A plastic part does not have to crack to fail in compression.
A connector housing can distort enough to lose alignment. A molded support may carry the required load but deform beyond its dimensional tolerance. A plastic spacer can shorten enough under assembly load to affect the components around it.
ISO 604 provides a controlled way to characterize this behavior at the material level.
At first glance, the test is simple: place a specimen between two platens and compress it. Getting useful data is more demanding. Specimen geometry, alignment, test speed, force range and strain measurement can all affect the result.
The same applies to equipment selection. Maximum machine capacity is only one parameter. A practical ISO 604 test system has to match the specimen, expected force, required property and measurement method.
This guide explains how to approach the test from that perspective—from the standard and test setup to force estimation and testing machine selection.
What Does ISO 604 Measure?
ISO 604, Plastics — Determination of Compressive Properties, specifies a method for determining the compressive behavior of plastics under defined conditions.
The method is primarily applicable to rigid and semi-rigid thermoplastic and thermosetting materials, including certain filled and reinforced materials.
During the test, a specimen is loaded in compression while force and deformation are measured. The resulting data can be used to determine properties such as:
- compressive stress;
- compressive strain;
- compressive strength;
- compressive modulus;
- and other characteristics of the compressive stress–strain relationship.
The required result should be defined before configuring the test.
For example, a laboratory interested mainly in compressive strength may not have exactly the same measurement priorities as one determining compressive modulus.
This distinction is particularly important with plastics because compression does not always end with an obvious fracture. A material may yield, barrel or continue deforming while carrying increasing load.
The curve—not simply the maximum force—often tells the more useful story.
From Compression Force to Material Data
The basic stress calculation is straightforward:
σ = F / A
where:
σ = compressive stress
F = compressive force
A = initial cross-sectional area of the specimen
Nominal compressive strain relates the change in specimen length to its initial length:
ε = ΔL / L₀
where:
ε = compressive strain
ΔL = change in length
L₀ = initial specimen length
The equations are simple. Obtaining trustworthy inputs is harder.
The force should represent predominantly axial compression. The initial specimen dimensions need to be measured correctly. The deformation measurement has to be suitable for the property being calculated.
That is why specimen preparation and test setup deserve as much attention as the calculation itself.
Specimen Geometry Can Change the Result
Cross-sectional area has an obvious effect on calculated stress, but it also determines how much force the testing machine may need to generate.
Specimen length introduces another consideration: stability.
ISO 604 recognizes that specimen length may need to be selected so that buckling does not interfere with the intended compression measurement.
If the specimen starts bending sideways, the test is no longer producing a clean axial compression condition.
Parallel Surfaces Matter
Consider a specimen whose end surface is slightly angled.
One side contacts the platen first. As the load increases, stress is initially concentrated on that side rather than being distributed uniformly across the specimen.
The resulting response can contain both compression and bending.
Platen parallelism, specimen end condition and centering are therefore part of the measurement—not cosmetic setup details.
Keep the Load Axial
An off-center specimen creates eccentric loading. A tilted specimen produces uneven contact. A slender specimen may buckle before the material response of interest is reached.
Increasing machine capacity fixes none of these problems.
If the specimen buckles or loads eccentrically, machine capacity is no longer the main issue.
The Machine Moves More Than the Specimen Does
Suppose the crosshead moves by 0.20 mm during a test.
That does not automatically mean the specimen shortened by exactly 0.20 mm.
The measured movement can contain contributions from the entire mechanical system:
Machine frame + load train + fixtures + specimen
For some compression measurements, this difference may have limited practical influence. For small-strain measurements and compressive modulus, it becomes much more important.
ISO 604 specifically addresses correction for test-machine compliance.
This is one reason strain measurement should be selected according to the result being reported rather than treated as an afterthought.
An appropriate extensometer, LVDT or other suitable measurement arrangement may be used depending on the test configuration. Where machine displacement contributes to the strain determination, the influence of machine compliance needs to be considered.
The useful question is not:
Does the machine measure displacement?
It is:
What displacement are we actually measuring?
A Practical ISO 604 Test Procedure
The exact test conditions should follow the applicable ISO 604 requirements together with any material, customer or laboratory specification. In practice, a good test sequence starts before the specimen reaches the machine.
1. Define the Required Property
First decide what the test needs to report.
Is the objective:
compressive strength, compressive modulus, stress at a defined strain, or the broader stress–strain response?
That decision influences the force range, strain measurement and data requirements.
2. Prepare and Condition the Specimen
Prepare and condition the specimens as required for the test program.
Plastics can be sensitive to environmental and preparation conditions, so comparable data depend on controlling these variables rather than treating them as background information.
3. Measure the Specimen
Measure the dimensions needed to calculate the initial cross-sectional area.
At the same time, inspect the specimen for:
- unsuitable end surfaces;
- dimensional irregularities;
- poor contact geometry;
- or a geometry that may be vulnerable to buckling.
4. Estimate the Working Force
Before choosing the load cell, estimate the expected force from the anticipated material stress and specimen area.
This immediately provides a more useful basis for equipment selection than the material name alone.
5. Install and Align the Compression Platens
The platens should accommodate the specimen and expected load while providing an appropriate compression interface.
Check the specimen position, contact condition and alignment before running the test.
Where a spherical seating arrangement is used, its purpose is to help establish appropriate platen alignment—not to compensate for poor specimen preparation.
6. Establish Contact and Preload
The beginning of a compression curve can contain seating effects as the specimen and compression surfaces establish contact.
Follow the applicable preload procedure so that the measurement starts from the intended condition.
7. Apply the Required Test Speed
Do not choose crosshead speed simply because it is convenient.
Plastic response can depend on loading rate. ISO 604 includes defined test-speed considerations, and the appropriate condition should be selected for the test being performed.
8. Record Force and Deformation
Collect the required force and deformation or strain data throughout the test.
The resulting measurements are then used to calculate and report the required compressive properties.
Where Does Plastic Compression Data Matter?
ISO 604 is a material characterization method. It does not reproduce every mechanical condition experienced by a finished product.
Its value is in generating controlled compression data that can support material comparison, design decisions, quality control and specification verification.
Rather than asking which industries use compression testing, it is more useful to ask what compression is doing to the component.
Connector and Electrical Housings
Plastic connector bodies and electrical housings may experience localized loads during terminal insertion, fastening, mating or assembly.
Failure does not necessarily mean fracture.
Dimensional distortion can be enough to affect alignment, retention or fit.
Molded Structural Components
A plastic bracket or support may carry load through a relatively small cross section.
In this situation, the design question may involve both load capacity and deformation.
A polymer that survives the force but deforms excessively can still be unsuitable for a dimensionally sensitive component.
Spacers, Supports and Insulators
For these components, stiffness may matter as much as ultimate compressive strength.
If a spacer shortens excessively under load, surrounding parts can move even though the plastic remains intact.
Engineering and Reinforced Plastics
Higher-performance polymers and reinforced materials can generate substantially greater forces during compression.
Material orientation and specimen preparation may also influence the measured response when the material is anisotropic.
These requirements appear across automotive, electronics, electrical equipment, aerospace, medical devices and industrial products.
The important distinction is that ISO 604 characterizes the material under defined conditions. Product-level performance still has to be evaluated against the actual component geometry, loading and service environment.
Calculate the Force Before Choosing the Testing Machine
Equipment selection should begin with the specimen—not the name of the polymer and not the largest load frame available.
A useful first estimate is:
Expected Force ≈ Expected Compressive Stress × Initial Cross-Sectional Area
Consider a plastic expected to reach approximately:
80 MPa
under the condition of interest.
Because:
1 MPa = 1 N/mm²
a specimen with a 10 × 10 mm cross section has an initial area of:
100 mm²
The estimated force is:
80 N/mm² × 100 mm² = 8,000 N
or approximately:
8 kN
Now keep the expected stress exactly the same and increase the specimen cross section to:
20 × 20 mm
The area becomes:
400 mm²
Therefore:
80 N/mm² × 400 mm² = 32,000 N
or approximately:
32 kN
Nothing about the assumed material stress changed.
The required load frame did.
This is why saying:
“I need a compression testing machine for engineering plastic.”
is not enough information for reliable equipment selection.
The specimen dimensions can change the required force capacity several times over.
The Calculated Maximum Is Only the Starting Point
The estimated force does not automatically become the final machine specification.
Also consider:
- expected variation between specimens;
- the force range where measurements matter;
- required measurement accuracy;
- load-cell capacity and working range;
- overload allowance;
- fixture configuration;
- and future testing requirements.
A bigger load cell is not automatically a better load cell.
The measurement system should fit the forces you actually need to measure.
How to Select an ISO 604 Plastic Compression Testing Machine
Once the expected working force is known, machine selection becomes much easier.
Capacity is still only the first filter.
Match the Load Cell to the Working Force
Imagine that a laboratory owns a machine capable of 50 kN but the plastic specimen generates only a few kilonewtons.
The frame may have more than enough capacity. That does not automatically make the highest-capacity load cell the best measurement choice.
The load cell should provide appropriate performance over the force range that actually matters in the test.
This becomes particularly useful when one universal testing machine handles several specimen sizes or material types.
Check the Compression Platens
The platen configuration should suit:
specimen dimensions, expected force, available test space and alignment requirements.
Platen area can become a practical limitation for larger specimens even when the load frame itself has sufficient force capacity.
Parallelism and fixture stiffness also contribute to the quality of the setup.
Decide How Strain Will Be Measured
If the main objective is compressive modulus, strain measurement deserves particular attention.
An appropriate extensometer or displacement transducer may be used depending on the required configuration. If machine displacement contributes to nominal strain measurement, machine compliance has to be accounted for appropriately.
The decision should come from the property being reported—not from whichever displacement channel is easiest to access.
Confirm Speed and Control
The universal testing machine should provide stable control over the required test-speed range.
Because plastics can respond differently as loading rate changes, speed is part of the test condition rather than simply a machine setting.
Select the Load Frame Last
Only after these questions are answered does maximum machine capacity become a useful final selection parameter.
Within the ITM-LAB universal testing machine range:
Suitable for lower-force plastic testing where the expected load remains within the available range.
Suitable for higher-force engineering plastics, larger cross-sectional specimens and other medium-force compression requirements.
Suitable for substantially higher-force compression requirements and larger or higher-strength material systems.
These ranges are not three automatic material categories.
The same polymer can require a very different machine when specimen area changes.
Select from the force and measurement requirement—not from the material label.
A Practical Selection Example
Suppose a laboratory needs to characterize an engineering plastic with the following test information:
Specimen cross section: 10 × 10 mm
Expected compressive stress: approximately 80 MPa
Required result: stress–strain curve and compressive modulus
Use: routine laboratory material testing
The first force estimate is:
80 MPa × 100 mm² ≈ 8 kN
That already tells us something useful: a 5 kN system would not provide sufficient capacity for the expected condition.
But choosing a frame above 8 kN does not complete the specification.
Because compressive modulus is required, the laboratory should also define how strain will be measured and how machine compliance will be handled.
The compression platens must suit the specimen geometry and maintain an appropriate loading condition. The load cell should match the actual working force range, not simply the maximum capacity of the frame.
That is the difference between selecting a machine and configuring a test system.
ISO 604 Compression Test Equipment Selection Roadmap
A reliable equipment specification works outward from the specimen.
ISO 604 Testing FAQ
What is ISO 604 used for?
ISO 604 is used to determine the compressive properties of plastics under defined test conditions. Depending on the test objective, the results can include compressive strength, compressive modulus and other aspects of the compressive stress–strain relationship.
What testing machine is used for ISO 604?
A universal testing machine configured for controlled compression can be used. The complete setup should include an appropriate load frame, load cell, compression platens, control system and deformation or strain measurement method for the required result.
Maximum force alone does not define a suitable ISO 604 test system.
How do I calculate the required compression tester capacity?
A useful starting estimate is:
Expected Force ≈ Expected Compressive Stress × Specimen Cross-Sectional Area
The final configuration should then consider the actual working force range, specimen variation, measurement requirements, fixtures and an appropriate capacity allowance.
Why is strain measurement important for compressive modulus?
Crosshead movement can include deformation from the machine, load train and fixtures in addition to deformation of the specimen.
That difference becomes particularly important when evaluating small strains for compressive modulus. The strain measurement method and any required machine-compliance correction should therefore be considered when configuring the test.
Is ISO 604 the same as ASTM D695?
Both standards address compressive properties of plastics, and the current ASTM D695 identifies the method as equivalent to ISO 604.
For actual laboratory work, use and report the standard specified by the customer, drawing, material specification or qualification requirement.
What Should You Provide When Requesting an ISO 604 Test System?
You do not need to select the machine before contacting the equipment supplier.
The most useful starting information is:
Plastic material — polymer type, grade or reinforced material if known.
Specimen dimensions — especially cross-sectional dimensions and specimen length.
Expected compressive stress or force — preferably the expected working range, if available.
Required result — compressive strength, stress, strain, modulus or complete stress–strain curve.
Applicable standard — ISO 604 plus any customer-specific requirements.
Testing frequency — occasional R&D work, routine laboratory testing or higher-volume QC.
These parameters allow the machine, load cell, platens and strain measurement system to be configured around the test rather than the other way around.
Selecting the Right ISO 604 Test System
ISO 604 compression testing is mechanically straightforward. Reliable measurement is less so.
The specimen has to remain stable. The loading condition should remain axial. The force range has to suit the measurement system. If modulus is required, deformation measurement becomes especially important.
For equipment selection, work in this order:
Specimen → Expected Stress → Expected Force → Load Cell → Compression Platens → Strain Measurement → Load Frame
This approach avoids two common specification mistakes: choosing machine capacity before estimating the actual force, and treating maximum force as the only important parameter.
Need to Configure an ISO 604 Plastic Compression Test System?
Our engineering team can use these parameters to recommend the appropriate machine capacity, load cell, compression platen and strain measurement configuration for your application.
