Post-shrinkage is a critical dimensional stability parameter for polyamide (PA, nylon) components, because continued crystallization, relaxation of molded-in stresses, and moisture exchange cause parts to change size long after they leave the mold. This article explains the principles, applicable standards, sample requirements, test items, and step-by-step procedures for PA nylon post-shrinkage measurement, helping engineers control tolerances and qualify materials with confidence.
Technical Overview
Post-shrinkage (also called after-shrinkage) refers to the additional dimensional reduction of an injection-molded plastic part that occurs after the initial post-molding measurement window has passed. While molding shrinkage is typically evaluated 16 to 24 hours after demolding, PA nylon parts can continue to shrink for weeks, months, or even longer once they are exposed to elevated service temperatures, dry environments, or thermal loading cycles.
Polyamide is particularly susceptible to post-shrinkage for two fundamental reasons. First, PA is a semi-crystalline polymer: if mold temperatures and cooling rates do not allow crystallization to reach its equilibrium level, post-crystallization will continue during storage or service, densifying the material and shrinking the part. Second, PA is strongly hygroscopic: nylon absorbs several percent of water at ambient humidity, and this moisture exchange produces swelling or drying shrinkage that overlaps with and complicates the purely thermal post-shrinkage effect.
In standardized testing per ISO 294-4 (and the equivalent GB/T 17037.4), three related quantities are distinguished:
- Molding shrinkage (Sp): shrinkage measured 16 to 24 hours after molding on specimens conditioned at 23 degrees C and 50% relative humidity.
- Total shrinkage (St): shrinkage measured after a defined thermal treatment (annealing) of the specimen.
- Post-shrinkage (Sa): the difference between total shrinkage and molding shrinkage, Sa = St minus Sp, expressed in percentage points.
Because post-shrinkage reveals how much additional dimensional change a part will undergo in service, it is an essential input for tolerance design, mold shrinkage compensation, annealing decisions, and long-term dimensional stability qualification of PA6, PA66, and their glass-fiber-reinforced modifications.
Testing Samples
Standard Molded Plates (Laboratory Specimens)
The reference specimen for shrinkage testing is a flat square plate, typically 60 mm by 60 mm with a thickness of 1 mm or 2 mm, injection-molded in a mold conforming to ISO 294-3. Measuring marks are applied to the plate surface in the flow direction and perpendicular to it, allowing directional shrinkage to be tracked from the earliest possible moment after demolding.
Glass-Fiber Reinforced PA Specimens
Glass-filled nylon grades exhibit strongly anisotropic shrinkage because fibers align with the melt flow. Specimens of PA-GF30 and similar compounds must always be measured in both the parallel and perpendicular directions, since the post-shrinkage can differ significantly between the two axes and may drive warpage in real parts.
Production Parts and Components
For qualification of actual products such as connectors, gears, housings, or structural brackets, functional dimensions defined on the part drawing are used instead of plate dimensions. Coordinate measuring machines (CMM) or optical measurement systems are usually required for these geometries, and gate position, wall thickness distribution, and fiber orientation should be documented because they influence local post-shrinkage.
Conditioning States of the Material
The moisture state of the specimen must be defined before testing. Samples can be evaluated in the dry-as-molded state, after standard atmosphere conditioning at 23 degrees C and 50% RH per ISO 291, or after accelerated conditioning to moisture equilibrium per ISO 1110 (for example, 70 degrees C and 62% relative humidity). Since PA6 and PA66 may absorb roughly 2.5% to 3% moisture at 50% RH, the conditioning state directly affects the measured dimensions.
Testing Items
A complete PA nylon post-shrinkage evaluation typically includes the following measured and derived parameters:
- Molding shrinkage (Sp) in the flow direction and perpendicular to flow, as the baseline.
- Total shrinkage (St) after the defined annealing treatment, in both directions.
- Post-shrinkage (Sa = St minus Sp), quantifying the additional dimensional change after thermal treatment.
- Shrinkage anisotropy ratio between flow and cross-flow directions, particularly for fiber-reinforced grades.
- Mass change during annealing or conditioning, used to track moisture loss or uptake and to verify treatment completion.
- Warpage and distortion assessment of plates or parts after treatment, reported qualitatively or as flatness deviation.
Testing Methods
Specimen Preparation and Baseline Measurement
Specimens are injection-molded under documented conditions per ISO 294-1, with melt temperature, mold temperature, injection speed, holding pressure, and holding time recorded, since all of these parameters influence crystallinity and residual stress. After demolding, specimens are conditioned at 23 degrees C and 50% relative humidity. The reference dimensions are measured 16 to 24 hours after molding using calibrated micrometers, dial gauges, optical comparators, or CMM equipment, always at the marked measurement positions and in the defined directions.
Annealing (Accelerated Post-Shrinkage) Method
To accelerate the slow post-shrinkage process into a practical laboratory timescale, the specimens are annealed in a forced-air circulating oven at a temperature agreed between the parties; for PA6 and PA66, temperatures around 110 degrees C are commonly applied, with a holding time sufficient to complete post-crystallization (frequently several hours, or until specimen mass is essentially constant). The annealing temperature must stay safely below the melting point and below any temperature that would induce unwanted degradation or severe oxidative discoloration. After annealing, specimens are cooled in a defined manner, reconditioned to the standard atmosphere, and remeasured at the same marked positions. The difference from the baseline dimensions represents the post-shrinkage.
Long-Term Storage and Conditioning Method
As an alternative or complement to annealing, specimens may be stored under controlled real-service conditions (for example, dry heat at 80 to 100 degrees C, or humidity cycling) and measured at defined intervals over days, weeks, or months. This approach captures moisture-driven dimensional change in addition to post-crystallization, and is useful when parts will serve in environments where humidity fluctuates rather than at constant elevated temperature.
Calculation and Reporting
Post-shrinkage is calculated from the measured dimensions. Using L0 as the dimension measured after the initial 16 to 24 hour conditioning and L1 as the dimension after annealing, the post-shrinkage equals (L0 minus L1) divided by L0, multiplied by 100 percent; equivalently, it can be reported as the difference St minus Sp from the ISO 294-4 evaluation. The test report should include the material designation, drying and molding conditions, conditioning state, annealing temperature and time, individual and mean shrinkage values in each direction, mass change, and any observed warpage, so that results are traceable and comparable between laboratories.
Applications
Post-shrinkage data for PA nylon are used across the entire value chain of thermoplastic component development and production:
- Automotive engineering: under-hood parts, air intake components, sensors, and connectors made from PA6 or PA66 operate at elevated temperatures where post-shrinkage directly affects fit, sealing, and long-term performance.
- Electrical and electronics: connector housings, terminal blocks, and switch components require tight pitch tolerances over years of service, making post-shrinkage a key qualification criterion.
- Precision mechanical parts: gears, bearings, and clips must maintain functional geometry, and post-shrinkage data guide tolerance allocation and assembly clearances.
- Mold and process engineering: measured shrinkage and post-shrinkage values determine the shrinkage compensation built into cavity dimensions and guide optimization of mold temperature and holding phase.
- Production stabilization: deliberate annealing of machined or precision parts, validated by post-shrinkage testing, pre-relieves stresses and pre-completes crystallization so that dimensions remain stable in service.
- Material qualification and incoming inspection: comparisons of post-shrinkage between resin lots or supplier grades support material specification and substitution decisions.
FAQ
What is the difference between mold shrinkage and post-shrinkage?
Mold shrinkage is the immediate reduction in size relative to the cavity, measured within one day of molding, while post-shrinkage is the additional reduction that occurs later, after annealing or during long-term storage and service. Post-shrinkage reflects ongoing post-crystallization and stress relaxation that were not completed during molding.
Why does PA nylon show higher post-shrinkage than many other plastics?
PA is semi-crystalline and hygroscopic. Incomplete crystallization at molding continues at service temperature and densifies the material, while moisture loss in dry or hot environments causes additional drying shrinkage. Unfilled PA6 and PA66 grades therefore demand careful post-shrinkage evaluation, especially for hot-service applications.
What are typical post-shrinkage values for PA?
Exact values depend on grade, molding conditions, and annealing severity, but properly molded unfilled PA6 and PA66 commonly show post-shrinkage in the range of a few tenths of a percent up to about one percent, with lower and more anisotropic values for glass-fiber-reinforced grades. Values should always be confirmed by testing under the agreed conditions.
How can post-shrinkage in PA parts be reduced?
Using a higher mold temperature promotes more complete crystallization during molding, transferring shrinkage from the post-shrinkage phase to the controlled mold shrinkage phase. Optimized holding pressure and time, correct gate sizing, glass-fiber reinforcement, and a deliberate stabilization annealing step before final machining or assembly are all effective countermeasures.
Which standards govern PA post-shrinkage measurement?
ISO 294-4 and GB/T 17037.4 define the determination of molding shrinkage, total shrinkage, and post-shrinkage on injection-molded plates, ASTM D955 covers mold shrinkage determination, and ISO 291 and ISO 1110 define the conditioning states of the specimens before and after measurement.