The right extrusion plastometer for every testing volume
Extrusion plastometers, also known as melt flow index testers are used to determine the melt mass flow rate (MFR) and melt volume flow rate (MVR) of a plastic melted mass. Measurement of the flow behavior of the melted material is used to compare different grades of the same polymer and provides an important parameter for quality assurance and incoming goods inspections, for material selection and the setup of processing machines by plastics processors. Demands on these melt flow indexers vary according to which plastics processing stage is involved.
ZwickRoell supplies extrusion plastometers for companies serving every phase of plastics production, including raw material producers, compounders, and processors. Find the right melt flow index tester for your needs!
- The Cflow is particularly suited for incoming goods inspections, when only a few tests need to be performed.
- The Mflow is modular and features a pneumatic weight application device and piston travel transducer.
- The Aflow, with its defined pre-compaction of polymers, automatic material-dependent parameterization, fast residual material purge with a force of up to 80 kg and cleaning at the push of a button helps make your test sequence more efficient and reliable.
MFR or melt mass flow rate (formerly melt flow index or MFI) measures the rate of extrusion of thermoplastics through a standardized orifice in a die at a specified temperature and load. While MFR is measured in mass per unit time, such as grams/10 minutes, MVR is measured in volume per unit time, such as cm3/10 minutes. The ratio of the two values, MFR divided by MVR, provides a measure of the melt density of a material in g/cm3.
Because the melt density of a plastic material is not the same as its solid density, knowing these two values is important for plastics processors, for example, to ensure that the molding machine they use has enough capacity to process the required part.
Injection molding is the most widely used method for processing plastics. Polymers for injection molding are usually characterized by high fluidity at a temperature that will allow precise filling of a mold at the specified injection pressure to obtain, in some cases, very complex injection molded products. It is therefore very important to use the right polymer grade with adequate melt density, as well as the right processing parameters such as temperature and pressure. Another plastics processing method is plastic extrusion, where the polymer grades are normally characterized by higher molecular weights and higher viscosity in the molten state. This allows for better control of the extrusion when forming profiles (rods, tubes), films, and products such as fishing line, extruded in a single, continuous filament.
- For testing, the die with the specified size orifice is placed in a heated barrel.
- Polymer granules are added and packed into the barrel with a piston.
- A defined heating period melts or softens the polymer.
- A weight is placed on the piston, and the molten polymer is forced through the die.
- The extrusion plastometer calculates the extruded mass per unit of time (MFR) or the distance the piston travels per unit of time (MVR).
At the end of the test, the die and barrel are cleaned.
Differentiation of test methods according to the standards:
Method A: Melt mass flow rate (MFR)
- With this method, the extrudate is cut at constant intervals and its mass is determined by an analytical balance. The test result is the extruded mass per unit of time. It is usually expressed in g/10 min.
- The accuracy of the cutting times and the weighed mass determine the quality of the calculated MFR result. Manual cutting is only recommended for low MFR values, provided that the cutting times are long. If multiple sections are to be determined from one barrel filling or if the MFR values are greater than 10g/10 min, we recommend using an automatic extrudate cutter.
Method B Melt volume flow rate (MVR)
- For this method, the melt flow index tester must be equipped with a displacement transducer. 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.
- A significant advantage of this method is the elimination of mechanical cutting. With this method, when synchronizing the displacement/time measurement values, high accuracy can be achieved within short measurement times and piston displacements. This enables up to 40 individual measurements from one barrel filling, depending on the material involved, accuracy requirements and MVR result.
Method C to ASTM D1238: Measurements with half-die
- Most commonly used are dies with a height of 8.00 mm and a bore-diameter of 2.095 mm. For measurement of MFR values greater than 75 g/10 min, the standards recommend a half-height/half-internal-diameter die with dimensions 4.00/1.050 mm. This is designated as Method C in ASTM D1238.
Method D to ASTM D1238: Multi-weight tests
- With many polyolefins it is common to state the MVR value for different load levels and determine the flow rate ratio (FRR).
- This requires measurements from several fillings when simple extrusion plastometers are used. Extrusion plastometers equipped with an automatic load-change unit can measure multiple load levels from a single filling.