The problem

A company making moulded tanks wanted a separate moulded base to support a new tank. Both the tank and base were to be moulded in HDPE and they wanted them welded together. The tank was designed to take an internal pressure and part of the job of the base was to support the bottom of the tank against considerable loads. It was not clear whether a plastic extruded weld would be strong enough or would fail.

Our solution

There were 2 welds to consider, running around an outer circumference and an inner one. The tank was asymmetric so we could not model a segment of it. Our consultants built an FEA model of the tank and base using shell elements and loaded them to understand the gross stresses and deflections. This allowed us to chose a plane to cut the tank, where there were no deflections and build a more detailed FEA model of the remaining portion using solid elements.

Having taken advice from the supplier of the weld gun, our consultants modelled the weld bead, including its outer face and its typical penetration into the melted surfaces below. We predicted the residual stresses from the welding process using a thermal boundary condition on the weld. Finally, the loads on the tank were added in, so the weld stresses could be predicted. The results showed that the weld stresses may lead to long-term cracking and failure. The design loads on the tank were then limited to a safe level, to give adequate performance from the welded joint.