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.
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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.
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8 Responses to “Regrind: Making the Most of Your Resin”
Many thanks for the interesting article. As a user of regrind can I just add a couple of things we did when we carried out initial trials to see if we could run 100% reground PA6.6?
Our barrel heat profile and plasticizing speed needed to be refined when we swapped from Virgin/Regrind mix (65/35). We found a higher barrel heat profile and lower plasticizing speed helped to reduce a huge variation in screw slippage. Our thoughts were that we were introducing too much sheer heat with a high plasticizing speed so we switched to higher barrel heats and lower plasticizing speed which greatly improved the screw slip, which in turn improved the melt quality.
Also worth consideration is the mould that will be used to make parts from 100% regrind. Balanced gates, melt flow paths into the cavities, shut out faces of pins/cores etc. and size of vents in the mould can all have an affect on whether or not the mould will run 100% regrind. We run fast cycling multi cavity moulds ( 5 second Cycle/64 cavities) so had to choose the mould carefully but we’ve been running 100% regrind successfully since January 2018 on one mould using regrind from other machines in the Plant.
A De-dusting system was an essential additional piece of equipment that helps ensure the quality of regrind material. The return of investment will be very short now that we are able to run with 100% reground material.
Thanks for sharing your insight, Ray!
As a UK based purchaser of PC PC-ABS mouldings used as housings in domestic electronic consumer products, we have had a blanket ban upon our suppliers use of regrind, should we reconsider this as a cost down / environmental opportunity? it sounds like we should, if yes, what is the UL stance upon product certification / approvals, ie would we need to conduct safety testing using the material containing regrind?
Very good information. The article mentions that too much regrind can cause degradation of the material. Have any studies been done on the impact on the finished material with respect to the UL ratings for the plastic or molded part? = per UL94 or UL746C? Could a finished product pass the tests with virgin material but not with a regrind mixed material? Do we need to test products with enclosures made with a maximum controlled level of regrind to insure compliance is maintained?
Using high density polyethylene fractional melt for blow molding black .
My name is Jaques and I am new to the bussines of regrind(hdpe) , weve just bought a secondhand single screw extruder and were busy commisioning the line now, weve put some material through and all seems to be working fine, accept when the strings come out of the die the seem to have hard pieces in them and they break easily, we do have a screen changer in place with a very dense screen, any advice on how to rectify this problem?
I want to use regrind as a dirt filler and then plant strawberries in it. Do you think this is safe, or will the regrind let off chemical during the process?
What is the statistical math of reusing X% of a shot. It must result in a certain increase in recycled content after Y amount of shots.