ASTM D1621 Rigid Cellular Plastics Compression Testing

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ASTM D1621 Rigid Cellular Plastics Compression Testing

ASTM D1621 specifies the test method for determining the compressive properties of strength and modulus of elasticity of rigid cellular expanded (hard foam) plastics. ASTM D1621 is technically equivalent to ISO 844 and EN 826.

Why is Testing Rigid Cellular Plastics Important?

Rigid cellular plastics are manufactured in different forms for a variety of applications. They are commonly used in the construction industry as insulation, wall sheathing, waterproofing, roofing, etc. The walls of a building insulated with rigid cellular plastic can provide a better thermal performance than those made from fiberglass. The lightweight nature of the polymer also makes it easier to transport and easier to install on site. Cellular plastic has a variety of features that make it an attractive option for those looking to build. This includes thermal insulation and sound absorption.

The most important properties of a cellular plastic product is its resin (polypropylene and polyethylene, its cellular structure (open cell or closed cell), and its density. The most common resins used in plastics are polyethylene (PE) and polypropylene (PP). Polyethylene is the most common resin for containers like cups and lids, while polypropylene is more often used in items like automobile parts.

ASTM D1621 Test Method

ASTM D1621 provides cellular materials property data and characterization regarding the behavior of a sample under compressive loads. The complete load-deformation curve can be used to calculate the compressive stress at any load (such as compressive stress at proportional-limit load or compressive strength at maximum load) and the effective modulus of elasticity.

ASTM D1621 compression tests provide important data for research and development, quality control, and material acceptance or rejection.

When measuring sample displacement, the standard allows for the measurement using a deflectometer or via crosshead displacement as long as the system compliance is eliminated from the data.

ASTM D1621 Test Procedure

The test speed in compress mode is set to 2.5 +/- 0.25 mm/min (0.1 +/- 0.01 in/min) for each 25.4 mm (1 in.) of specimen thickness. The test continues until a yield point is reached or until the specimen has been compressed 13% of its original thickness, whichever occurs first.

ASTM D1621 Test Specimens

The test specimen cross section is square or circular with a minimum of 258 mm2 (4 in.2) and maximum of 2320 mm (36 in.2) in area. The minimum sample height is 25.4 mm (1 in.) and the maximum height should be no greater than specimen width or diameter.

It is important to take care in the placement of the sample between the platens. The loaded ends of the specimen should be parallel with each other and perpendicular to the sides.

ASTM D1621 Test Machine Requirements

  • The test machine must be capable of constant velocity control (constant rate of extension), while measuring load per ASTM E4 with an accuracy of at least ± 1% of the measured value or better
  • Meet the standard requirement for acceptable displacement control using our test machine crosshead displacement sensor to control test speed

Our test machines meet or exceed the ASTM D1621 requirements.

ASTM D1621 Fixtures

The test machine is equipped with compression platens sized larger than the largest sample size. Our G223 spherical seated upper platen compensates for non-parallelism of the sample. A fixed G23 compression platen acts as the lower platen.

There are several approaches to measuring direct compression platen-to-platen displacement. The most common is through use of an optional E3540 deflectometer calibrated to ASTM E2309. The E3540 is magnetically mounted to the base with an extension tip contacting the upper platen. It's easy to use and affordable.

An optional digital sliding caliper gauge is used to measure sample dimensions. They meet ASTM D1621 requirements and measure specimen dimensions to +/-1 %.

Important Considerations When Testing Cellular Plastics per ASTM D1621

Cellular plastics are not perfectly elastic materials, and the compressive modulus can actually appear significantly different, depending on varying test conditions, such as measurements of test thickness. It's important that all data that are to be compared should be obtained using consistent or common testing conditions. The standard requires batch testing conditions to be maintained.

If the foam is suspected to be anisotropic, the direction of the compressive loading must be associated with the suspected direction of anisotropy.

The rigid foam is inherently viscoelastic or time-dependent by its plastics nature so ASTM D1621 compression tests should not be used for engineering design when an application differs from the load - time scale of the test.

Sample conditioning is important. Sample dimensional measurement is also important. It's very important to read the actual ASTM standard prior to testing. When measuring sample displacement, the standard allows for the measurement using a deflectometer or via crosshead displacement as long as the system compliance is eliminated from the data.