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The most valuable knowledge an extruder can have is a thorough and accurate understanding of the extrusion process.  This statement may appear patently obvious, but many extruders still consider both die design and the extrusion production process to be an art as well as a science.  This is no longer true.

There was once a time when the extrusion of aluminum was perhaps almost as much an art as it was a science.  Usually the die designer learned his trade from his own experience.  This was an empirical process.  What worked for him before was likely to work again.  He didn’t really understand or use the laws of physics that govern the flow of metal through the die.  His success depended on his experience, his talent, and on the close cooperation of the extruder’s die corrector.

Those days have gone forever.

Extrusion is basically a simple process.  If the die is properly designed, and impeccable alignment, thermal as well as physical, maintained throughout the production process, good product should result from every billet.  This doesn’t always happen.  But it no longer depends on the personal skills of the die designer, the die corrector and the press operator.  Over the years, light metal extrusion technology has improved to an extent that now, with computer-assisted die design and die cutting, the extruder should be able to get good product from the first billet every time.  If he doesn’t, and his alignment is correct, he may be well advised to consider another die maker.

I have been asked a few times this week about container to die alignment.

Container to die alignment should be controlled to be within +/-2.5mm.  This can best be checked and monitored by not shearing the butt from the die on the last billet of any order.  After allowing the die and butt to cool to room temperature the distance from the edge of the die to the edge of the butt can be measured in each of the 12, 3, 6 and 9 o’clock positions.  Knowing the top of the die when it is located in the press, misalignment and direction of misalignment can be easily reported to maintenance department for correction if out of tolerance.  This should be done on both die slide pockets, and at a frequency depending on the extent of the problem.  Initially it can be done daily for each die slide pocket. Establish SPC control charts for each die slide pocket.
This is an important practice to overcome poor performance from dies, and avoid correcting dies that don’t need correction.

For any extruder, the three basic rules of productivity are:

First, fill the container as much as possible.

Second, empty the container as fast as possible.

Third, repeat rules one and two as often as possible.

In other words, productivity depends on container utilization,

ram-speed, and contact time.

If the Extrusion Ratio is in the ideal range of 40 – 60, it should be possible to utilize most of the container length most of the time.

Cut length at the saw will restrict using full length billets most times, but you should be able to operate within 75% at all times.

An opportunity exists for most presses to increase the container length, usually by about 3 inches (75 – 100 mm).  There’s usually enough available daylight to do this.

And remember that it’s always possible to improve your productivity

We continue to be asked questions about shear blades. The good news is that extruders are starting to understand the importance of a good die shear. We have categorized shears blades into 3 categories based on butt thickness.

A sharp knife-edge blade works best for soft alloys such as 1XXX and 3XXX
alloys.  The tendency is to run to very short butts (because of minimal billet shell zone there is no need to leave a longer butt) and the butt tends to naturally curl with a simple knife-edge blade.  There is therefore no need for a radius to encourage curling – in fact because of the stickiness of these alloys it is arguably best to avoid prolonged contact with the blade.

Hard alloys like 2XXX and 7XXX need to run to longer butts (typically 15% of the upset billet length).  Because of this the butt will never curl and it is necessary to simply shear the butt from the die entry.  A sharp blade doesn’t promote a straightforward shear, and therefore typical blade designs for these hard alloys are more of a blocky shear design.  But do not encompass all 7XXX into this category.

The leaner 7XXX alloys such as 7003, 7005, 7020 fall into the same group as the harder 6XXX alloys such as 6061, 
are run to shorter butt lengths typical of 6061 and therefore can use the scoop type design.  Indirect extrusion of hard 2XXX and 7XXX alloys should be employing much shorter butt lengths than for direct extrusion, and in this instance it makes good sense to use the typical scoop type blade.

So in essence, the most effective blade design tends to more depend upon the butt length used rather than the alloy itself (of course, the alloy dictates the butt length).  It’s worth noting that some plants employ rather long butt lengths to some critical 6061 alloy products (e.g. automotive), and do tend to use the blockier blade design to ensure a good shear.  However having said the alloy itself is not instrumental in driving the optimum blade design, in the case of really soft 1XXX and 3XXX alloys, alloy stickiness does come into reckoning and it’s best to keep away from the curved scoop type blades.