There is no universal regrind percentage for injection molding. Start with the resin grade and its processing history, the finished part’s requirements and any customer or application restrictions. An approval may limit regrind or prohibit it altogether. A plastic granulator can cut scrap into regrind, but cutting does not restore polymer properties or approve the material for a particular part.
Agree on the application before the ratio
Identify the part and drawing revision, the resin grade and lot, and where the proposed regrind came from. A runner from a controlled production stream and material with a different or uncertain history do not automatically belong in the same qualification. The material-stream guide helps describe the material before granulation.
Restrictions can be application-specific. For example, Avient’s overmolding guidance notes that regrind is not an option for many two-material applications. Separately, WITTMANN describes a factory that defined ratios for individual products after molding and quality trials. Neither example supplies an approved ratio for your part.
What does the regrind percentage divide by?
For a percentage of the total blend by mass, divide the regrind mass by the mass of all ingredients in that blend, then multiply by 100. The two hypothetical examples below contain only regrind and virgin resin. They illustrate different ways to describe a quantity; they are not recommended recipes.
| Statement | Percentage of total blend mass |
|---|---|
| 20 kg regrind + 80 kg virgin resin = 100 kg blend | 20 ÷ 100 × 100 = 20% |
| 20 kg regrind per 100 kg virgin resin = 120 kg blend | 20 ÷ 120 × 100 = 16.7%, rounded |
“20 kg per 100 kg virgin” therefore does not describe a 20% share of the total blend. If the actual blend also contains colorant or other additives, include their mass in the denominator. Write both the numerator and denominator in the recipe record so that purchasing, production and quality use the same regrind ratio.
Keep a qualification record that another shift can use
Agree which results matter for the actual part and how the decision will be made. Depending on the application, these may concern functional performance, dimensions or appearance. The following is a practical record, not a fixed test suite; use the requirements agreed for the part.
| Record | What to identify |
|---|---|
| Part and permission | Part, drawing revision, customer or application restrictions and the person or process responsible for approval. |
| Material and recipe | Resin grade and lot, regrind source and processing history, proposed regrind mass and total blend mass. |
| Conditions and results | Handling, drying and molding conditions; sample identification; actual results against the agreed relevant requirements. |
| Decision | Approval or rejection, limits and conditions, and the agreed review process when grade, source, history, ratio or application changes. |
Keep the evidence with the decision. A ratio approved for one part and material history should not silently become the default for another. Eastman’s processing guidance for its own polymers discusses clean, uncontaminated and dried regrind, minimizing fines and overheating, and maintaining a consistent feed ratio. Use the instructions applicable to your own grade and process.
Separate approval from repeatable delivery
Once the application permits a recipe, equipment must deliver its ingredients in the intended proportions. A plastic granulator prepares cut regrind; the GB gravimetric batch blender weighs recipe ingredients before mixing. Neither action by itself establishes that the resulting material is acceptable for a part. Compare where ingredients are measured and mixed in the blending and dosing guide.
For an equipment discussion, share the scrap form, intended material route, ingredients and the mass basis of the recipe. Set the regrind percentage only within the approval and operating conditions for the actual material and application.