Rotational molding combines design flexibility with stress-free strength. To maintain precise dimensional accuracy, however, designers need to understand how rotomolding materials behave during processing. Unlike injection or blow molding, rotomolding produces plastic parts without pressure. Rotomolded parts also undergo significant shrinkage during cooling.
Thermal shrinkage affects part tolerances, acceptable variations in size for part features and overall dimensions that are expressed as plus or minus (±) values in millimeters or inches. Yet temperature alone doesn’t explain tolerance issues. Keep reading to learn more and contact Gregstrom for value-added rotomolding backed by 75+ years of plastics manufacturing experience.
Understanding Shrinkage and Tolerance Factors
According to the Association of Rotational Molders (ARM) Design Guide, part dimensions are heavily influenced by how plastic cools inside the mold. Factors that affect dimensional tolerances include:
- Outside vs. Inside Sections: Outside dimensions (free-formed surfaces) draw away from the mold wall as the plastic cools, making them subject to greater variance. Conversely, inside sections shrink down onto mold cores or inserts.
- Tooling Construction: CNC-machined aluminum molds deliver the tightest dimensional repeatability. Cast aluminum molds offer good accuracy but introduce minor casting variances. Fabricated sheet metal tooling exhibits the highest dimensional variation.
- Process and Material Consistency: Variations in resin grade, polymer quality, pigment loading, and the use of mold releases affect shrinkage rates. Rotomolders who maintain strict cycle controls and perform regular mold maintenance minimize these variables.
Best Practices for Rotomolded Part Design
For best results, a part designer and rotomolder both have a role to play.
- Design for Uniformity: Maintain a consistent nominal wall thickness throughout the part to encourage uniform cooling and minimize warpage.
- Use Post-Molding Fixtures: For critical dimensions on large flat surfaces, shrinkage jigs (cooling fixtures) can be used immediately after demolding to hold tolerances as the part reaches room temperature.
- Specify Realistic Tolerances: Avoid applying tight overall tolerances to an entire assembly. Focus precision callouts only on critical mating features or interface surfaces.
As an experienced rotational molder, Gregstrom can help you to achieve part tolerances that are both realistic and cost-effective. To discuss your rotomolding application, contact us.
