During the successful AEC webinar, "It’s About the SHAPE: Designing with Aluminum Extrusions," hosted by Design News, there were a number of great questions that we didn’t have time to answer. Some of the questions and answers should prove interesting and useful to a wider audience, so I’ve decided that blogging might be the correct venue to spread the knowledge! I've chosen one particular question here that requires a longer answer than you might expect.
Question: We have found dramatic differences in extruders' willingness to commit to tolerances. Twist is a big deal for us. Are the differences a function of technique, skill level, process, or just the optimism of the extruder?
Answer: Yes, and a bit more. Extruders have varying levels of skill and experience. Sometimes doing the same thing 1,000 times doesn’t yield the same experience as doing 10 different things 100 times each. Even within some large companies, various manufacturing facilities may have a different willingness to take on challenging extrusions depending on their experience, their booking level, etc. In addition, different extrusion lines utilize equipment of varying vintages and capabilities.
The ability to hold tight tolerances (including twist) along the long extruded length is affected by many variables, and there are many variables which need to be controlled to make exceptional extrusions. Luckily, there are many AEC member companies with great equipment, personnel, and experience (see examples at AECGuide.org). Several critical variables come to mind as illustrative examples:
In an ideal world, the centerline of the press frame, the press container, the press stem and dummy block, the tool carrier, and the actual extrusion tooling share a common centerline. In reality, brass press ways or die slide ways wear or are misaligned, dies may be loaded into tool carriers with some debris in them, etc. Excellent extruders use SPC techniques to monitor and drive action on alignment issues, and they keep their tooling and tool carriers clean.
When a die is misaligned to the other press components, the “centerline” of the billet and pressure is off center from the die. Picture squeezing toothpaste out of a tube -- if the hole you are squeezing the toothpaste through is offset, the emerging “extrudate” will bow in one direction or another (a similar phenomenon occurs with twist).
Thermal control and thermal “alignment” of the equipment:
Same as press alignment, but if the die, press container, or other components aren’t evenly heated (or the difference in heat, top to bottom in the container, for example, isn’t taken into account), flow issues can occur, causing bow, twist, or other anomalies.
This refers to both the overall thickness of the stack (from where the billet enters the tooling to where it exits), as well as the stack’s configuration. The extrusion process occurs under tremendous forces and pressures.
- The 2,750-ton press that we used (a medium-sized press), generated 5,500,000 lbs of force, enough to lift almost several thousand automobiles.
- A typical extrusion press die to make the step extrusions of a stepladder is about two inches thick, producing two steps extrusions from each push (about 120 ft long).
- If 5,500,000 lbs were placed onto the die without support, the die would instantly deflect and break.
- When the press was purchased in 1983, a typical tool stack was 12 inches thick, but we specified the press with a 16-inch stack. The stack we used for solid dies like steps was a die ring containing the die and backer and a bolster.
- The die has an aperture (die bearing) that defines the shape.
- The backer is about five inches thick and has an aperture larger than the step profile to provide clearance (the ability of the extrudate not to rub on the backer).
- The bolster was about nine inches thick with an even larger aperture. It still form fit in the same basic shape as the step.
- By using a thick tool stack with form fit tooling, we minimized die deflections (impossible to eliminate deflection), which gave us very consistent dimensions (and low twist) hole to hole and along the length of the extrusion.
- This, coupled with excellent die design, preparation, great press alignment, and temperature control/alignment, yielded great productivity and quality.
- The “cost” to achieve this was the up-front equipment, the ongoing excellent technical support and maintenance, including alignment, a long tool stack (more money in steel cost for each profile),the purchase of form fit backers and bolsters, and the overall higher cost of longer extrusion tooling. Other extruders might have foregone these costs, even using a combination of sub-bolsters and bolters, but at cost of tolerances and productivity.