By: Bozilla Corporation
My customer doesn’t like using any recycled material.
Working with a Tier 1 supplier to a major OEM, there was a discussion with the molders on the floor, people molding the parts on the injection molding machine, and their personal experiences.
Their experience with recycled resins is the inability to maintain a controlled injection molding process.
Because they are responsible for the quality of the part they are molding, they make decisions (call an audible) and replace the recycled resin with a locally available equivalent virgin resin. This protects them from producing unacceptable.
Losing Control of the Process
Recycled plastic goes through several processes when it is reclaimed. In layman’s terms, it gets beaten up.
Here is the rundown:
In the first phase of the plastic recycling process, the material goes through the injection molding process, which partially degrades the material (molecular weight reduction). Then, it is potentially exposed to UV, temperature fluctuations, or chemicals, which contributes to the degradation process. Next, the plastic is ground up and typically cleaned with a thermal or chemical process, further degrading the material. Finally, the reclaimed plastic is ready to, again, be beaten up in the injection molding machine.
Loss of injection molding process control
It is essential to know that the reclamation process breaks the polymer’s molecular structure down, making it lesser quality because the properties of that original polymer have been degraded. Recycled resin has a smaller molecular weight( length of chains) and varying viscosity, making it unpredictable. Virgin resin has molecular weight consistency/control with much less variation, giving it consistent properties.
This lack of consistency, or better stated, lack of control of the recycled resin will affect the quality of your part.
Do you see some reasons why you may reconsider using recycled plastic for your part?
Industry Response
Now, the response in the industry and all industries to using recycled materials is, when possible, to take a certain percentage of recycled material and blend it in with the virgin resin.
However, it is essential to note that the percentage of added recycled material vs. part quality is not a 1:1 ratio, e.g., adding 20% of recycled resin does not equate to 20% loss of properties. For instance, you can test the recycled resin of 1 lot of material, and it may meet specs, but the next lot is completely different, thus knocking the process out of control. This lack of conformity may continue throughout the entire recycled resin being used.
The typical response to this situation is to combine a certain percentage of recycled material with virgin resin. By combining the recycled materials (uncontrolled variation of molecular weight) with virgin resin, which is very controlled, the blend will allow the process to maintain control, thus allowing the part to meet specifications. There may be a loss of 5% of the properties, so it will still make and meet standards.
However, you are still using recycled material, and 20% of the material has a history of being degraded. Unfortunately, properties will be lost, influencing the quality of the part.
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The $10 million question is, how do you start using more recycled and less virgin resin?
This is typically not an easy question to answer, but there are ways the industry can address it.
The industry can adapt by designing parts to accept more variation, allowing increased content of recycled material. For example, the part could be globally or locally thickened in order to allow for more process variation without loss of part quality.
This is just one example, but the industry has the potential to address this in many other ways.
That’s the question, and that’s the caution.
Currently, material suppliers are working with recyclers to have test methods put in place to address these issues with recycled materials. The caution is that these test methods reveal the problem vs. fix the problem. How do the recyclers/material suppliers create quality control with regard to molecular weight variation? Suppliers might say they can test the material with an MFR flow test. Unfortunately, an MFR flow test is a zero viscosity test, which does not capture the true shear rate the material will experience during the injection molding process.
However, there are better ways to measure Rheology since it is necessary to know the molecular weight variation. But, there is no way to correctly measure this vital detail on recycled material because there is so much fluctuation in its properties.
So, with recycled material, even though you may be able to create a good test method, there’s no way to create consistency because of the history of the recycled material.
In other words, there is still no consistency in recycled materials. Until this is addressed, using recycled materials for injection molding must be managed smartly.
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