Thermoplastic resins are indispensable in our daily lives because they can be mass-produced at low cost and are used in all kinds of products, including home appliances, automotive parts, and household goods.

Thermoplastic resin can be processed by injection molding, extrusion molding, blow molding, and other methods. In all of these molding methods, the flowability of the resin in the molten state is an important indicator.

Melt flow rate is widely used as an indicator of the flowability of molten resins, and is standardized by ISO, JIS, and ASTM.

Generally, the higher the melt flow rate, the better the flowability, and the lower the melt flow rate, the poorer the flowability. Also, when the same type of resin is measured, the larger the value, the lower the molecular weight, and the smaller the value, the higher the molecular weight.

However, since the measurement is basically performed in an almost static state, the shear rate generated in the resin during measurement is very small compared to the shear rate during processing, such as injection molding, and there may be no correlation with the actual behavior during processing. Therefore, it is mainly used for quality control of materials.

1. Measuring principle

Resin placed in a cylinder is heated at a certain temperature and loaded with a piston from above, and the amount of resin extruded from the die at the bottom of the cylinder is measured.

There are two measurement methods, Method A and Method B, depending on the method used to measure the amount of resin.

There are also two ways to express the results: MFR (Melt mass-flow rate) and MVR (Melt volume-flow rate).

 Method A Measurement

The resin extruded from the die is cut off at a fixed time to measure its mass, and the mass per 10 minutes [g/10min] is calculated from the following formula.

• m：Weight of cut resin [g]
• t：Resin cutout time interval [s]

Method B Measurement

Measure the distance the piston moved in a given time or the time it took the piston to move a given distance and calculate the volume per 10 minutes [cm3/10min] from the following formula.

• A：Nominal average cross-sectional area of cylinder and piston head [cm²]
• l：Piston travel distance [cm]
• t：Piston travel time [s]

This volume per 10 minutes [cm³/10min] is called MVR. If the melt density of the measured resin at the test temperature is known, the MFR can be calculated from the following equation

• ρ：Melt density of the measured resin at the test temperature [g/cm³]

For method B measurements, the testing machine must be equipped with a device to measure the distance traveled by the piston and a timer to measure the travel time.

2. Measurement Procedure

Basic measurements are made in the following steps

1. Insert the piston into an empty cylinder and allow it to stabilize at the test temperature.
2. Fill the cylinder with the resin to be measured and compress and degas the sample from above using a filling rod. (This process takes less than 1 minute.)
3. Immediately set the piston and start preheating. Load weights according to the test conditions. (The preheating time is adjusted according to the amount of resin filled and the weight loaded during preheating. (For low melt viscosity resins, an outflow prevention mechanism may be necessary to prevent the resin from flowing out during preheating.)
4. Measurement is started when the piston descends to the starting position.
5. In the case of method A measurement, the resin is cut and collected. In the case of method B measurement, the tester automatically measures the piston travel or travel time. (Depending on the standard or regulation, multiple measurements are taken and the average value is calculated.)
6. The measurement is completed by the time the piston descends to the end position.

3. Factors of variation in measurement

Melt flow rate measurement is prone to measurement errors caused by procedures and measurement methods.

For example, degassing during the filling process may result in differences in degassing conditions and filling time depending on the person performing the measurement. In addition, preheating time varies depending on the amount of resin filled and the way the load is applied during preheating, and this effect is greater for resins that are easily affected by thermal history or have low thermal conductivity.

Therefore, in order to ensure stable measurements, the measurement process should be made uniform by the tester and the test conditions, including the appropriate preheating time, should be constant, or a tester with an automated measurement time function can be used to reduce the cause of data variation.