How to control shrinkage and warping during rotational molding?

How to control shrinkage and warping during rotational molding?

Many variables used to control rotomolding are interrelated; when tuning the rotomolding process, it can be difficult to isolate the effects of individual factors. Central to these interactions are shrinkage and warpage. Because shape, size, and appearance are often crucial criteria for product conception, nearly every factor impacts the final product in some way.

Crystalline materials (such as polyethylene) are the most difficult to control because their crystalline structure is influenced by numerous variables during development. Amorphous materials, lacking a simpler structure, are less susceptible to some variables. Currently, parameters that directly influence final product shape and warpage include:

Material selection—determines final product properties and shrinkage range.

Pigments and additives—affect shrinkage and final product properties.

Mold design—surface treatment and tolerances influence final product dimensions.

Mold release system—the part's release point can affect dimensions and warpage.

Vents—improper venting can cause warpage—applying pressure can reduce warpage.

Rotation—affects wall thickness distribution and color uniformity.

Heating process—affects the plasticization of the part, which in turn affects density and, consequently, shrinkage.

The cooling process—it has a major impact on bulk density and residual stress.

Demolding temperature—Controlling the product's demolding temperature helps control final product dimensions.

Secondary processing—Using cold-free jigs helps maintain flatness and tight dimensional tolerances.

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