ASTM D1238 – Melt Flow Rate of Thermoplastics
ASTM D1238 is a standard test method for measuring the melt flow rate (MFR) of thermoplastic materials by extrusion plastometer. A measured amount of material is heated to a set temperature, pushed through a small die under a fixed load, and the mass that flows out in ten minutes is reported in grams per 10 minutes. The result gives a quick, repeatable indication of how easily a plastic flows when molten.
Watch: ASTM D1238: What Does Melt Flow Rate Tell You? (Melt Flow Testing)
Illustrative visuals in this video are AI-generated.
Short: Higher melt flow is always better? ASTM D1238 in under a minute
How to read a melt flow result
- A higher melt flow rate means the material flows more easily. A lower rate means it is stiffer when molten.
- Higher is not automatically better. The right value depends on the process (for example injection molding versus extrusion) and the part.
- Results are only comparable when the temperature and load are the same. Always check the test conditions on a data sheet before comparing two grades.
- Melt flow rate is a screening and quality-control number. It does not predict final part strength or service life on its own.
Watch short explainers on rubber and plastic tests in our Rubber Testing Video Library. For plastic strength, see ASTM D638 tensile properties.
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