Testing standard
Standard Test Method for Compressive Properties of Rigid Cellular Plastics
Written and technically reviewed by Dak System Inc. engineeringLast reviewed
ASTM D1621 measures the compressive properties of rigid cellular plastics — insulation boards, structural foams and foam cores. A prism is compressed between parallel platens at a rate of about 10 % of its height per minute. Where the specimen yields, compressive strength is the stress at yield; where it does not, the reported figure is the stress at 10 % deformation.
A prism of rigid cellular plastic, at least 25.8 cm² in section and between 25.4 and 50.8 mm high, is stood between two parallel compression platens with the upper one free to self-align. The platens close at a rate that deforms the specimen at about 10 % of its original height per minute — 2.5 mm/min for a 25.4 mm specimen. Force is recorded against platen separation. The test continues past yield where the material yields, or to 10 % deformation where it does not. The direction in which the specimen is loaded relative to the foam's rise direction is recorded, because it changes the answer substantially.
The method reports compressive strength at yield, or stress at 10 % deformation where there is no yield, and compressive modulus where required. These figures govern every application in which a rigid foam carries load rather than merely occupying space: insulation under screeds and in flat roofs, cold-store flooring, foam cores in sandwich panels, and packaging designed to protect by crushing in a controlled way. In a sandwich panel the core's through-thickness compressive strength is what resists local loads and fastener crushing, and it is often the property that limits the panel rather than the facings. Compressive strength also tracks density and cell quality closely, which makes it a sensitive production control.
Rigid foam is usually anisotropic — it is stronger along the direction the cells grew — so the orientation of the specimen is part of the result rather than a detail of preparation.
A result quoted without the loading direction is close to meaningless for a rigid foam. Rise-direction and transverse strengths differ enough that they are effectively different materials.
σ = F / A₀
ε = Δh / h₀ × 100
E = Δσ / Δε over the initial straight portion
Only meaningful once any toe from imperfect seating has been corrected out. On a foam with rough-cut faces the uncorrected slope can be badly low.
Direct compression platens with a self-aligning upper face. Self-alignment matters more here than on rigid plastics — a cut foam face is rarely perfectly parallel, and a fixed platen would load one edge first.
SpecificationsA frame with force measurement to ASTM E4 across a range that may run from a few hundred newtons for lightweight insulation to tens of kilonewtons for dense structural foam, and a crosshead that holds a slow rate accurately — 2.5 mm/min for the standard specimen. Platens must be parallel, larger than the specimen, and the upper one free to self-align, which matters more here than for rigid plastics because a cut foam face is rarely perfectly parallel. Deformation is taken from platen separation, so the contact datum must be set under light load rather than from a gap, and frame compliance is part of the measured travel on stiffer grades.
The most damaging error is omitting the loading direction, because rise-direction and transverse strengths can differ by a factor of two and a mixed data set then looks like an unstable process. The second is confusing the two possible outputs — a strength at yield and a stress at 10 % deformation are different quantities, and only one exists for any given specimen. The third is a toe on the curve from roughly cut faces, which depresses modulus and shifts the deformation axis; a fine-tooth blade or hot wire and a self-aligning platen remove most of it. Leaving the skin on inflates strength substantially, and running all specimen heights at one crosshead speed strains them at different rates.
| ASTM D1621 | ISO 844 | |
|---|---|---|
| Specimen | Minimum 25.8 cm² area, 25.4–50.8 mm high | Commonly a 50 mm cube or comparable prism |
| Rate | About 10 % of height per minute | Rate set to give a comparable relative deformation |
| Where no yield occurs | Stress at 10 % deformation | Stress at 10 % relative deformation |
| Anisotropy | Direction recorded | Direction recorded |
The two methods agree in intent and usually in result, but the specimen geometries are not identical and rigid foams are geometry-sensitive. Where a specification names one, run that one.
It is the ASTM test method for the compressive properties of rigid cellular plastics — insulation boards, structural foams and foam cores. A prism is compressed between parallel platens at about 10 % of its height per minute, and the method reports compressive strength at yield where the specimen yields, or the stress at 10 % deformation where it does not.
So that specimens of different heights are deformed at the same relative rate. Rigid foams are rate-sensitive, and a 25.4 mm specimen and a 50.8 mm one run at the same crosshead speed would be strained at quite different rates and give different strengths. Ten per cent of the original height per minute works out at 2.5 mm/min for the shorter specimen and about 5 mm/min for the taller.
More than almost anything else in the test. Most rigid foams are anisotropic because the cells elongate along the rise direction as the foam expands, and the compressive strength along the rise can be double the transverse value or more. A result quoted without the direction is close to meaningless, and comparing a rise-direction figure with a transverse one looks like a material difference when it is a geometry difference.
Then there is no compressive strength to report and the method says so explicitly: report the stress at 10 % deformation instead, identified as such. Some rigid foams crush progressively with no clear yield point. The two quantities are not interchangeable, and mixing them within a data set produces scatter that has no material cause.
Almost always a toe on the curve from imperfectly cut faces. Foam is cut rather than machined, and a ragged or non-parallel face makes contact progressively rather than all at once, so the early part of the curve is soft. That softness is included in the slope unless it is corrected out. A fine-tooth blade or hot-wire cut and a self-aligning platen remove most of it.
Yes, unless the specification requires otherwise. The skin on a moulded or lamination-faced board is much denser than the core and can carry a large share of the load at a thickness of a millimetre or two, which makes the specimen look far stronger than the foam is. Where the board is used with its facings, that is a different test on a different article.
Chiefly for insulation boards that carry load — under screeds, in flat roofs, in cold-store floors — and for foam cores in sandwich panels, where the core must resist the through-thickness compression that local loads and fastenings apply. It is also the routine production control for rigid foam density and cell quality, since compressive strength tracks both closely.
Dak verifies against whichever standard the method names, and where a class applies our frames sit a class tighter than it asks.
| The method asks for | Dak supplies | |
|---|---|---|
| Capacity | Low to moderate — a 50 mm square specimen of insulation-grade foam commonly yields between 0.3 and 3 kN | Load cells from 1 kg to 60 ton on the Series 7200, and 0.5 to 100 kN on the Series 9000 |
| Force accuracy | ASTM E4 | Verified to ASTM E4, and to ISO 7500-1 Class 0.5 |
| Gripping | Flat parallel compression platens larger than the specimen, one self-aligning | Our compression anvils, built to the specimen |
| Environment | Standard laboratory atmosphere, 23 ± 2 °C and 50 ± 10 % RH | 3009 series chambers, −150 °C to +400 °C — temperature only |
This page describes the method as practised. The governing text is the current edition from the issuing body. Tell us what you are testing and we will answer with the machine, the fixture and a quotation.
