This is a quick collection of some basic extrusion design rules to provide tips to achieving both good tool and product design.
Wall thickness and stability
This is the desired requirement but variations will occur whilst production is in progress and concerns can arise in producing even this simple section.
This illustrates the problem with thin walled tubes, the haul-off can deform the section because the pressure necessary to produce the grip. The solution is to form the belts of the haul-off to mirror the form of the section thus increasing surface area contact.
The wall thickness may vary around the section if the centre pin of the die is incorrectly set. This may cause:
Excessive shrinkage in the area that is thickest.
Bending of the profile because the thick and thin sections will cool at different rates.
If the wall thickness varies then when gripped in haul-off the thinner wall may deform uncontrollably.
Squares and rectangles
The square or rectangle can be seen in most extrusion sections.
The internal and external corners are shown as sharp but this condition is not always desired nor always possible to achieve.
This illustrates the more acceptable result, small internal radii, slightly larger external radii and straight sides. The wall thickness again should be constant or as near constant as possible to avoid shrinkage and bending of the section (in length).
The sides are convex and concave, to counteract this condition the former or calibrator must pre-form the section the opposite way. This condition is very difficult to predict so a certain amount of testing is necessary, as is with all extrusion tooling.
Internal walls and chambers
The introduction of internal walls and features to produce chambers can produce other problems.
This shows the required result but this must be modified to produce a trouble free extrusion.
This illustrates the sinkage problem encountered with thick internal walls. Where the two walls meet there is a large mass of material at the junction and the larger mass of material will shrink more and create a sink mark along the length of the extrusion.
The thickness of the internal wall needs to be reduced to keep it in proportion with the outer wall. Normally two thirds the outer wall is the maximum thickness for inner walls.
This is the ideal solution. Small radii (0.15 - 0.25mm) in the junction corners and a reduced thickness internal wall.
This illustrates that the chamber produced is to be used for a fitment, because the internal wall cannot be calibrated it is therefore uncontrollable and distortions as shown in the lower figures can result.
This illustrates a solution, the chamber is initially produced larger than required. When the die has been tuned and the section is running in a stable manner, small pips are added to the walls to position the fitment correctly and ensure the required fit.
When external fitments are to be incorporated certain areas may present a problem. For example perhaps a packer or spacer is to locate with the extrusion.
This shows the desired internal fitment.
The result is that the haul-off crushes the section because of the recessed key area being weaker. The internal wall is distorted and the fitment location is reduced. This may tighten the key fit or make it completely unacceptable.
One remedy is to reduce the depth if this is possible for new fitments. For existing fitments this may not be possible.
By introducing two small bridge walls it is possible to support the key area and retain both the depth and dimensional accuracy of the fitment location.
Where small protrusions are necessary they may also present a concern.
The outer wall behaves like an internal wall and sinkage marks are present on the flat face. Tool tuning can help to overcome the problem.
These show the clearance the former or calibrator uses, this allows the protrusion to 'float' within the confines of the calibrator. If the clearance is not available then the extrusion may become tight and jam in the former.
Areas within the chambers
Areas within the chambers are difficult to control.
This design for a screw port is not difficult to produce, but it is not sufficiently strong enough to accommodate a screw. Two solutions are shown below.
Solution 1: This will add sufficient strength to the port, but manufacturing the die is more complicated.
Solution 2: This shows the best solution, strong screw port and simpler die manufacture.
In this case there is an internal screw port and the wall thickness variations will again produce shrinkage.
The rules regarding two thirds wall thickness should be applied.
This illustrates a simple and effective functioning clip. However as with the screw port it is difficult to manufacture the die and to measure the extrusion simply. Possible solutions are:
(a) To thicken up wall thickness. This may lead to the legs cooling slower therefore distorting more and possibly sinkage occurring.
(b) To bridge the top of the protrusions.
This develops the spring clip but it is much simpler to produce the die, very much simpler to measure the extrusion and easier to control the formers.
This further develops the spring clip to provide a positive location for the insert.
Last edited: 17/02/15
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