Brackets and load-bearing parts

What characterises the part

A bracket has a load path. Force enters at one point, travels through the section and out into the fixing points. Everything else about the part is secondary. The most common design error we see is a bracket drawn as a folded plate and then converted to a casting by adding draft angles. The result is a part heavier than it needs to be, with a load path that runs through a sharp internal transition.

A cast bracket should be drawn as a casting from the start: thin, even wall in the shell, ribs where the bending moment is, generous radii in every internal corner, and material concentrated where the stress actually is.

Why casting is right here

• The load path can run in three dimensions. A bent plate has two. A cast bracket can carry force around a corner, down into a foot and out into four bolt holes without a single weld.
• No heat-affected zone. A welded bracket has a zone beside the weld where the material is weakened. A cast bracket does not.
• Hole positions come from the tool. Part number 40,000 has its holes in the same place as part number 1. That repeatability is what keeps the customer assembly line running.
• Ribs are almost free. In sheet metal every stiffener costs an operation. In casting it costs nothing beyond a little metal and a little cycle time.

When casting is not right here

• The bracket is in practice a folded plate and you need 500 a year. Laser cutting and press braking is cheaper, both in tooling and per part.
• You need a few hundred pieces and the part will never be made again. Machine it from billet.
• The requirement is documented fatigue strength in a safety-critical structure with certified material data. Forged, or machined from extruded profile, is often more appropriate than a casting.
• The part must be stainless steel. We cast aluminium and zinc.

Method choice: pressure or gravity

This is the most important choice for a load-bearing part, and it is decided by whether the part is to be heat treated.

High pressure die castings normally cannot be heat treated. Metal is injected at high pressure and some air travels with it. During a solution treatment near 500 degrees the entrapped gas expands and raises blisters under the surface. If you need tensile strength through a T6 temper, the part has to be gravity die cast.

Gravity die casting gives a slower cycle, thicker walls and a rougher surface — but it opens up heat treatment. We run both and choose according to what the part has to withstand.