A plastic does not have to break to become unsuitable for a product.
Consider two materials being evaluated for an electrical housing. Both survive a bending test. Both show acceptable strength. But under the same moderate load, one bends noticeably more than the other.
Which one is better?
The answer depends on what the design needs.
If resistance to deformation matters, maximum strength alone does not tell the whole story. Flexural stiffness matters too.
This is one of the reasons manufacturers use ISO 178 flexural testing when evaluating rigid and semi-rigid plastics.
ISO 178:2019, Plastics — Determination of flexural properties, describes a three-point bending method for determining flexural behavior under defined test conditions. The results can support material development, supplier comparison, incoming inspection and production quality control.
But there is an important point that is easy to overlook:
ISO 178 testing is not simply a matter of putting a plastic specimen on a fixture and measuring the maximum force.
Specimen thickness, support span, test speed, conditioning, force range and deflection measurement can all influence the result.
So when selecting equipment, the better question is not:
“Which machine is for ISO 178?”
It is:
“What measurement system does this specimen require?”
That question leads to a much better testing solution.
Standard status: ISO 178:2019 remains the current published edition as of September 2026. A new edition is under development.
What Does ISO 178 Actually Measure?
The basic test principle is straightforward.
A rectangular plastic specimen rests on two supports. A loading nose applies force at the midpoint, creating a three-point bending condition.
During the test, the system records the applied force and the corresponding deformation.
From this data, the relevant flexural properties can be determined, including:
Flexural stress · Flexural strain · Flexural strength · Flexural modulus · Flexural stress-strain response
These results answer two different engineering questions.
How much bending stress can the material withstand?
and:
How strongly does the material resist bending deformation?
Those questions are related, but they are not interchangeable.
Strength and Stiffness Tell Different Stories
Suppose Material A and Material B are being considered for the same molded component.
Both eventually reach a similar flexural strength.
Material A, however, bends considerably before reaching that point. Material B shows much less deformation under the same earlier-stage loading.
For a product where dimensional stability matters, that difference may be more important than the final failure load.
Flexural Strength
Flexural strength describes the stress associated with the defined maximum or failure response under bending.
It is useful when comparing materials, formulations, suppliers or production batches.
Flexural Modulus
Flexural modulus describes stiffness in bending.
It helps engineers understand how strongly the material resists deformation in the relevant initial region of its flexural response.
This leads to a simple but important point:
Same strength does not mean same stiffness.
For many product-development decisions, both values matter.
Where Is ISO 178 Used?
ISO 178 is primarily a material characterization method.
That distinction matters when discussing applications.
An automotive manufacturer does not simply place an assembled connector or complete dashboard component on a bending fixture and call it an ISO 178 test.
Instead, the standard is typically used to characterize suitable test specimens prepared from or representative of the plastic material.
A more realistic workflow is:
PRODUCT REQUIREMENT
↓
PLASTIC MATERIAL
↓
MOLDED / MACHINED SPECIMEN
↓
ISO 178 FLEXURAL TEST
↓
MATERIAL DECISION
This makes the method relevant to several industries.
Automotive — Before the Material Becomes a Component
Automotive suppliers use engineering plastics in connector housings, interior structures, control-module enclosures, brackets, covers and other molded components.
Imagine two glass-filled PA grades being evaluated for a connector support.
The final component will have its own product-level validation requirements. But earlier in development, flexural testing can help engineers compare the underlying materials.
Questions may include:
Which grade is stiffer?
Which provides higher flexural strength?
Has a formulation change altered the mechanical response?
Is the latest batch comparable with the approved material?
ISO 178 can therefore sit upstream of final component validation as part of material selection and QC.
Electronics — When Thinner Does Not Mean Better
Electronic products continue to move toward thinner walls and lighter structures.
That creates a familiar engineering trade-off.
Reducing material can save weight and space, but insufficient stiffness may allow housings, internal frames or covers to deform excessively.
Typical applications may involve plastics used in:
- device housings;
- charger enclosures;
- connector bodies;
- internal structural plastics;
- covers and support components.
Here, flexural modulus can become particularly useful.
The strongest resin is not automatically the resin that provides the required stiffness at the desired geometry.
Electrical Components — Is the Batch Still the Same?
Terminal blocks, sockets, switches, electrical housings and insulating components frequently use rigid engineering plastics.
For these manufacturers, the question is sometimes less about discovering a material property and more about maintaining it.
A QC laboratory may already know the nominal flexural modulus on the supplier's datasheet.
Its practical question is:
Has today's production batch shifted from our approved reference?
ISO 178 testing can support incoming material comparison, supplier qualification, material substitution and ongoing production control.
Appliances — Following Material and Process Changes
ABS, PP, PC and other plastics are widely used in appliance housings, panels and internal structures.
Changes in resin supplier, recycled-content ratio, filler content or molding conditions can affect mechanical performance.
A controlled flexural test gives the manufacturer another way to compare those changes before they appear as a product-level problem.
Plastics Manufacturers and Compounders
For resin manufacturers and compounders, flexural testing can support both R&D and production.
Applications include:
Formulation Development
Filler and Reinforcement Comparison
Grade Development
Supplier Comparison
Batch QC
Technical Data Generation
Typical materials can include ABS, PC, PP, PA, POM, PVC and various filled or reinforced compounds within the applicable scope.
A laboratory handling many material grades may therefore need more than one load-cell range or test configuration.
Is ISO 178 the Right Standard for Your Material?
Not every plastic or composite bending test belongs under ISO 178.
The method covers relevant rigid and semi-rigid thermoplastic and thermosetting materials, including certain filled and reinforced plastics.
Other material constructions may require another standard.
For example, some long-fibre-reinforced composites may fall under ISO 14125, while cellular plastics and sandwich structures can require other flexural methods.
This should be established before equipment selection begins.
A request saying:
“We need an ISO 178 machine.”
is useful, but it is not enough.
We still need to know what material and specimen will actually be tested.
How the Three-Point Bending Test Works
The mechanical setup contains three main contact points:
one central loading nose and two lower supports.
The specimen is positioned across the supports and loaded at the center.
As the loading nose moves, the system records:
FORCE
＋
DEFLECTION
↓
FLEXURAL RESPONSE
Simple principle. Sensitive measurement.
Changes in specimen geometry, span, speed or deformation measurement can change the result even when the material name on the datasheet remains the same.
Thickness Enters the Calculation Twice
This is one detail worth paying attention to.
For a rectangular specimen in three-point bending, the flexural stress relationship includes:
where:
σf = flexural stress
F = applied force
L = support span
b = specimen width
h = specimen thickness
Look at the final term:
Thickness is squared.
For example:
while:
The measured thickness changed by only 0.10 mm, but the squared thickness term changed by approximately 5%.
That does not mean the reported flexural result automatically changes by exactly 5%—force and deformation behavior still matter.
It does show why specimen measurement deserves attention.
A 0.1 mm thickness difference is not simply a 0.1 mm problem once thickness enters the equation as h².
When flexural results start drifting, specimen preparation and measurement are worth checking before assuming the testing machine is responsible.
Support Span Is More Than a Fixture Setting
The distance between the two lower supports defines part of the bending geometry.
Changing that span changes the relationship between load and specimen response.
So the supports should not simply be moved until:
“The specimen fits.”
They need to be positioned according to the applicable test configuration and specimen geometry.
For laboratories testing different specimens, this also affects fixture selection.
The bending fixture needs an appropriate adjustment range and repeatable support positioning.
This is why we prefer to confirm the specimen before finalizing the fixture.
Test Speed Matters More With Plastics Than Many Users Expect
Polymers can be sensitive to loading rate.
The same material tested at different speeds may show different mechanical behavior.
Therefore, using a crosshead speed simply because it is familiar from another tensile or bending method can compromise comparability.
The applicable ISO 178 procedure, specimen geometry and required property should determine the programmed test conditions.
This is also one reason ISO 178 and ASTM D790 results should not be casually mixed.
Similar bending principle does not mean identical test conditions.
Do You Need Flexural Modulus?
This question can change the equipment configuration.
A customer may initially tell us:
“We need to measure plastic bending strength.”
If maximum flexural response is the only objective, the system requirements may be relatively straightforward.
But if the laboratory also needs flexural modulus, deformation measurement becomes much more important.
Machine crosshead movement contains more than specimen deformation. The frame, load cell, fixture and other mechanical elements all contribute some compliance to the overall system.
Depending on the applicable ISO 178 procedure and measurement approach, appropriate deflection measurement or system-compliance correction therefore needs to be considered.
So a complete system is not simply:
Machine + Bending Fixture
It is:
Machine + Load Cell + Bending Fixture + Force Measurement + Deflection Measurement + Software
Machine capacity alone does not define an ISO 178 test system.
Estimate the Force Before Choosing the Machine
Customers often ask us:
“Should we buy a 1 kN, 5 kN or 50 kN machine?”
Before answering, we need to understand the specimen.
The flexural stress relationship can be rearranged to make a preliminary estimate of the expected force:
This can help with preliminary machine and load-cell sizing.
It is not an ISO 178 specimen requirement, acceptance limit or substitute for the standard procedure.
An Illustrative Example
Assume:
Specimen width: 10 mm
Specimen thickness: 4 mm
Support span: 64 mm
Expected flexural stress: 100 MPa
Because:
then:
giving approximately:
The useful part of this example is not the exact number.
It is what the number tells us about equipment.
A specimen expected to produce around this force is fundamentally a low-force test.
Choosing a 50 kN or 300 kN frame simply because “higher capacity is better” would not automatically create a better measurement system.
Why a 5 kN Machine Can Make More Sense Than a 50 kN Machine
Machine capacity and useful measurement range are not the same question.
If most specimens generate a few hundred newtons, the goal is to configure a system appropriate for that range while maintaining suitable capacity margin.
A practical selection process looks more like this:
Expected Force
↓
Required Capacity Margin
↓
Load Cell Range
↓
Measurement Requirement
↓
Future Materials
↓
Machine Platform
This is why lower-force precision universal testing machines are frequently logical for routine plastic flexural work.
It is also why we do not recommend selecting the frame before understanding the sample.
ITM-LAB Solution: RS-8010A + Three-Point Bending Fixture
For many routine lower-force plastic flexural applications, the ITM-LAB RS-8010A Universal Testing Machine is the first platform we would evaluate.
The RS-8010A provides a 50 N–5 kN capacity range and supports tensile, compression and bending applications.
For an ISO 178-oriented configuration, the complete system can include:
RS-8010A Universal Testing Machine
↓
Appropriate Load Cell
↓
ITM-LAB Three-Point Bending Fixture
↓
Required Deflection / Displacement Measurement
↓
Test Software & Data Acquisition
The exact configuration should be based on the specimen and required result.
We therefore prefer the wording:
RS-8010A can be configured as part of an ISO 178-oriented flexural testing system after the specimen geometry, expected force and measurement requirements are confirmed.
rather than simply treating one machine model as universally suitable for every ISO 178 application.
The Bending Fixture Is Not a Generic Accessory
The three-point bending fixture determines how force is introduced into the specimen.
For an ISO 178 application, the fixture configuration should therefore be checked against the actual:
Specimen dimensions
Support span
Support geometry
Loading nose geometry
Expected force
Required adjustment range
ITM-LAB's bending fixture can be configured together with the testing platform after these requirements are confirmed.
This becomes particularly useful in plastics laboratories where one machine may be used for several resin grades or specimen geometries.
The machine produces the motion and force.
The fixture determines how that force reaches the specimen.
When Should You Consider the RS-8000?
Not every plastic flexural test is low force.
Thicker specimens, stronger engineering plastics, reinforced compounds or broader future test requirements may push the expected load higher.
This is where the ITM-LAB RS-8000 becomes relevant.
Instead of drawing an arbitrary line and saying:
“Below 5 kN = RS-8010A”
we prefer to evaluate the expected working range and leave appropriate margin.
The selection path should be:
Plastic Material
↓
Specimen Dimensions
↓
Support Span
↓
Expected Flexural Performance
↓
Estimate Test Force
↓
CONFIRMED LOWER-FORCE APPLICATION
RS-8010A
or
HIGHER-FORCE / ENGINEERING PLASTIC
RS-8000
↓
Select Load Cell
↓
Configure Bending Fixture
↓
Verify Deflection Measurement
↓
COMPLETE TEST SYSTEM
For routine plastics, this will often point toward the RS-8010A.
For higher-force engineering materials, the RS-8000 provides a higher-capacity platform to evaluate.
What About Flexural Testing at Different Temperatures?
Room-temperature data does not always describe how a polymer behaves in service.
An automotive or electrical plastic may become stiffer, softer or more brittle as temperature changes.
Where a research program specifically requires mechanical testing under controlled temperature conditions, an environmental mechanical-testing configuration such as the ITM-LAB RS-8000GDW may be evaluated.
This is an extended application rather than a universal ISO 178 requirement.
In other words:
Do not add an environmental chamber because the standard number says ISO 178. Add it because the material research program requires temperature-controlled mechanical testing.
That distinction keeps the test system focused on the actual application.
ISO 178 vs ASTM D790: Similar Principle Does Not Mean Identical Data
ISO 178 and ASTM D790 are often compared because both are widely used for plastic flexural testing.
Both commonly use three-point bending.
Both can provide flexural strength and modulus information.
But they are separate test methods.
For example, commonly referenced specimen geometries differ, with ISO 178 frequently associated with a preferred 4 mm specimen thickness and ASTM D790 commonly using a preferred 3.2 mm depth for relevant configurations.
The standards also differ in test-speed procedures and details related to modulus determination.
This matters.
A laboratory should not perform an ASTM D790 configuration and simply change the report heading to ISO 178.
Similar test principle does not mean interchangeable test data.
If your customers require both standards, the same configurable universal testing platform may be capable of supporting both—but the specimen, fixture configuration, test program and calculation method still need to follow the applicable standard.
Related Guide: ASTM D790 Flexural Testing of Plastics →
Add an internal link to your existing ASTM D790 article here.
Why Do Flexural Results Drift Between Batches?
The testing machine is often the first suspect.
It should not always be the first conclusion.
If a calibrated system suddenly shows greater scatter, check the entire test chain.
Specimen thickness
Because thickness enters the stress equation as , dimensional variation matters.
Support span
A changed span changes the bending geometry.
Specimen position
The specimen and loading nose need repeatable alignment.
Test speed
Polymers can respond differently when loading rate changes.
Conditioning
Temperature and moisture history may influence material behavior.
Load-cell range
The selected sensor should suit the expected force.
Deflection measurement
Especially when modulus is required, the deformation measurement method matters.
Specimen preparation
Molded specimens and specimens machined from finished products can have different processing histories and material orientation.
A useful troubleshooting principle is:
Repeatability starts before the loading nose touches the specimen.
Before Buying a Flexural Testing System, Check These Five Things
A surprisingly common purchasing sequence begins with:
“Please quote a 5 kN machine for ISO 178.”
That may eventually be the correct choice, but several questions should come first.
1 — Have you estimated the expected force?
Without the specimen dimensions and approximate material performance, machine capacity is largely a guess.
2 — Is the load cell appropriate for the normal working range?
One large-capacity sensor is not automatically ideal for every low-force plastic.
3 — Do you need flexural modulus?
If yes, deflection measurement and system compliance deserve attention before purchase.
4 — Does the bending fixture match the specimen?
Support adjustment, loading geometry and specimen dimensions need to be considered together.
5 — Will the laboratory test stronger materials later?
Future requirements may justify additional capacity—but they should be identified deliberately rather than solved by automatically buying the largest machine available.
This usually leads to a better system and a more useful quotation.
What Information Should You Send ITM-LAB?
Instead of sending only:
“We need an ISO 178 tester.”
send us the test itself.
Material
ABS, PC, PP, PA, POM, reinforced plastic or another material?
Specimen
Length × Width × Thickness
Expected Flexural Performance
If you have a material datasheet or previous test result, include it.
Standard
ISO 178, ASTM D790 or both?
Required Result
Flexural strength?
Flexural modulus?
Full force-deflection / stress-strain data?
Environment
Room temperature or controlled temperature?
Test Frequency
Occasional R&D or regular production QC?
Future Materials
Will stronger, thicker or reinforced specimens be added later?
From this information, we can evaluate the machine, load cell, bending fixture and measurement configuration together.
FAQ
- What does ISO 178 measure?
ISO 178 determines flexural properties of applicable rigid and semi-rigid plastics using a three-point bending method. Depending on the test requirement, results can include flexural stress, strain, strength and modulus.
- Is ISO 178 the same as ASTM D790?
No. Both are widely used for plastic flexural testing, but they have different specimen, procedure, speed, calculation and measurement requirements. Results should not automatically be treated as interchangeable.
- What machine capacity is required for ISO 178?
There is no single required capacity. It depends on the material, specimen dimensions, support span and expected flexural response. Estimate the expected force first, then select an appropriate load cell and machine with sufficient margin.
- Do I need to measure deflection for flexural modulus?
Deflection measurement is particularly important when determining modulus. The applicable ISO 178 requirements for deformation measurement and system compliance should be considered when configuring the system.
- Can the RS-8010A be used for ISO 178?
For many lower-force plastic flexural applications, the RS-8010A combined with an appropriate load cell, three-point bending fixture and required deformation measurement configuration is a practical platform to evaluate. Final suitability should be confirmed from the actual specimen and test requirements.
- Build the Test Around the Plastic
There is no reason to begin an ISO 178 project by choosing the biggest machine.
Begin with the plastic.
Material
↓
Specimen
↓
Support Span
↓
Expected Force
↓
Strength / Modulus Requirement
↓
Deflection Measurement
↓
Load Cell
↓
Bending Fixture
↓
Machine
For many routine lower-force plastic tests, that process may lead naturally to the RS-8010A.
Higher-force engineering plastics may point toward the RS-8000.
Temperature-dependent mechanical research may require a configuration such as the RS-8000GDW.
The model number comes near the end of the decision—not the beginning.
The machine creates the load. The complete measurement system creates the result.
Configure Your ISO 178 Flexural Testing System
Planning a new plastic flexural testing system?
Send ITM-LAB your:
Material + Specimen Dimensions + Required Standard + Expected Flexural Performance + Strength/Modulus Requirement + Test Environment
Our engineering team can evaluate the required:
Machine Capacity
Load Cell
Three-Point Bending Fixture
Deflection Measurement
Test Configuration
for your application.
Contact ITM-LAB to configure a plastic flexural testing solution for your laboratory.
