PET sintering shrinkage test data provides critical quantitative insight into the dimensional behavior of polyethylene terephthalate powder during laser sintering and thermal consolidation processes. Accurate shrinkage characterization enables engineers to apply dimensional compensation, optimize sintering parameters, and guarantee part accuracy in additive manufacturing and powder processing applications. This article presents a comprehensive overview of sample preparation, testing items, measurement methods, typical data ranges, and practical applications of PET shrinkage testing.
Technical Overview
PET (polyethylene terephthalate) is a semi-crystalline thermoplastic widely used in powder-based processing routes, including selective laser sintering (SLS), hot powder compaction, and sinter-based manufacturing. During the sintering cycle, PET particles undergo particle rearrangement, viscous flow, neck growth, and crystallization, all of which reduce the overall part dimensions. This dimensional reduction, known as sintering shrinkage, must be quantified through controlled laboratory testing.
PET sintering shrinkage test data typically documents linear shrinkage along the X, Y, and Z axes, volumetric shrinkage, warpage, densification, and related crystallinity changes. Because PET exhibits pronounced anisotropic behavior in layer-based processes, shrinkage along the build direction (Z) is generally higher than in the in-plane directions. Reliable shrinkage data forms the scientific basis for pre-compensating CAD models, designing tooling, and defining stable sintering process windows.
Testing Samples
Virgin PET Powder
Untouched PET powder with a nominal particle size distribution of 20 to 100 micrometers (D50 of approximately 55 to 65 micrometers) and sphericity above 0.90 is conditioned at 23 ± 2 °C and 50 ± 5% relative humidity for at least 24 hours prior to testing. Key powder characteristics recorded before sintering include:
- Apparent powder density, typically 0.45 to 0.55 g/cm³
- Bulk density and tap density per standardized flow and packing metrics
- Melt flow index to verify molecular weight and flowability
- Initial crystallinity, usually below 10% for amorphous feedstock
Recycled and Aged PET Powder
Reused powder blends containing 30% to 70% recycled fractions are tested to evaluate how thermal aging affects shrinkage behavior. Aged PET powder tends to show reduced melt flow and slightly elevated shrinkage scatter, so test data distinguishes lot-to-lot variation and establishes refresh-rate guidelines.
Sintered Test Specimens
Standardized geometry coupons are fabricated and measured, including 10 × 10 × 10 mm cubes for volumetric shrinkage, 100 × 100 × 10 mm flat plates for warpage assessment, and tensile bars for combined dimensional and mechanical evaluation. A minimum of five replicate specimens per parameter set is recommended for statistical validity.
Testing Items
The core measurement program for PET sintering shrinkage test data covers the following items:
- Linear shrinkage in the X, Y, and Z directions, expressed as percentage of the nominal green dimension
- Volumetric shrinkage calculated from the combined orthogonal dimensional changes
- Warpage and flatness deviation on plate specimens
- Sintered density and relative density by the Archimedes method
- Open and closed porosity derived from density measurements
- Degree of crystallinity by differential scanning calorimetry (DSC)
- Edge rounding, surface growth, and feature resolution loss
Testing Methods
Sintering Procedure
Laser sintering trials are conducted with a powder bed temperature of 190 to 215 °C (below the PET crystalline melting point of approximately 250 °C), laser powers of 20 to 35 W, and scan speeds of 2000 to 4500 mm/s. For oven sintering studies, green specimens are heated at 5 to 10 °C/min to 240 to 260 °C, held for 30 to 60 minutes, and cooled at a controlled rate of 3 to 5 °C/min to minimize thermal gradients and warpage.
Dimensional Measurement
Green-state dimensions are recorded before sintering using a coordinate measuring machine (CMM) with a stated accuracy of ±1.5 micrometers or calibrated digital calipers for larger features. After sintering and 24-hour stabilization at laboratory conditions, identical measurement points are re-measured. Linear shrinkage is calculated with the standard formula:
Linear shrinkage (%) = (L0 − L1) / L0 × 100, where L0 is the green dimension and L1 is the sintered dimension. Volumetric shrinkage is determined analogously from pre- and post-sinter volumes obtained by optical scanning or geometric reconstruction.
Density and Structure Characterization
Sintered density is measured by the Archimedes immersion method, and relative density is referenced against fully dense PET at 1.38 to 1.40 g/cm³. DSC analysis quantifies the crystallinity increase that accompanies shrinkage, while microscopy of cross-sections documents pore distribution and interlayer bonding quality.
Typical Data Ranges
Representative PET sintering shrinkage test data from controlled studies generally fall within the following intervals:
- In-plane linear shrinkage (X/Y): 1.0% to 2.0%
- Build-direction shrinkage (Z): 2.0% to 3.5%
- Volumetric shrinkage: 4% to 7%
- Sintered relative density: 88% to 96%
- Crystallinity increase: from under 10% to 30% to 40% after sintering
Reported results should always include mean values, standard deviations, sintering parameters, and specimen orientation to ensure traceability and comparability between data sets.
Applications
PET sintering shrinkage test data is applied across several engineering functions:
- Dimensional compensation of CAD models by applying inverse scaling factors prior to production
- Process window definition for laser power, bed temperature, and scan strategy in SLS manufacturing
- Powder lot qualification and acceptance criteria for incoming material inspection
- Recycled powder blend management and refresh ratio optimization
- Tooling and mold design for sinter-based and compression-assisted PET processes
- Quality control traceability in automotive, packaging, aerospace ducting, and consumer product manufacturing
FAQ
What is the typical shrinkage of sintered PET parts?
Linear shrinkage commonly falls between 1.0% and 2.0% in the plane of sintering and between 2.0% and 3.5% in the build direction, giving a volumetric shrinkage of roughly 4% to 7%. Exact values depend on powder quality, thermal profile, and degree of crystallization.
Why is shrinkage higher in the Z direction?
The Z direction reflects the powder bed packing density, which is lower than the in-plane consolidation density. Layer-by-layer bonding and gravitational settling during sintering further amplify dimensional contraction along the build axis.
Which standards govern the testing?
Specimen conditioning follows ISO 291 or ASTM D618, density measurement follows Archimedes-based methods such as ASTM B962 or ISO 2738, and tensile specimen geometry follows ASTM D638 or ISO 527. Shrinkage itself is computed from direct dimensional metrology per laboratory-validated procedures.
How can PET shrinkage be reduced?
Effective countermeasures include lowering peak holding temperature within the acceptable densification range, slowing the cooling rate, optimizing scan strategy to equalize energy input, preheating uniformly, and incorporating glass fiber or mineral fillers, which can cut linear shrinkage to below 0.8%.
How many specimens are needed for reliable data?
A minimum of five replicate specimens per parameter combination is recommended, with critical applications requiring ten replicates so that mean values, standard deviations, and confidence intervals adequately represent process variability.
