PC (Polycarbonate) Polymerization Shrinkage Test: Principles, Methods, and Standards

Polycarbonate (PC) polymerization shrinkage testing measures the dimensional and volumetric changes that occur in PC materials during polymerization, curing, and molding processes. Accurate shrinkage characterization is essential for controlling part dimensional accuracy, minimizing internal stresses, and ensuring product quality across automotive, electronics, optical, and medical applications. This article explains the technical principles, sample requirements, testing methods, and industry standards for PC polymerization shrinkage evaluation.

Technical Overview

Polycarbonate (PC) is a high-performance engineering thermoplastic widely used in applications requiring excellent transparency, impact resistance, and dimensional stability. During polymerization, curing, and subsequent molding or solidification, PC materials undergo a reduction in volume and linear dimensions—a phenomenon known as polymerization shrinkage. This shrinkage results from the reduction of free volume between polymer chains as monomer units convert into covalently bonded polymer networks, decreasing the mean intermolecular distance.

For polycarbonate, the degree of shrinkage is influenced by multiple factors, including molecular weight distribution, residual monomer content, processing temperature, cooling rate, holding pressure, filler loading, and crystallinity. Although PC is primarily amorphous and exhibits relatively low shrinkage (typically 0.5%–0.8% for injection-molded parts), uncontrolled shrinkage can still lead to warpage, sink marks, internal residual stresses, dimensional deviation, and reduced optical performance in precision components. The PC polymerization shrinkage test quantifies these dimensional changes, providing critical data for material qualification, mold design, process optimization, and quality control.

Testing Samples

PC Resin and Pellet Samples

Raw polycarbonate resin pellets, powders, or granules can be tested to evaluate baseline polymerization and molding shrinkage characteristics before processing. These samples are typically conditioned and molded into standardized test plaques or bars under controlled conditions.

Molded Test Specimens

Standardized specimens such as injection-molded plaques, dumbbell bars, or rectangular plates (e.g., 100 mm × 100 mm × 3 mm or per ISO 294-3 standard plates) are used to determine mold shrinkage. Specimen geometry must be documented precisely, as shrinkage varies with thickness, flow direction, and gate location.

PC Blends and Composites

PC/ABS blends, glass-fiber-reinforced PC, and filled PC compounds require dedicated shrinkage evaluation because fillers and second-phase polymers significantly alter shrinkage behavior, often introducing anisotropy between the flow and transverse directions.

Finished Parts and Components

Actual manufactured components—such as optical lenses, housings, medical device parts, and automotive glazing—can be tested for post-mold shrinkage to verify long-term dimensional stability after storage or thermal exposure.

Testing Items

• Volumetric polymerization shrinkage: the total percentage reduction in specimen volume from the uncured or molten state to the solid state
• Linear shrinkage (mold shrinkage): dimensional change between mold cavity dimensions and molded part dimensions, measured in flow and transverse directions
• Post-mold shrinkage (annealing shrinkage): additional dimensional change occurring after molding due to relaxation, aging, or thermal conditioning
• Anisotropic shrinkage ratio: the difference between shrinkage in the flow direction and perpendicular to flow, critical for fiber-reinforced grades
• Density change: measured before and after polymerization or solidification to derive volumetric shrinkage
• Shrinkage uniformity: dimensional consistency across multiple specimens or locations within a single part

Testing Methods

Volumetric Shrinkage Determination

Volumetric shrinkage is commonly determined by the density method based on ISO 3521 or ASTM D2566 principles. The density of the liquid or uncured resin is measured using a pycnometer, and the cured polymer density is determined via the Archimedes immersion method. Volumetric shrinkage (Sv) is calculated as Sv = (ρc − ρl) / ρc × 100%, where ρc is the cured density and ρl is the liquid density. Gas pycnometry may also be used for higher precision measurements.

Linear Mold Shrinkage Test

Linear shrinkage is measured according to ASTM D955 or ISO 294-4. A standardized specimen is molded under specified temperature, pressure, and cooling conditions. After a defined conditioning period (typically 16–24 hours at 23°C and 50% relative humidity), the specimen dimensions are measured with calibrated instruments such as optical comparators, coordinate measuring machines (CMM), or micrometers. Mold shrinkage is calculated as S = (Lmold − Lpart) / Lmold × 100%, where Lmold is the cavity dimension and Lpart is the molded part dimension.

Post-Mold Shrinkage Test

Post-mold shrinkage is evaluated per ASTM D6289 or ISO 294-4 Annex. Molded specimens are measured immediately after molding, then subjected to thermal aging or extended storage at controlled conditions, and re-measured. The difference in dimensions indicates the post-mold shrinkage, which reflects long-term dimensional stability and internal stress relaxation behavior.

Dilatometric and Optical Methods

For research-grade characterization, dilatometry can continuously monitor volume changes during polymerization or cooling, providing shrinkage kinetics data. Laser displacement sensors and digital image correlation (DIC) techniques enable non-contact, real-time measurement of linear and warpage behavior during curing and solidification.

Standard Testing Conditions

Tests are conducted in standard laboratory atmospheres of 23°C ± 2°C and 50% ± 10% relative humidity unless otherwise specified. Molding parameters such as melt temperature, mold temperature, injection pressure, holding pressure, and cooling time must be recorded and reported, as they directly influence measured shrinkage values.

Applications

• Automotive industry: dimensional control of PC glazing, headlamp lenses, interior trim, and electronic enclosures where optical clarity and tight tolerances are required
• Electronics and electrical: shrinkage data for connectors, housings, LED components, and precision insulating parts produced from PC and PC blends
• Optical and display: lens manufacturing, optical waveguides, and display panels requiring minimal distortion and birefringence
• Medical devices: qualification of PC used in syringes, IV components, surgical instruments, and sterilizable equipment where dimensional accuracy is safety-critical
• Mold and tooling design: shrinkage compensation factors derived from test data are applied directly to cavity and core dimensions
• Material development and qualification: comparing shrinkage behavior of new PC grades, recycled PC, blends, and reinforced compounds against specifications
• Quality control and incoming inspection: verifying that production lots meet contractual shrinkage tolerances

FAQ

What is the typical shrinkage value for polycarbonate?

Unfilled polycarbonate generally exhibits mold shrinkage in the range of 0.5% to 0.8%, which is relatively low compared with semi-crystalline plastics. Glass-fiber-reinforced PC grades may show lower shrinkage in the flow direction (0.1%–0.3%) but higher anisotropy, requiring directional measurement.

Why does PC show lower shrinkage than semi-crystalline polymers?

PC is an amorphous polymer that does not form an ordered crystalline structure upon solidification. Since crystallization is a major contributor to shrinkage in semi-crystalline materials, amorphous PC experiences only smaller volume changes driven by thermal contraction and packing effects.

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

Mold shrinkage is the immediate difference between the mold cavity dimensions and the molded part dimensions measured shortly after molding. Post-mold shrinkage is the additional dimensional change that develops during storage or thermal aging due to residual stress relaxation and continued physical aging of the polymer.

How should specimens be conditioned before measurement?

Specimens should be conditioned at 23°C ± 2°C and 50% ± 10% relative humidity for at least 16 hours (typically 16–24 hours) after molding before taking reference measurements. For post-mold shrinkage testing, longer conditioning or thermal exposure periods are applied according to the relevant standard or customer specification.

Which measurement instruments are used for shrinkage determination?

Linear dimensions are typically measured using micrometers, calipers, optical comparators, height gauges, or coordinate measuring machines depending on the required accuracy. Volumetric shrinkage is derived from density measurements using pycnometers and analytical balances based on the Archimedes principle.

What factors most strongly influence PC shrinkage test results?

Key factors include melt and mold temperatures, injection and holding pressures, cooling time, specimen thickness and geometry, gate design, filler content and orientation, residual moisture in the resin, and ambient conditioning conditions. Consistent control of these parameters is essential for reproducible and comparable results.

Can recycled or blended PC materials be tested with the same methods?

Yes. The same test standards apply to PC blends (such as PC/ABS), recycled PC, and filled compounds. However, blended or recycled materials may exhibit greater variability and anisotropy, so directional measurements and larger sample sizes are recommended for statistical reliability.