Undercuts are internal or external protrusions that can impede the removal of the part from the mould. Compared to other plastic material manufacturing processes, rotational moulding is less restricted by the presence of undercuts.
This is because the moulded material is free to shrink away from to the internal surface of the mould during cooling: it could happen that the part shrinkage is equal to or more than the size of the undercut.
Usually, for articles in polyethylene, positive everted undercuts with dimensions that may reach up to 50% of the expected linear shrinkage are normally considered acceptable.
For flexible materials like PVC, larger undercut dimensions can be considered (for example,
dolls heads are extracted from the neck zone).
More rigid materials, such as PA and PC, have less flexibility and therefore less tolerance to undercuts.