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General knowledge of plastic testing: melt flow rate

Source：Chengde Jinjian Testing Instrument Co., Ltd　  　Views：565　  　Time：2021-01-26

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

What is the purpose of measuring melt flow rate?

Different members of the plastics industry use melt flow rate tester for different purposes. Resin suppliers use it for quality inspection in the hope of finding changes in melt flow rate due to different polymerization and / or synthetic materials.

Marketing and sales personnel use it to distinguish different grades of materials. The resin buyer will check the purchased resin to ensure that the materials they receive are in accordance with the order requirements. Some users also use it to test the amount of renewable materials used in their products while maintaining the same specifications.

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

In fact, the melt flow rate is helpful to analyze the relative value of material properties and predict the relative difficulty of resin flow during processing. Mi is inversely proportional to the molecular weight. With the increase of molecular weight, the melt flow rate decreases, and vice versa. The strength of polymer is related to molecular weight, so MI can be used as an index of polymer strength.

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. Relatively speaking, the rigid modulus is not affected by the increase of melt flow rate.

For HDPE, the increase of melt flow rate improves glossiness, but has no effect on transparency. If all other parameters (such as molecular weight distribution) remain unchanged, the processing becomes easier with the increase of melt flow rate.

Test method and specific process of melt flow rate

First, the plastic particles are melted into plastic fluid in a certain time (10 minutes), under a certain temperature and pressure (different material standards), and then the number of grams (g) flowing out through a round pipe with a diameter of 2.1 mm. 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, and the measuring instrument of this test standard is melt indexer. Unit: g / 10min

Taking polyethylene as an example, the specific operation process of the test is as follows: the mfi-2232s melt flow rate meter is heated to 190 ℃ and kept at constant temperature for 20 minutes, and 3-4g PE raw material to be tested is loaded into the mfi-2232s melt flow rate meter (there is a thin tube at the end of the tank, with a diameter of 2.095mm and a length of 8mm). The upper end of the raw material is extruded downward by 2.16 kg applied by the piston. When the lower measuring line reaches the notch, press start, the instrument will cut automatically, and measure the weight of the raw material extruded within 10 minutes, which is the flow index of the plastic. Sometimes you can see the expression: Mi 12.3g/10min, which means that 25 grams of the plastic will be extruded in 10 minutes. The MI value of commonly used plastics is about 1 ~ 25. The higher the MI is, the lower the viscosity and molecular weight of the plastic material will be. Conversely, the higher the viscosity and molecular weight of the plastic material will be.

In addition to MFR, MVR can also be used to measure MFR.

Melt flow rate, formerly known as melt index, is defined as the amount of thermoplastic material extruded under specified conditions within a certain period of time, that is, the mass of melt passing through the capillary tube of standard die every 10min, expressed by MFR, in g / 10min. Melt flow rate can be used to characterize the viscous flow characteristics of thermoplastic in molten state, which has important guiding significance for ensuring the quality of thermoplastic and its products and adjusting the production process.


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