As thermoplastic resin prices and availability continue to fluctuate, it makes sense to make the most of the resin you have. One common way to do that is with regrind. Once a project is complete, the resulting excess material and rejected parts can be reclaimed and repurposed as regrind, which can be mixed with new resin, or used on its own. As regrind needs to be used properly and strategically, we turned to plastics expert John Bozzelli for further insight.
What is important to keep in mind when regrinding plastic?
When thermoplastic is exposed to thermal and mechanical stress, it becomes weak and brittle. This degradation is called “heat history.” Both the heat history of processing, and the grinding process itself may degrade physical, chemical and flow properties of the thermoplastic resin, and anything made from the regrind.
Temperature or heat history is commonly believed to be the biggest issue in polymer degradation, though if treated properly in processing, many resins can hold their physical properties for a short number of regrind passes.
What is a good ratio of regrind to virgin to use?
Generally, the molding community targets 20 to 25 percent or less for blending regrind into virgin resin, though one can see everything from 100 percent virgin to 100 percent regrind during production. Variances can be caused by:
- Improper training of the resin handlers
- Improper calibration of either (or both) the virgin and regrind feeders
- Unbalanced blending of the materials
- Lack of shop floor discipline or adherence to established procedures
Appropriate shop floor procedures and discipline must be in place to avoid potentially catastrophic results.
Another point to keep in mind: while the first blending may be 20 percent regrind, all subsequent passes always contain some of the previous regrind blend. Resin from the first pass never leaves the resin stock. Ask your resin supplier how many times their plastic can go through a molding process before properties begin to decrease by more than 10 percent. Also confirm which properties show signs of degradation first.
What are some potential problems to watch for?
Make sure resins such as nylon, polycarbonate, poly(butadieneterephthalate) (PBT), and poly(ethyleneterephthalate) (PET) are dried properly before initial processing. If not, they will undergo hydrolysis in the barrel of the molding machine. This chemical reaction significantly lowers the polymer chain length and causes degradation. Blending degraded regrind into virgin at 25 percent levels may significantly alter subsequent part performance and function.
You also need to monitor temperature – processing the virgin resin at higher-than-recommended temperatures is a sure way to accelerate polymer degradation.
Consistent granule size can also be an issue. If a grinder doesn’t receive regular maintenance, there is the potential to get a wide range of granule sizes. Everything from fine, dust-like particles, to ¼-inch or larger chunks. During plasticizing or screw rotation, the screw does not melt these different-sized granules at the same rate. This can potentially compromise completed part properties.
Repelletizing will eliminate this problem; the regrind is melt-filtered to remove non-plastic contamination. Unfortunately, this adds $0.12 to $0.20 per pound. For process stability, perform regular grinder maintenance: sharpen the blades, clean the machine, and make sure the screen is working properly.
What do you consider the biggest problem during regrinding?
In my experience, contamination, of both foreign plastic and foreign materials, is the biggest problem. Consider how often production stops because of a plugged hot tip. You can save money by running only virgin resin in hot runner tools and use the regrind for cold runner tools. Not many have this luxury but when possible, it is a winning strategy.
How can you solve these issues?
Instead of blending regrind with virgin resin, consider using 100 percent regrind. None of the scenarios above will be an issue. Using this alternative approach, you use all the virgin resin and then feed the regrind into the machines at 100 percent.
This strategy decreases the chance of contaminating the virgin resin, plus you won’t need to buy any blending equipment.
Are there any problems when using 100 percent regrind?
Yes – watch the fiber length in fiber-reinforced resins, and verify color matching. Excessive “fines” (small plastic dust particles) can also still be an issue, just as they are with regrind-virgin blends. Adding a deduster can help with that.