Overview
Melt index meters, often referred to as melt flow rate meters, are critical equipment in thermoplastic resin processing and quality control. Its core function is to measure the mass of the resin extruded through a standard mouth die per unit time at a specific temperature and load, which is the melt flow rate (MFR), or the melt volume flow rate (MVR) when expressed as volume. MFR/MVR is an important index to characterize the processing fluidity of thermoplastic resins, which is directly related to the behavior of materials in injection molding, extrusion and other processes, and has guiding significance for product performance consistency and process parameter setting.
Measurement principle
The measurement is based on the principle of capillary rheology. A certain amount of resin sample is added to the barrel of the instrument and fully melted at a set temperature. Subsequently, a defined weight load is applied to the top of the piston so that the melt is extruded from the standard size capillary mouth die at the bottom of the barrel. The volume calculated by measuring the mass of the specified extrusion or the piston displacement and converting it to the extrusion volume per 10 minutes gives the MFR (in g/10min) or MVR (in cm³/10min). The basic calculation formula is as follows:
MFR = (m × 600) / t
where m is the average mass of the cut spline (g), and t is the slice time interval (s). For MVR, the formula is:
MVR = (A × L × 600) / t
where A is the cross-sectional area of the barrel (cm²), and L is the displacement of the piston in time t (cm).
The widely adopted standards at home and abroad mainly include ISO 1133 and ASTM D1238. These standards specify in detail test conditions (e.g., combination of temperature and load), instrument specifications, sample pretreatment, and testing procedures to ensure comparability of data between different laboratories.
Typical applications:
The choice of test conditions depends on the type of material being tested and the intended application. A range of temperature and load combinations are provided in the standard, and the common conditions are shown in the table below. Choosing the right conditions is a prerequisite for meaningful, repeatable data.
| Examples of material categories | Typical Test Conditions (Temperature/Load) |
| Polyethylene (PE) | 190°C / 2.16 kg |
| Polypropylene (PP) | 230°C / 2.16 kg |
| Polystyrene (PS) | 200°C / 5.00 kg |
| Acrylonitrile-Butadiene-Styrene Copolymer (ABS) | 220°C / 10.00 kg |
| Polyamide (PA) | 275°C / 0.325 kg or 5.00 kg |
Operational points
A typical melt index meter consists of the following systems: a precision temperature control heating system (including barrel), a loaded piston rod and weight, a standard-sized mouth die, a cutting device, and a timing/weighing or displacement measurement system. Modern instruments often have automated data acquisition and processing capabilities.
In order to ensure accurate measurement, it is necessary to pay attention to the following during operation: the sample should be fully dried to avoid moisture vaporization interference; The barrel temperature needs to be uniform and stable and achieve thermal equilibrium before testing. After feeding, there should be an appropriate preheating time to make the sample completely melted; The cutting time interval should be accurate, and the average value of multiple consecutive cutting segments should be taken for calculation. After the test, the barrel and mouth die should be thoroughly cleaned in time.
Influencing factors
The melt index value reflects the fluidity of the resin at low shear rates. Higher values usually indicate better material fluidity, relatively low molecular weight, or low molecular chain branching. However, it should be noted that MFR/MVR is a conditional parameter, and it is meaningless to only report the value without specifying the test conditions.
The measurement results are affected by a variety of factors: the instrument includes the clearance between the barrel and the piston, the finish of the inner wall of the mouth die, and the temperature control accuracy. The sample includes moisture content, the presence of fillers or reinforcements, molecular weight distribution, etc.; The operation includes warm-up time, piston rod cleanliness, sample standardization, etc. Strict adherence to standard procedures is key to controlling these variables.
Conclusion
Melt index meters provide a relatively quick and easy way to assess the flow of thermoplastic resins. Although it cannot fully simulate the complex shear and thermal histories in actual processing, the measured MFR/MVR plays an important role as a basic and standardized data in resin grade identification, incoming material inspection, process stability monitoring, and providing a preliminary processability reference. Understanding its principles, adhering to standard methods, and interpreting data together are fundamental to the effective use of this technology.
References
ISO 1133-1:2022, Plastics — Determination of the melt mass-flow rate (MFR) and melt volume-flow rate (MVR) of thermoplastics — Part 1: Standard method.
ASTM D1238-23, Standard Test Method for Melt Flow Rates of Thermoplastics by Extrusion Plastometer.
"Plastic Testing Technology", edited by Zhang Shimin and others, Chemical Industry Press.