ASTM D1238 Melt Flow Rates of Thermoplastics by Extrusion Plastometer

Get Certified ASTM D1238 Testing for Reliable Melt Flow Rate Data

Thermoplastic resins have to demonstrate consistent flow to ensure reliable extrusion, injection molding, and other forms of melt processing, and hence even minor differences in material flow can cause issues during manufacturing. ASTM D1238 provides manufacturers with an extrusion plastometer-based standard, which allows them to verify the incoming material’s properties, conduct quality control tests, and compare the characteristics of various thermoplastic resins during material development.

The scope of ASTM D1238 includes:

• Material type: Unfilled and filled thermoplastic resins suitable for melt-flow testing.
• Purpose: Determination of melt mass-flow rate (MFR), melt volume-flow rate (MVR), and flow rate ratio (FRR) as quality-control and comparison indicators.
• Properties evaluated: Rate of extrusion of molten resin through a die under fixed temperature and load.
• Testing method: Procedure A uses manual extrudate cutting and weighing; Procedure B uses automatic timing to determine both MFR and MVR; Procedure C uses automatic timing with a modified half-die for higher-flow polyolefins, and Procedure D uses multiple test loads on one charge to determine MFR values and flow rate ratio. 
• Results: MFR reported in g/10 min; MVR reported in cm³/10 min, convertible to a mass basis using the material’s melt density; flow rate ratio (FRR) reported as a dimensionless value under Procedure D.
• Test limitations: Procedure A is generally used for materials with melt flow rates from 0.15 to 50 g/10 min, while Procedure B is generally used from 0.50 to 1500 g/10 min. Procedure C is generally used as an alternative to Procedure B for polyolefins above 75 g/10 min. Procedure D uses multiple test loads on one charge, and its results should not be directly compared with results from Procedures A or B. Materials below 0.15 or above 900 g/10 min may also be tested, but precision data have not been developed for those ranges.
• Use conditions: Technicians perform the test at the specified temperature, load, die dimensions, preheating time, and piston-position conditions applicable to the material and selected procedure. 
• Applications: Quality control, incoming-material verification, product development, and material comparison for thermoplastic resins.

What are the Uses of ASTM D1238 Testing? 

ASTM D1238 testing provides manufacturers with a repeatable way to verify that a thermoplastic resin flows between production lots. Formulators and processors of thermoplastic resins use ASTM D1238 to ensure that changes in their material’s flow characteristics are not going to interfere with their manufacturing process. This standard-

• Verifies that an incoming thermoplastic resin meets a defined flow-rate specification.
• Supports routine quality control across production batches.
• Helps engineers compare thermoplastic materials during product development.
• Detects deviations in the material’s flow-rate characteristics, which could indicate thermal degradation or impurity contamination of the reprocessed material
• Distinguishes between the melt mass flow rate, melt volume flow rate, and flow rate ratio based on a specification’s requirements.

Which Materials Can Be Tested Under ASTM D1238?

ASTM D1238 covers thermoplastic resins, including unfilled and filled materials, which can be subjected to testing under the conditions described in the standard. Technicians select the procedure, test condition, temperature, and load combination that matches the resin’s expected flow-rate range and the specification governing the material.

Why is ASTM D1238 Important?

A thermoplastic’s melt flow rate provides manufacturers with an early practical indicator of processing behavior and batch-to-batch uniformity without requiring a full production run to reveal a problem. ASTM D1238 is particularly useful for quality control on thermoplastics, since the data it produces flags flow-rate drift that can point to thermal degradation, contamination, or inconsistent resin manufacturing before the material ever reaches a mold or extruder.

ASTM D1238 Testing Process and Data Collection

The technician loads a representative sample of the thermoplastic resin into the barrel of the plastometer and allows it to preheat at a defined temperature for a specified period. After the preheating cycle, the plastometer applies a specified load and extrudes the material through the die. The technician manually cuts and weighs the extruded plastic at specified time intervals during Procedure A or times the extrusion process automatically during Procedures B and C. According to Procedure D, the technician also evaluates the same sample of thermoplastic resin at the given multiple loads, and the analyst calculates the flow rate ratio based on the obtained flow values. The technician also calculates the melt mass-flow rate in g/10 min or the melt volume-flow rate in cm³/10 min in accordance with Procedure B and reports the results along with the temperature, procedure, and load.

Common Challenges and Troubleshooting

Accurate ASTM D1238 results depend on consistent preheating time, a properly calibrated plastometer, and precise extrudate cutting or timing. Contaminated or incorrectly conditioned resin, incorrect piston positioning, or incorrect extrudate cutting technique during Procedure A can affect the repeatability of test results. Technicians reduce these risks by following the specified preheating period, verifying die and barrel cleanliness between runs, and checking the balance, timing equipment, temperature system, and plastometer condition according to the applicable procedure.

• The analyst reports melt mass-flow rate (MFR) in g/10 min when the selected procedure requires an MFR result.
• The analyst reports melt volume-flow rate (MVR) in cm³/10 min for Procedure B, according to the requirements of the specification.
• Analysts calculate the flow rate ratio (FRR) as a dimensionless value under Procedure D, based on the MFR values obtained at the specified test loads.
• Engineers compare the measured flow rate against the resin’s target specification to confirm batch consistency.
• A flow rate that differs significantly from the expected specification can prompt engineers to investigate material variation, processing history, contamination, or degradation.

Link to ASTM D1238