Melt index: ASTM D – 1238 (Viscosity), or the measurement of the melt flow of a material, measures lot – to – lot resin uniformity precisely. This test, which provides a good indication of how the material will fl ow in the tool under typical molding conditions, is a good test for determining acceptance or rejection of materials.

It is important that resins are dry before testing, as moisture can cause variations in fl ow rate. Therefore, a moisture test should always be run prior to a melt test. If the resins are dried prior to processing, this should also be done before performing this test. This will correctly classify the material ’ s fl ow for identifi cation and aid the oduction people in understanding how the resin will process.
The test requires an extrusion plastometer (funnel – shaped device), which is a heated container with a die opening of specifi ed diameter and length that has a piston to fi t the barrel. To perform the test, resin is loaded into the funnel.

Under specifi ed conditions of temperature, piston weight, and position in the barrel, an amount of polymer is extruded during a specifi ed time period. The sample is then weighed, and the results compared with standards and previous lot melt index data for acceptance or rejection.

In performing these or any other tests, at least three tests should be run to determine the slope of the data curve. Usually, fi ve samples are run from each incoming lot to determine the results and make sure the data are reliable.

Before and after each test, check that the equipment is clean and in calibration. The failure to do this can result in erroneous fl ow data. These data are useful in many ways. A minor shift up or down may not always be cause for rejection, but it will enable the production setup team to adjust processing conditions, barrel heats, screw speed,  ack pressure, and mold temperatures to produce the product required.

Another test, similar to melt index, is called the capillary rheometer test (ASTM D – 3835). It measures fl ow (viscosity), plus shear rate and shear stress on the resin. This rest is used for more viscous resins used to produce sheet, fi lm, tubing, and profi le shapes, where lot – to – lot variability could cause more and different problems for Injection Molding.

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