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The topic of regrind use is a hot topic in today’s processing world.  We will be discussing its impact on plastic part production on this blog.  Regrind offers a solution to minimizing plastic use and reducing plastic waste. It is crucial to understand the technical considerations and challenges that come from using regrind effectively.

Challenges with Regrind

The use of regrind can lead to various issues like possible contamination, inconsistent pellet configuration, glass fiber breakage, and polymer degradation. These problems can affect the final part dimensional control, the overall quality of the final plastic part and processing consistency.

Contamination is often the result of improper material handling. To avoid this, it is essential to establish specific and detailed material handling procedures. Properly cleaning grinders and hoppers and keeping material containers covered are just a few of the essential steps that need to be taken to maintain the purity of the regrind. Another method of avoiding contamination is to do the grinding press side and adding the regrind back into the process at the established ratio.  This will avoid any possible contamination in a separate part of the plant.

Glass-filled materials commonly exhibit some degree of fiber breakage during processing and regrinding. Using such reground material can lead to reduced mechanical properties, particularly tensile and impact strength. Moreover, maintaining dimensional control becomes more challenging due to changes in shrinkage rates. The extent of these issues depends on the amount of fiber breakage and the quantity of regrind utilized. Low rpm grinders and press side cutting while the runners or parts are warm is an effective method of reducing fiber breakage.

One common misconception about regrind is that it might process better than virgin material. While this may be true in certain cases, it is most likely that moisture or degradation of the material is the cause of the change in viscosity.  This is the case when dealing with hygroscopic resins with hydrolytic degradation or thermally degraded materials.  Please note that the mechanical properties of the regrind are compromised in these cases and the final parts molded with regrind will have diminished mechanical attributes, which can impact their performance of the parts.

Consideration for Regrind Particle Size

Another crucial factor to consider is the particle size of the regrind. The size of the particles, whether large particles, small particles, or fines, can significantly affect the performance of the regrind. Inconsistent sizes, especially large variations, may lead to non-uniform melting and variable drying rates for hygroscopic resins. The larger the particle, the longer it will take to dry.  This can cause issues such as variations in shrinkage, part weights, dimensional results, warpage, functional deficiencies, and even non-melt in the parts, particularly for semi-crystalline materials. Additionally, fines can cause unwanted black specs or discoloration streaks in transparent or translucent materials because of degradation of the fines. Consider using screens with your grinders to greatly reduce adding the fines into the process or separating large particles before they are introduced into the hopper.

Methods of Regrind Use

There are several ways of managing the use of regrind in plastic part production:

1. The 75-25 Blend Ratio: This method, commonly recommended by many material suppliers, suggests using a blend of 75% virgin material and 25% regrind. This approach typically works well if the regrind is consistent in pellet size, free from contamination or degradation, and blended properly. However, it's essential to evaluate the mechanical property retention for your application if using this method, especially when using glass-filled materials. The regrind will receive numerous heat histories but it is considered to be limited in this percentage of regrind.
2. Cascade Regrinding: This innovative method involves using 100% virgin material in the first production run. The regrind generated from this run is marked as "first generation" and used entirely in the second production run. The process continues for subsequent runs until all the regrind is utilized. While this approach may work effectively for certain applications, it might not be ideal for hygroscopic materials due to moisture uptake and drying concerns. It may also pose challenges in demanding applications that have specific mechanical requirements, especially for glass-filled materials. It is also important to know that hot material will absorb more moisture so it is imperative to effectively dry the material before each use.
3. Application-Specific Usage: The third method focuses on tailoring the use of regrind to the specific requirements of each part. Some parts may tolerate using 100% regrind, while others demand 100% virgin material to meet stringent product and performance criteria.

Regrind undoubtedly has a significant role to play in reducing waste and cost in the plastics industry. However, its successful implementation relies on establishing proper procedures and disciplines on the production floor. Regardless of the chosen method, it is crucial to ensure that parts produced with regrind are fully qualified to minimize any potential performance risks.

We hope you found this Tech Tip informative and useful for your plastic part production processes. Stay tuned for more insights into the world of plastics in our future blog posts!

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