In today’s world, sustainability is quickly going from being a marketing advantage to an actual requirement. Because of these changes, processors are increasingly looking to include post-consumer recycled (PCR) plastics in their material streams. Mixing a little recycled plastic with virgin resin is easy. However, running 100% PCR is harder. It affects how your screw and barrel perform.
Recycled material behaves differently from virgin resin. Not only are you introducing variability and contamination into your process, but you’re also faced with unstable melt flow and unexpected shear sensitivity. If you want to successfully run 100% PCR, you not only need to adjust your process, but you also need to understand how your screw and barrel interact with that material.
This article will discuss how to process 100% PCR and overcome its challenges.
Things to Consider When Processing 100% PCR
The major difference when processing PCR is that you don’t know what you’re getting.
Recycled plastic comes from countless sources, which means that the material you are processing can have widely varying molecular weight, moisture content, and contamination. These differences will impact how the material flows when melted and how it behaves under shear.
Such variability creates a couple of key problems.
1- Melt instability
The biggest one is melt instability. PCR will degrade faster than the new resin. It creates inconsistent viscosity and poor flow properties. The other challenge is contaminants. Oftentimes, you will find bits of paper, metal particles, or degraded polymer mixed into your material. These contaminants can create surface defects in your parts and damage your equipment.
2- Moisture sensitivity
It’s also important to note that moisture is a huge factor when processing 100% PCR. Most recycled material is susceptible to moisture absorption. As the material melts, any moisture creates steam and creates bubbles, voids, and surface defects in your parts. You can have the best screw design in the world, but if your material is not dried properly, you’re screwed.
Dealing with these issues is part process, part screw and barrel design. Let’s dive into some of the critical things to look out for when processing 100% PCR.
Why Screw and Barrel Design Matters More with PCR
100% PCR cannot be processed using a general-purpose screw. PCR melt flows can vary due to melt flow index, contamination, degradation, etc. A screw designed to stabilize the melt can benefit processing PCR.
Barrier screws may help to alleviate melt instability by design, forcing solid and molten polymer to be separated during processing. Barrier screws have a barrier flight, which, rather than allowing partially molten material to surge forward into the next section of the screw, forces material into two distinct channels.
For PCR materials, this separation becomes critical. Recycled plastics often contain particles that melt at different rates. When processing PCR, this is important because recycled plastics can have particles that melt at different rates than the majority of the material.
Without splitting the melt from the solids, flow instability, unmelted granules, and melt surface defects can occur. Distributive mixing and dispersive mixing sections, such as Maddock mixers or spiral mixing elements, greatly improve the mixing of additives, colorants, and melt streams to produce a more uniform melt, fewer visual defects, and better part performance.
Processing PCR materials needs extra attention to mixing to help with contaminants, color variations, degraded polymer fractions, etc. It is important to note that screw designs for PCR should not shear the material too much. While mixing helps with homogeneity, it can also increase polymer degradation. A barrier screw with some light mixing would be ideal.
Role of Optimized Screw Design
Having the right screw design can mean the difference between stable processing and constant fluctuations. Your screw can help combat some of the material inconsistencies you see when running PCR. A properly configured screw can help melt your material more consistently and mix it more evenly.
When running PCR, you’ll usually want a screw with some type of barrier or mixing section. Barrier and mixing sections help separate your solid material from your molten material. It allows for more complete melting prior to injection/extrusion. Better mixing also improves color dispersion and limits the amount of unmelted particles.
Compression ratio refers to how much the material is being compressed throughout the melting, pressurizing, and conveying action of the screw. It’s a comparison of how deep the channel is in the feed section vs the metering section. Many screw designers will use a high compression ratio to create more shear and attempt to force melt 100% PCR.
The problem is that most recycled material is degraded and heat sensitive due to thermal history. High compression ratios force the material through more and more shear as it travels down the screw creating more shear heat. Shear heat, in turn, further degrades the polymer.
Once you start degrading the melt, it typically gets worse. Visually, the melt will appear to darken. The viscosity will drop at an uncontrollable rate. The mechanical properties of your end product will begin to degrade. You can even end up generating gases and burned particles, which translate to black specks and surface defects.
By running a screw with a lower compression ratio, you decrease the amount of shear being forced onto the material. It allows for a more stable melt because you are not aggressively melting the material with shear heat. You are gradually melting the material.
Again, most PCR applications will run better with a lower compression ratio in the 2:1–2.5:1 range. This range can vary based on the polymer being processed and the contamination of the material.
The compression ratio is just one variable that needs to work in harmony with the barrier, mixing, and other features of the screw. A well-designed screw will melt, mix, and convey the material without subjecting it to excessive thermal and mechanical degradation. Working with Deer Machine will help dial in your screw geometry for your specific PCR material.
How to Adjust Temperature Profiles for Stability
One of the trickiest parts about processing PCR is temperature control.
Unlike virgin resin, PCR doesn’t tolerate high temperatures. Running too hot can cause your polymer to degrade, resulting in discoloration, odor, and decreased strength. Running too cold can cause poor melting, resulting in flow problems and surface defects.
Ideally, you want to gradually increase your temperatures down the barrel. You also want to make sure you’re monitoring actual melt temperature and not just your set points. In some cases, running lower temperatures with a better screw design will yield better results than running hotter.
Installing filtration and monitoring moisture content will also go a long way when processing PCR.
Controlling Contamination, Filtration, and How to Address Moisture and Material Preparation
Filtration is another huge factor to consider when running PCR. Any amount of contamination can cause huge disruptions when processing 100% PCR. Small amounts of metal or paper can cause defects in your parts and cause major headaches when it comes to cleaning the material out of your machine.
Make sure you select the appropriate screen pack or melt filter based on the level of contamination in your material. You also want to regularly change your filters. A clogged melt filter can cause major pressure build up and yield flow issues.
Controlling Material Moisture
Moisture can be a major cause of defects when running PCR. Any recycled material can absorb moisture from the air, especially if the material is stored in a drum or warehouse that’s not climate-controlled.
Your material would be dried before it even reaches your hopper. Each material type has different drying times and temperatures. Oftentimes, dehumidifying dryers are used to achieve a consistent moisture reading. While you can run PCR in the dryer, any moisture will create issues downstream.
Another thing you can do is blend your PCR batches. Even if you may be running 100% PCR, blending your material can help achieve more uniform material properties. It can help you maintain a more stable process.
Adjust Processing Parameters
Running lower screw speeds when processing PCR creates less shear, which limits material degradation. You also want to back off on back pressure. Too much back pressure can cause mixing issues. You may even need to adjust your injection/extrusion pressure to compensate for the inconsistent melt viscosity.
Make sure, when processing PCR, you want to take small steps when adjusting processing parameters. PCR is extremely sensitive to changes, and small adjustments can have major impacts.
The Role of Equipment Quality in PCR Processing
Processing 100% PCR can put additional stress on your screw and barrel due to mechanical and chemical processes. Fillers/additives and contaminants degrade the screw and barrel faster than processing virgin material.
One of the first things you will encounter when processing PCR is abrasive wear. Many fillers/additives and contaminants are very abrasive, including glass fibers, mineral fillers, dirt, sand, and others that are ground up in the recycling process. PCR fillers/additives and contaminants find their way into the melt and eventually work their way up and around your screw.
These abrasive particles slowly work their way through your screw and barrel, essentially grinding away at your metal, causing flash erosion. This flash erosion slowly decreases your screw flight height and increases screw-to-barrel clearance. As your screw wears, your process stability is compromised, which could lead to melt backflow, unmixed material, surging, etc.
Depending on the polymer(s) you process, you may also encounter corrosive wear when processing PCR. Additives/degraded materials left behind from recycled products can cause corrosive wear. As you may know, certain materials will corrode at high temperatures when they come into contact with metals. Corrosive wear can cause pitting and weaken the surface of both your screw and barrel, causing long-term damage.
Wear down your tolerances, which will decrease process stability and product quality. To reduce wear during PCR processing, wear-resistant materials should be used. These materials come in many forms, including bimetal barrels, hardened alloys for screws, and coating options. These materials help fight against abrasive particles while trying to keep your original screw and barrel tight for longer.
Regular maintenance and inspection are also important when working with PCR materials. Look for common signs of wear and take your screw and barrel out of service before they become noticeable during production. Check melt pressures, monitor motor load, observe the melt for consistency, and watch your outputs for any slow changes. All of these things can help identify wear before it becomes a huge problem.
When working with Deer Machine, we have a variety of wear-resistant options and can help design applications that are specific to your needs. We will help you choose the best application for your materials and budget while keeping your screws and barrels running longer.
Common Mistakes to Avoid
There are a lot of things that go into processing PCR, but the biggest mistake people make is running their 100% PCR like virgin resin.
People try to force the material through the machine by turning up temperatures, which just ends up degrading their material even faster.
Others neglect to keep up with wear on their screws and barrels. Worn screws and barrels exaggerate any material inconsistencies. Evaluate where you can improve, as small changes can make a big difference.
Conclusion
As the availability of recycled material increases, running 100% PCR is only going to become more common. While there are challenges associated with running PCR, they can all be overcome with some careful planning.
Try to understand how your screw and barrel interact with your material and make adjustments accordingly. Make sure you control your temperature settings, manage contamination, and dry your material properly. These are just a few things you can do to successfully process 100% PCR.
