1. What Compression Testing Measures
Compression testing measures how a material responds when a force pushes it together. The test shows how much compressive stress the material can carry and how its thickness or length changes under load. Results can include compressive strength, compressive stress at a specified strain, yield behavior, modulus, and deformation characteristics that support material selection, design, and quality control.
ASTM D695 applies to rigid plastics. It evaluates solid polymer specimens that resist compression through the bulk material. Engineers use it to characterize compressive properties of molded, machined, or fabricated rigid plastics. ASTM D1621 applies to rigid cellular plastics. Cellular plastics deform through mechanisms such as cell-wall bending, buckling, crushing, collapse, and densification, so their stress-strain response differs substantially from that of a solid plastic.
The standards answer related but different questions. D695 focuses on how a solid rigid plastic carries compressive load, while D1621 characterizes the compressive response of a material whose cellular structure controls deformation. Specimen dimensions, loading rate, conditioning, density, cell structure, and loading direction can affect results. Use D695 when the product is a solid rigid plastic and D1621 when rigid foam or cellular construction defines performance. Follow the governing specification when it sets preparation or test conditions.
2. Full Comparison
Material structure matters more than density alone. A solid plastic and a cellular foam can use the same polymer chemistry but require different compression standards.
| PARAMETER | ASTM D695 | ASTM D1621 |
|---|---|---|
| Material structure | Solid rigid plastic | Rigid cellular plastic or foam |
| Typical specimen | Standard rigid plastic compression specimen | Foam block or cellular specimen |
| Deformation behavior | Elastic compression, yielding, crushing, or fracture | Cell wall bending, buckling, collapse, and densification |
| Primary outputs | Compressive strength, yield stress, modulus as applicable | Compressive stress at specified deformation, maximum strength, effective modulus |
| Design context | Rigid plastic parts and material data | Foam cores, insulation, cushioning, structural cellular products |
| ISO relationship | Equivalent to ISO 604 | Technically equivalent to ISO 844 |
3. Materials Covered
D695 targets solid rigid plastic materials. D1621 targets rigid cellular structures where pores or cells dominate the compression response.
Standard Test Method for Compressive Properties of Rigid Plastics
- Rigid thermoplastics and thermosets
- Reinforced rigid plastic materials within scope
- Machined or molded solid plastic specimens
Standard Test Method for Compressive Properties of Rigid Cellular Plastics
- Expanded polystyrene and rigid polyurethane foams
- Rigid cellular plastic insulation
- Structural foam and sandwich-core cellular materials
4. How Each Test Is Run
Both methods compress a specimen between platens, but they track different deformation behavior and use material-specific specimen requirements.
Standard Test Method for Compressive Properties of Rigid Plastics
- Prepare the required solid plastic specimen with flat, parallel loading surfaces.
- Center the specimen between compression platens and apply load at the specified rate.
- Record force and deformation to calculate compressive properties.
Standard Test Method for Compressive Properties of Rigid Cellular Plastics
- Prepare a representative rigid cellular specimen and measure its dimensions and orientation.
- Center the foam specimen between platens and compress it under the specified conditions.
- Use the load-deformation curve to calculate the required compressive stress, strength, or effective modulus.
5. Testing Equipment
| EQUIPMENT | ASTM D695 | ASTM D1621 |
|---|---|---|
| Universal testing machine | Required | Required |
| Compression platens | Rigid, parallel platens | Rigid, parallel platens sized for cellular specimen |
| Deformation measurement | Crosshead or suitable strain measurement | Crosshead or deformation measurement suited to foam compression |
| Specimen preparation | Machining or molding for solid plastic | Cutting tool that avoids crushing or damaging cells |
6. Applications
Standard Test Method for Compressive Properties of Rigid Plastics
- Rigid plastic resin qualification
- Compression behavior of molded plastic components
- Engineering data for solid thermoplastics and thermosets
Standard Test Method for Compressive Properties of Rigid Cellular Plastics
- Insulation foam qualification
- Structural foam and sandwich-core evaluation
- QC for expanded plastics and rigid cellular products
7. Strengths and Limitations
| STANDARD | STRENGTH | LIMITATION |
|---|---|---|
| ASTM D695 | Provides standardized compressive properties for solid rigid plastics and supports engineering material comparison. | It does not represent the collapse and densification behavior that controls rigid cellular materials. |
| ASTM D1621 | Captures the characteristic compression response of cellular materials and supports foam QC and material selection. | Foam properties can vary strongly with density, cell structure, orientation, specimen size, and loading history. |
8. Which Standard Should You Run?
| IF THIS DESCRIBES YOU | RUN THIS | WHY |
|---|---|---|
| Your sample is a solid rigid plastic or reinforced plastic | ASTM D695 | D695 directly covers compressive properties of rigid plastics. |
| Your sample is an expanded or rigid cellular plastic | ASTM D1621 | D1621 addresses the cell collapse and deformation behavior of rigid cellular materials. |
| Your product combines a solid skin with a cellular core | Define the property you need before selecting the method | A skin-core product may require specimen preparation or additional test methods to isolate the relevant structure. |
Still unsure? Infinita Lab can review your material, specimen form, governing specification, and reporting goal before you submit samples for ASTM D695 or ASTM D1621 testing. Talk to our team.
9. Frequently Asked Questions
Can I use ASTM D695 for foam?
Use D1621 for rigid cellular plastics. D695 targets solid rigid plastics and does not describe foam-specific deformation behavior.
What does ASTM D1621 report for foam?
The method can derive compressive stress at selected loads or deformations, compressive strength at maximum load, and effective modulus from the load-deformation response.
Does foam orientation matter?
Yes. Cell orientation and manufacturing direction can change compressive response, so the report should identify specimen orientation when relevant.
Are D695 and D1621 values directly comparable?
No. The material structures and deformation mechanisms differ. Compare results within the same method and material class.
