Controlling costs is a huge part of any project. Materials can be expensive, especially advanced or specially-engineered resins, so you want to get as much bang for your materials buck as you can. One way to do this is the proper use of regrind.
There is always some unused thermoplastic material that is left over from injection molding, typically taken from mold components such as gates, flash, runners, and sprues. What's the point in wasting it.
Even though this material has been processed once, it can be used again (within certain guidelines) and blended into a future virgin plastic melt. This excess material is collected and ground into much smaller granules called “regrind.” Most material manufacturers agree that up to 30 percent of compatible regrind can be mixed in with virgin material and not lessen or compromise the mechanical properties this primary material is supposed to have.
Benefits & Typical Use
Using regrind is a great way to reduce cost, optimize material usage, and reduce demand on natural resources. But like everything else about injection molding, mixing in regrind is a science that requires high precision. The amount of regrind that can be used, and how it may impact the characteristics of primary material, varies from project to project—variables indicate resin type, how the regrind was originally processed, granule size of the regrind, and any contamination on the regrind like dirt, dust, water, etc.
It is critical to fully understand to what degree the heat history and processing of the regrind has degraded its physical, chemical, and flow behavior, and how much these changed properties may affect these same properties of the primary melt when the regrind is added. Regrind will never behave exactly the same way as the virgin resin. How much regrind can be used depends on the performance specifications of the final product and the tolerances that are required.
Important Considerations of Regrind
The level of degradation for most polymers during injection molding is well known and, for the most part, degradation in the regrind does not negatively impact the quality of the virgin resin if regrind is kept to 30 percent or less of the melt. The biggest cause of degradation is heat—it is important not to overheat the material during processing or subject it to too much shear. If the heat history of regrind is overlooked, it is possible that products made from a regrind blend could fail in the field. The injection-molding process may also result in shorter polymer chains in the regrind, which can greatly impact quality and performance.
Leftover plastic (including any rejected parts or products) of varying sizes, including tiny dust-like granules are collected and passed through a grinder, which reduces the range of sizes from dust-sized to up to half an inch. These, however, do not melt at same rate and create some variations in the molded product; however, these are typically not serious-enough flaws to impact quality or performance. This can be more of a problem for products where aesthetics are important because regrind can cause tiny surface blemishes or discolorations.
Of course, clean material handling and clean operations are essential for every injection-molding project. All plastics (including regrind) must be free of dust, dirt, oil from the skin, or foreign particulates such as metal or plastic residue from other products. Moisture is a big issue too—if regrind is not properly dried, its physical properties can change significantly during injection molding, which compromises the quality and performance of the final product.
Controlled use of materials is a key component in reducing overall cost. When the same percentage of regrind can be used for a product line, the process can be modified to always accept this blend ratio; however, if the percentage changes, be sure to adjust the injection-molding process accordingly.
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