Wall

The rotational moulding process can be used to mould hollow items, but this does not mean that by the term “hollow item” it is intended to have a large hollow internal volume. It is, in fact, possible to mould “hollow items” that have two walls very close to each other which form apart that has both the external section and the internal section produced by the mould surface. Moulds that permit the moulding of a double wall part can be designed with a “core” that defines the inner part.

Shrinkage

In the rotational moulding process the mould only restrains the external surface of the moulded part. The decrease in volume that takes place during the solidification process causes shrinking that is distributed in all directions, even as a function to the restrictions. A moulded part that is very restricted (due to the presence of metal inserts, stiffening ribs or other) will show shrinkage that is less than average in the direction of the restraint and higher than average in the directions where the shrinkage can take place freely.

Flatness

Wide flat surfaces are usually difficult to produce in all plastic moulding processes. In rotational moulding the difficulty is higher because the polymer is only bonded on the external part (to the mould). Therefore, it is usual experience that in flat zones warpage often takes place: a wide concave or convex deformation of the part walls.

Distortion

Some of the main limitations are regarding the minimum distance between walls, in order to allow the material to flow freely to the more remote zones of the mould and the need to avoid acute internal angles less than approx 45°, where the thickness could decrease to an unacceptable degree. As with other moulding processes, it is better to follow all the usual rules regarding the size of radiuses, the requirement for draft, and stiffenings ribs.

Vent

A tube in the mould tool which extends into the inside of the moulding to allow the equalisation of pressure. Air moves out of the tool during the oven cycle and in during cooling helping to prevent mouldings with excessive flash, blow holes, faces distorted inwards.

Tickness

The internal angles tend to have less thickness as the time in contact with the material is less: it is for this reason that radius on internal corners have to be greater in relation to those of external corners. Large radiuses improve the flow of the material inside the mould and help to distribute the stress over a wider area, increasing the strength of the part. In the external corners there is a greater material thickness. This can be useful in zones where stress and wear are greater.

Undercuts

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. neck zone).

Ribs

Stiffening ribs are used mainly for reinforcement and increase the stiffness without increasing the part wall thickness. In rotational moulding ribs with fins cannot be used as the way they are used in injection moulding to stiffen thin surface areas. The powder would be unable to enter into the deep cavities of the mould and the resultant stiffening would be poor.