Plastic Granules Tensile Strength Testing: Standards, Specimen Preparation, and Test Methods

Plastic granules are the fundamental raw material for injection molding, extrusion, and blown film processes, and their tensile properties directly determine the mechanical reliability of finished products. Because pellets themselves cannot be gripped and pulled, tensile strength testing of plastic granules is performed on standardized dumbbell specimens molded from the pellets in accordance with ISO 527, ASTM D638, and GB/T 1040. This article explains the test principle, specimen preparation, measured parameters, test procedures, and typical industrial applications of plastic granule tensile strength testing.

Technical Overview

Plastic granules (also called pellets or resin pellets) are the primary feedstock for virtually all thermoplastic processing operations. However, an individual pellet is far too small to be clamped into a tensile fixture, so the tensile properties of a granule batch are evaluated by first converting the granules into standardized test specimens and then pulling those specimens to failure on a universal testing machine. This combined "molding plus testing" workflow is what the industry refers to as plastic granule tensile strength testing.

The test principle is straightforward: a dumbbell-shaped specimen with a defined cross-section is gripped at both ends and extended at a constant crosshead speed. The machine records the force versus extension curve, from which the stress-strain behavior of the material is calculated. Key results include the tensile strength (maximum stress before failure), stress and strain at yield and at break, and the tensile modulus (Young's modulus) in the initial elastic region.

The governing standards most commonly applied by accredited third-party laboratories and in-house QC labs include:

• ISO 527-1 and ISO 527-2 — Determination of tensile properties of plastics, general principles and test conditions for moulding and extrusion plastics
• ASTM D638 — Standard Test Method for Tensile Properties of Plastics
• GB/T 1040.1 and GB/T 1040.2 — Chinese national standards, technically aligned with ISO 527
• JIS K 7161 and JIS K 7162 — Japanese standards corresponding to the ISO 527 series
• ISO 293, ISO 294-1, ISO 20753, and ASTM D4703 — specimen preparation standards for compression molding, injection molding, and multi-purpose test specimen geometry

A typical test setup consists of a universal testing machine (UTM) with a suitable load cell matched to the expected force range, pneumatic or mechanical wedge grips, an extensometer (contact or non-contact video type) for accurate strain measurement in the modulus region, and a controlled laboratory atmosphere of 23 °C and 50% relative humidity.

Testing Samples

Injection-Molded Dumbbell Specimens

Injection molding is the preferred preparation route for most engineering thermoplastics because it reproduces the processing conditions of real production parts. Granules are dried if required by the material data sheet, melted, and injected into a dumbbell cavity following ISO 294-1 or ASTM D4703. The resulting specimens are then tested per ISO 527-2 (specimen types 1A and 1B) or ASTM D638 (specimen Type I). The ISO 1A specimen has a gauge length of 50 mm, an overall length of 170 mm, and a width of 10 mm in the parallel section.

Compression-Molded or Die-Cut Specimens

For materials that are sensitive to molecular orientation induced by injection molding, such as certain polyolefins or materials being compared on a purely compositional basis, sheets are first compression molded per ISO 293 or ASTM D4703. Dumbbell specimens are then machined or die-cut from the sheet. This route minimizes orientation effects and often produces lower but more isotropic strength values than injection-molded specimens.

Granule Types Commonly Submitted for Testing

Accredited third-party laboratories regularly receive the following sample categories:

• Virgin commodity resins: PE (LDPE, LLDPE, HDPE), PP (homopolymer and copolymer), PS, EPS, and PVC compounds
• Engineering resins: ABS, PA6, PA66, PC, POM, PBT, PET, PMMA, and PPO blends
• High-temperature and specialty resins: PPS, PEEK, PEI, and PSU
• Modified and reinforced compounds: glass-fiber or carbon-fiber filled, mineral filled, impact modified, and flame-retardant grades
• Recycled and reprocessed pellets, post-industrial and post-consumer regrind, and masterbatch carriers
• Thermoplastic elastomers such as TPU, TPE, and TPV

Testing Items

The tensile test delivers a complete stress-strain characterization of the granule lot. Typical reported parameters include:

• Tensile strength (σM) — the maximum tensile stress sustained by the specimen before rupture
• Tensile stress at yield (σY) — the stress at the first point where strain increases without a stress increase, relevant for ductile materials
• Tensile stress at break (σB) — the stress at the moment of specimen separation
• Tensile strain at yield (εY) and tensile strain at break (εB), expressed in percent
• Tensile modulus (Et) — determined as the slope of the stress-strain curve between strains of 0.05% and 0.25% per ISO 527, or as chord modulus per ASTM D638
• Nominal tensile strain and strain at maximum force when an extensometer is not used beyond the extensometer limit
• Failure mode and location documentation, which is important for result validity assessment

Optional measurements on the same equipment include Poisson's ratio (per ISO 527-1 Annex and ASTM D638 appendices using bi-axial extensometry) and stress-strain curves at multiple temperatures for material database generation.

Testing Methods

A standard test flow carried out by accredited third-party laboratories consists of the following steps:

• Sample receipt, drying, and conditioning: granules are dried per the material specification, molded into specimens, and then conditioned at 23 ± 2 °C and 50 ± 10% relative humidity for a minimum of 88 hours per ISO 291 or ASTM D618, unless a shorter period is justified by documented equilibrium
• Specimen preparation and marking: dumbbells are checked for flash, voids, and sink marks; width and thickness are measured at three points in the gauge section with a precision of 0.02 mm or better, and the minimum cross-section is used in stress calculations
• Gripping: the specimen is centered in the grips with a gauge length of 50 mm for ISO 1A/1B and ASTM Type I specimens, with a small preload (typically 0.1 MPa to 1 MPa) applied to remove slack
• Modulus determination: the crosshead speed is set to 1 mm/min and strain is measured with an extensometer to capture the initial linear region (0.05% to 0.25% strain)
• Strength determination: the test speed is increased, commonly to 50 mm/min for ISO 1A specimens or 50 mm/min (Method A) for ASTM D638 Type I, and the specimen is pulled until it yields or breaks
• Calculations: stress σ = F / A (force divided by original cross-sectional area), strain ε = ΔL / L0 (extension divided by original gauge length), and modulus Et = (σ2 − σ1) / (ε2 − ε1) between the defined strain limits
• Replication and statistics: a minimum of five valid specimens is required per ISO 527 and ASTM D638; results are reported as mean values with standard deviation, and specimens that fail outside the gauge section are normally discarded and retested

The test report should clearly identify the material and lot, the specimen preparation method (injection or compression molding and molding conditions), specimen type, conditioning atmosphere, test speeds, individual and mean results for each parameter, and the specific standard edition applied. This traceability is essential because both molding conditions and test speed have a measurable influence on the measured tensile strength, particularly for strain-rate-sensitive polymers.

Applications

Tensile strength testing of plastic granules supports decision-making across the entire polymer value chain:

• Incoming material inspection: injection molders and extruders verify that each delivered lot meets the certificate of analysis before production release, protecting against off-spec or adulterated resin
• Supplier qualification and batch release: resin producers and compounders use tensile data as part of routine quality control and certification of conformity
• Recycled material evaluation: recyclers and buyers of regrind or regranulated material rely on tensile testing to grade recycled pellets and demonstrate that properties remain within an acceptable percentage of virgin material
• Formulation development: R&D teams quantify the effect of fillers, glass fiber content, impact modifiers, stabilizer packages, and compatibilizers on mechanical performance
• Processing window studies: comparing specimens molded at different temperatures and cooling rates reveals process sensitivity and supports scientific molding studies
• Failure analysis and complaint investigation: when a molded part fails in the field, tensile testing of the retained granule lot helps determine whether the raw material or the processing conditions contributed to the failure
• Regulatory and customer compliance: automotive, electrical, packaging, and medical specifications frequently cite ISO 527 or ASTM D638 tensile requirements as acceptance criteria

FAQ

Why can the granules not be pulled directly in the tensile machine?

A single pellet is only a few millimeters in size and cannot be gripped without premature failure at the jaws. Standards therefore require converting the granules into standardized dumbbell specimens by injection or compression molding first; the specimen is representative of the granule lot provided molding parameters are controlled and documented.

Which standard should be chosen, ISO 527 or ASTM D638?

Both standards measure the same fundamental properties, but they use slightly different specimen geometries, gauge lengths, and modulus definitions, so results are not directly interchangeable. European and Asian supply chains typically specify ISO 527, while North American specifications often cite ASTM D638. The choice should match the customer specification or the region of the end market.

How many granules are needed for one test program?

Depending on specimen size and molding method, approximately 100 to 500 grams of granules are usually sufficient to mold the five or more dumbbell specimens required for a single condition, plus spares. For multi-condition programs (for example, dry-as-molded and conditioned states of polyamide), a larger quantity of 1 to 2 kilograms is recommended.

Do injection-molded and compression-molded specimens give the same results?

No. Injection molding introduces molecular and fiber orientation along the flow direction, which typically raises strength and lowers elongation for filled grades. Compression-molded specimens are more isotropic. Comparison between datasets is only valid when the preparation method is identical, which is why the report must always state how the specimens were produced.

What are typical tensile strength values for common resins?

Indicative ranges at 23 °C include approximately 20 to 35 MPa for polyethylene, 30 to 40 MPa for polypropylene, 40 to 60 MPa for ABS, 60 to 85 MPa for unfilled PA66 (dry), 60 to 70 MPa for polycarbonate, and 100 to 200 MPa for glass-fiber reinforced engineering grades. Exact values depend on grade, filler content, and conditioning, so specification limits should always be agreed against the specific material data sheet.

How long does a standard tensile test take?

Excluding specimen molding and conditioning, the mechanical test itself normally runs 2 to 10 minutes per specimen depending on the ductility of the material and the selected test speed. Including molding, conditioning, and reporting, a complete program at an accredited third-party laboratory is typically completed within three to seven working days.