Profile shape problems

Profile shape problems




Angle or square-ness of a profile is one of the most common defects encountered in

extruded profiles. The problems are directly related to an imbalance in the flow or feed to

part of the profile within the die itself. This may be from the bearings or flow related

factors, for example, a deep or offset recess or pocket.

A balanced flow is the first step in making a successful correction to any die; it is also the

best starting point from which to view the die. The die can be said to be in balance when the

front ends (nose-ends) resemble a saw cut, i.e. at 90 degrees.

The probable course of action will be to reduce bearing lengths where ever possible to

balance the flow to the extrusion. This is of course assuming that all bearings are producing

their optimum amount of friction, in other words that the bearings are square and flat or have

a minute amount of choke. This will be discussed later in the handbook.

Should this be the desired course of action then the die maker should be informed about the

changes if the tool is a new or replacement and has not been used for many tones of

production, which may mean that the tool has become deflected.

Corrections to flat dies that have an integral recess that is contoured to the profile may be

used to make a correction with the help of a milling machine. The usual method employed is

to increase the flow or volume of aluminum to a certain part of the profile (norm ally the

slow area) to balance the flow to the extrusion. The normal areas are the ends of legs that are

attached to the main body of the extrusion, these are usually positioned towards the outer

reaches of the feed to the die.

The normal practice is to manufacture a die that has bearing lengths that become shorter

as you approach the end of a given leg (as it gets progressively further from the centre of the

die). This is to allow for the reduced flow or lower velocity of aluminum to this point. By

increasing the velocity and volume of aluminium there will be a change to the angle of the

profiles leg. Legs that may have some detail attached to them, such as a brush path slot will

often extrude with the leg angled towards the detailed area. This is due to the extra friction

imparted from the detail and possibly insufficient bearing friction from the adjacent wall.

Multi-cavity dies may suffer from square-ness problems when the run-out lengths are

disproportionate. S mall differences in angularity will be exaggerated on the slow running

cavities by the pullers force which will be exerting a somewhat larger force to the slow

cavities. The problem of angularity can frequently disappear when a correction is made to

improve the run-out lengths.

Any profile with an unusual angularity problem (especially if the change in angle is acute)

should be examined closely to see if there is any evidence of the profile rubbing or fouling

the backing out or indeed any support tooling. Further checks may be carried out to see if the

angular problem has been generated from an outside source for example the run-out table or

graphite pegs or slats that may have been used to separate the extrusion. Look for any tell tail

signs on the extrusion that may be apparent as black graphite generated lines in the area of

the fault.

Marks on the profiles surface (orange peel) may show the profile has been given a larger

than normal stretch for some reason like the profile has distorted badly during cooling. With

the maximum amount of information at hand a correction can be arrived at.