Effective Strategies for Incorporating PCR in Injection Molding

This is the first of a two-part series that will provide insight into the adjustments needed in your molding operation to process post-consumer recyclate (PCR).

Part I: Material Considerations

To dive into a new circular economy, we need to learn how to process PCR, or post-consumer recyclate, into conventional applications.

We stand at the dawn of a new era, facing numerous challenges and unanswered questions. However, our industry’s sustainability depends entirely on our ability to achieve circularity. From the molder’s perspective, the most crucial advice is clear: prepare yourself.

You can also read: Additives to enhance post-consumer recycled (PCR).

Molding PCR is by no means a drop-in solution. You will likely need to adjust every process within your organization. And yet, the earlier you start, the faster you will be able to compete.

Evaluate Flowability

Typically, when you purchase new material for your molding operation, your material supplier will provide you with a data sheet that you can trust. Rarely does a molder conduct any quality control checks on the material they acquire.

However, the supply of PCR is often inconsistent in terms of quality and quantity. The flow of recyclate materials is generally not pure, and therefore, even the best sorting effort will leave you with the possibility of cross-material contamination. So, the first change you will need to implement is installing a quality control process for the incoming material.

Controlling flow properties, such as MFI, is always advisable. If possible, a more advanced flow check is performed using the spiral mold method .This mold will measure the flow length of a given melt at constant fill and temperature conditions. The test is more accurate and closer to reality of what you will need to evaluate in a molding operation.

Oxidation in PCR is the first cause for degradation, and it will cause cross-linking between molecules, reducing flowability and tenacity. It is therefore advisable to check for changes in flow behavior.

Check Material Performance

Also, if possible, test mechanical properties. A simple tensile test can give you a lot of valuable information regarding ductility and ultimate tensile resistance, which tend to drop when incorporating PCR content.

To start molding, it may be a good idea to formulate compounds and to incorporate a percentage of PCR content into virgin material, instead of going all-in with 100% PCR at once. A good start may be to incorporate 15% at first.

Evaluate how the part behaves and conduct your regular quality control over the molded component. If the part behaves well, start adding 5% more to the mixture. There is a boundary of typically 30% of PCR where things start to change; at this boundary, it may be necessary to start adjusting the properties of the base material to tailor your PCR-Virgin compound.

For example, if the part is too brittle, you may want to add a copolymer to the blend. If the component lacks rigidity, it may be necessary to select a virgin material with a higher modulus to compensate for this loss in properties.

Educate Yourself About Additives

In PCR, it is normal to find cross contamination from different material sources, and if the material acting as contaminant is not compatible with the polymeric matrix it will form an immiscible new phase. These phases may act as stress concentrators and will lower the component strength. The use of compatibilizers is then necessary.

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

It is important to make sure that your component will perform properly when incorporating PCR. As the material normally has lower properties than its virgin counterpart, it may be necessary to make some adjustments to your part design.

Be aware of the location of weld lines. While weld lines may not be an issue with virgin materials, the presence of phases of one material inside the other or impurities in the PCR will account for stress concentrators and will lower the strength of your part.

Also, some thin walls may need to include flow leaders to help the material achieve better part filling, in case you may have to work with materials with lower MFI. Currently the sorting technologies barely account for the type of material but not the process flowability, so you will probably end up with grades that where formulated for extrusion and blow molding, and have therefore way lower MFI than the ones you would normally specify for injection molding.

Document Results

Finally, a basic recommendation: take note of all your results. A diligent documentation of your testing process may be the best invested time in your way to circularity. Make sure to diligently save test procedures, photos, curves of your testing and to make sure to add your own comments and observations. Remember that we are all starting a new era, and the knowledge you need to process these new materials is still not available out there. You need to create it.

In the next article we will explore adjustments in the injection molding process and in your tooling to incorporate PCR.