ASTM D1621: Compressive Testing of Rigid Cellular Plastics — Method Guide
What Is ASTM D1621?
ASTM D1621 — Standard Test Method for Compressive Properties of Rigid Cellular Plastics — measures the compressive stress-strain behaviour of rigid foam materials under uniaxial compression. The test determines compressive strength (stress at yield or 10% deformation, whichever occurs first) and compressive modulus (initial linear stiffness), which are the primary mechanical design properties of structural foam core materials and thermal insulation products.
What Are Rigid Cellular Plastics?
Rigid cellular plastics — commonly called rigid foams — include:
- Polyurethane (PU/PUR) foam: Insulation panels, refrigerator cores, structural sandwich core
- Polyisocyanurate (PIR) foam: Roofing insulation, wall panels
- Extruded polystyrene (XPS): Below-grade insulation, road subgrade
- Expanded polystyrene (EPS): Packaging, below-grade foundation insulation
- Phenolic foam: High fire resistance insulation for HVAC ductwork
- PMI foam (Rohacell): Aerospace structural sandwich core
- PET foam: Wind turbine blade and marine sandwich core
Compressive properties govern the structural performance of these materials under floor loading, soil pressure, structural sandwich applications, and impact absorption.
ASTM D1621 Test Procedure
Cubic or disc-shaped specimens are cut from the foam material in the rise direction (perpendicular to the foam board surface) and in the transverse direction (parallel to the board surface), as foam compressive properties are typically anisotropic — higher in the rise direction due to elongated cell structure.
Specimens are compressed between rigid parallel platens at a constant crosshead speed of 2.5 mm/min per unit specimen height (i.e., a strain rate of approximately 10%/min). The compressive stress-strain curve is recorded continuously until 10% strain is achieved.
Compressive strength is reported as the stress at the yield point (if the stress-strain curve exhibits a distinct yield drop) or as the stress at 10% compressive strain (if no yield point is observed).
Compressive modulus is the slope of the initial linear portion of the compressive stress-strain curve.
Anisotropy in Rigid Foam Compressive Properties
Most rigid foams exhibit orthotropic mechanical behaviour — compressive strength and modulus differ between the foam rise direction and the two transverse directions. In polyurethane and PIR foams, rise-direction compressive strength is typically 1.5–3× higher than transverse compressive strength due to the elongated cell geometry in the rise direction. ASTM D1621 testing in both rise and transverse directions characterises this anisotropy for structural design.
Industrial Applications
In structural sandwich panels for building, transportation, and marine applications, ASTM D1621 compressive strength determines the load-carrying capacity and wrinkling resistance of the panel. For EPS and XPS insulation used in below-grade foundations and road subgrade applications, compressive strength determines the maximum superimposed load the insulation can carry without permanent deformation. For aerospace PMI foam core, compressive modulus data feeds into sandwich panel structural analysis to calculate core shear and face sheet wrinkling allowables.
Conclusion
ASTM D1621 is the standard test method used to determine the compressive properties of rigid cellular plastics, including materials such as polyurethane (PU), polyisocyanurate (PIR), EPS, XPS, PMI, and PET foams. It measures the compressive strength (typically at yield or 10% deformation) and compressive modulus, which are essential design parameters for insulation boards, structural foam cores, and sandwich composite panels.
Because rigid foams often show directional differences in strength due to cell orientation, ASTM D1621 is especially important for characterising anisotropy in both the rise and transverse directions. The data generated is critical for applications such as building insulation under compressive loads, below-grade foundation systems, wind turbine blades, aerospace sandwich cores, and marine composite structures.
Why Choose Infinita Lab for ASTM D1621 Foam Compressive Testing?
Infinita Lab provides ASTM D1621 compressive property testing for all rigid cellular plastic materials — polyurethane, PIR, EPS, XPS, phenolic, PET, and aerospace foam grades — through our nationwide accredited materials testing laboratory network.
Looking for a trusted partner to achieve your research goals? Schedule a meeting with us, send us a request, or call us at (888) 878-3090 to learn more about our services and how we can support you.
Frequently Asked Questions (FAQs)
Which materials are tested using ASTM D1621? It is commonly used for PU foam, PIR foam, EPS, XPS, PMI foam, PET foam, and other rigid expanded plastics.
Why is 10% deformation important? For many rigid foams, a clear yield point may not appear, so the stress at 10% compressive strain is used as the standard comparison value.
Is the foam tested in multiple directions? Yes, rigid foams are often tested in both the rise direction and transverse direction because their properties are usually anisotropic.
Which industries use ASTM D1621? Construction, aerospace, marine, wind energy, packaging, refrigeration, and transportation industries widely use this test.
What equipment is used? A universal testing machine (UTM) with parallel compression platens is used to compress the specimen under controlled speed.
