ASTM D696 measures the coefficient of linear thermal expansion of plastics from -30 °C to 30 °C by means of a vitreous silica dilatometer. The sample is enclosed in a silica tube, while a silica rod measures its length changes as temperature varies. MatX Lab applies the test to rigid thermoplastics, thermosets, and filled materials in accordance with ASTM D696.
What Is ASTM D696?
ASTM D696 is the standard test method for coefficient of linear thermal expansion of plastics with a vitreous silica dilatometer. Vitreous silica expands very little as it heats, making it a stable reference against which much larger polymer movement can be measured. The specimen is held in the tube, and its length change is read by a dial gauge or displacement transducer as the assembly is taken through the temperature range. The apparatus itself expands slightly. That contribution is subtracted using a correction determined from a reference specimen of known expansion.
This leads to a coefficient of units 1/°C, expressed in µm/meter/°C. Rigid plastics normally fall within the range of 50 to 100 of those units, while unfilled polyolefins tend to be larger.
ASTM D696 Scope and Applications
The method covers rigid plastics tested across the -30 °C to 30 °C interval, which is the ambient service range where dimensional mismatch between plastic and metal causes most of its trouble.
Applications:
- Dimensional design. Determination of clearance/interference in assemblies in which a component made of plastic contacts metal or glass or another type of polymer.
- Filler assessment. Measurement of the effect of glass, mineral, or carbon filling on shrinkage, as this is often why the filler was added.
- Mismatch investigation. Explaining a failure where a bonded or fastened joint cracked as the assembly moved through its temperature range.
- Material selection. Comparing candidate resins against an expansion limit set by the design. Expansion above 30 °C or over a broad temperature range is done according to ASTM E831 using thermomechanical analysis, while ASTM E228 involves solid materials through a push-rod dilatometer.
Materials having a glass transition temperature within the testing range should have that transition temperature plotted first, since the coefficient varies drastically around it.
ASTM D696 Test Procedure
The measurement is a subtraction of two small numbers, so the discipline lies in preparation and in temperature control.
| Step | What happens |
| Specimen preparation | Bars are machined with flat, parallel ends square to the length axis. Uneven ends produce seating errors that read as expansion. |
| Annealing | Specimens are annealed as required so molded-in stress does not relax during the run and add to the movement. |
| Conditioning | Specimens are conditioned at standard laboratory conditions, and moisture-sensitive materials are dried or conditioned to a stated level. |
| Length measurement | Specimen length at room temperature is measured and recorded as the baseline for the calculation. |
| Assembly | Place the specimen in the silica tube, with the silica rod resting on its upper end and the displacement sensor zeroed. |
| Cold stage | Bring the assembly to -30 °C and hold until the reading stops moving. |
| Warm stage | Bring the assembly to 30 °C and hold it again until stable. The length change between the two stable readings gives the expansion. |
| Correction and calculation | Subtract the dilatometer’s own expansion, then calculate the coefficient from the corrected length change, the original length, and the temperature interval. |
Limitations: The method assumes the material behaves linearly across the interval, and a polymer with a transition inside that window will not. The molded-in stress is relieved during initial heating and thus contributes to the observed deformation, so an annealing-free sample shows higher values initially and lower values after. As the hygroscopic plastic heats up, moisture evaporates and causes shrinkage along with expansion, so an unconditioned polyamide will show a value related to its drying process.
Filled and fiber-reinforced materials are directional, and a specimen cut along the flow direction of a molded part will not match one cut across it. Very soft materials deform under the measuring rod’s contact load and read low.
ASTM D696 Specimen Requirements and Test Conditions
| Parameter | Typical requirement |
| Specimen form | Machined bar or rod with flat, parallel, square ends |
| Length | 50 mm to 125 mm, matched to the dilatometer tube |
| Cross section | Up to about 12.5 mm, sized to move freely inside the tube |
| Replicates | Three per material and orientation |
| Condition | Annealed where required, conditioned to a stated moisture level |
| Temperature range | -30 °C to 30 °C |
| Orientation | Stated for every specimen cut from molded or extruded stock |
Tell us the orientation you need when the material is filled or reinforced. A glass-filled polyamide can expand two to three times as much across the fiber direction as along it, and testing the wrong axis produces a number that will not match the assembly it was meant to describe.
Send stock long enough to machine the specimen properly. Short offcuts force compromises in end squareness that show up directly in the result.
ASTM D696 Test Results and Reporting
Your report gives the coefficient of linear thermal expansion across the -30 °C to 30 °C interval for each specimen, along with the mean across replicates.
We always provide specimen information with each value: initial length, cross section, orientation with respect to flow/fiber, annealing/conditioning history, and dilatometer correction. When the data are scattered, we present the actual value along with its average.
We state the temperature interval on every result. A coefficient quoted without its interval is not usable, since expansion behavior changes across the range and a single number only means something when the range is attached.
ASTM D696 FAQs
Why does my result differ from the resin datasheet?
The datasheet values may come from a different temperature range, use a different approach, or be measured on a sample in a different orientation. Investigate all three before declaring the problem a property issue.
Should specimens be annealed first?
Anneal when the part will see service above its molding stress-relief point, or when you want the polymer's behavior rather than its molding history. We will follow your instruction and record it either way.