Aluminium trainingExtrusion die correction

Testing a new aluminium extrusion die

 

The die is rightly considered the heart of the aluminum extrusion press. New dies or dies, after major adjustments, are almost always subjected to a preliminary press test.

Checking and testing of a new extrusion die

Testing a new extrusion die includes the following steps [3]:

  • The interpretation of the die drawing and the die specifications relevant to die trial
  • Liaising with die designer
  • Finishing off die, if required
  • Analysing trial extrusion
  • Identifing appropriate remedy for out of specification results
  • Re-testing die after correction

Die corrections

Once manufactured, the die is sent to the press for testing and production, or for testing only, depending on the critical features of the extrusion [1]:

  • If the produced extruded product  is successful in the first trial, production continues with proper checks and measures.
  • If the extrusion does not succeed, the die needs to be corrected based on the report of the test run along with the front piece of the extrusion.

Die corrections or maintenance could be required due to many reasons, such as [1]:

  • improper metal flow
  • dimensional variation
  • surface finish
  • any interference with runout table.

Die correction procedures require considerable practical experience. There are many ways to correct the same problem based on the skill and experience of the die corrector.

Two types of die correctons

Die corrections can be subdivided into two types [2]:

  • pre-service corrections
  • post-service corrections.

Pre-service corrections are the ones carried out on new dies after the initial trial runs. These corrections are mainly focused on:

  • adjusting the geometry or
  • feed design.

Post-service corrections are done every time the die is dismounted after being used for extrusion. They are mainly focused on correcting [2]:

  • the flow
  • damage and wear
  • fractures
  • deflections
  • loss of hardness.

Principles of die correction

The general principles that should be followed in carrying out any die correction operation are as follows [2]:

  • Corrections should be usually carried out at the back end of the die. Working at the front end can reduce the die life.
  • A particular die should be corrected by one specific corrector, as he/she is familiar with the die and leaves a kind of mark or handwriting.
  • All corrections must leave sufficient room for corrections in the future.
  • Most of the corrections are done by removing some die material, so the selection and the extent of the operation should be determined appropriately to avoid a permanent damage of the die.
  • Even though corrections are done, it should be kept in mind that problems can recur later.
  • All correction details and design changes must be properly documented.

Means of die  correction

Typical means of die  correction include [4]:

  • short or long chokes and reliefs
  • milled reliefs
  • grinding on the face of the die
  • grooving the die face
  • welding dams on the die face
  • peening
  • changing spider dimensions.

Trial extrusion

When running trial extrusion of new dies, the following steps are performed (Figure 1) [5]:

  • The lead ends of the extrusions are marked properly and cut.
  • The centers of the length of the extrusion must be marked and cut in pieces of about 300 mm in length.
  •  Studing the tendency of the flow of aluminum from the lead ends—that is, the areas of the extrusion that tend to be slow or too fast.
  • From the center cut you check all your overall and wall dimensions.
  • After checking your dimensions from the center piece, you should open all walls that require opening before doing anything else.
  • Very often, just by opening the walls to desired dimension, the flow of aluminum will change and the extrusion will come out with the correct angularity.
  • If after opening the walls to allow commercial toierance, the extrusion is stiii out of angularity, then you will proceed to correct the die openings according to the lead ends.


Figure 1 — It must be always sure that the lead ends of
the extrusions are marked properly and cut.
The centers of the length of the extrusion must be marked and
cut in pieces of about 300 mm in length [5]

The factors to be checked

Although techniques for correcting dies are many and varied, most of them have direct bearing on one or more aspects of metal flow in the actual extruding process. It is often possible to use only operating techniques to control flow.

Some of these include [4, 5]:

  • rotating die to change position of orifice
  • alignment (Figure 2)
  • adjustment
  • use of lubricant
  • taper billet heating
  • adjusting billet heat in relation to container temperature
  • extrusion speed change,
  • use of guides and canisters (Figure 3).

Figure 2 – The principle of extrusion press alignment [6]

Figure 3 – There is the tendency of multiple-hole extrusion dies to twist.
This can be lessened by using what are known in the industry as canisters.
Canisters are used mainly on multiple-hole dies
such as the three-hole extrusion die shown in this Figure [5]

Temperature and speed

Temperature and speed when testing dies are very important. The die should be run for production at the same temperature and extrusion speed that were used at the time it was corrected. Often a die is corrected at a given rate of speed, and when it comes up for production the extrusion speed is increased. The increased speed will increase the temperature of the aluminum as it passes through the bearings, changing its flow on critical openings where the flow should be slower to keep up with the rest of the extrusion. This is more often found when extruding hollow dies, where you have screw holes or other critical designs on the mandrel [5].

The alignment of the press

Another thing that has to be considered in testing dies is the alignment of the press. A die which has been corrected for a press that is properly aligned will not rerun properiy if the press is out of alignment. The die shouid be reworked to compensate for the press
being out of alignment (Figure 4).

In sketch “A”, the die was corrected for a press that was properiy aligned. The flow of aluminum was balanced evenly. In sketch “B”, the same die was set up to run on the same press, at the same billet temperature, with the same rate of speed, but this time. the press was out of  alignment. Consequently, the flow of metal will have changed. This, in most cases, will require a slight correction to be able to run acceptable metal.

Also in sketch “B”, you will notice that by the container being off to the left the billet is off just the same. This condition will move a left leg of the shape toward the center of the diameter of the billet, where the flow of aluminum is at a faster rate. When the flow of aluminum is upset in such a manner, it is clear that this die must be reworked at the press to compensate for the unbalanced flow brought about by the press being out of alignment.

A – The container and the die in properly aligned extrusion press.
B – The press is out of alignment: the container and
the die are off relative to each other.
Figure 4 [5]

One correction in this particular case would be to speed up the leg on the right which has been placed to the edge of the billet (Sketch “B”) where the flow is less due to the drag of the billet against the container. The alternative correction would be to choke up or slow up the flow of metal on the left leg (Sketch “B”) which has been placed towards the center of the billet by the press being out of alignment.

Before correcting a die that you have already corrected, be sure that the press where the die was run was properly aligned.

Non-uniform temperature of the container

Other conditions that can cause changes in flow on well-corrected dies are burned-out elements in the container on the side of the container where the elements are not working properly and are cold. These will cause the flow of aluminum here to be slower than on the hotter side of the container [5].

Figure 5 – Containers with heating elements:
a – external, b – inserted into the mantle [6]

Non-uniform temperature of the billet

Also, bad elements in the billet heater can cause one side of the billet to be at higher temperature than another. This will result in the cold side of the billet being slower; and even if the die was corrected properly, a billet in this condition will make the flow change [4].

Sources:
1. Aluminum Extrusion Technology / P.K. Saha – ASM International – 2000
2. Die Defects and Die Corrections in Metal Extrusion /S.Z. Qamar, T. Pervez and Jo.Ch. Chekotu –  Metals 2018, 8, 380
3. Test a new aluminium extrusion die – MEM07044A – Manufacturing Scills Australia – 2013
4. Process Factors to Consider before Modifying the Die /H. Glicken //Aluminum Extrusion Technology Seminar, Chicago, 1977
5. Die Corrections for Changing Flow Characteristics /Luis Bello //Aluminum Extrusion Technology Seminar, Chicago, 1977
6. Press Alignment – Chapter 2 in Extrusion Press Maintenance Manual /Al Kennedy – 2023