Keep the wall thickness equal wherever possible

Different thickness walls in a section can affect the running of a die. As the billet is pushed through the die, material will flow at different speeds. This causes an imbalance in the die, which may result in certain parts of the extrusion not being fully formed due to material starvation (ripping). By changing the porting at the entry face of the die (rear), die correctors can alter the flow characteristics of the die. To some extent these alterations are dependent on trial and error based upon the expertise of the individual die corrector.

A deep channel feature with walls of differing thickness can lead to die breakage. Similarly, if a hollow section has a thick wall at one side, an imbalance occurs. This can lead to movement from the mandrel (die centre), which in turn can produce thick and thin walls around the hollow part. This could cause the collapse of the section.

The example on the right shows the "nose end" of an extrusion, where the differing wall thicknesses and speeds have caused the section to distort.

The differing wall thicknesses can be seen in fig 5.3.01.This can be rectified by altering the porting details. Wall thickness is very important to the outcome of a predicted shape. When producing a wide section, thin wall thickness' can result in problems with denting, bending, convexity, and concavity. Advice from the Design Department on section shape and size can be given in order that the extrusion can be produced to the tolerance required.

Simple shapes make for the most cost-effective extrusions. A complicated profile, incorporating several features such as channels, screw-ports or hollow parts, can suffer from increased die costs, slower running speeds, and lower recovery, increasing the overall cost to the customer. Some times, the reduced speeds and recovery resulting from complicated sections can lead to rejection at the enquiry stage.

Big hollow sections, particularly ones that are close to the limits of press capacity require careful deliberation. Larger sections serve to intensify the effect of problems covered in this section. The dies themselves are also very expensive compared to smaller dies.

Symmetry in a section is a big advantage; the resultant pressures created on the die plates are all equal. This adds to the strength of the die, and allows the die manufacturer to alter the layout of the die plate, to achieve a more predictable outcome when the section is produced.

Channels in sections are one of the most common problem areas for extruders as they cause weaknesses in dies. The policy regarding the entry gap related to the depth of channels (or tongue ratio) is an industry standard, where the gap across the top to area ratio should equal 3:1 or lower.

One method of reducing the problem caused by several channels in a heat sink feature is shown in figure 5.3.04. Every second prong in the heat sink is shortened considerably, this has the effect of decreasing the gap to area ratio to an extrudable level, whilst maintaining the heat-dispersing properties required of the section. Full radii have also been incorporated at the peaks and troughs of the feature.

It is essential that screwports are broken out with a 60° angle (see fig 5.3.05). This makes the tongue ratio almost exactly 3:1. If the angle were tighter, an un-extrudable tongue would be created. Sometimes this 3:1 ratio is near impossible for customers to meet. In these cases, it may require a complete section redesign. Hollow dies are sometimes introduced, incorporating tear-out strips. These are thin webs of metal that cover the end of the tongue ratio creating a hollow section. These tear-out strips can be clearly seen in figure 5.3.06.

The left-hand section shows the part after extrusion, whilst the right-hand section shows the same extrusion with the tear out strip removed. The tear-out strip can be removed at Kaye, or customers may prefer to do it. The tolerances associated with channels are often not taken into consideration by designers. These tolerances are very important, as they are tighter than often realised.

On deep narrow channels, a lead-in radius is generally required at the entry, with a full radius at the bottom. These will to add to the strength of the finished section, reducing the chance of die breakage.

Sharp corners create stress points on dies, and should be avoided wherever possible. If a sharp internal corner is required for a fit, then waste the corner with a radius. On unspecified external corners the policy is to use a 0.4mm minimum radius. Radii can be reduced to an absolute minimum of 0.2 in specific positions, for example, on the ends of screw threads.

Every endeavour should be made to terminate legs on extrusions with a full radius. This reduces wear, increasing die life and making the section easier to run and fill. The chances of the shape coming out fully formed will then be increased. On all corners where a sharp internal corner is not required, a fillet radius should be added.

The optimum weight at the press head for press 1 is 1.8 kg per metre, whilst that for press 2 and 3 it is 1.2 kg per metre. All sections need to be run through a bolster, having 1, 2, 3, 4, 6 or 8 cut outs or cavities in them (see picture). The required section must fit through the bolster with at least 8mm clearance all around it to achieve the desired material feed to the die plate.

If these criteria are not met at the enquiry stage because of a poor size-to-weight ratio, and the desired weight is not compatible with the bolster, the relevant shape could be rejected, unless necessary modifications are carried out. These modifications can involve reducing the overall size of the section so it will fit the bolster, or increase wall thickness to increase the weight at the press head. With the increased weight, fewer cavities can be used, resulting in larger holes in the bolsters.

If the size-to-weight ratio is not ideal, the estimated figures for scrap and running speeds can turn out low, in some cases making the section unviable.

When an extrusion runs at the press it is fed out onto a specially designed padded table, known as a "run-out table". Marking can occur due to friction between the surface of the extrusion and the surface of the pads. This can be in the form of a black deposit or scratches on the surface. This marking is usually only significant when the section is to be anodised, as any surface abnormalities are made more apparent. Damage and marking incurred on heavy sections is generally worse than on lighter sections. Also, if a die has multiple cavities the material can fall on top if itself, causing more surface damage.

Time can be saved, during the enquiry process, if the customer specifies any important or visible surfaces on a section. The face that the material will be run-out on can then be identified.

There are several tried and tested designs of joints that can be applied to join two sections together. Jointing is a complicated process to go into without looking at a few examples.

The basic clip detail has a clipping base with matching section featuring two legs. The legs need to be of suitable length and thickness to deflect, thus giving the desired clip action. At the enquiry stage, specific information should be obtained about the clip-fit required. If the clip-fit is too tight, a new die or new die plate might be required to loosen it. Loose clips can be tightened with a simple on-site modification. Other methods used are double clips, hinges and hinge and clips.

The basic information required is:

Aluminium can be joined by welding, riveting, mechanical fixing, bonding with tapes or adhesives, or sliding fits. Material that is in mill finish condition has a high co-efficient of friction and will not readily slide without some form of lubrication. A paint finish applied to the section can act as a lubricant.

Surface finish must always be considered for the eventual applications of sections (see section SURFACE FINISHING ). Surface defects and abnormalities such as weld marks and blend marks can be covered by a paint finish, but the anodising process will highlight these defects.

The environment where the extrusion will be used should be considered. The performance of the material, together with the type of coating or protection used, can be affected.

An anodised finish is suitable for high wear usage, e.g. hand rails.

In a non-wear exterior use, a paint finish would be more applicable.

Careful consideration must be given to material that is to be fitted in areas where chemicals are used as some chemicals will react with certain types of aluminium.

Adding a tolerance to important dimensions on a section can effect its viability. Industrial standard tolerances are used conforming to BS 1474 & DIN 17615/1748.

In some circumstances tolerances are required to be tighter than the relevant standard. These cases should be discussed with internal die correctors, so as not to incur higher scrap and cost penalties. On some less-complicated sections, tighter tolerances can be consistently achieved, and tolerances can be reduced to 2/3rds of the associated BS 1474 value.