Sharper Dies Make for Smoother Extrusions

Although it’s wise to use soft tools to clean the die, it should be noted that all metals that have a “gold” color are not necessarily soft. Typical low-carbon steel has a Rockwell Hardness (Rb) of less than 71. Pure copper has a typical Rb of <10, but some copper alloys are as hard as 98 Rb while brass alloys can be 63 Rb. It’s not uncommon to observe operators using razor knives, steel spatulas, hardened drills, steel-wire brushes, stainless feeler gauges, and even screw drivers to work on the die. These types of tools are often harder than the die-exit edges unless the edges are made of heat-treated, high- carbon steel, and as a result can wear them quickly.

The “breakaway” effect for a sharp edge is contrasted by one with a radius in the accompanying figure. The polymer tends to follow the radius because of its adhesion to the wall, and then swell when it needs to break away cleanly from the die face and move downstream. This can cause micro-tearing of the surface and leave polymer on the radius or above it on the die face as die buildup. The combination of tearing along with die buildup is often a largely invisible cause of poor surface finish on the extruded part. A radius as small as 0.010 in. has been found to result in surface tearing and die-face buildup with certain polymers.