The plastic shrinkage rate determination test quantifies the dimensional change that occurs when molten thermoplastics cool and solidify inside an injection mold, delivering critical data for tooling design, tolerance analysis, and process validation. Performed in accordance with ISO 294-4, ASTM D955, and GB/T 15585, the test measures molding shrinkage, post-molding shrinkage, and shrinkage anisotropy in both amorphous and semi-crystalline materials. Accredited third-party laboratories use these results to help manufacturers predict final part dimensions, optimize molding parameters, and ensure reliable assembly fit in mass production.
Technical Overview
Plastic shrinkage rate refers to the relative reduction in dimensions between the mold cavity and the molded plastic part after cooling to room temperature and stabilizing under a standard atmosphere. It is expressed as a percentage and calculated by comparing the cavity dimension measured at 23 °C with the corresponding dimension of the molded specimen. Shrinkage is a fundamental material and process characteristic that directly determines mold cavity compensation, dimensional tolerances, and the risk of warpage, sink marks, and internal stress in finished parts.
The physical origins of shrinkage include thermal contraction of the polymer as it cools from processing temperature to ambient temperature, density increase during crystallization of semi-crystalline resins, relaxation of molecular and filler orientation, and volumetric recovery during and after the packing phase. Amorphous plastics such as PS, ABS, PC, and PMMA typically exhibit low, nearly isotropic shrinkage of roughly 0.2 % to 0.8 %, whereas semi-crystalline plastics such as PE, PP, POM, PBT, and PA display higher shrinkage, often between 1.0 % and 3.0 % or more, because crystal formation significantly increases density upon solidification.
Shrinkage is also direction-dependent. In glass-fiber or mineral-filled compounds, filler orientation aligned with the melt flow suppresses shrinkage in the flow direction while transverse shrinkage remains comparatively high, producing pronounced anisotropy that must be evaluated in both principal directions. The plastic shrinkage rate determination experiment therefore provides the quantitative basis for mold design, computer-aided engineering simulation validation, incoming material qualification, and serial production quality control.
Testing Samples
ISO 294-4 Plate Specimens
ISO 294-4 specifies small square plates, commonly Type D1 (60 mm × 60 mm × 1 mm) and Type D2 (60 mm × 60 mm × 2 mm), molded in a single-cavity mold with a film gate feeding one full side of the cavity. This configuration produces a uniform, well-defined unidirectional flow field, allowing shrinkage to be measured accurately both parallel and perpendicular to the melt flow direction. The cavity dimensions are precision-machined and certified so that the reference lengths are known to within a few micrometers.
ASTM D955 Bar Specimens
ASTM D955 evaluates shrinkage from mold dimensions using rectangular bars of approximately 127 mm × 12.7 mm × 3.2 mm (5 in. × 0.5 in. × 0.125 in.) or flat plaque molds with documented cavity dimensions. Because the standard emphasizes comparison against the actual tool dimensions, it is widely applied in North American practice for thermoplastic material data sheets and mold shrinkage qualification.
Production Parts and Custom Plaques
For engineering and troubleshooting purposes, accredited third-party laboratories also measure shrinkage on actual production parts, prototype tools, or custom plaques with negotiated geometry and gating. In these cases, measurement locations, gating scheme, and molding conditions are agreed upon in advance, and results are interpreted in the context of the specific part geometry, wall thickness distribution, and process window.
Regardless of specimen type, a sufficient sample quantity, typically a minimum of three and preferably five specimens per material lot and molding condition, should be molded consecutively under stabilized conditions to ensure statistical validity. Specimens must be free from flash, sink marks, voids, and deformation, and they should be marked to identify the flow direction before measurement.
Testing Items
A comprehensive plastic shrinkage rate determination experiment typically covers the following measurement items:
- Molding shrinkage parallel to the melt flow direction, measured after initial conditioning.
- Molding shrinkage perpendicular to the melt flow direction, measured on the same specimen.
- Shrinkage anisotropy, expressed as the ratio between flow-direction and cross-direction shrinkage.
- Post-molding shrinkage, determined after extended conditioning or annealing at an agreed elevated temperature.
- Total shrinkage, combining molding shrinkage and post-molding shrinkage.
- Effect evaluation of key process parameters, such as mold temperature, melt temperature, holding pressure, and holding time, on the measured shrinkage values.
Testing Methods
Specimen Preparation and Molding
Specimens are injection molded on a calibrated molding machine using the reference tool geometry specified by ISO 294-4, ASTM D955, or GB/T 15585. The material is dried according to the supplier's recommendations, and all molding parameters, including melt temperature, mold temperature, injection speed, holding pressure, holding time, and cooling time, are documented in the test report. For materials with pronounced moisture sensitivity, such as polyamides and polyesters, residual moisture content is verified before molding because it strongly influences both processing stability and shrinkage.
Conditioning Requirements
After ejection, specimens are conditioned in a standard laboratory atmosphere of 23 ± 2 °C and 50 ± 10 % relative humidity in accordance with ISO 291 or the equivalent conditioning clauses of the applicable test standard. Molding shrinkage is conventionally determined between 16 h and 24 h after molding, which allows the part to reach thermal equilibrium while capturing the as-molded dimensional state. For post-molding shrinkage, specimens are stored for an agreed additional period, commonly 48 h to 168 h under standard atmosphere, or annealed at an elevated temperature defined by the material standard or by customer agreement, after which dimensions are remeasured.
Dimensional Measurement
The mold cavity reference dimensions are measured at 23 °C using calibrated instruments with an accuracy of 0.01 mm or better. Specimen dimensions are then measured with appropriate devices selected according to the required precision and geometry:
- Micrometers and digital calipers for routine length and thickness measurements at clearly marked gauge positions.
- Optical measuring systems and video measuring machines for non-contact measurement of delicate or flexible specimens.
- Coordinate measuring machines (CMM) for complex contours, production parts, and multi-point dimensional mapping.
Measurements are performed at defined locations in both the flow direction and the transverse direction, and each measurement is repeated to confirm repeatability. Instruments must be calibrated and traceable to national or international measurement standards.
Shrinkage Calculation and Reporting
Molding shrinkage is calculated using the following expression:
S = (L0 − L1) / L0 × 100 %, where L0 is the mold cavity dimension measured at 23 °C and L1 is the corresponding specimen dimension measured at 23 °C after the specified conditioning period. A positive value indicates contraction, and a negative value would indicate growth, which can occasionally occur transverse to flow in highly filled compounds.
Post-molding shrinkage is calculated as Sp = (L1 − L2) / L1 × 100 %, where L2 is the specimen dimension after additional conditioning or annealing. The total shrinkage is derived from the combined dimensional change relative to the mold cavity. The final report presents individual and mean values for both flow directions, the anisotropy ratio, the complete molding parameter record, conditioning history, measurement instrumentation, and the applicable standards. For reference, typical molding shrinkage ranges of common plastics are as follows:
- PS (polystyrene): 0.4 % to 0.7 %
- ABS: 0.4 % to 0.7 %
- PC (polycarbonate): 0.5 % to 0.8 %
- PMMA (acrylic): 0.3 % to 0.8 %
- Rigid PVC: 0.2 % to 0.6 %
- PP (polypropylene): 1.0 % to 2.5 %
- PE-HD (high-density polyethylene): 2.0 % to 5.0 %
- POM (acetal): 1.8 % to 2.5 %
- PA6 and PA66 (polyamide, dry state): 0.5 % to 1.5 %
- PBT and PET (unfilled): 1.5 % to 3.0 %
- Glass-fiber reinforced grades: typically 0.2 % to 0.8 %, with strong anisotropy between flow and transverse directions
Applications
Shrinkage rate data from the plastic shrinkage determination experiment are applied throughout the product development and manufacturing lifecycle:
- Mold design and cavity compensation, where measured shrinkage values are used to scale cavity dimensions so that production parts meet drawing tolerances.
- Material selection and comparison, enabling engineers to balance dimensional stability, mechanical performance, and cost among candidate resins.
- Injection molding process optimization, identifying holding pressure, mold temperature, and cooling settings that minimize and stabilize shrinkage.
- Warpage and deformation analysis, where direction-dependent shrinkage data validate finite element simulation predictions and guide gating and cooling strategies.
- Incoming material inspection and supplier qualification in accredited third-party laboratories, confirming that resin lots conform to data sheet specifications.
- Quality control for regulated industries such as automotive, electronics, medical devices, packaging, and consumer appliances, where dimensional conformance is contractually or legally required.
- Troubleshooting of field defects such as undersized features, poor assembly fit, sink marks, and distortion after heat exposure.
FAQ
What is the formula for calculating plastic shrinkage rate?
Shrinkage rate is calculated as S = (L0 − L1) / L0 × 100 %, where L0 is the mold cavity dimension and L1 is the molded part dimension, both measured at 23 °C. The result is expressed as a percentage of the cavity dimension, and separate values are reported for the flow direction and the transverse direction.
What is the difference between molding shrinkage and post-molding shrinkage?
Molding shrinkage is the dimensional change measured shortly after molding, conventionally between 16 h and 24 h after ejection under standard atmosphere. Post-molding shrinkage is the additional dimensional change that occurs during extended storage or annealing, caused by continued crystallization, stress relaxation, and physical aging. Total shrinkage is the sum of both contributions and is critical for parts that experience elevated temperatures in service.
Why do semi-crystalline plastics shrink more than amorphous plastics?
When semi-crystalline polymers such as PP, PE, POM, and PBT solidify, their molecular chains organize into ordered crystalline regions that are significantly denser than the amorphous melt. This crystallization-driven densification adds to ordinary thermal contraction, producing shrinkage values several times higher than those of amorphous materials, which contract thermally without major structural densification.
How does glass fiber reinforcement affect shrinkage?
Glass fibers have a very low coefficient of thermal expansion and become oriented along the melt flow during filling. This orientation restrains contraction in the flow direction while leaving transverse shrinkage largely unaffected, so filled grades show lower overall shrinkage but a much higher anisotropy ratio, which is a common cause of warpage in flat or thin-walled parts.
Which standards govern the plastic shrinkage rate determination experiment?
The principal standards are ISO 294-4 for plate specimens and mold shrinkage of thermoplastics, ASTM D955 for shrinkage measured from mold dimensions of molded thermoplastics, and GB/T 15585 as the corresponding Chinese national standard. Conditioning of specimens is controlled according to ISO 291 or equivalent clauses within each method standard.
What process variables most strongly influence measured shrinkage?
Holding pressure and holding time govern how much material is packed into the cavity during solidification and are among the most influential parameters. Mold temperature and melt temperature control the cooling rate and degree of crystallization, while wall thickness, gate design, and flow path determine pressure transmission and orientation. Because of these dependencies, a complete and reproducible shrinkage report must always document the molding conditions under which the specimens were produced.