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Regrinding Plastics

By John Bozzelli, Scientific Molding

Maximizing the Value of Regrind: Do's & Don'ts!

With today's upward spiral of resin prices, it has become ever more important to optimize resin usage. A significant part of optimizing resin usage is proper use of regrind. Regrind is material that has undergone at least one processing method such as molding or extrusion and the subsequent sprue, runners, flash, rejected parts etc are ground or chopped. Both the heat history of processing and the grinding may lead to degraded physical, chemical and flow properties for the thermoplastic resin and subsequent parts made from this regrind. Normally it is considered that the heat history is most detrimental as stabilizers and antioxidants get consumed.

For perspective, note that even virgin resin may have one or more heat histories as it must be compounded or extruded into pellets when made. Virgin resins can have additional heat history as additives such as color, fillers (fibers, talc, mica, etc) or a host of additives may require a second compounding step. This may be done by the resin supplier or at a separate compounding facility. This is not considered regrind but it is interesting to note that virgin resin right out of the bag, box or rail-car can already have a "Heat History".

To develop a strategy and optimize the value of regrind it is important to know the issues involved. Appropriate shop floor procedures and discipline must be in place to avoid potentially catastrophic results. The list of issues is significant and there are subtleties under each.

  1. Level actually used
  2. Degraded polymer in the regrind
  3. Broad of granule sizes in the regrind; should you repelletize?
  4. Possibility of contamination, both from another plastic or metal, wood, dirt etc.
  5. Excessive fines in the regrind
  6. Tracking the actual level of regrind in a plastic part.
  7. Testing worst case situations using regrind. Remember that if you keep using regrind some of the regrind stays in the main stream. Some plastic molecules will go through the system 100's of times.

The Details

Level Actually Used
Generally the molding community targets 20 to 25% or less for blending regrind into virgin. The emphasis is on "targets" for if one watches what really happens at the press during production one can see everything from 100% virgin to 100% regrind going through the feed throat. The subtleties here involve:

    • A: Proper training of the resin handlers, usually an entry level position with little attention to training.
    • B: Improper calibration of the virgin and regrind feeders. Some of these blenders are fairly sophisticated, calibrations get fouled up, and hopper geometry (angle of repose) often encourages hang-up or bridging. With 2, 3, and 4 separate hoppers all feed a single feed throat, hopper size and geometry is limited.
    • C: Improper blending of the virgin and regrind. Even when one uses weigh feeders the virgin, regrind and color can be dumped together but not proper mixed before reaching the feed throat.
    • D: Lack of discipline on the shop floor. The above details need attention on a regular basis.

Degraded Polymer in the Regrind
Most consider that using up to 25% regrind does not significantly compromise virgin resin properties. Unfortunately this statement can get you in trouble. For example: Resins such as nylon, polycarbonate, poly(butadieneterephthalate) (PBT), poly(ethyleneterephthalate) (PET), etc, if not dried properly before initial processing will undergo a chemical reaction, called hydrolysis, in the barrel of the molding machine which causes significant lowering of the polymer chain length. This is not visible to the naked eye; parts look good but have lower physical or chemical properties. Often the problem is not found until the parts are in service, which can be a costly issue. Blending regrind of this quality into virgin at 25% levels may significantly alter subsequent part performance and function. Further each processing history may influence the virgins additive levels and if fiber filled the length of the fibers. Processing grinds up the fibers to shorter lengths. Then there is the problem with processing the virgin at higher than recommended temperatures, a sure way to accelerate polymer degradation. These are only a sample of the potential problems of using regrind and all of them can narrow the processing window.

Temperature or heat history is commonly believed to be the biggest issue in polymer degradation. However a study done by Sue Janicki et al. Antec 1992 pgs 1201-1204; "The Material Cascade: An Alternative Form of Regrind Utilization" (pdf) provides data showing excellent retention of physical properties through five passes through an injection molding machine. The study included High Impact Polystyrene (HIPS), Ignition Resistant HIPS and Polycarbonate (PC). This indicates that if treated properly in processing that many resins can hold their physical properties for a short number of regrind passes.

Range of Granule Sizes
When rejected parts, sprues, runners etc go through a grinder there is the potential to get a wide range of granule sizes. Everything from fines, small dust like particles, to ¼ inch or larger chunks. Preventive maintenance on grinders is often rarely done in many shops. During plasticizing or screw rotation the screw does not melt these different size granules the same. Some start to melt earlier than others and this leads to non-uniformly melted plastic forming the part, again with potentially compromised part properties. Further, processing is less robust due to this inconsistency. Repelletizing will eliminate this problem and it allows for the regrind to be melt filtered to remove non-plastic contamination. Unfortunately this will add $0.12 to $0.20 per lb. For process stability grinders should be well maintained, blades sharp, cleaned properly, and screen working properly.

Possibility of Contamination
It has been my experience that the biggest problem with regrind is not excessive heat history but contamination, both foreign plastic and foreign materials. How many of us have seen production stop due to a plugged hot tip. This has forced many to use nozzle filters to prevent downtime and the significant cost of cleaning out the hot runner system. A nozzle filter creates a pressure drop and further reduces process robustness. This is the reason why you can save money by running only virgin 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.

Further if there is contamination how do you purge the resin system? The only sure way is to sell all of your regrind and start over. There is no way to purge the resin stream of contamination with 20% regrind.

Excessive Fines in the Regrind
Excessive fines are often a special problem for they melt differently than larger granules. If you have a clear application, fines are the source of both black and white speck development. There are numerous applications that require fines removal before processing. Fines separators can be bought. Fines are generated in the both grinding and resin conveying, again reasons to do preventive maintenance on grinders.

Tracking the Actual Level of Regrind in a Plastic Part
While this seems possible on paper when using a 25% regrind stream, practically out on the shop floor it is impossible. First there is confusion in use, is it 80% virgin and 20% regrind or do you add 20 lbs. of regrind to 100 lb. of virgin. While the first blending may be 20% regrind all subsequent passes always have some of the previous regrind blend. Resin from the first pass never leaves the resin stock. In fact it does not take much to calculate that after a few months of processing some molecules have gone through the injection molding cycle hundreds of time. Ask your resin supplier how many times his plastic can go through a molding process before properties begin to decrease by more than 10%. Also which properties are the first to show signs of degradation.

Testing Worst Case Situations Using Regrind
If the application has any liability to it, how can you test a worst case situation? To prove you are OK in using regrind. It is impossible, and many application use only virgin for safety's sake.

An Alternative Approach

Instead of blending regrind and encountering all the above problems, one may want to evaluate using 100% regrind. That is there will be no blending. The regrind is held until all the virgin resin is used up and then the regrind is fed into the machines at 100%. The benefits of this approach are several:

  1. There is no question about amount of regrind or if it is properly blended.
  2. If there is polymer degradation the machine will tell you if you are using velocity control and watching actual pressure at transfer.
  3. Range of granule size may affect drying time but this can be addressed with a good dryer and appropriate drying conditions.
  4. Possibility of contamination still exists but this strategy is self-purging, it will be removed from the resin stream.
  5. Excessive fines are still a problem, no change here.
  6. Tracking the regrind becomes simple. Parts are labeled with the resin lot number and the number of passes regrind is used. Even if bad parts are made, if coded correctly only these have to be pulled from inventory or discarded. There is far less chance of contaminating virgin resin.
  7. Testing worst case situation becomes possible. Test parts made from virgin and 5 pass 100% regrind. See how they compare. If all properties are good stay use on a maximum of 4- passes.
  8. Capital costs are lower because you do not need to buy blending equipment.

There are issues with using 100% regrind. As with most strategies there are compromises to be made. One has to watch fiber length in fiber reinforced resins and there is the issue of color matching.