Using recycled materials in Injection plastic mold remains a focus of protecting resources and the environment. However, although recycled plastic benefits applications in different sectors, including it for injection molding can create manufacturing and product-quality challenges.
As specialists in sustainable injection molding, Jinhong offers customers extensive knowledge of working with recycled material in plastics. Because of our hands-on experience, we’re familiar with how recycled materials may be friendlier to the environment than to injection molding.
A first hurdle in U.S. recycling is that our nation is still maturing in its practice of it. Other countries are more diligent in gathering and recycling sustainable materials. They also have lower costs and stricter standards for recycling programs. The greater expense of U.S. recycling appears in product and material pricing, including for injection molding.
Beyond recycling-related costs, injection molding further contends with recycled plastics’ differing properties and melting points, which can cause cross-contamination, procedural complications and variations affecting part quality.
Before Jinhong uses recycled plastic for injection molding, it is first extruded at another facility. During the extrusion process, the material is fed into a heated cylinder where a rotating auger pushes the melting plastic through a screen. Once the molten form cools, it is cut into recycled plastic pellets.
Pellets for injection molding of a polypropylene plastic closure or fitment might include recycled material with both polypropylene and polyethylene terephthalate (PET) from the extrusion. This is because polypropylene melts at 450°F and PET at 475°F. As the recycled material is processed at the facility, one might assume the melting-point difference would allow the polypropylene to go through the heated-cylinder screen at 450°F
This often proves to not be the case. While the melting polypropylene passes the sitting PET, it creates a shear heat with it, melting the PET enough to force it through the screen and cross-contaminate the product.
Jinhong then receives the pellets for the current closure or fitment project, which will involve re-melting them and injecting fixed quantities of the plastic at a high pressure into a mold. The problem on our end is that as we’re melting the pellets compromised by the recycled plastics, the mixed material may not pass through certain gates that allow the plastic into the mold; our gates have different openings from the facility’s screens.
Similarly, some recycled plastics may contain bits of metal, which doesn’t melt as plastic does. When the melted plastic is pushed into the mold, those bits can plug our gates, forcing an uneven flow or, worse yet, restricting the flow. Both scenarios can impair the molded part’s quality. Either the part will be incomplete or it will lack the proper density, which can later cause product problems or failures.
Fixing such contamination typically calls for downtime by requiring either that the mold be removed from the press or the press be shut down to clean the part and clear the foreign material. The cost for this correction ranges from $500 to $3,000 or more.
Yet another concern with recycled materials for injection molding is the popular use of post-consumer scrap. Some recycled plastics carry chemicals absorbed from their former containers, such as those for lawn care or pest control. The recycled material can then introduce these chemicals into the new product.
In one instance, Jinhong was filling a mold with material that included plastic from recycled battery cases. The material had been extruded, filtered and pelletized, but enough battery acid remained to complicate the injection process. As the pellets were pushed in, heated and melted on our end, the hot, acidic gases that escaped through the mold vents began etching the molds, which allowed them to start rusting.