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Plastic part with regrind 25%

Plastic part with regrind 25%

(OP)
What impact with using 25% regrind in plastic? How do identify the plastic part defect is due to regrind?

RE: Plastic part with regrind 25%

(OP)
a bit difficult to identify this way.
any test can identify the part have regrind?

RE: Plastic part with regrind 25%

How easy it is to detect regrind will depend on the type of polymer. PE and PP won't change much due to an extra processing step. Something like PET, PLA or other polyesters will.

It's not that easy to detect in the case of PE and PP. You can try:

1. FTIR to look for oxidation as seen in the carbonyl band (probably not sensitive enough though)
2. Have an expert lab measure the antioxidant levels and their degradation products. Each pass through an extruder consumes around 200ppm of process stabilizer(this method is most likely to work)

If you go for the second option the only lab I know of that can do it is www.stabilization-technologies.com or maybe www.norner.no

Chris DeArmitt - PhD FRSC

Plastics & Materials Consulting
www.phantomplastics.com

Plastic Training Seminars
www.plastictraining.com

RE: Plastic part with regrind 25%

It would help to know what plastic you are looking at. What I would do is get virgin material, mold some test specimens, and test for tensile, izod impact, and melt flow. Then add 10% regrind and repeat. Then add 20% and repeat, and so on. Look at your mechanicals and determine where your properties are no longer suitable for your application. You would then have a correlation between mechanicals and melt flow which you could use for pre-production QC. Of course the source of your regrind is critical. If this is generated in-plant and you can control it, you will get better results than if you are buying regrind. There will be more variables with purchsed regrind, and the correlation with melt flow will not be as good

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Plastic part with regrind 25%

(OP)
the plastic is ABS , ABS +GF , PC+GF, the defect has shown not immeadiate fail but in sometime of application, mostly the part use has gone to the market. it has given us to trace what happen, it seen no way to specify a lab test method check if it due to regrind or over regrind added

RE: Plastic part with regrind 25%

What is the failure? Is the part molded, extruded, or? What is the source of the regrind?

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Plastic part with regrind 25%

(OP)
the source of regrind likely from the runner, the mold supplier seen doing re-grind themself. the failure is crack intermittent during drop, the design have some weakness on it, but it have not seen any crack during NPI phase.

RE: Plastic part with regrind 25%

Plastics undergo a reduction in molecular weight with every molding cycle. The heat and shearing result in chain scission which reduces molecular weight, which results in a loss of ductility and impact resistance. For this reason, regrind is not usually a good idea for parts subjected to impact. The test I suggested above would help determine the level of regrind at which mechanical properties are degraded beyond an acceptable level. If your part has a flat straight section, you could machine an impact specimen straight from a molded part. This is a good idea anyway as you get the exact processing you part will see in production. The specimen does not need to be to exact ASTM dimensions, as long as it is the same for every test. Find a good lab and do impact tests on specimens with different levels of regrind. Melt flow testing would be beneficial if you have a rheometer in house and would allow you to do some preventative QC.

An issue with regrind is contamination. Any foreign matter that gets into the plastic can have a bad effect on impact resistance. If you know you are going to use reground runners, take care to keep them clean. Store them in covered containers, make sure the grinder is clean and free of dirt debris and other plastic, and keep the hopper covered. Only use runners from properly molded molded parts. Make sure that the regrind particles stay mixed with your virgin pellets. If they segregate because of different shape or size, you may get some parts made with very high regring content.

It may be that the parts a marginally acceptable in impact even without regrind, and the addition of regrind tips them over the edge. Make sure your process is optimum. There is one school of thought that says use a melt temp near the bottom of the recommended range and inject fast. The idea here is to get your reduction in melt viscosity from shear thinning rather than temperature. I saw data where two samples of I think PP were processed by molding parts ad grinding them up, checking melt flow, remolding, rechecking, and comparing results. One sample was molded fast and cool, the other slow and hot. Slow and hot had the same melt flow after one cycle as the fast and cool did after five molding cycles. Heat is the killer here. I think the suggestion about testing for antioxident levels is a good one if ABS is used. If the butidiene oxidizes, is will cross link and get brittle and you loose impact resistance.

Another issue with glass filled material is damage to the fiber from excessive shearing. I worked on a problem once where a glass filled polyester part was breaking at some hardened steel pins held captive in the assembly. The molder was inexperienced with glass filled part and was using a very fast screw speed. This was chopping the glass fiber into glass particles. We set the screw speed to a level recommneded by the resin supplier, and the part was so strong that the failure mode was now fracture of the steel pins. It may be that grinding the runners and the additional molding cycle are breaking the fibers.

If the failure is at a sharp corner, do what you can to remove the corner. PC is notch sentitve, meaning its a normally ductile material that will undergo brittle fracture in the presence of a stress concentration.

Rick Fischer
Principal Engineer
Argonne National Laboratory

RE: Plastic part with regrind 25%

ABS is very sensitive to extruder passes. I've measure mechanical properties before and after extrusion and they do change. It only takes a tiny change in the cross-link sensity of the impact modifier to change the impact resistance of the ABS. This would be hard to test for.

In glass fiber filled systems the glass fiber dominates properties like modulus and strength and to some degree impact resistance. Just one extruder or molding pass can really break the fibers down into shorter ones dramatically reducing properties. A test for this would be to ash the sample (e.g. 500C in air) and look at the fibers to see if they are like new or ground up. A look at the fiber length will tell you if it's regrind or not.

Chris DeArmitt - PhD FRSC

Plastics & Materials Consulting
www.phantomplastics.com

Plastic Training Seminars
www.plastictraining.com

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