The problem

Rotational moulding in high density polyethylene, HDPE, can be very cost effective for large mouldings.  It is often used for tanks, street furniture and industrial casings, as well as marine products.  The design details, particularly structural ribs and reinforcing features, are heavily constrained by the manufacturing process.  This is because the HPDE enters the mould as a powder and it gets distributed around the inside as it is tumbled by the mould.  The wall thickness is not even and may be too thin or thick in corners and parts of the mould where the material isn't evenly distributed leading to mechanical failure. 

Our consultants were approached by a company making casings for an industrial product, after a casing suffered a mechanical failure.  The casing had cracked along a corner when the casing had been under a negative pressure.

Our solution

At the outset it wasn't clear whether the mechanical failure (crack) was a due to a design fault or a manufacturing fault.  Our consultants built a finite element analysis model of a portion of the casing, with a fine mesh in the area of the crack.  FEA showed that the casing would not have cracked if it was the correct thickness and it was unstressed when it left the factory.

A sample casing was cut up and this showed that the local thickness was not correct.  Our consultants then modelled the cooling process and found a high residual stress at the cracked corner, due to the manufacturing process.  The combination of the residual stress and incorrect thickness were enough to cause mechanical failure under negative pressure.

Minor changes to the structural details can often solve this sort of problem.  Metal plates or reinforcing bars can sometimes be incorporated into rotational mouldings.  In this case, the client took the opportunity to design an improved product and our consultants assisted with the structural design of an improved casing, which has not failed.