Front end and back end defects
Back end defects
One of the defects that can occur in extrusion is known as a “back end defect”. These can arise during the end of the pressing cycle if the inlet opening in the tool is too large or if not enough of the “butt end” of the ingot is saved, causing the high oxide content ingot “skin” to be extruded. This results in surface defects and a loss of durability in the finished extrusion.
To avoid this, steps must be taken to ensure that the inlet into the tool is max 90% of container diameter. For an 8″ ingot, container diamater is approximately 190 mm. In addition, “butt size” must be adapted to the relevant ingot length.
Front end defects
Front end defects, also known as Charge Weld Faults, occur when residual material in the tool mixes with the new ingot at the start of each new press cycle. This presents particular durability problems for heavy duty extrusions and, as a rule, 1–5 metres are cut away at each ingot seam if the extrusion is to be processed, if it is to be subject to durability requirements or if the cross-section is to be anodised.