The Effect of Regrind Material Ratios on Processing Speed

Regrind, or recycled sprues, runners, and rejected parts, is commonly reintroduced into the virgin material stream to reduce waste and material costs in injection molding. While economically beneficial, the proportion of regrind used can influence processing speed. Regrind particles often have a different shape (more irregular) and potentially altered molecular weight compared to virgin pellets due to thermal history during initial processing. This can affect how the material flows and packs in the barrel and mold, potentially requiring adjustments that impact cycle time.

High regrind ratios can lead to variations in melt viscosity and flow characteristics. This inconsistency makes it harder to establish a stable, optimized process. To compensate for potential flow variations, processors might need to reduce injection speeds or increase back pressure to ensure proper mixing and homogenization. This conservative approach, while ensuring quality, inevitably slows down the overall process. Additionally, regrind can contain more air or volatiles, necessitating more rigorous drying or venting, which can add time to the preparation or cycle phases.

Degradation is another concern. Each reprocessing cycle subjects the polymer to heat and shear, which can lead to chain scission and a reduction in molecular weight. Lower molecular weight material tends to have a higher Melt Flow Rate (MFR) and flows more easily, which might seem beneficial for speed. However, it often results in poorer mechanical properties and increased shrinkage, potentially leading to quality issues that require slower, more controlled processing or longer cooling times to compensate.

To mitigate these effects and maintain speed, it's crucial to carefully control the amount and quality of regrind used. Limiting regrind to a specific percentage (often 10-25%, depending on the material and part requirements) helps maintain consistency. Blending regrind thoroughly with virgin material and ensuring it's properly dried are essential practices. Some processors use dedicated dosing units for precise regrind introduction. By managing regrind usage effectively, manufacturers can reap its economic benefits without significantly compromising processing speed or part quality.