PCB News - PCB problem possible causes of high plastic MFI value

I have previously introduced that the quality control of plastic injection strength can control the MFI (Melt Flow Index, also known as Melt Index, or Melt Flow Rate) value of plastic pellets (resin). In addition, if the plastic products are brittle or insufficient in strength, the first thing we test is the MFI value, because MFI is the cheapest preliminary test basis for the quality of plastic products, but it can only be used as a reference at present, not direct evidence.

Whether the plastic pellets are re-pumped or dyed.

If the plastic pellet dyeing is not in the original factory, the problem of increased MFI value is likely to occur.

Re-grind or re-grind is added too much.

The plastic raw materials of the MFI to be tested have not been dried and dehumidified according to the requirements of various plastics.

Some plastics are particularly sensitive to moisture, such as PC materials. If these plastic raw materials are directly used for MFI without pre-baking and dehumidification as required, some unexpected results may occur.

Use different MFI test machines or use different test conditions.

This most often occurs in different sites or between different laboratories. Because the sites are different, the test instruments used are usually different. If the calibration is not done, the test results will be different. Another thing to pay attention to is the temperature setting and load. Although ASTM D1238 specifies the temperature and load, sometimes the liquid after the plastic melts is either too fast or it can’t flow out, so the tester often changes the temperature by himself. And the load, resulting in a difference.

Another point is the setting of time. ASTM D1238 generally stipulates that the total weight of plastic that flows out in 10 minutes is used as a standard, but most of the testers have experienced some reasons, such as insufficient raw materials, saving time, saving money... etc. The test time was shortened. Some only took 1 minute or even 30 seconds, using this as the basis for 10 minutes and multiplying it by a certain multiple. This should be explained during the test.

Bake the material for too long or stay in the barrel of the injection machine for too long, resulting in degradation of the plastic pellets.

The temperature of the injection machine is set too high, causing degradation of the plastic pellets.

The screw speed of the injection machine is set too fast, causing degradation of the plastic pellets.

Plastics are usually formed by long chains of molecules, and injection machines usually use screws to push liquid plastics into filaments. This process is very likely to break the long molecular chains of the original plastic into shorter molecular chains. Plastic molecules that are easily entangled become more fragile.

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