PA6 shrinkage data determination quantifies the dimensional change that polyamide 6 (nylon 6) undergoes during injection molding and subsequent conditioning, yielding the mold shrinkage, post-molding shrinkage, and total shrinkage values required for tooling compensation and tolerance design. Because PA6 is a semi-crystalline, hygroscopic polymer, its shrinkage depends strongly on crystallinity, filler content, moisture uptake, and processing parameters, making accurate measurement essential for part quality. This article outlines the test samples, measured parameters, standard test methods, typical data ranges, and practical applications of PA6 shrinkage determination.
Technical Overview
PA6 (polyamide 6, also known as nylon 6) is a semi-crystalline engineering thermoplastic widely used in injection-molded components. During cooling in the mold, PA6 undergoes thermal contraction and crystallization-induced densification, causing the molded part to be smaller than the cavity that produced it. Shrinkage data determination measures this dimensional difference under standardized conditions and expresses it as a percentage of the corresponding cavity dimension.
Three quantities are typically distinguished in PA6 shrinkage determination: mold shrinkage, measured within a short time after ejection; post-molding shrinkage, which occurs during annealing or long-term conditioning; and total shrinkage, the sum of the two. For unreinforced PA6, mold shrinkage generally falls between 1.0% and 2.0%, while glass-fiber-reinforced grades shrink less but exhibit pronounced anisotropy, with shrinkage in the flow direction often being several times smaller than shrinkage transverse to flow. Because PA6 absorbs moisture from the environment and swells as it equilibrates, humidity conditioning is an integral part of any rigorous shrinkage measurement program.
Testing Samples
Unfilled PA6 Resins
Natural and lubricated unreinforced PA6 injection molding grades are tested to establish baseline shrinkage behavior. These samples show nearly isotropic shrinkage, typically 1.0% to 2.0%, which is strongly influenced by mold temperature and wall thickness.
Glass-Fiber-Reinforced PA6 Grades
Compounds containing 15% to 50% short glass fiber are tested in both the flow direction (machine direction) and the transverse direction. A typical PA6-GF30 shrinks about 0.2% to 0.5% along the flow and 0.5% to 1.0% across it, and the anisotropy ratio is itself an important reported datum.
Mineral-Filled and Impact-Modified PA6
Grades filled with minerals, glass beads, or impact modifiers display intermediate shrinkage, commonly 0.7% to 1.2%, with reduced anisotropy compared with fiber-reinforced compounds.
Specialty Compounds
Flame-retarded, heat-stabilized, pigmented, and high-regrind-content formulations are also tested because additives and colorants can shift crystallization behavior and therefore alter shrinkage values.
Testing Items
A complete PA6 shrinkage data determination typically includes the following measured and calculated items:
- Mold shrinkage in the flow direction, measured 16 to 24 hours after molding
- Mold shrinkage transverse to flow
- Anisotropy ratio, defined as transverse shrinkage divided by flow-direction shrinkage
- Post-molding shrinkage after annealing at an agreed elevated temperature
- Total shrinkage, combining mold shrinkage and post-molding shrinkage
- Moisture-related dimensional change after humidity conditioning
- Mass change and moisture content, recorded alongside dimensional data for correlation
Testing Methods
Specimen Preparation
Test plates or bars are injection molded in accordance with ISO 294-1 or the equivalent national standard. A common geometry is a 60 mm by 60 mm plate with a 2 mm thickness, molded from resin dried to below 0.2% moisture. Typical processing windows for PA6 include melt temperatures of 240 °C to 280 °C and mold temperatures of 60 °C to 100 °C. To study process sensitivity, holding pressure, holding time, melt temperature, and mold temperature may be varied systematically across a designed experiment, since each parameter measurably affects final dimensions.
Conditioning
Mold shrinkage is determined after specimens equilibrate in a standard atmosphere of 23 °C and 50% relative humidity per ISO 291, typically 16 to 24 hours after ejection for the initial measurement. Post-molding shrinkage is evaluated after annealing, commonly 2 to 4 hours at 100 °C to 120 °C for PA6, or after accelerated humidity conditioning per ISO 1110. Because PA6 parts absorb moisture and swell, dimensional readings are always paired with moisture-state documentation.
Dimensional Measurement
Cavity dimensions are measured first using calibrated gauges, optical comparators, or coordinate measuring machines. Specimen dimensions are then measured at defined gauge points in both principal directions using micrometers or optical systems with resolution sufficient to detect differences of 0.01 mm or better. All instruments are calibrated with traceable reference standards, and multiple specimens per molding condition are measured to establish statistical confidence.
Calculation
Mold shrinkage is calculated as the cavity dimension minus the molded specimen dimension, divided by the cavity dimension, multiplied by 100%. Post-molding shrinkage is calculated as the specimen dimension before conditioning minus the dimension after conditioning, divided by the initial specimen dimension, multiplied by 100%. Total shrinkage is the mold shrinkage plus the post-molding shrinkage, or equivalently the cavity dimension minus the final conditioned dimension, divided by the cavity dimension. Results are reported separately for the flow and transverse directions, together with molding conditions and conditioning history.
Applicable Standards
- ISO 294-4, covering the determination of molding shrinkage from injection-molded test plates
- ISO 294-1, governing the preparation of injection-molded multipurpose test specimens
- ISO 291 and ISO 1110, covering standard and accelerated conditioning atmospheres
- ASTM D955 and ASTM D6991, historically used for thermoplastic mold shrinkage evaluation in North America
- ISO 62, used to characterize water absorption that drives dimensional change in PA6
Applications
PA6 shrinkage data is applied throughout the product development and manufacturing cycle. Tooling engineers use the measured shrinkage values to scale cavity steel, applying directional corrections for glass-filled grades where anisotropy is significant. Design engineers feed the data into tolerance stack-up analyses and geometric dimensioning calculations to ensure interchangeability of molded parts. CAE engineers use measured shrinkage and crystallization data to validate packing and warpage simulations before steel is cut. Quality teams compare incoming resin lot shrinkage against established data windows to detect formulation drift. Typical end-use sectors include automotive under-hood components, gears, fasteners, connectors, housings, industrial parts, and PA6 film for shrink packaging applications.
FAQ
What is the typical shrinkage of unfilled PA6?
Unfilled PA6 generally shows mold shrinkage between 1.0% and 2.0%, with thicker walls, higher mold temperatures, and lower packing pressures all pushing shrinkage toward the upper end of the range.
Why does glass-filled PA6 shrink less but more anisotropically?
Glass fibers restrict polymer contraction along their orientation, which is predominantly the flow direction. The matrix still contracts strongly across the fibers, so flow-direction shrinkage can drop to 0.2% to 0.5% while transverse shrinkage remains near 0.6% to 1.0%.
How does moisture affect measured dimensions?
PA6 is hygroscopic, and moisture absorption causes dimensional swelling that partially offsets molding shrinkage. Conditioning state must therefore be recorded with every measurement, since a dry-as-molded part and a fully equilibrated part can differ by roughly 0.2% to 0.7% in linear dimensions.
How can shrinkage of PA6 parts be reduced?
Effective measures include increasing holding pressure and holding time until gate freeze-off, lowering mold temperature, reducing wall thickness, selecting a glass- or mineral-filled grade, and ensuring adequate drying of the resin before molding.