Why Extrusion  Dies Perform Differently

It has long been a mystery as to why some dies turn out to be a major problem for the

Technician and the Production team. These particular dies can be to produce a relatively

simple profile but can be the cause of many discussions and possibly heated exchanges

between Technicians and Production staff.

Let us consider the various scenarios involved.

1.regularly running die that has been hitherto free of problems.

The most likely cause is that the die has worn and thereby gone out of correction. By this I

mean the die has repeatedly used and re-polished and nitride on many occasions and the

bearings have become rounded thus changing it’s extrusion characteristics. The surface

quality may deteriorate and blend lines may appear. The changes in friction may result in

length differences in the case of multi-cavity dies.

The solutions are to study the information and front-ends and check the die thoroughly. The

die can then be brought back into correction providing the tool has sufficient tolerance left

in the die openings. Otherwise a replacement tool should be ordered and the Die

manufacturer advised of any problems. If any improvements to the die design can be made

these should be suggested at this time.

2. A frequently problematic die.

These are the bane of all technicians life. The problem must be approached in a methodical

and scientific way. The solution to such a problem is more often than not in the die design.

This statement is not meant to detract from the die designer as they have done the best they

can at the time. The onus falls now on the die technicians to arrive at an improved design

based on the results obtained. It is all too easy just to dismiss the design as a bad one

and try another die from a different manufacturer. Factors that should be considered are

m any and include die size, bearing lengths, integral recesses, mandrel depth, web

thickness, port openings, flow availability, extrusion breakthrough pressures and extrusion

ratio to name but a few.

The solution is to involve the technicians and the manufacturer and discuss the problems

originating from the die. If the problems are dramatic then a dramatic re-think in design

must follow. If all that can be suggested is a minor change or two of the bearing lengths

then another manufacturer should be consulted and the problems encountered outlined.

3. Surface quality problems.

The quality of the bearing surface has an effect on the overall surface of the extrusion. If

the bearing surface is pitted with spark-erosion marks or has wear from over use then the

solution is obvious. Namely, file and check the bearing for flatness, square-ness and then

re-harden.

If the quality problems are in the form of different colours or banding then the underlying

cause must be detected. The position of any banding in relation to any bearing changes on

the die design should be looked at. If these areas match then the probable cause is at hand.

The solution is to naturally advise the die supplier of the problem and discuss with them

any solution for the next die. The bearing changes can be reduced in many cases without

causing any problems to the die, especially if they are only localised and incorporated in

the design to " hold back a small mass ". The bearing may be removed completely and a

match to the adjacent bearings obtained.

Sharp bearing changes should be present on a modern die and the exit corners of such a

bearing should be smoothly curved with as large a radius as possible.

Bearing changes that extend past the mass area and into the thinner adjacent bearings

should be moved and this will help eliminate the banding on either side of the mass area.

Wherever possible the tapers of the bearing changes should be as long as possible so as to

prevent any sudden localised differences in friction. If the bearing changes are longer then

the likelihood of surface problems will be greatly reduced.