PA6 Shrinkage Rate Determination Method: Standards, Procedures, and Best Practices

PA6 (Polyamide 6) is a semi-crystalline engineering thermoplastic whose dimensional shrinkage during and after injection molding directly affects part tolerances, tooling compensation, and product quality. This article provides a comprehensive guide to PA6 shrinkage rate determination methods, covering test standards such as ISO 294-4 and ASTM D955, specimen preparation, measurement procedures, calculation formulas, and key factors influencing shrinkage results.

Technical Overview

PA6 (Polyamide 6, also known as Nylon 6) is one of the most widely used semi-crystalline engineering thermoplastics, applied extensively in automotive, electronics, and industrial components. Because PA6 crystallizes during cooling from the melt state, molded parts undergo measurable dimensional reduction relative to the mold cavity. This dimensional change, known as molding shrinkage, typically ranges from 1.0% to 1.5% for unfilled PA6 and from 0.3% to 0.8% for glass-fiber-reinforced grades such as PA6-GF30.

Shrinkage in PA6 arises from two combined mechanisms: thermal contraction as the material cools from processing temperature to ambient temperature, and volume reduction associated with crystallization of the polymer matrix. In addition, PA6 is hygroscopic, absorbing moisture after molding, which produces dimensional growth that can partially offset or mask shrinkage. Accurate determination of the shrinkage rate is therefore essential for mold design, tolerance control, and quality assurance in PA6 processing.

• Unfilled PA6 typical shrinkage: 1.0% to 1.5%
• Glass-fiber-reinforced PA6 (GF30) typical shrinkage: 0.3% to 0.8%
• Shrinkage varies with mold temperature, holding pressure, wall thickness, gate design, and crystallinity level
• Glass fiber introduces anisotropy, with lower shrinkage in the flow direction and higher shrinkage transverse to flow

Testing Samples

Injection-Molded Square Plates

The preferred test specimen for shrinkage determination is a flat plate, typically 60 mm x 60 mm x 2 mm as specified in ISO 294-4, molded with a film gate to ensure unidirectional flow. The plate geometry allows simultaneous measurement of shrinkage both parallel and perpendicular to the flow direction, which is critical for characterizing the anisotropic behavior of reinforced PA6 grades.

Multipurpose Test Specimens

Dumbbell-shaped multipurpose specimens conforming to ISO 3167 or ASTM D638 Type I may also be used, particularly when shrinkage data is required alongside mechanical test results. Measurements are taken in the narrow parallel section, and the corresponding cavity dimensions are used as the reference.

Glass-Fiber-Reinforced PA6 Specimens

For reinforced grades, fiber orientation during filling produces strongly direction-dependent shrinkage. Specimens must be measured in both the machine direction (MD) and transverse direction (TD) to fully characterize anisotropy, and gate type must be documented since it strongly influences fiber alignment.

Conditioning States of Specimens

Because PA6 absorbs moisture rapidly, specimens should be tested in a defined state, either dry-as-molded or moisture-conditioned. Standard conditioning is performed at 23 ± 2°C and 50 ± 5% relative humidity until mass stabilization is achieved. The conditioning state must be reported together with the shrinkage value, as moisture uptake can cause dimensional changes of 0.2% or more.

Testing Items

A complete PA6 shrinkage characterization typically includes the following measurement items, each addressing a different aspect of dimensional behavior:

• Molding shrinkage in the flow direction (machine direction, MD)
• Molding shrinkage perpendicular to flow (transverse direction, TD)
• Anisotropy ratio, calculated as the ratio or difference between TD and MD shrinkage
• Post-molding shrinkage after annealing at elevated temperature
• Total shrinkage, combining molding shrinkage and post-molding shrinkage
• Moisture-dependent dimensional change between dry and conditioned states

Testing Methods

Specimen Preparation

Test specimens are injection molded under documented and reproducible processing conditions. For PA6, typical processing parameters include a melt temperature of 240°C to 270°C and a mold temperature of 60°C to 90°C. Prior to molding, the material must be dried to a moisture content below 0.1% to prevent hydrolytic degradation and void formation. The molding machine, screw configuration, holding pressure profile, and cooling time must be recorded, since all of these parameters influence the measured shrinkage.

Conditioning Before Measurement

Molded specimens are conditioned in a standard laboratory atmosphere of 23 ± 2°C and 50 ± 5% relative humidity. Molding shrinkage is commonly measured 24 hours after molding, by which time the majority of room-temperature post-molding dimensional change has occurred. For moisture-conditioned results, specimens are stored until weight equilibrium is reached, which may require several days depending on specimen thickness.

Dimensional Measurement

The mold cavity dimensions are measured directly using calibrated instruments such as coordinate measuring machines (CMM), optical comparators, or precision micrometers. Specimen dimensions are then measured at marked reference positions, using the same measurement locations for all specimens to ensure repeatability. Instrument resolution should be 0.01 mm or better, and at least three specimens should be measured with results averaged. Optical and non-contact measurement methods are preferred for reinforced grades to avoid surface deformation.

Calculation of Shrinkage Rate

Molding shrinkage is calculated according to the following formula:

S = (L0 − L1) / L0 × 100%, where L0 is the mold cavity dimension and L1 is the corresponding specimen dimension measured after conditioning. A positive value indicates contraction relative to the cavity. Post-molding shrinkage after annealing is calculated as Sp = (L1 − L2) / L1 × 100%, where L2 is the specimen dimension after heat treatment. All results are reported as percentages, rounded to two decimal places, together with the measurement direction, conditioning state, and molding parameters.

Post-Molding Shrinkage (Annealing) Test

To determine post-molding shrinkage, specimens measured after the initial conditioning period are annealed in a circulating air oven or oil bath at an agreed temperature, commonly 100°C to 150°C for PA6, for a defined holding period such as 4 to 48 hours. After annealing and reconditioning to room temperature, dimensions are remeasured. This test simulates service exposure to elevated temperatures and reveals additional crystallization-related shrinkage that would otherwise appear as part distortion in the field.

Relevant Standards

• ISO 294-4: Injection moulding of test specimens — Determination of moulding shrinkage
• ASTM D955: Standard Test Method for Measuring Shrinkage from Mold Dimensions of Molded Plastics
• GB/T 17037.4: Chinese national standard adopting the ISO 294-4 methodology
• ISO 291: Standard atmospheres for conditioning and testing of plastic specimens

Applications

Shrinkage rate data determined by these methods is applied throughout the product development and manufacturing cycle for PA6 components:

• Mold design and tooling compensation, where cavity dimensions are scaled up by the anticipated shrinkage to achieve target part dimensions
• Dimensional tolerance analysis for precision automotive parts such as gears, brackets, fuel system components, and under-hood modules
• Material selection and comparison between unfilled, mineral-filled, and glass-fiber-reinforced PA6 grades
• Process optimization, evaluating the effect of mold temperature, holding pressure, and cooling time on part dimensions
• Quality control and incoming material inspection for injection molding operations
• Qualification of parts for elevated-temperature service through post-molding shrinkage testing
• Warpage prediction and gate and cooling channel design for parts with tight flatness requirements

FAQ

What is the typical shrinkage rate of PA6?

Unfilled PA6 typically exhibits a molding shrinkage of 1.0% to 1.5%, while glass-fiber-reinforced PA6-GF30 grades shrink approximately 0.3% in the flow direction and up to 0.8% or more in the transverse direction. Actual values depend strongly on processing conditions and part geometry.

Why does glass fiber reduce PA6 shrinkage but increase anisotropy?

Glass fibers have very low thermal contraction and restrict the crystallization-related contraction of the polymer matrix. However, fibers align with the melt flow during injection, so the restraining effect is strongest in the flow direction and weakest perpendicular to it, producing a large difference between MD and TD shrinkage known as anisotropy.

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

Molding shrinkage is the dimensional reduction measured shortly after molding, typically 24 hours after demolding under standard laboratory conditions. Post-molding shrinkage is the additional shrinkage that occurs when the part is exposed to elevated temperatures during service or annealing, caused by further crystallization and stress relaxation of the polymer.

How does moisture affect shrinkage measurement of PA6?

PA6 absorbs up to several percent of water at saturation, and moisture uptake causes dimensional swelling rather than shrinkage. If specimens are measured in a partially dried state, the apparent shrinkage will be overestimated compared with equilibrium-conditioned parts. For this reason, the conditioning state must always be reported with shrinkage data.

What are the key differences between ISO 294-4 and ASTM D955?

ISO 294-4 prescribes a specific 60 mm x 60 mm x 2 mm plate specimen with a film gate and defines both molding shrinkage and post-molding shrinkage procedures. ASTM D955 is more flexible regarding specimen geometry and defines shrinkage measurement from mold dimensions within a specified time after molding. Results from the two standards are not directly interchangeable and the method used must always be cited.

How can PA6 shrinkage be reduced in production?

Effective measures include increasing holding pressure and holding time, lowering mold temperature within the range required for surface finish and crystallinity, reducing wall thickness variations, optimizing gate size and location, using mineral-filled or glass-fiber-reinforced grades, and applying annealing where dimensional stability at elevated temperature is required.