Plastic test tips: melt flow rate

- Apr 25, 2019-

Melt flow rate (MFR), also known as melt flow index (MFI), is a common material performance test in the plastics industry.The test is used to determine the melt flow performance of the resin under specified shear stress and temperature (unit: g/10min) (related to applied load).The test is carried out by a extrusion plasticizer, commonly known as a "melt flow rate tester" (formerly known as a "melt index tester").It is used for testing natural, composite and treated thermoplastics.

What is the purpose of measuring melt flow rate?

Different members of the plastics industry use melt flow rate meters for different purposes.The resin supplier USES it for quality inspection, hoping to detect changes in melt flow rates due to differences in polymerization and/or synthetic materials.

Marketing and sales people use it to distinguish between different grades of material.Resin buyers use it to inspect the resins they purchase to ensure that the materials they receive are in accordance with the order requirements.There are also users who use it to test the amount of renewable materials in their products while maintaining the same specifications.

What is the effect of melt flow rate on material properties?

In fact, melt flow rate helps to analyze the relative value of material properties and predict the relative difficulty of resin flow in processing.MI is inversely proportional to molecular weight. When molecular weight increases, melt flow rate decreases, and vice versa.The strength of the polymer is related to the molecular weight, so MI can be used as an indicator of the strength of the polymer.

With the increase of melt flow rate, tensile strength, tear strength, stress cracking resistance, heat resistance, weather resistance, impact strength and shrinkage/warpage all decrease.In contrast, the rigidity modulus is not affected by the increase of melt flow rate.

For HDPE, the increase in melt flow rate increases gloss but has little effect on transparency.If all other parameters, such as molecular weight distribution, are constant, processing becomes easier as the melt flow rate increases.

Melt flow rate test method and specific process

First let the plastic particles in a certain time (10 minutes), a certain temperature and pressure (different material standards), melting into plastic fluid, and then through a diameter of 2.1mm round tube by the number of grams (g).The larger the value is, the better the processing fluidity of the plastic material is, and vice versa.

The most commonly used test standard is ASTM D 1238, which is measured by melt flow rate meter (MeltIndexer).Unit: g / 10 min

With polyethylene as an example, the test of specific operation process is: MFI - 2232 - s melt flow rate instrument temperature to 190 ℃ and constant temperature for 20 minutes, the PE material under test 3-4 g into MFI - 2232 - s in the melt flow rate instrument (at the end of the tank by a tube, the tube diameter is 2.095 mm, length is 8 mm).The upper end of the raw material is extruded out by applying 2.16kg downward pressure on the piston. When the measuring line reaches the groove, press "start" and the instrument will automatically cut it. The weight of the raw material extruded in 10 minutes is measured as the flow index of the plastic.Sometimes you'll see a representation like this: MI 12.3g/10min, which means that the plastic is extruded in 25 grams in 10 minutes.Commonly used plastics have MI values between 1 and 25.The larger MI means the smaller viscosity and molecular weight of the plastic raw material, and vice versa means the larger viscosity and molecular weight of the plastic material.

In addition to the melt mass flow rate (MFR), the melt volume flow rate (MVR) can also be used for measurement.

Melt flow rate, formerly known as melt index, is defined as: the amount of thermoplastic material extruded in a given time under specified conditions, that is, the mass of melt passing through the capillary tube of standard die every 10min, expressed by MFR, unit is g/10min.Melt flow rate can be used to characterize the viscous flow characteristics of thermoplastics in the molten state.

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