Injection Molding Virgin vs. Regrind

Most processing operations generate regrind. Therefore, it is necessary to be able to reprocess the regrind back into the part. Some processes naturally generate regrind. For example, in blow molding, 35 to 50% of the regrind is generated just from the flash and trim.

In injection molding, some molds will generate more regrind than others. For example, a hot runner system will typically produce less regrind than a process using a cold runner system making multiple small parts. Sometimes a mold of this type will actually generate more regrind by weight than the weight of the parts it produces.

Therefore, when regrind is used in an injection molding process, the viscosity will be different depending on the percentage of regrind being blended in the feedstock. The accompanying graph shows the difference between Surlyn (DuPont’s tradenamed ionomer-class molding and extrusion materials created from its proprietary acid copolymers) virgin material and regrind.

The chart shows that the regrind resin is more viscous than the virgin material. In order for the regrind resin to have a similar flow characteristic as the virgin, it would be necessary to increase the melt temperature of the regrind. The slope of the curve may not be exactly the same, but in the area where the resin is being processed in the screw and filling the mold, it can be conditioned to be very close to the virgin material.

This change in viscosity can easily be determined by the injection molding machine using the following method.

Method:

Step 1: Start by processing the virgin resin at the resin manufacturer’s recommended processing temperature. Mold some parts and record the injection pressure.

Step 2: Now, fill the hopper with 100% regrind and duplicate Step 1.

Step 3: Based on what was shown in the above chart, it will be necessary to increase the melt temperature, so increase all of the barrel zones, endcap, and nozzle by 5°C (10°F). Repeat Step 1, measure the melt temperature of an air shot sample, and record all of the data for later reference.

If the injection pressure is still higher than it was when processing the virgin resin, then it will be necessary to continue to increase the machine’s zone temperatures until the injection pressure matches the injection pressure of the virgin resin.

The resin being processed may not necessarily always require the melt temperature to be increased but instead may require the melt temperature to be reduced. The method will be the same, except if at first by increasing the melt temperature the injection pressure is lower than when the virgin resin is processed, instead of increasing the zone temperature setting it will be necessary to decrease the zone setting to produce a lower melt temperature.

By matching the injection pressures between the virgin resin and the regrind, the flow characteristics should be very similar and therefore, fill the mold the same.

All of this being said, even when various ratios of virgin/regrind are processed, the flow characteristics will also be different; so it is best to do this simple procedure in order to determine what process temperatures need to used in order to match the same rheological characteristic as the virgin resin for which the mold design.

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Tim Womer is a recognized authority in plastics processing and machinery with a career spanning more than 35 years. He has designed thousands of screws for all types of single-screw plasticating. He now runs his own consulting company, TWWomer & Associates LLC. Contact: (724) 355-3311; tim@twwomer.com; twwomer.com