Key Points on Rotation

Recent ARM calls and committee meetings have sparked discussions on rotation ratios. As a result, we’re developing multiple items to share with members on the subject. Here are some key points for now.

1. Size Sensitivity to Rotation Ratios: Small molds are less sensitive to rotation ratios than larger molds.
2. Preference for Non-Integral Ratios: Non-integral ratios (e.g., 1:4.1 instead of 1:4) are favored as they significantly increase flow paths.
3. Limited Impact of Rotation Ratio: The rotation ratio matters for only about 15% of the cycle; once the powder is laid up, it has no effect.
4. Rotation Speeds: Some advocate for maximum rotation speeds to create more flow paths before powder lays up, but others argue against it citing potential issues like static build-up and mechanical/safety limits.
5. RPM Theory and Evidence: While higher RPMs may theoretically lead to more even wall thickness distribution, there’s a lack of hard evidence. Counterarguments include static build-up and mechanical/safety limitations.
6. Visual Checks: Visual measurements are preferred over relying solely on machinery manufacturers’ data. Modern machines often display RPMs directly, but manual checks, especially on older machines, are recommended due to wear and accuracy issues.
7. Arm Balance: It’s crucial to balance arms to avoid excessive bearing wear mechanically.
8. Mechanical Drag and Creep: Gearboxes may have mechanical drag or “creep,” where the plate may still turn slowly even when set to zero rotation. Negative RPMs may be needed to keep the plate fixed.

Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.