The thermoplastics materials industry uses the Melt Flow Index (MFI) as basic tool for quality control and acceptance of incoming products. This measure of the flow properties is contained into the identity card of polymers and it is used to verify if the plastics grade is within the required fluidity range. This is commonly used for polyolefins (polyethylene HDPE, LDPE, LLDPE, polypropylene PP).
ASTM D1238 covers the measurement of the rate of extrusion of molten resins through a standard die (2.095 x 8 mm) under prescribed conditions of temperature, load, and piston position in the barrel as the timed measurement is being made. The standard includes four different test procedures, with procedures A and B common with ISO 1133. Procedure C is related to polyolefins with a high flow rate and prescribes the use of a die with half dimensions (length and diameter), which reduces the flow rate. High fluidity samples can also be tested by using a die plug, inserted before the material charge and removed prior to removing the piston/weight support. Procedure D is related to the application of different weights to the same sample charge in order to obtain the melt mass-flow rate (MFR) data under different test conditions. This can easily be performed with our Multiple Weight Melt Flow Testers that are designed to have up to 5 weights in one test. This allows the automatic calculation of the Flow Rate Ratio (FRR), which is the ratio of the average flow rate at the higher load to the average flow rate at the lower load. FRR is commonly used as an indication of the way in which rheological behavior is influenced by the molecular mass distribution of the material.
We have tested the melt flow properties of a High Density PE sample at 190° C at different masses (2.16, 5, and 21.6 Kg) with our CEAST MMF Multiweight tester. It is equipped with an encoder, motorized weight lifter with automatic handling and application of the masses, and automatically cuts the extruded material at precise intervals. Our unique load cell feature for material compacting, with controlled force up to 750 N, defines and accurately monitors the desired level of material softening before the test, since the material pre-heating phase is strongly affecting the repeatability of the measurement. Additionally, we suggest using a 50 measurement point acquisition of the encoder to obtain the highest accuracy for each weight. The user can then use the melt mass-flow rate obtained with the different weights to calculate the flow ratio rate for each weight combination.
At the end of the test, we recommend cleaning the instrument thoroughly with our automatic barrel cleaning device to reduce the testing time and to optimize the quality control process. For chemically aggressive materials, such as PVC or Fluoropolymers, we developed a special corrosion resistant kit (barrel, die, piston) that can be provided with all our instruments.