Getting PCR Right in Injection Molding

This is the second in a two-part series discussing adjusting your injection molding process to incorporate PCR.

Our first article discussed what you could evaluate in your incoming material when processing PCR. In our first delivery, we recommended checking flowability, mechanical performance, and formulation, specifically regarding additives to improve part performance.

You can also read: Effective Strategies for Incorporating PCR in Injection Molding.

I would like to discuss what I have learned regarding process adjustments to incorporate PCR in this issue. Whenever the viscosity conditions vary, several parameters are affected. Knowing beforehand what to expect from the material point of view can help you achieve better results in the molding machine.

Changes in Plasticizing

It is normal to find many blow-molded articles as PCR sources. Because blow-molded bottles are abundant and easy to identify, they usually account for a broad proportion of the PCR available. Blow molding grades have a far lower MFI (melt flow index) than injection molded materials. To stabilize the parison in blow molding, you may find MFI between 0.2 and 0.8 gr/10 min at 210°C. Under the same testing conditions, the MFI for injection molding grades ranges between 10 and 45, extending to 80 and 100 in thin-walled applications. This means that blow molding grades are more viscous and resistant to flow than injection molding grades. Therefore, you may find that when molding PCR you will face a high resistance to plasticizing. Sometimes you may even hear your machine “roar”, which is a sign of alarm. Try to decrease viscosity by increasing the temperature along the plasticizing unit – I have found that sometimes you need to go up about 40°C over the entire profile! Also, it increases back pressure to enhance homogenization. And expect plasticizing time to increase.

Material Shape

Some will say it is the same to incorporate flakes than pellets and that you do not need the hassle of pelletizing. While pelletizing involves costs and induces a subsequent thermal cycle in the material, standard injection molding machines are designed to feed pellets. It is possible to feed flakes, but it is less efficient, it may present variations in dosage and melting and some blockages may occur. Also, when having fine powder amongst the flakes, you will endure the nightmare of black specks. I personally have made a far better experience from working with PCR pelletized in a double screw pelletizer, where you can formulate a blend, incorporate enhancing additives and even filter the melt before it comes back into the injection molding machine.

Changes in Filling Behavior

If your PCR source is mostly injection-molded material, molecular weight degradation from processing and environment may improve flowability.
This happens because shorter molecules, formed through degradation, can act as lubricants, which helps the material flow more easily. Consequently, average molecular weight decreases, while the range of molecular sizes increases, reducing the material’s resistance to flow overall. Thus, lower injection pressures and even lower processing temperatures may be sufficient for mold filling with degraded PCR material. However, this benefit only applies when the PCR’s melt flow index closely matches what you normally use in injection molding. More often, PCR viscosity varies widely, since sorting technologies don’t distinguish processability, only material type or general source. Therefore, using unchanged parameters like clamping force may lead to incomplete filling or flashing due to inconsistent viscosity.
To solve this, some machines include software that adjusts settings in real time to match the material’s flow behavior. For instance, Engel has showcased tools that detect PCR viscosity changes and adapt the process to minimize defects and waste.

Cooling Time

Another often-overlooked phenomenon when working with PCR is the need to increase cooling time during processing. In some cases, processors have observed a clear decrease in thermal diffusivity. This likely results from molecular weight degradation, which further reduces the already low thermal diffusivity of polymers. As a result, the molded component takes longer to transfer heat to the mold surface. However, this effect isn’t consistent across all materials and can vary depending on the type of PCR and the polymer matrix involved.

Machine and Tooling Maintenance

Another adjustment you may need to prepare for is the frequency of maintenance and machine inspection. Because PCR materials may have contaminants it is possible that nozzles in the machine or the hot runners will block. In some cases, you may even see some pigmentation of the screw and barrel, as well as the mold, due to migration of volatiles during processing. And, in some particular cases, you may notice even some odor. The presence of volatiles contaminants, both from pigments, inks and organic contamination, is common in PCR. Cascade extrusion may lower this effect, as well as a proper venting system installed in the extruder (preferably when pelletizing). Also, it is sometimes evident that both screw and barrel may wear earlier, due to the abrasive behavior of lower MFI materials.

Educate Yourself About Additives

Be aware of the large availability of additives to support the processing of PCR. There are several solutions out there able to give a boost to your PCR-Virgin compounds and help you compensate for the losses in flowability and mechanical properties. Antioxidants are available to reduce the degradation of the basic materials, and they should be a part of every formulation. They will prevent yellowing and fragility of the blends, but they will be consumed after a couple of years of use and they must be reincorporated every new processing cycle. To improve compatibilization, some suppliers provide tie agents; they are molecules able to compatibilize mixtures which normally do not blend due to chemical incompatibility. They are extremely valuable to improve ductility in material mixtures, such as PP and PE.

Also, there are several waxes and other additives that can be added to compensate for loss in flowability. These additives increase the processability of the melt and reduce the energy consumption and wear demanded in the injection molding machine.

Validate Design

When incorporating PCR into your component, it is essential to ensure proper performance, since its properties are usually lower than virgin materials.
Therefore, you may need to adjust your part design to compensate for potential weaknesses introduced by the recycled content. In particular:

• Weld lines: Be mindful of weld line locations, as these areas may become critical stress points when using PCR-based materials. While weld lines are typically manageable with virgin plastics, PCR may introduce impurities or phase separation that reduce mechanical strength. Consequently, these inconsistencies can act as stress concentrators, making your part more vulnerable to failure under load.
• For thin-wall designs: You may need to incorporate flow leaders to assist filling with lower MFI materials. This is especially important because current sorting technologies seldom account for material flowability, focusing mostly on type or source. As a result, you might receive PCR grades designed for extrusion or blow molding, not optimized for injection molding processes. These grades typically have much lower melt flow indices than the ones you would normally select for your mold design. Hence, understanding and anticipating these differences will help you avoid processing issues and achieve better product consistency.
• Document the process: To support this adaptation process, documenting all testing steps, results, and observations becomes a crucial best practice.
  Indeed, carefully recording procedures, test curves, images, and your own comments will be time well invested in future design work.
  Moreover, since this field is still developing, the knowledge you generate will fill gaps that current industry data cannot cover.

Ultimately, we are entering a new era in circular materials, and your own data will become your most valuable asset. In our next article, we will explore how to modify your injection molding process and tooling to accommodate PCR more effectively.