PA Nylon Shrinkage Coefficient Testing: Standards, Methods, and Technical Guidelines

PA nylon shrinkage coefficient testing quantifies the dimensional change that polyamide materials undergo during and after injection molding, providing essential data for tool design, tolerance planning, and quality control. This technical guide explains the testing principles of ISO 294-4 and ASTM D955, specimen preparation, measurement procedures, and typical shrinkage values for PA6, PA66, and glass-fiber reinforced grades. Understanding these parameters helps manufacturers predict part dimensions accurately and reduce costly tool modifications.

Technical Overview

PA (polyamide), commonly known as nylon, is a semi-crystalline engineering thermoplastic widely used in automotive, electrical, electronic, and industrial applications. During injection molding, PA undergoes volumetric contraction as the melt cools, crystallizes, and solidifies within the tool. The shrinkage coefficient, also referred to as mold shrinkage, expresses this dimensional reduction as a percentage of the corresponding mold cavity dimension.

The basic calculation is defined as: Shrinkage (%) = (Lm − Ls) / Lm × 100, where Lm is the mold cavity dimension and Ls is the dimension of the molded specimen measured under specified conditions. Because polyamide is semi-crystalline and hygroscopic, its shrinkage behavior is influenced by crystallinity, glass fiber orientation, moisture absorption, and processing parameters, making systematic testing essential.

• Unfilled PA6: typical mold shrinkage of 0.8% to 1.5%, reaching up to 2.0% in thick sections
• Unfilled PA66: typical mold shrinkage of 0.8% to 1.8%
• Glass-fiber reinforced PA (30% GF): shrinkage of 0.2% to 0.9%, with strong directionality
• Mineral-filled PA grades: shrinkage of 0.7% to 1.2%, with reduced anisotropy
• PA11 and PA12: shrinkage of approximately 0.8% to 1.2% due to lower crystallinity

Shrinkage in PA is inherently anisotropic, particularly in fiber-reinforced grades. Shrinkage measured parallel to the flow direction (MD, machine direction) can differ significantly from shrinkage measured perpendicular to flow (TD, transverse direction). For glass-filled PA, transverse shrinkage may be two to three times higher than flow-direction shrinkage, which is a critical consideration for mold design and warpage control.

Testing Samples

Unfilled PA Granules

Natural PA6 and PA66 resins in pellet form are molded into standard test plaques to establish baseline shrinkage data. These samples are dried to the recommended moisture content before molding, typically below 0.1% to 0.2%, since residual moisture causes silver streaking, hydrolysis, and inconsistent dimensions.

Glass-Fiber Reinforced PA Compounds

PA6 and PA66 compounds containing 10% to 50% glass fiber are tested to characterize both the magnitude of shrinkage reduction and the degree of anisotropy. These samples require special attention to gate design and molding conditions because fiber orientation dominates the differential shrinkage behavior.

Mineral-Filled and Impact-Modified Grades

Compounds filled with talc, glass beads, or minerals, as well as impact-modified and flame-retardant PA grades, are tested to verify that additive packages deliver the expected isotropic shrinkage profile for tight-tolerance applications.

Molded Production Parts

Actual components, such as gears, connectors, clips, and housings, can be measured directly against tool CAD dimensions to validate shrinkage compensation factors and monitor process stability over production runs.

Testing Items

A comprehensive PA shrinkage evaluation program typically includes the following measurement items:

• Mold shrinkage in flow direction (MD) and transverse direction (TD)
• Post-molding shrinkage after 48 hours of storage at standard atmosphere
• Total shrinkage, defined as the sum of molding shrinkage and post-molding shrinkage
• Annealing shrinkage after heat treatment at elevated temperature
• Anisotropy ratio, calculated as TD shrinkage divided by MD shrinkage
• Moisture-induced dimensional change after humidity conditioning
• Warpage and flatness deviation of test plaques

Post-molding and annealing shrinkage are particularly important for PA materials. Unfilled polyamides can exhibit an additional 0.1% to 0.5% of post-molding shrinkage as crystallization continues at room temperature or during service at elevated temperatures. Moisture absorption also causes dimensional growth of roughly 0.2% to 0.3% for every 1% of absorbed water, which must be separated from true thermal shrinkage during data interpretation.

Testing Methods

Specimen Preparation

Test specimens are produced according to ISO 294-4, which specifies a 60 mm × 60 mm × 2 mm square plaque molded with a film gate, or according to ASTM D955, which uses molded bars or 102 mm discs. Molding is performed on a calibrated injection molding machine with documented parameters, including melt temperature, mold temperature, injection speed, holding pressure, holding time, and cooling time. For PA6, typical conditions include a melt temperature of 230 °C to 260 °C and a mold temperature of 60 °C to 90 °C; for PA66, melt temperatures of 270 °C to 295 °C and mold temperatures of 70 °C to 100 °C are common. All molding variables are recorded because even small deviations in holding pressure or mold temperature can shift shrinkage results measurably.

Conditioning Requirements

Specimens are allowed to stabilize after ejection before measurement. Initial shrinkage is measured 16 to 24 hours after molding. Post-molding shrinkage is determined by remeasuring the same specimens after storage for at least 48 hours at 23 ± 2 °C and 50 ± 5% relative humidity, or after annealing at an agreed temperature, commonly 100 °C to 160 °C depending on the PA grade and application requirements. Consistent conditioning is critical because PA dimensions are highly sensitive to ambient moisture.

Measurement Procedure

Cavity dimensions of the mold are measured first using calibrated tools or a coordinate measuring machine to establish reference values. Specimen dimensions are then measured along both the flow direction and the transverse direction at defined positions, using optical comparators, vision measuring systems, CMMs, or calibrated micrometers with a measurement resolution of at least 0.01 mm. Each measurement is typically repeated on multiple specimens, often five, and the results are averaged. Shrinkage values for MD, TD, post-molding, and annealed conditions are calculated and reported, together with the anisotropy ratio and complete molding parameters.

Equipment and Instrumentation

• Hydraulic or electric injection molding machine with process monitoring
• ISO 294-4 or ASTM D955 compliant test mold
• Coordinate measuring machine (CMM) or optical vision measurement system
• Calibrated micrometers and digital calipers for verification
• Standard climate chamber maintained at 23 °C and 50% relative humidity
• Annealing oven with precise temperature control
• Moisture analyzer and desiccant dryer for sample preparation

Applications

Shrinkage coefficient data for PA materials directly supports engineering decisions across many industries:

• Automotive: tool design for under-hood components, air intake manifolds, radiator end tanks, sensors, and interior structural parts where dimensional stability under heat is mandatory
• Electrical and electronics: connector housings, terminal blocks, switches, and PCB supports requiring tight pitch tolerances and stable insulating properties
• Industrial machinery: gears, bearings, bushings, rollers, and guide elements where dimensional accuracy affects mesh clearance and noise
• Consumer goods: power tool housings, appliance components, fasteners, and cable ties produced in high-volume molds
• Quality control: incoming resin verification, comparison of competing material suppliers, and validation of regrind or reprocessed content
• Mold engineering: calculation of cavity compensation factors and simulation input data for warpage and flow analysis software

FAQ

What is the difference between PA6 and PA66 shrinkage values?

Both unfilled PA6 and PA66 shrink in the range of approximately 0.8% to 1.8%, but PA66 generally shows slightly higher crystallinity-driven shrinkage and faster crystallization. In glass-filled grades, PA66 GF30 typically exhibits 0.2% to 0.4% in the flow direction and 0.5% to 0.9% in the transverse direction, while PA6 GF30 shows marginally higher flow-direction values. Exact figures depend on the specific compound, so testing of each commercial grade is always recommended.

Why does glass fiber reduce shrinkage but increase anisotropy?

Glass fibers have near-zero thermal contraction and restrict polymer chain mobility during crystallization, which lowers overall volumetric shrinkage. However, fibers align parallel to the flow direction near cavity walls and in the core flow stream, restraining shrinkage mainly along that axis. This produces low MD shrinkage and comparatively high TD shrinkage, and the resulting differential can drive warpage in flat parts.

Which standard should be used, ISO 294-4 or ASTM D955?

ISO 294-4 is the current international reference method and is preferred for new programs, particularly in global supply chains, because it uses defined 60 × 60 × 2 mm plaques with film gating. ASTM D955 remains widely referenced in North American specifications. Because the two methods differ in specimen geometry and gating, shrinkage values are not directly interchangeable, and the report must always state the standard used.

How does moisture affect shrinkage test results?

PA absorbs up to 2.5% moisture at 50% relative humidity and as much as 8% to 9% when saturated in water, causing dimensional swelling. Specimens must therefore be dried before molding, and measurements must be taken at defined humidity and elapsed-time conditions. Comparing measurements made at different moisture contents leads to erroneous conclusions, since moisture expansion can partially mask true mold shrinkage.

When should shrinkage measurements be taken after molding?

Initial shrinkage is measured 16 to 24 hours after ejection, when most of the rapid post-ejection contraction has stabilized. Post-molding shrinkage is assessed after at least 48 hours at 23 °C and 50% relative humidity, or after annealing at the temperature relevant to the intended service environment. For parts used at elevated temperatures, annealed total shrinkage is the most meaningful design value.

How should mold designers apply shrinkage test data?

Cavity dimensions are typically scaled by the measured shrinkage value in each principal direction, with separate compensation for MD and TD in fiber-reinforced grades. A safety margin, tolerance analysis, and prototyping or steel-safe correction cycles are strongly advised, because part geometry, wall thickness variations, gate location, and processing drift all influence final dimensions beyond the nominal material coefficient.