WHY WEB (BRIDGE) GET CRACKED EXTRUSION DIE

After several production runs the mandrel will begin to deflect with the resulting

cracks appearing on the weakest part of the webs ( maybe noticeable more quickly on

dies with a high extrusion ratio ). The cracks if left unattended will worsen and in severe

cases small pieces of the mandrels web may become loose or break off and are then very

likely to be extruded through the die bearing. The result of this at best may be a dramatic

decrease in the surface quality of the extrusion, or at worst permanent damage or

breakage to the die bearing. The chances of this occurring are greatly increased when

the die has been nitrided to harden the bearing surface (especially if nitrided many

times ).

The nitrided layer though very hard is also very brittle and will not tolerate any

local deformation without shattering or chipping taking place. This damage is most likely

to occur at the base of a web in the welding chamber.

A degree of success in preventing this from becoming a major problem is at hand.

The removal of the nitrided layer in the area of highest deformation by dressing back the

nitrided layer to reveal the softer steel, which is more resilient to pressure deformation

without the likelihood of the same damage occurring.

For a badly affected area with the cracks penetrating the webs very noticeably the

solution is a little more drastic. Firstly the webs will be cut back as much as 5mm and

then welded. The then welded area is then dressed back to the original height and

cross-section using cutters and mounted points.

The possibility of webs cracking initially may be reduced by the introduction of

good radii to all the parts of the webs base and also good fillet radii at the point where the

web joins to the main body of the die or mandrel. The sharp corners will act as a starting

point for the fracture to begin.

The cracking to the webs adjacent to the mandrel. Here it is only minor and

inspection is sufficient. When the cracks become more of a problem the some

refurbishment will be necessary.