PET Sintering Shrinkage Determination: Testing Methods, Standards, and Technical Guidelines

PET sintering shrinkage determination quantifies the dimensional change that polyethylene terephthalate (PET) powder undergoes as it consolidates during thermal or laser sintering. Accurate shrinkage data are essential for dimensional compensation in selective laser sintering (SLS), powder coating, and other powder-based manufacturing processes. This technical guide explains the measurement principles, sample preparation requirements, testing methods, key test items, and practical applications of PET sintering shrinkage evaluation.

Technical Overview

PET sintering shrinkage refers to the reduction in linear or volumetric dimensions that occurs when PET powder particles fuse, densify, and eliminate inter-particle porosity during a sintering cycle. As the material is heated through its glass transition region (approximately 75 to 80 degrees Celsius for PET) and approaches its melting range (approximately 250 to 260 degrees Celsius), particle necking, coalescence, and pore closure drive macroscopic contraction of the powder mass or compact. Determination of this shrinkage is a critical measurement for any process in which PET powder is thermally consolidated, because uncorrected shrinkage directly translates into dimensional inaccuracy, warpage, and part rejection.

Linear sintering shrinkage is conventionally expressed as a percentage according to the relationship: Shrinkage (%) = (L0 minus L1) divided by L0, multiplied by 100, where L0 is the green (pre-sintered) dimension and L1 is the sintered dimension measured under standardized conditions. Volumetric shrinkage is calculated analogously from the change in specimen volume or from measured density before and after sintering. Depending on powder grade, packing density, and process parameters, PET sintering shrinkage typically falls within a range of roughly 0.5 to 4 percent, although laser-sintered parts may exhibit direction-dependent (anisotropic) values.

Numerous material and process factors influence the measured shrinkage, including particle size distribution and morphology, powder bed or green density, moisture content, thermal history and crystallinity, heating rate, peak sintering temperature, holding time, cooling rate, part geometry, and build orientation. A controlled, standardized determination procedure is therefore required to generate data that are reproducible, comparable between laboratories, and suitable for dimensional compensation and material qualification.

Testing Samples

Virgin PET Powder

Untreated PET powder intended for laser sintering, powder coating, or compaction studies is sampled in accordance with powder sampling standards to ensure representative particle size distribution. Because PET is strongly hygroscopic, all powder must be dried and conditioned in a desiccated environment before testing; residual moisture promotes hydrolytic degradation, bubble formation, and artificially high or erratic shrinkage values. Powder is typically characterized for median particle size, span, bulk density, and flowability prior to specimen preparation.

Green Compacts and Compression-Molded Coupons

For dilatometric and furnace-based shrinkage tests, PET powder is formed into standard geometry specimens such as rectangular bars, cylinders, or cubes using uniaxial compaction or compression molding at controlled pressure and temperature. Standard bar specimens, for example 30 by 6 by 6 millimeters, allow shrinkage measurement along multiple axes. Green dimensions are recorded immediately after specimen preparation, and specimens are weighed to determine green density, which is a key covariate in shrinkage interpretation.

Laser-Sintered (SLS) Test Specimens

Cubes, tensile blanks, and step gauges produced on a selective laser sintering system are used to determine effective in-process shrinkage. Specimens are built in multiple orientations, typically flat, on-edge, and vertical, so that directional shrinkage anisotropy in the X, Y, and Z axes can be quantified. Build parameters such as laser power, scan speed, hatch spacing, layer thickness, and powder bed temperature are documented because they materially affect the measured result.

Recycled and Aged Powder Blends

Reused SLS powder undergoes gradual thermal aging, increased molecular weight, and changed melting and crystallization behavior, all of which shift shrinkage. Test coupons made from aged powder, virgin powder, and defined virgin-to-recycled refresh blends are measured to establish acceptable recycling ratios and to control batch-to-batch dimensional stability.

Testing Items

A comprehensive PET sintering shrinkage determination program typically includes the following measurable quantities:

• Linear shrinkage along each principal axis (X, Y, and Z), expressed as a percentage of the green dimension.
• Volumetric shrinkage, derived either from dimensional measurements or from the density change between green and sintered states.
• Shrinkage anisotropy ratio, describing directional differences relevant to part orientation strategy.
• Shrinkage onset temperature, shrinkage rate, and total shrinkage as a function of temperature, obtained by thermomechanical analysis.
• Densification and residual porosity of the sintered specimen, determined by density measurement or microscopy.
• Warpage, distortion, and out-of-flatness resulting from non-uniform or anisotropic shrinkage.
• Repeatability metrics, including mean values, standard deviations, and measurement uncertainty across replicate specimens.

Testing Methods

Thermomechanical Analysis (TMA)

A thermomechanical analyzer applies a small static force to a powder compact or small specimen while programming a controlled heating rate, commonly 5 to 10 degrees Celsius per minute, through the glass transition and melting regions of PET. The instrument continuously records dimensional change as a function of temperature, yielding the shrinkage onset temperature, the shrinkage rate curve, and total linear shrinkage at the test endpoint. Testing is performed under an inert nitrogen purge to suppress oxidative degradation. TMA methods are generally aligned with ISO 11359 and ASTM E831 practices adapted for polymer powder compacts, and only a few tens of milligrams of material are required.

Push-Rod Dilatometry

A dilatometer with a suitable furnace and inert atmosphere monitors the length of a larger standard bar specimen across the full sintering thermal cycle, including ramp, hold, and cooling segments. This method captures shrinkage behavior under realistic thermal profiles and distinguishes sintering contraction from thermal expansion and post-sintering crystallization effects. The measurement approach follows the general principles of ISO 4492, adapted to thermoplastic PET materials.

Furnace Sintering with Dimensional Metrology

Green specimens are measured with calibrated micrometers, calipers, or a coordinate measuring machine, then sintered in a calibrated furnace under a documented thermal cycle and nitrogen atmosphere. After controlled cooling and conditioning at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity per ISO 291, sintered dimensions are re-measured at identical datum points, and percentage shrinkage is calculated for each axis. This gravimetric-plus-metrological approach is the reference method for material data sheets and process qualification.

Density-Based (Archimedes) Evaluation

Green and sintered densities are determined by the Archimedes immersion method in accordance with ISO 3369 or ASTM B962. Because mass is conserved during sintering, the volumetric shrinkage can be derived from the density ratio, providing an independent cross-check on dimensional measurements and an indication of densification and residual porosity.

Optical Scanning and Full-Field Mapping

Structured-light or laser 3D scanning captures the complete surface geometry of specimens before and after sintering. Comparison against the nominal CAD model or the green scan produces full-field shrinkage and distortion maps, which are particularly valuable for complex laser-sintered geometries where localized shrinkage gradients and warpage cannot be captured by discrete point measurements.

Applications

Determined shrinkage data for PET materials support a wide range of industrial and research activities:

• Dimensional compensation in selective laser sintering, where measured shrinkage factors are applied to CAD models or machine scaling parameters to meet tolerance requirements.
• Process window development, correlating laser energy density, bed temperature, and layer thickness with dimensional stability.
• Powder coating formulation and curing optimization, ensuring target film thickness and coverage after consolidation.
• Recycled powder management, defining refresh ratios and acceptance criteria that keep part dimensions within specification.
• Material qualification and batch release testing for powder suppliers and additive manufacturing service bureaus.
• Calibration input for sintering simulation models used to predict distortion and to plan pre-compensated geometries.
• Research and development of modified PET grades, filled compounds, and nucleated blends, where shrinkage response indicates changes in crystallization and densification behavior.

FAQ

What shrinkage value is typical for laser-sintered PET?

Effective in-plane shrinkage for laser-sintered PET powders commonly lies between about 1 and 3 percent, while vertical (Z-axis) shrinkage may differ due to layer-by-layer consolidation and powder packing effects. Exact values depend strongly on the powder grade, process parameters, and part geometry, which is why direct determination is always recommended over nominal literature figures.

Which standards govern PET sintering shrinkage determination?

No single standard addresses polymer powder sintering shrinkage exclusively. Practice is built from a combination of ISO 4492 for dimensional change of powder compacts, ISO 11359 and ASTM E831 for thermomechanical shrinkage measurement, ISO 3369 or ASTM B962 for density-based densification, and the ISO/ASTM 52900 series for additive manufacturing process qualification context.

Why is moisture control so important in this test?

PET absorbs moisture readily, and even small amounts of water cause hydrolytic chain scission and steam generation during heating. This degrades the polymer, generates porosity, and produces erratic or inflated shrinkage results, so powders and specimens must be dried and stored in a controlled environment before measurement.

How can measured shrinkage be compensated in production?

Common strategies include geometric pre-scaling of the CAD model using the measured shrinkage factor, adjusting laser power, scan speed, hatch spacing, and bed temperature within a qualified process window, optimizing build orientation to minimize anisotropic distortion, and applying distortion-compensation software driven by validated simulation models.

How is sintering shrinkage different from injection mold shrinkage?

Mold shrinkage, covered by standards such as ASTM D955 and ISO 294-4, describes dimensional change of a melt solidified under pressure in a mold and is dominated by thermal contraction and crystallization. Sintering shrinkage describes the densification-driven contraction of a powder mass fusing without external pressure, and it depends on packing density and particle fusion kinetics, making it a distinct property requiring its own determination method.