ASTM D1238 Polymer Melt Flow Rate Testing by Extrusion Plastometer

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ASTM D1238 Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer

ASTM D1238 test method test method covers the determination of the rate of extrusion of molten thermoplastic resins using an extrusion plastometer. it is particularly useful for quality control tests on thermoplastics. The data produced by ASTM D1238 test serves to indicate the uniformity of the flow rate of the polymer as made by an individual process. It is not to be used as an indication of uniformity of other properties without valid correlation with data from other tests.

The flow rate obtained with the extrusion plastometer is not a fundamental polymer property. It is an empirically defined parameter critically influenced by the physical properties and molecular structure of the polymer and the conditions of measurement. The rheological characteristics of polymer melts depend on a number of variables. It is possible that the values of these variables occurring in this test will differ substantially from those in large-scale processes, which would result in data that does not correlate directly with processing behavior.

Test Principle of ASTM D1238:

The core principle is empirical measurement of extrusion rate under controlled thermal and mechanical conditions:

A thermoplastic sample is heated in a vertical cylinder (barrel) to a specified temperature (e.g., 190°C for polyethylene) until molten.

A weighted piston forces the melt through a standardized die (orifice) at a constant load (e.g., 2.16 kg).

The rate of extrusion is quantified as:

MFR: Mass of material extruded in 10 minutes (g/10 min), measured by weighing cut extrudate (Procedure A) or converting volume to mass (Procedures B/C).

MVR: Volume of material extruded in 10 minutes (cm³/10 min), measured via piston displacement (Procedures B/C).

Procedure D extends this by comparing MFR across multiple loads to assess flow behavior under varying shear stress.

Specific Test Methods:

Procedure	Description	Application Range	Key Features
Procedure A	Manual cut-off method for MFR measurement	Low flow rates (0.15-10 g/10 min)	Extrudate cut at constant time intervals; mass determined manually
Procedure B	Automatic timed measurement for MFR/MVR	Medium to high flow rates (0.50-1500 g/10 min)	Uses standard die (2.095 mm diameter × 8.000 mm length); measures volume displaced during fixed time
Procedure C	Half-Die for high flow polyolefins	Very high flow rates (>75 g/10 min)	Half-sized die (1.048 mm × 4.000 mm); measures MVR over 25.4 mm piston stroke
Procedure D	Multiple load method for flow rate ratio (FRR).	Materials requiring viscosity curve data	Determines MFR at different loads to calculate FRR (shear sensitivity indicator); Measures MFR at 2–3 different loads (e.g., 2.16 kg → 21.6 kg) in a single test. Calculates the Flow Rate Ratio (FRR) (higher-load MFR / lower-load MFR), a dimensionless parameter for characterizing shear sensitivity. Requires an automatic weight-lifting/lowering device.

Test equipment and device required for ASTM D1238 Melt flow index test:

The primary apparatus is an extrusion plastometer (also called a melt flow indexer), comprising:

ASTM D1238 Polymer Melt Flow Rate Testing by Extrusion Plastometer

Heated Cylinder: A thermostatically controlled steel barrel (50±10 mm diameter, 115–180 mm length) with a smooth bore (9.5504±0.0076 mm diameter). Equipped with a die at the bottom.

Die (Orifice):

1), Standard Die: 2.095±0.005 mm diameter, 8.000±0.025 mm length (for Procedures A/B/D).

2), Half-Die: 1.048±0.005 mm diameter, 4.000±0.025 mm length (for Procedure C). Made of tungsten carbide or hardened steel to resist wear.

Piston: Steel rod with a 9.4742±0.0076 mm diameter “land” (foot) and reference marks (4 mm apart) for positioning. Includes an insulating layer to minimize heat transfer.

Load System: Dead weights (e.g., 2.16 kg, 21.6 kg) or a load-cell-based drive system. Total load (piston + weights) must be accurate to ±0.5%.

Temperature Control: Oil bath or electric heater maintaining cylinder temperature within ±0.5°C of the setpoint (e.g., 125–300°C). Calibrated via a platinum resistance thermometer (RTD) with a brass tip.

Timing System: Manual stopwatch (Procedure A) or automatic sensor (Procedures B–D) measuring piston displacement (6.35±0.025 mm for low MFR, 25.40±0.102 mm for high MFR).

Auxiliary Tools: Level (for barrel alignment), go/no-go gauges (verify die diameter), cleaning kits (cotton patches, brushes), and balances (0.001 g accuracy).

Materials Commonly Tested use ASTM D1238, ISO 1133, Specimen information:

It is critical data for thermoplastics such as polyethylene,polypropylene, polystyrene, and polycarbonate that allow manufacturers to optimize their production parameters for injection molding and extrusion processes according to their behavior under heat and pressure.

Specimens must be representative of the material and avoid thermal degradation during preparation:

Forms: Powder, granules, film strips, molded slugs, or pellets.

Preparation:

PVC plastisols: Gel into 0.5 mm thick sheets (oven), then cut into ~2 mm squares.

Pellets/extrudates: Grind/cut to pass a 2.0 mm sieve (ISO 565) with >80% material retained; keep some on a 1.4 mm sieve.

Films/sheets: Cut into squares/cubes with sides ≤2 mm.

Coatings: Separate from substrates, then process as above.

Cable insulation: Cut into thin slivers (per 6.2 dimensions).

Mass: Suggested sample masses (Table 2) range from 2.5–8.0 g, depending on expected MFR (e.g., 2.5–3.0 g for 0.15–1.0 g/10 min).

Conducting the Melt Flow Rate Test

ASTM D1238 test method involves heating a polymer sample in a test chamber to a specified temperature. Once the polymer reaches the desired temperature, it is forced through a die of known diameter under a specified load using a piston. The mass of the polymer extruded through the die is collected over a defined period and weighed. The melt flow rate is then calculated by dividing the mass by the elapsed time, typically expressed in grams per 10 minutes.

There are two procedures in ASTM D1238: Procedure A (manual) and Procedure B (automated).

Procedure A involves manually loading the sample and timing the test, while Procedure B uses automated equipment to perform these tasks.

The choice of procedure usually depends on the available equipment and the level of precision required.

Similar standard of ASTM D1238:

ISO 1133: Plastics — Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics — Part 1: Standard method

BS 2782-7.720A: METHODS OF TESTING PLASTICS - RHEOLOGICAL PROPERTIES - DETERMINATION OF THE MELT MASS-FLOW RATE (MFR) AND THE MELT VOLUME-FLOW RATE (MVR) OF THERMOPLASTICS

For the Rheological Properties Method 720A: Determination of the Melt Mass-Flow Rate (MVR) of Thermoplastics, while it does not specifically focus on melt flow index testing like ISO 1133 or ASTM D1238, BS 2782 includes procedures for testing other key properties of thermoplastics, such as tensile strength, impact resistance, and thermal properties. This standard provides a comprehensive framework for assessing plastic materials’ performance, which is valuable for quality assurance and product development in industries ranging from automotive to construction.

JSA - JIS K 7210-1 Plastics-Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics-Part 1: Standard method

JIS K7210 is a Japanese industrial standard established by the Japanese Industrial Standards Committee (JISC) for determining the melting rate of thermoplastic materials. This standard outlines the procedures for conducting MFR tests using a melt flow indexer apparatus, similar to ASTM D1238 and ISO 1133. JIS K7210 specifies the test conditions, equipment requirements, and calculation methods for determining thermoplastic MFR. The procedure outlined in JIS K 7210:2016 involves heating thermoplastic samples and extruding them through a mold under a specified load. Flow rate (MFR) is calculated by measuring the amount of material flowing through the mold within a given time. This test is crucial for understanding the processing characteristics of thermoplastic materials and ensuring manufacturing consistency.

GB/T 3682.1: Plastics—Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics—Part 1:Standard method

DIN 53735 German - TESTING OF PLASTICS; DETERMINATION OF THE MELT FLOW INDEX OF THERMOPLASTICS

Melt flow index tester is for melt mass-flow rate (MFR) and the melt volume-flow rate (MVR),to measure the melt mass flow rate of thermoplastic plastics under specific temperatures and pressures, reflecting the flow performance of materials through standardized testing.

Q1. What does ASTM D1238 actually measure?

A: It measures the Melt Flow Rate (MFR, g/10 min) and Melt Volume Flow Rate (MVR, cm³/10 min) of molten thermoplastics under fixed temperature and load. It reflects how easily a plastic melts and flows through a small die.

Q2. Why is the ASTM D1238 test important?

A: The melt flow rate is a key parameter that reflects the viscosity of a polymer melt. It is an indication of how easily the material can flow when subjected to heat and pressure. A higher MFR suggests a lower viscosity, meaning the polymer flows more easily, while a lower MFR indicates higher viscosity, suggesting more resistance to flow. Understanding this behavior is essential for processes like extrusion, injection molding, and blow molding, where the flow characteristics of a polymer directly impact the efficiency and quality of the final product.

Q3. Can ASTM D1238 be used for dry-blend PVC compounds?

A: No. The document explicitly states: “It is recommended for compounded materials and products only. It is not recommended for compounds in the form of dry blends since such materials may not be sufficiently homogeneous.”

Q4. How to test high-flow-rate materials (MFR >75 g/10 min)?

A: Use Procedure C (automated, “half-die”) for polyolefins. The half-die (1.048 mm diameter, 4.000 mm length) reduces flow rate to improve reproducibility. Results are not directly comparable to Procedures A/B.

Q5. What are common sources of test error?

A: Based on the document’s Troubleshooting Guide (Appendix X2), key errors include:

Temperature fluctuations: Affect melt viscosity (high temp = high MFR).

Die contamination/damage: Residue or scratches alter extrusion rate.

Piston wear: A worn “land” (foot) allows melt to bypass, causing low MFR.

Sample moisture: Accelerates degradation at high temps.

Incorrect load weight: Higher loads = higher MFR.

Poor extrudate cutting: Ragged ends or incorrect timing skew results.

Q6. What is the relationship between MFR and molecular weight?

A: MFR is inversely related to molecular weight. A low MFR indicates a high molecular weight (stronger material but harder to process), while a high MFR means a low molecular weight (easier to process but weaker).

Q7. What are the most commonly used test conditions?

A: Typical standard conditions include:

Polyethylene (PE): 190 °C / 2.16 kg

Polypropylene (PP): 230 °C / 2.16 kg

Other polymers use material-specified temperature and load combinations.

Q8. What do high and low MFR values mean?

A: Higher MFR: Lower viscosity, better flowability, easier to process, often lower molecular weight.

Lower MFR: Higher viscosity, poorer flow, higher mechanical strength but harder to mold.

Q9. Why must we preheat the sample before testing?

A: Preheating ensures the plastic is completely and uniformly melted, eliminates temperature gradients, and stabilizes flow, which ensures repeatable and accurate results.

Q10. Can ASTM D1238 be used for all plastics?

A: Only for thermoplastics that melt and flow without significant thermal degradation. It is not suitable for thermosets, elastomers, or highly filled materials that do not flow smoothly.

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