PA6 Dry Shrinkage Rate Testing: Standards, Methods, and Technical Analysis

PA6 dry shrinkage rate testing quantifies the dimensional contraction of injection-molded polyamide 6 specimens in their dry, as-molded state, delivering essential data for mold cavity sizing, tolerance design, and dimensional quality control. This technical guide explains the testing principles, applicable international standards, sample preparation, measurement procedures, and the key material and processing factors that influence PA6 shrinkage behavior.

Technical Overview

PA6 (polyamide 6, also known as nylon 6) is a semicrystalline engineering thermoplastic widely used in automotive, electrical, and consumer applications. During injection molding, PA6 parts undergo dimensional contraction caused by thermal contraction of the melt and, more significantly, by crystallization of the polymer structure as the part cools in the mold. The resulting shrinkage directly determines the final part dimensions and must be accurately characterized before a production mold is cut.

The dry shrinkage rate specifically refers to the dimensional change measured on specimens in the dry, as-molded state, before atmospheric moisture absorption has altered the part dimensions. This distinction is critical because PA6 is strongly hygroscopic: the material can absorb up to approximately 2.5–3% water by weight at saturation, and absorbed moisture causes measurable swelling. Measuring shrinkage in the dry state isolates the true molding shrinkage and improves reproducibility between laboratories.

Typical dry mold shrinkage values for PA6 range from approximately 1.0% to 1.8% for unfilled grades, while glass-fiber-reinforced grades such as PA6-GF30 typically show 0.2% to 0.8% in the flow direction and 0.4% to 1.2% in the transverse direction. Accurate knowledge of these values enables steel-safe mold design, reliable tolerance stacks, and dependable warpage prediction.

Testing Samples

ISO Standard Plates

Square plates of 60 mm × 60 mm × 2 mm molded in a single-cavity test mold in accordance with ISO 294-1 are the preferred specimen geometry for shrinkage testing under ISO 294-4 and GB/T 17037.4. Both the length parallel to flow and the length perpendicular to flow are measured to capture directional shrinkage behavior.

ASTM Standard Plaques

For North American specifications, plaques of approximately 127 mm × 127 mm × 3.2 mm or rectangular bars of 12.7 mm × 127 mm × 3.2 mm are molded and measured in accordance with ASTM D955.

Production Parts and Special Grades

Actual production components may also be tested at defined checkpoint sections for validation purposes. Sample programs frequently cover multiple material grades, including unfilled PA6, glass-fiber-reinforced PA6 (GF15, GF30, GF45), mineral-filled grades, and impact-modified grades, since filler type and loading strongly affect shrinkage.

Specimen Conditioning for Dry-State Measurement

To maintain the dry state, specimens are stored in a desiccator or sealed with desiccant immediately after molding, or measured within a short, defined window after ejection at 23 ± 2 °C. The moisture content of representative specimens is verified using a moisture analyzer or Karl Fischer titration, with dry-state moisture typically confirmed below 0.2%.

Testing Items

A comprehensive PA6 dry shrinkage test program typically includes the following measurement items:

• Mold shrinkage in the flow direction (machine direction, MD)
• Mold shrinkage in the transverse direction (TD, perpendicular to flow)
• Anisotropy ratio, calculated as the ratio of transverse shrinkage to flow-direction shrinkage, which is an important indicator of warpage tendency in glass-filled grades
• Post-molding shrinkage, measured at defined intervals such as 24 hours and 48 hours after molding under standard laboratory conditions
• Shrinkage after annealing at an elevated temperature, used to assess long-term dimensional stability and total shrinkage potential
• Shrinkage as a function of wall thickness, since thicker sections cool more slowly and develop higher crystallinity and greater shrinkage
• Moisture content verification to confirm the validity of dry-state measurements
• Shrinkage sensitivity studies across varying mold temperatures, melt temperatures, holding pressures, and holding times

Testing Methods

The standard test procedure is performed in the following sequence:

• Material preparation: PA6 pellets are dried in a desiccant dryer or vacuum oven, typically at approximately 80 °C, until the moisture content is below 0.1–0.2% to prevent hydrolytic degradation and moisture-related dimensional errors.
• Specimen molding: Test plates are injection molded under controlled and documented conditions, typically with a melt temperature of 240–280 °C, a mold temperature of 80 °C or as specified by the grade datasheet, and defined holding pressure and holding time profiles, in accordance with ISO 294-1.
• Cavity calibration: The mold cavity is measured with calibrated instruments before the campaign to establish the reference dimensions L0.
• Specimen measurement: Part dimensions are measured at the specified time points using calibrated digital calipers, micrometers, an optical comparator, or a coordinate measuring machine (CMM), with at least three specimens measured per condition and multiple measurement points averaged.
• Dry-state verification: Representative specimens are weighed on a moisture analyzer to confirm that the moisture content remains within the defined dry-state limit.
• Calculation: Mold shrinkage is calculated as Sm = (L0 − L1) / L0 × 100%, where L0 is the cavity dimension and L1 is the specimen dimension in the dry as-molded state. Post-molding shrinkage is calculated as Sp = (L1 − L2) / L1 × 100%, where L2 is the dimension after the defined storage or annealing period.
• Reporting: The test report records the material grade and lot, molding conditions, specimen geometry, measurement times, individual and mean shrinkage values for each direction, moisture content, and environmental conditions of the laboratory.

Key processing parameters must be held constant and documented, because mold temperature, holding pressure, and holding time have a first-order effect on PA6 shrinkage. Higher mold temperatures increase crystallinity and therefore increase shrinkage, while higher holding pressures and longer holding times reduce shrinkage by compensating for volumetric contraction during solidification.

Applications

PA6 dry shrinkage test data supports a wide range of engineering and manufacturing activities:

• Mold cavity sizing and steel-safe mold design, ensuring that dimensional corrections are made by removing metal rather than adding it during mold trials
• Tolerance analysis and dimension chain calculations for precision components such as automotive interior trim, under-hood parts, connectors, cable ties, gears, and pump housings
• Material grade selection and comparison, including evaluation of unfilled, glass-filled, and mineral-filled PA6 compounds for dimensional stability
• Generation and verification of supplier datasheet shrinkage values, and incoming resin quality control inspections
• Calibration and validation of injection molding simulation models (CAE) used for shrinkage and warpage prediction
• Process window optimization, balancing shrinkage against residual stress, cycle time, and part appearance
• Part qualification and PPAP-type dimensional validation in automotive and electrical industry supply chains

FAQ

What is the typical dry shrinkage rate of PA6?

Unfilled PA6 generally shows dry mold shrinkage of approximately 1.0–1.8%, depending on wall thickness, mold temperature, and holding conditions. Glass-fiber-reinforced PA6-GF30 typically shows 0.2–0.8% in the flow direction and 0.4–1.2% in the transverse direction.

Why must PA6 shrinkage be measured in the dry state?

PA6 absorbs moisture rapidly from the atmosphere, and absorbed water causes the part to swell and grow in dimensions. Measuring specimens in the dry, as-molded state isolates the true molding shrinkage, eliminates humidity-related variability, and ensures that results are comparable between different laboratories and testing dates.

What is the difference between dry shrinkage and conditioned shrinkage?

Dry shrinkage is measured before significant moisture uptake, while conditioned shrinkage is measured after specimens have been equilibrated at 23 °C and 50% relative humidity or otherwise moisture-conditioned. Conditioned values typically appear lower because moisture swelling partially offsets molding contraction.

Which standards govern PA6 shrinkage testing?

The principal standards are ISO 294-4 (determination of molding shrinkage and post-molding shrinkage), ASTM D955 (mold shrinkage of thermoplastics), and GB/T 17037.4. Specimen preparation is controlled by ISO 294-1, and moisture conditioning methods follow ISO 62 or ISO 1110 where applicable.

How does glass fiber affect PA6 shrinkage?

Glass fibers reduce the overall shrinkage of PA6 by constraining polymer contraction, but they also introduce strong anisotropy because the fibers align with the melt flow. This produces significantly lower shrinkage in the flow direction than across it, which raises the anisotropy ratio and can increase warpage in flat or large parts.

How soon after molding should dry shrinkage measurements be taken?

Measurements are commonly performed within a defined short window after ejection, such as 2–4 hours at 23 °C, and repeated at 24 or 48 hours to record post-molding shrinkage. Whatever interval is selected, it must be documented and kept consistent so that results remain comparable across studies.