Nowadays, it is often necessary to regrind and recycle materials in projects, but for many moldmakers, the word "R" can trigger painful memories.

I started working in this industry in 1994 in the control equipment injection molding department of Cherry Electrical Products as a three-shift machine operator. The title operator stretched it a bit. In essence, like many operators in this industry, I am nothing more than a packer and sorter. Soon after I started, I had the opportunity to become a service worker. This is what Cherry refers to as a material handler.

In that role, my job is to make sure that the material is dry and the hopper is full. All Cherry products allow the use of up to 25% recycled materials; since almost all of our molds are cold runners, we never lack recycled materials. Now, when I say 25% recycle, we are not talking about weight mixers, as we see today, to provide loaders with almost exactly specified amounts of recycle. We are talking about mixing by hand the recycled material that looks close to a quarter of a 5-gallon bucket with the original resin poured on it.

Recycled materials present unique challenges to the molding process, but its use will only increase in the next few years. (Photo: )

Before pouring it into the machine or dryer hopper, I will reach into the bucket and mix as much as possible. Then I would take the same bucket, slide it under the hopper of the dryer, open the door, and fill it with a mixture of freshly dried recycled materials and virgin materials. When I go to the grinder and empty them, I will start to notice that the grinding material is powdery.

The plastic starts to look less like granules, but more like colored powder. Of course, at this time we have not yet generated the concept of control-tracking the heat and processing history of the plastic-we only know that each grinder needs to be emptied and mixed with the new material at the end of our shift. With us Introducing more and more of this powdery material into the process, I started to notice an increase in what we call "hot shots".

This is a situation where the mold falls off due to degradation of the resin. It may take several hours to clean up the heat shot, and sometimes it is necessary to pull the mold and send it to the tool room. Remember, these are engineering grade resins, not as forgiving as polyethylene or polypropylene. Hot shots are usually blamed on process technicians not sufficiently cleaning the machine between cycle interruptions. This is correct to a certain extent, but I noticed that as time goes by, when the amount of shredded material is higher, the length of time the machine stops before the mold is hot shot will decrease.

This is when I started to throw away powdered materials. When I was told that I could not do this, I decided that I would not use it. We used gaylords and we put the excess regrind there, so I will throw in any powdered regrind. This does not completely eliminate flash from molds with degradable materials, but it significantly reduces flash.   

There is a strong push to reduce waste as much as possible, but giving recycled materials a more interesting name has not changed many of the problems we encountered before increasing attention to recycling.

Within a few months of entering this industry, I have witnessed how much recycled material affects the molding process. All these effects have a negative impact on part quality, mold conditions, and process repeatability. We may all have dozens of first-hand examples of what might go wrong when running regrind. Over the years, I have found mobile phones (many pieces), brass rods, hammers, boots, various beverage containers, etc. in the grinder.

In the medical field, there is not much work for regrinding, but in those jobs that deal with most foreign objects, filtration and metal detection systems can be found. However, outside of the medical industry, I have not seen many molders apply this filtration technology to their recycled materials. This is why most of the interesting things I find in grinders are found when trying to solve part quality problems.   

Just like for many people in our industry, regeneration is a bad word for me, and I will spend a lot of time proving the benefits of not operating it from a cost and quality perspective. I remember that I have been trying to find a way to track the algebra of recycled materials, but when using a mixture like 25%, it is almost impossible to do so. From a cost point of view, it's just not worth it, but this is the wrong way to look at it. Dealing with the environmental impact of all these resins should be the first priority. Finding ways to dispose of used materials and keep them away from the landfill should definitely be my motivation, but like many people, I focus on the problems I can solve. 

Post-consumer resins face more challenges than post-industrial resins. (Photo: PT)

Fast forward to 2021, and everyone is now trying to integrate some kind of recycled resin into their products. Customers who have made it clear that regrind is not allowed in their products are now trying to use recycled resin in all their production lines. PCR (Post-Consumer Recycling) and PIR (Post-Industrial Recycling) have become the main focus of our industry.

Recycled material will be classified as PIR, which usually (but not always) means that it is recycled in a manufacturing facility. This is different from PCR, which is a resin recycled after consumer use. All of us in this industry are vigorously pushing to reduce waste as much as possible, but giving recycled materials a more interesting name has not changed many of the problems we encountered before the focus on recycling increased.

Within a few months of entering this industry, I have witnessed how much recycled material affects the molding process. All these effects have a negative impact on part quality, mold conditions, and process repeatability.

What needs to be clear is that the regrind in the factory is only one type of PIR. Not all PIRs are just ground scrap parts and runners. PIR can also be reprocessed from multiple industrial sources. There are several recycling facilities that can convert your waste into nearly original resin. This is the same process as PCR, in which recycled plastic is reprocessed and pelletized with additives through an extruder to provide pelletized resin that meets the original resin specifications. This is obviously a simplified 30,000-foot view of the process, but the point is that this material should be treated the same as the original material. Of course, this will increase the cost of running recycled materials, but it should still be much lower than the original material itself. 

I recently spent some time in a facility that fits exactly what I described above and was impressed by the level of recycling that is taking place. The filtration system there ensures that no foreign matter (FM) is introduced into the system, and it can separate the metal from the grinding process or the FM from the extrusion process. The recycler also uses newspapers and glass fibers as fillers for some of its resins, which means that the reprocessing of used resin not only prevents plastic from entering the landfill, but also prevents other materials from entering the landfill. The whole reprocessing process from beginning to end left a deep impression on me. My only concern about regranulated materials-like any regrind-is the ability to have some kind of generational control.  

Molders should use virgin resin to establish process boundaries that can be used for scrap. (Photo: PT)  

Every thermal cycle that plastic undergoes, including compounding, is a degradation step. Each of these steps will reduce the characteristics of the resin, and we will lose some of the required material functions. I once wrote: "As a processor, our job is to minimize degradation", and running recycled materials through the extruder to return to pellet form does require an additional degradation step. Most importantly, not knowing the occurrence of regrind means that we don’t know how many times the resin has been reground, and it is almost impossible to track this effectively, if it goes from virgin to regrind-the ratio of use. So how can we ensure that the severely degraded repelletized resin will not be used for molded parts?

This reminds me that the same is true for PIR and PCR. How do I know that these materials have material properties comparable to the original materials in the case of PIR, or whether the PCR I am running meets the original material specifications? These problems will be more challenging than the ones we faced many years ago, because we are currently under pressure to reduce the impact on the environment by using recycled resins.

I think if someone has a well-equipped laboratory to test recycled materials, it will increase the possibility that these materials are comparable to the original materials. But, of course, you can't test it all, so even if the material is tested, there is no guarantee that some contamination will not be introduced. The laboratory cannot test 100% of the material, but the injection molding machine can.  

As I wrote before, the first step is to develop a robust, repeatable process. This should be done using 100% original materials. You cannot use recycled resin for process development, nor can you try to build a separate process using recycled resin if it is intended to replace the original material. If you have read any of my other columns, you will know that I strongly support the establishment of strict process limits for all outputs after the process is established-this is no exception.

Just because the machine is able to increase or decrease the injection pressure according to the viscosity of the material does not mean that it should be uncontrolled.

Allows you to develop a process to run several different batches of raw resin and record your output during the entire run to identify normal changes in the machine when it runs the raw material. The process limits should be large enough to account for normal variations, and they should be established for all process outputs (including injection pressure). I have said many times that we need to stop thinking that our injection pressure “should change”. It should not change outside of the normal deviations we have established. Just because the machine is able to increase or decrease the injection pressure according to the viscosity of the material does not mean that it should be uncontrolled. When this happens, it means that something has changed. Something in the whole process causes a significant change in the viscosity of the material, and we should not ignore it just because the machine can adjust it. 

If our recycled resin runs within the limits of the process we have established without making any changes to the process, we can be confident that its properties are within the range of the original material. If our injection pressure increases or decreases significantly, we will know that the viscosity level of our resin has changed, making it easier or more difficult to fill. This does not necessarily mean that our recycled resin is the problem, but with some quick troubleshooting, we can eliminate other causes and narrow it down to the material. At that time, this batch can be separated from other batches, and testing can confirm our suspicions. If I have the ability to automatically transfer injections beyond the established process limits and issue an alarm after this happens a predetermined number of times, I can use my injection molding machine to ensure that the recycled resin I am running is what we expect it is.

It is important that all of us should do everything we can to reduce the environmental impact of our industry, but we don't have to sacrifice the quality of parts for this, as long as we run a robust and repeatable process.  

About the author: Robert Gattshall has more than 22 years of experience in the injection molding industry and holds multiple certifications in scientific injection molding and Lean Six Sigma tools. Gattshall has developed a number of "best-in-class" Poka Yoke systems with third-party production and process monitoring functions, such as Intouch Monitoring Ltd. and RJG Inc. He has worked in multiple management and engineering in the automotive, medical, electrical and packaging production industries Position. Gattshall is also a member of the Public Policy Committee of the Plastics Industry Association. In January 2018, he joined IPL Plastics as a process engineering manager. Contact: (262) 909-5648; rgattshall@gmail.com.

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