Semi-open extrusions impose requirements on tool design as the partially sealed section of the extrusion must be formed by a tongue in the tool. This is only located in the “opening” and the surface is exposed to material pressure during the pressing cycle. Consequently, there is a significant risk of damaging the tool.

The graph below can act as a guide for determining how deep a tongue can be sited in relation to the opening. If you keep below the solid black line, it is probable that the tool will maintain its integrity, although the total design of the extrusion may force you to reduce the depth or increase the size of the opening.

Different types of tongue present different problems to tool designers. It is always preferable to use tongues with good supports in any circumstance, but problems that arise can be resolved by making adaptations to the design of the tool.