Polycarbonate (PC) linear shrinkage testing quantifies the dimensional change that occurs in PC materials during polymerization processing and injection molding, providing essential data for mold design, tolerance control, and part quality assurance. Accurate shrinkage measurement ensures that finished components—from automotive lenses to electronic housings—meet strict dimensional specifications. This guide explains the principles, applicable standards, sample requirements, test methods, and industrial applications of PC linear shrinkage testing.
Technical Overview
Linear shrinkage is defined as the reduction in a linear dimension between the mold cavity and the corresponding molded plastic part, measured after the part has cooled to ambient temperature under standard conditioning. For polycarbonate (PC), a high-performance amorphous engineering thermoplastic, linear shrinkage testing is a fundamental quality control and material characterization procedure used throughout polymer production, compounding, and part manufacturing.
Because PC is an amorphous polymer, it exhibits relatively low and isotropic shrinkage—typically between 0.4% and 0.8%—compared with semi-crystalline materials such as POM or PA, which can shrink by 1.5% to 3%. Nevertheless, even small variations in shrinkage can compromise the fit, function, and assembly of precision components. Linear shrinkage in PC arises from several mechanisms, including thermal contraction during cooling, relaxation of molecular orientation induced during filling, volumetric compensation during the packing phase, and stress relief after ejection from the mold.
Linear shrinkage data is used to correct mold cavity dimensions, validate processing windows, compare resin grades and filler formulations, and predict long-term dimensional stability. The most widely applied standards include ASTM D955 (mold shrinkage of molded thermoplastics), ISO 294-4 (determination of molding shrinkage of injection-molded specimens), GB/T 17037.4 (Chinese national equivalent of ISO 294-4), and ASTM D6991 (post-molding shrinkage).
Testing Samples
Injection-Molded Square Plaques
Flat plaques with dimensions of approximately 60 mm × 60 mm × 2 mm, molded in accordance with ISO 294-4, are the standard specimens for determining molding shrinkage. These plaques are gated at one edge so that shrinkage can be evaluated in both the flow direction and the transverse direction, revealing any anisotropic behavior.
Multipurpose Test Specimens
Dumbbell-shaped specimens conforming to ISO 3167 or ASTM D638 Type I geometry may be used when shrinkage data must be correlated with mechanical property testing. Measurements are typically taken in the narrow parallel-sided gauge section.
Compression-Molded Sheets
For resin producers evaluating polymerization consistency or pellet lots, compression-molded plaques provide a low-orientation baseline. Shrinkage measured on compression-molded sheets primarily reflects material composition rather than injection-induced orientation.
Production Components
Finished or prototype parts can be measured directly against certified mold cavity dimensions (verified with a coordinate measuring machine) to validate mold compensation calculations and production process stability.
Sample Conditioning
All specimens must be conditioned in a standard laboratory atmosphere of 23 °C ± 2 °C and 50% ± 10% relative humidity for a minimum of 24 hours, and preferably 48 hours, before measurement, following ISO 291 or ASTM D618 practice.
Testing Items
A comprehensive PC linear shrinkage test program typically includes the following measurable parameters:
- Mold shrinkage in the flow (machine) direction, expressed as a percentage of the cavity dimension
- Mold shrinkage in the transverse (cross-flow) direction
- Differential (anisotropic) shrinkage, calculated as the difference between flow and cross-flow shrinkage values
- Post-molding shrinkage after annealing or long-term storage, determined per ASTM D6991
- Total shrinkage, combining immediate mold shrinkage and subsequent post-molding shrinkage
- Relevant molding parameters recorded during specimen preparation, including melt temperature, mold temperature, injection speed, holding pressure, and holding time
Testing Methods
Specimen Preparation
Test specimens are injection molded under closely controlled conditions using a documented processing window appropriate for the PC grade—for example, melt temperatures of 280–320 °C and mold temperatures of 80–120 °C. Because packing pressure and holding time strongly influence shrinkage, these parameters must be held constant and reported with the results. PC pellets are dried to below 0.02% moisture content before molding to prevent hydrolytic degradation and bubble formation.
Dimensional Measurement
After conditioning, specimen dimensions are measured with instruments of sufficient accuracy, typically offering a resolution of 0.01 mm or better. Depending on the required precision, suitable instruments include digital calipers, micrometers, optical comparators, video measurement systems, or coordinate measuring machines (CMM). Mold cavity dimensions are measured under the same conditions to establish the reference values.
Shrinkage Calculation
Linear shrinkage is calculated for each direction using the following relationship:
S = (L0 − L1) / L0 × 100%, where S is the linear shrinkage in percent, L0 is the mold cavity dimension, and L1 is the corresponding conditioned specimen dimension. Each result is reported as the average of at least three, and preferably five, replicate specimens, together with the standard deviation.
Post-Molding Shrinkage Testing
To evaluate dimensional stability, specimens are re-measured after annealing at an elevated temperature below the glass transition range (commonly 100–120 °C for PC) or after extended storage. Post-molding shrinkage is calculated as the additional dimensional change relative to the initially measured specimen size, following ASTM D6991.
Applications
PC linear shrinkage test data supports critical engineering decisions across many industries:
- Automotive: headlamp lenses, glazing, interior trim, and electronic control housings where appearance and precise assembly tolerances are mandatory
- Electronics and electrical: connector bodies, switch housings, battery enclosures, and charger casings requiring tight dimensional control
- Optical components: LED light guides, lenses, and display windows where warpage and uneven shrinkage degrade optical performance
- Medical devices: transparent surgical instrument housings, IV components, and diagnostic equipment parts that must meet stringent regulatory dimensional specifications
- Construction and glazing: corrugated and multiwall PC sheets, where controlled shrinkage affects installation fit and thermal behavior
- Mold and tooling design: applying measured shrinkage values to scale cavity dimensions during tool design and mold compensation
FAQ
What is the typical linear shrinkage of polycarbonate?
Unfilled PC generally exhibits linear shrinkage of 0.4% to 0.8%, with 0.5% to 0.7% being most common under standard molding conditions. Glass-fiber-reinforced PC grades can show lower shrinkage in the flow direction but greater anisotropy between flow and transverse directions.
Why does PC have lower shrinkage than many other thermoplastics?
PC is an amorphous polymer, meaning it does not form an ordered crystalline structure upon cooling. Semi-crystalline polymers shrink more because crystal formation packs molecules into a denser state, producing an additional volume reduction beyond simple thermal contraction.
Which standards govern PC linear shrinkage testing?
The principal standards are ASTM D955 for mold shrinkage, ISO 294-4 and GB/T 17037.4 for molding shrinkage of injection-molded specimens, and ASTM D6991 for post-molding shrinkage. Specimen conditioning is governed by ISO 291 or ASTM D618.
What processing factors most strongly affect PC shrinkage?
Holding (packing) pressure and holding time have the greatest influence, as they compensate for volumetric contraction during solidification. Mold temperature, melt temperature, wall thickness, gate design, and cooling time also significantly affect the final shrinkage value.
How can shrinkage variation be minimized in PC molding?
Recommended measures include optimizing packing pressure and time to ensure adequate cavity compensation, maintaining uniform wall thickness, using proper drying procedures, controlling mold temperature within a narrow range, and validating the process with shrinkage tests on each production lot of resin.