ASTM D1238 Melt Flow Rate Determination on Plastics

The determination of flow rates is a quick and easy method to measure rheological properties of polymer melted mass at specified temperatures.

Unlike in a rheometer, the measurement takes place under constant load and thus also under largely constant shear stress. The rate at which the mass is extruded through a standardized die is set according to the viscosity of the polymer melt. The shear rate for this method is usually much lower than in injection molding operations. Nevertheless, the method offers good comparison possibilities, as they are especially needed in quality assurance. Multiple measurements in method D can be used to determine different points on an apparent viscosity curve. These measurements are presented as quotients of the MFR values under different loads.

In the case of polymer melts with homogeneous density distribution, in particular unfilled polymers, it is possible to convert theMVR value (cm³/10 min) into an MFR value (g/10 min) if the melt density at the test temperature is known. By combining methods A and B, the density of a polymer melt can be determined with a good level of accuracy.

Method A to ASTM D1238 for measurement of the MFR value is ideal for individual tests with with relatively low required testing frequency.

• In method A, the extrudate is cut at constant time intervals and its mass is determined with an analytical balance.
• The measurement is performed using the standard die, which is 8 mm high and has a hole diameter of 2.095 mm.
• The initial weight of the polymer to be tested is between 2.5 g and 8.0 g, depending on the expected flow rate.
• The starting point of the measurement lies within a piston position window of 46 ± 2 mm above the die and a preheating time of 7 ± 0.5 min. In the last two minutes before starting the measurement, no additional force may be applied to the piston.
• The time interval of the measurement is also dependent on the flow rate to be measured. It ranges from 6 minutes for low flow rates to 15 seconds for flow rates higher than 25 g/10 min.
• The test result is the extruded mass per unit of time, which is indicated in g/10 min.
• An operator must be present for the entire test sequence, and can therefore only be automated to a limited extent.
• Method A, according to ASTM D1238, covers a measurement range of approximately 0.15 g/10 min up to approximately 50 g/10 min. The limitation is a result of the requirement to cut the extrudate at regular intervals.

• For MVR measurements in method B according to ASTM D1238, the extruded volume of the polymer melt is determined at regular time or travel distance intervals. The measurement can be visually controlled through markings on the piston, or more commonly by using a piston travel distance measurement and time tracking system, which enables precise volume determination, even in the case of fast flowing materials.
• As is the case in method A, a standard die, 8 mm high with a hole diameter of 2.095 mm, is used. The starting point specifications are also identical to method A.
• The test is controlled in a travel-dependent manner. For MFR values up to 10 g/10 min, measurements are performed over a piston travel distance of 6.35 mm. Higher MFR values are measured over a piston stroke of 25.4 mm, equivalent to 1 inch.
• The MVR result is the extruded material volume per unit of time. It is specified in cm³/10 min and is calculated from the distance the piston travels per unit of time.
• In the case of polymer melts with homogeneous density distribution, the melt density can be used to convert the MVR value to an MFR value.
• A significant advantage of this method is the elimination of mechanical cutting. The entire test sequence can be performed without influence by the operator.
• Method B covers a significantly larger measurement range that, according to ASTM D1238, reaches from 0.5 g/10 min to approximately 1500 g/10 min. High-quality extrusion plastometers, however, are able to measure an even wider range with a high degree of reproducibility.
• Extrusion plastometers with displacement sensor and automatic test weight lifting device allow for a more automated control of the test sequence, which can be easily and conveniently set by entering a target MFR or MVR value.

Method C to ASTM D1238 is another variant of method B, in which a die with half the height and half the diameter is used.

• The measurement is started after 7 minutes of preheating time and at a piston position of 50 mm above the die. The MVR value is measured over a piston stroke of 25.4 mm.
• This method is used for polyolefin applications, which have an MFR value greater than 75 g/10 min. By using the “half” dies, the shear strain remains equal in the polymer melt, however the MVR value is reduced by a factor of 4 and is therefore easier to measure.

The flow rate ratio, FRR, is the quotient obtained from MFR measurements under two different loads.

• The measurements are usually carried out with a single filling of the extrusion barrel.
• After the preheating phase of 7 minutes, the test sequence is started with the first test weight at a piston position of 46 mm.
• As soon as the first measurement has ended, the test weight is changed and after a short stabilization phase, the next MFR measurement begins.
• Two or more test loads can be measured within one measurement sequence, either in ascending or descending order.
• The Aflow extrusion plastometer from ZwickRoell is commonly used for this test. The instrument allows for convenient test control, measurements in ascending or descending order of the test weights, as well as impact-free change of test loads applied by force control, without the effects of mass acceleration that often occur with deadweight instruments.

ASTM D3364 – measurements on PVC with unstable flow rates

• The method is an expansion of ASTM D1238 and is used to measure the MFR value and the instability of PVC compounds.
• In the standard configuration, measurements are performed at a temperature of 175 °C and a load of 20 kg. For hard PVC, it may be beneficial to use larger test loads up to approximately 50 kg. Semi-hard or soft PVC is also measured at 150 °C / 50 kg.
• The die is 25.43 mm (1 inch) long, and like the standard die, has a hole diameter of 2.095 mm and a cone-shaped inlet.
• Compared to the standard die, this die produces an approximately 10-fold reduction in MVR.
• The filling volume is 2.15 g and must be precisely adhered to.
• MFR values are calculated in mg/min and the instability of the melt material in %.
• With this test method to ASTM D3364, instabilities caused by temperature effects, shear with breakage of polymer bonds or due to rheological effects such as uneven distribution in viscosity or molecular weight can be detected.

Special test precautions must be taken for MFR measurements on PET:

• First, the PET must be sufficiently dried and poured into the extrusion barrel in the dry state.
• An optional nitrogen blanket at the extrusion barrel prevents direct contact by the material with the ambient air.
• In accordance with ASTM D1238, the test is run in precisely defined time sequences that are recorded by our testXpert testing software.

ZwickRoell manufactures simple extrusion plastometers for measurements according to method A, as well as instruments with automated displacement measurement that enable testing to methods A, B, C and D.