperature family 0.50–0.80 Confirm the specific commercial grade, filler system and supplier-reported test method.
PEEK Polyetheretherketone Semi-crystalline Unfilled family reference 1.00–1.50 Unfilled, glass-filled and carbon-filled PEEK grades can differ substantially. Use the exact grade's directional data where specified.
How to Use This Chart

Treat each percentage above as a family-level comparison range, not as a final mold-shrinkage setting. Before releasing steel dimensions, replace the generic family assumption with the exact commercial resin grade and its current supplier data. For reinforced materials, confirm filler percentage and separate flow / transverse values where reported. Supplier data may reference methods such as ASTM D955 or ISO 294-4; use the method and conditioning stated for the selected grade rather than mixing values from different test conditions.

Important: molding shrinkage and post-molding dimensional change are not necessarily the same measurement. Moisture uptake, conditioning and later dimensional stabilization are addressed separately in this guide.

Why Shrinkage Behavior Differs

Amorphous vs Semi-Crystalline Resin Shrinkage

Polymer structure is one of the main reasons that molding shrinkage differs across resin families. Amorphous and semi-crystalline plastics solidify differently as they cool, so their typical shrinkage ranges—and the amount of variation engineers should expect—are not the same.

Amorphous plastics cool without forming a highly ordered crystalline structure. Because the polymer chains remain relatively disordered, their family-level molding shrinkage is often lower and narrower than that of many semi-crystalline materials. ABS, polycarbonate and PMMA are common examples. This does not mean every amorphous grade has the same shrinkage value; additives, reinforcement and the exact commercial formulation still matter.

Semi-crystalline plastics undergo an additional structural change as crystalline regions form during cooling. This crystallization contributes to volumetric contraction, which is one reason materials such as PP, polyethylene, POM and unfilled nylon often show higher or broader shrinkage ranges than typical amorphous resins.

Why the Difference Matters When Reading a Shrinkage Chart

The distinction helps explain why a generic resin-family percentage should be treated as a comparison value rather than a universal tooling factor. A relatively narrow amorphous-resin range may provide a useful early reference, while a semi-crystalline resin often requires closer attention to the exact grade and the conditions under which the supplier measured shrinkage.

Reinforcement introduces another layer of behavior. Glass-filled grades may show lower overall shrinkage than their unfilled counterparts, but the shrinkage can become strongly directional because fibers align with the melt flow. That difference between flow and transverse shrinkage is covered separately in the reinforced-resin sections below.

Engineering Interpretation

Amorphous does not mean “no shrinkage,” and semi-crystalline does not mean “uncontrollable.” The distinction explains the general behavior of the polymer family. The exact commercial resin grade, reinforcement level and supplier data still determine the appropriate project-specific reference value.

Reinforcement Effect

Unfilled vs Glass-Filled Resin Shrinkage

Adding glass fiber can reduce the overall molding shrinkage of many thermoplastics, but it also changes how that shrinkage is distributed through the part. For reinforced grades, the direction of material flow becomes just as important as the headline shrinkage percentage.

Comparison of unfilled and glass-filled resin shrinkage behavior in injection molding
Reinforcement can reduce overall resin shrinkage while increasing the importance of flow direction and anisotropic behavior.

Unfilled grades rely primarily on the shrinkage behavior of the polymer itself. Their reported molding-shrinkage range is still affected by the exact commercial formulation and molding conditions, but the family-level value is often easier to use as an early comparison reference than it is for a strongly reinforced grade.

Glass-filled grades often show lower overall shrinkage because rigid fibers restrict polymer contraction. For example, PA66-GF30 or PBT-GF30 may have a lower headline shrinkage range than the corresponding unfilled resin. However, the reinforcement also introduces anisotropy: fibers tend to orient with melt flow, so shrinkage measured parallel to flow can differ from shrinkage measured across the flow direction.

Lower Total Shrinkage Does Not Mean One Uniform Value

This is why a filled grade should not be represented by one universal percentage when more specific supplier data are available. A notation such as “PA66-GF30: 0.30–0.80%” is useful for family-level comparison, but the selected commercial grade may provide separate longitudinal and transverse values. Those directional values are more informative for final tooling assumptions than a single averaged number.

The same principle applies to other reinforced materials, including glass-filled PBT, PPS and PEEK. Filler type, filler percentage and commercial grade should therefore be recorded alongside the shrinkage value rather than treating “GF30” as a complete material specification.

Unfilled Grade

Polymer-Dominated Shrinkage

Family-level ranges mainly reflect the base polymer's cooling and solidification behavior. Exact commercial grade data still take precedence.

Examples: PA66, PBT, POM, PP, PEEK

Glass-Filled Grade

Lower Overall, More Directional

Glass fiber can restrain overall contraction while creating different shrinkage behavior parallel and transverse to material flow.

Examples: PA66-GF30, PBT-GF30, PPS-GF, PEEK-GF

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Next distinction: the following section explains why flow-direction and transverse shrinkage should be reviewed separately for reinforced grades.

Directional Shrinkage

Flow vs Transverse Shrinkage in Reinforced Plastics

Reinforced thermoplastics can shrink differently parallel and perpendicular to melt flow. This directional behavior is why one generic shrinkage percentage may be insufficient for glass-filled nylon, PBT, PPS, PEEK and other reinforced grades.

In an unfilled resin, a family-level shrinkage value is often presented as one broad reference range. In a fiber-reinforced grade, that simplification becomes less reliable because reinforcement tends to align with the moving melt during cavity filling.

As fibers align, they restrict polymer contraction more strongly in some directions than others. The result is anisotropic shrinkage: the measured shrinkage parallel to flow can differ from the value measured transverse to flow. The difference depends on the exact resin, reinforcement type, filler percentage and supplier test conditions.

Why One Average Shrinkage Number Can Be Misleading

A single range such as “0.3–0.8%” can still be useful for early comparison, but it does not describe how a reinforced grade behaves in every direction. When an exact supplier datasheet reports separate flow and transverse values, those values should be retained separately rather than combined into one average number.

This distinction is particularly important when comparing different reinforced commercial grades. Two materials may have a similar overall shrinkage range while showing very different directional behavior. For that reason, the material name alone—such as “PA66-GF30”—is not enough to define a final shrinkage assumption.

Data Field What It Describes How to Use It
Flow Shrinkage Dimensional contraction measured parallel to the primary material-flow direction. Keep as a separate grade-specific reference when the resin supplier publishes it.
Transverse Shrinkage Dimensional contraction measured perpendicular to the primary material-flow direction. Compare directly with the flow-direction value to understand directional behavior.
Generic Family Range Broad reference range used for early resin-family comparison. Useful for screening, but replace it with exact-grade data before final tooling decisions.
Scope Boundary

Directional shrinkage helps explain why reinforced grades may not contract uniformly, but this section does not attempt to predict final part distortion. For the separate question of how material behavior can contribute to bow, twist or flatness risk, see the Injection Molding Warpage Risk by Material .

Dimensional Change After Molding

Mold Shrinkage vs Post-Mold Dimensional Change

The dimensional change measured as a molded part cools is not the same as every dimensional change that may occur later. Separating mold shrinkage from post-mold dimensional change prevents moisture uptake, conditioning, stress relaxation or later crystallization from being incorrectly treated as one shrinkage number.

Mold shrinkage describes the dimensional reduction associated with the molding and cooling process. It compares the mold cavity reference with the molded part after the specified cooling and conditioning period used by the applicable test method or resin supplier.

Post-mold dimensional change occurs after that initial molding-shrinkage measurement. Depending on the polymer, later dimensional movement may result from moisture absorption, moisture loss, stress relaxation, conditioning, thermal exposure or continued structural stabilization.

Why Nylon Requires Special Attention

Nylon is a useful example because moisture uptake can change dimensions after molding. A nylon part may therefore have one reported molding-shrinkage value and still change size later as it approaches a different moisture condition. That later movement should not simply be added to or described as the original mold shrinkage without stating the conditioning basis.

Semi-crystalline materials can also show additional dimensional evolution as crystallinity or internal stress stabilizes after molding. The importance of this effect is grade- and condition-dependent, which is why the selected resin datasheet and test conditions matter more than a generic family number.

Mold Shrinkage

Cavity-to-Molded-Part Change

Dimensional contraction associated with cooling and solidification under the stated molding and measurement conditions.

Typical data basis: supplier molding-shrinkage value, flow / transverse value, defined test method and specimen condition.

Post-Mold Change

Later Dimensional Movement

Dimensional change occurring after the initial molding stage because the material continues to interact with moisture, temperature or internal structural changes.

Examples: moisture conditioning, stress relaxation and later dimensional stabilization.

Engineering Interpretation

Keep these two effects separate when reviewing resin data. First identify the supplier's molding-shrinkage basis; then identify any later conditioning or environmental effect that may change part dimensions. The next section explains how to turn these reference inputs into a practical tooling and validation plan .

From Reference Data to Tooling

How to Turn a Shrinkage Range into a Tooling Validation Plan

A shrinkage chart is the starting point of a tooling decision, not the final answer. The engineering task is to move from a broad resin-family range to an exact-grade, direction-aware and trial-confirmed assumption before final steel dimensions are locked.

The first step is to use a family-level chart only for screening and initial comparison. It can indicate whether a resin family generally falls into a lower, moderate or higher shrinkage range, but that number should not automatically become the tooling value.

The next step is to identify the exact commercial resin grade. Supplier data should take precedence over a generic ABS, POM, nylon, PBT or PEEK family value because reinforcement, additives and formulation can materially change the reported shrinkage.

Preserve the Conditions Behind the Number

Record the filler percentage, test method and measurement direction together with the shrinkage value. For reinforced materials, keep flow and transverse shrinkage separate when both are available. For moisture-sensitive materials, also distinguish the molding-shrinkage basis from later conditioned dimensions.

Only after these inputs are defined should the project use a provisional tooling assumption. Final confirmation should come through a structured mold trial and validation process using actual molded parts produced under controlled conditions. Trial results can then show whether the selected resin grade and initial compensation assumption are consistent with the required part dimensions.

Level 01

Generic Reference

Use the resin-family range for early comparison only. Do not release final steel from this level.

Level 02

Exact-Grade Data

Confirm manufacturer, commercial grade, reinforcement level and current supplier shrinkage data.

Level 03

Directional Basis

Retain flow / transverse values and conditioning basis instead of compressing them into one universal number.

Level 04

Trial-Confirmed Value

Compare molded results with the intended dimensions and use project evidence to confirm or revise the initial assumption.

Reference Boundary

This page explains how shrinkage data should be qualified; it does not prescribe gate design, cooling-channel layout, packing profiles or detailed dimensional-inspection methods. Those decisions belong to the relevant mold-design, process-analysis and validation workflows.

Data Quality & Traceability

Shrinkage Data Sources, Test Methods and Limitations

A shrinkage percentage is only useful when its material identity, test direction, measurement condition and source are known. Generic resin-family ranges are useful for comparison, but exact tooling decisions should be based on traceable grade-specific information.

The preferred source for project-level shrinkage data is the current technical datasheet for the exact commercial resin grade. A generic designation such as “PA66-GF30” or “PBT-GF30” does not uniquely identify a formulation, even when the nominal filler percentage is the same.

The test method also matters. Shrinkage values generated from standardized specimens provide comparable material data, but they do not reproduce every feature of an actual molded component. Flow path, wall thickness, pressure and temperature gradients, reinforcement orientation and measurement conditions can all differ from the standardized specimen.

Record the Conditions Behind Every Shrinkage Value

When grade-specific data are available, retain the manufacturer, exact grade, reinforcement or filler content, measurement direction, test method and source revision. Do not combine a flow-direction value from one source with a transverse value from another grade and present the pair as one material dataset. For an active project, these inputs can be captured in the Part & Resin Data Sheet so the selected grade and shrinkage basis remain traceable through the tooling review.

Field to Record Why It Matters Preferred Entry
Manufacturer Resin families may contain many formulations from different suppliers. Exact resin manufacturer
Commercial Grade Additives, viscosity, reinforcement and other formulation differences can change reported shrinkage. Full supplier grade designation
Filler / Reinforcement Glass, mineral or carbon reinforcement can alter both total and directional shrinkage. Type and nominal percentage where specified
Measurement Direction Reinforced materials can report different values parallel and transverse to flow. Flow / transverse kept as separate fields
Test Method Different datasets should be interpreted with the method used to generate them. ASTM D955, ISO 294-4 or supplier-stated method
Condition / Timing Initial molding shrinkage and later dimensional change are not necessarily the same quantity. Record the stated measurement and conditioning basis
Source Revision Supplier formulations and technical documents can be revised. Datasheet revision, publication date or last verification date
ASTM D955

A test method for measuring thermoplastic shrinkage from mold dimensions under specified molding conditions. Standard-specimen results are useful for comparison but should not be treated as absolute predictions for every production part.

ISO 294-4

Covers determination of molding and post-molding shrinkage for injection-molded thermoplastic test specimens, including measurements parallel and normal to the direction of melt flow.

Reference Limitation

The family-level percentages in this guide are intended for comparison and early screening. They should not be interpreted as certified values for every grade or as direct instructions for final cavity dimensions. Where a project is active, the exact supplier datasheet and project-specific evidence take precedence.

Frequently Asked Questions

Plastic Resin Shrinkage Rate FAQ

These answers clarify the most common interpretation questions behind a resin shrinkage chart. Exact commercial-grade data should still take precedence over generic family-level ranges.

01

What is a typical injection molding shrinkage rate?

Typical molding shrinkage varies widely by resin family. Many amorphous thermoplastics fall in relatively lower ranges, while many semi-crystalline resins show higher or broader shrinkage. There is no single “normal” percentage for all plastics. Use the main chart for family-level comparison, then confirm the exact commercial grade and supplier data before using a value for tooling.

02

Can a shrinkage chart be used directly for mold dimensions?

No generic chart value should be treated as an automatic final cavity-compensation factor. A chart is useful for early comparison, but project decisions should move from the resin-family range to the exact supplier grade, reinforcement level, test method and directional data where applicable. Actual molded results are then used to confirm whether the provisional shrinkage assumption is appropriate.

03

Why do glass-filled plastics have different flow and transverse shrinkage?

Glass fibers tend to orient with the moving melt during cavity filling. Because the reinforcement restrains polymer contraction differently along and across that orientation, the material can show anisotropic shrinkage. For reinforced grades, flow-direction and transverse shrinkage should therefore be kept as separate values whenever the resin supplier publishes both.

04

What is the difference between mold shrinkage and post-mold dimensional change?

Mold shrinkage describes dimensional contraction associated with molding, cooling and the stated measurement condition. Post-mold dimensional change occurs later and can be influenced by moisture uptake, conditioning, stress relaxation, thermal exposure or continued structural stabilization. The two effects should be recorded separately rather than combined into one generic shrinkage number.

Reference Note

FAQ answers summarize the concepts above rather than replace grade-specific data. For an active project, confirm the manufacturer, exact commercial grade, reinforcement, measurement direction, test method and current supplier datasheet.

Project-Specific Review

Need a Grade-Specific Shrinkage Review?

Generic shrinkage charts are useful for screening, but final tooling decisions should be based on the exact commercial resin grade, reinforcement level, directional data and project requirements.

If you are preparing an injection mold or reviewing a dimensional issue, send the available part and resin information. SPI can review the project inputs before the tooling assumptions are finalized.

Request a DFM Review 2D / 3D + resin grade preferred