It’s said that you can’t please all the people all the time but when it comes to using extruded section we’d beg to differ. Regular readers will know we’re quite taken with how material in this form can benefit both your fabrication designs and your bottom line. Inevitably though, pleasing both your customers and your stockholders requires some background knowledge. Here we’ll share some thoughts on extrusion and die design.
Benefiting Design and Cost
Just as an “I” beam resists bending, so too do extruded sections. That lets designers add stiffness while taking away weight, benefitting metal fabrication applications as varied as trailer construction and architectural canopies. (If it’s not obvious, the weight savings come in two ways: by putting metal only where it’s needed, and by allowing a switch from steel to aluminum.)
Extruded section benefits aesthetics as well by help creating an attractive finished appearance. For instance, texture can mask surface imperfections or add visual interest while ribs can conceal joins.
Used appropriately, extruded section also lowers the final cost of a metal fabrication. There’s often less machining needed and pieces assemble quickly and without welding. Most aluminum grades don’t need the same corrosion protection as steel, and compared with die casting molds, extrusion dies are relatively inexpensive.
Custom or Off-the-Shelf
Resist the temptation to design the ideal aluminum extrusion as it’s likely something similar already exists in a catalog. That saves you the cost of die manufacture and the lead time in getting tooling made.
It’s not just about the geometry though. Not every grade of every material is available in every shape. Consider aluminum: the 6000 series grades, especially 6063 and 6061, are widely available as they extrude readily. The 7000 grades? Not so much. So start by looking for stock material, but be prepared to pay for a custom die.
Custom Die Design
While there’s no space here to cover every aspect of die design we can address four of the most important points. These are:
- Circumscribing circle diameter (the smallest circle that your section design fits inside.) This drives the size of the die and hence the extrusion machine needed, so keep it as small as possible.
- Symmetry. Keeping your section as symmetrical as possible helps minimize the circumscribing circle, but it also ensures the metal flows out at about the same speed all around the die. That keeps the section straight and reduces the chance of imperfections forming.
- Hollow (completely enclosed) sections. These add complexity and therefore cost. If a hollow section is needed (perhaps to reduce weight or material consumption,) use only one and consider what’s called “semi-hollow.” That’s when there’s a small opening into the section along one side.
- Wall thickness. When the width of the openings in the die vary – say one leg is 3mm wide and another 6mm –the metal flows unevenly, affecting straightness and possibly creating flaws. A good rule-of-thumb is to make the thinnest section more than half the thickness of the thickest. (Also note that the minimum wall thickness is proportional to the circle diameter.)
High-volume extruders and die designers use computer modeling to optimize die design. Lacking these tools, there are a couple of quick calculations you can do to compare designs.
- “Factor” calculation. This is the ratio of the circumscribing circle diameter to the perimeter of the section. The bigger this is, (the more perimeter within a given circle size,) the harder the shape will be to extrude.
- Extrusion ratio. Similar to the “Factor” this is a comparison of the area of the section to the area of the die. Bigger is harder to extrude.
Keep Everyone Happy
Here are three points to take away. First, extruded section is a great way to improve your fabrication design while lowering cost. Second, if you can use a standard, catalog section rather than designing something new, do so. And third, design dies for optimal metal flow.