Design for Casting Manufacture Part 3


The third and last article on design for casting manufacture.  This section covers concerns such as porosity, casting leakage and design for assembly.

Porosity and Leakage Concerns
A large percentage of die cast components must be leak tight.  Die casting parts are often used as housings for power train components, meters, pumps, etc.  In all of these cases, the casting must prevent liquids or air from passing from one chamber to the outside of the part, or other chambers.

In many cases, die castings are machined before usage or assembly.  When a die casting is machined, the opportunity for opening up internal porosity exists.  This porosity may cause a casting to fail due to leakage in several manners.  It may create a path from one side of the casting to another.  It also may exist on a sealing, gasket or o-ring surface, which may cause the seal component to not function as intended.

Porosity, both from gas entrapment and shrinkage, is most prevalent in thick sections of a casting.  Minimizing the amount of material removed during a machining operation is an effective method for reducing found porosity.  Obviously, machining stock must be kept at a level such that positional variation does not cause a non clean up, creating a balancing act.  Beyond that, the die casting company must be aware of critical surfaces and take steps in the casting process to minimize porosity in those areas.  An agreed upon porosity standard on the finished product is often a helpful tool.

Beyond porosity concerns, there are other factors that can cause a casting to leak.  Gate leakers are a frequent cause of failure in castings.  Proper gate placement can minimize this risk.  Wall thickness must be sufficient to be filled while thick sections should be minimized to prevent solder and solder tearing.

Design for Assembly

Very few die casting parts are used as is by the end consumer.  For the most part, die cast components are machined, painted, etc. then assembled with other components to form a functional device.  It is critical that all of the assembly requirements of a casting are taken into account during the design phase of a part.

Today’s engineering software allows designers to ensure that all components fit properly.  The primary concern in this task is ensuring that enough clearance exists through the mating components to ensure that dimensional variation cannot create interference.  As an example, two components that are designed to be 1mm away from each other will intermittently interfere if their total stack up tolerance is greater than 1mm.

Also of concern is proper placement of parting lines, gates, etc.  As discussed before, these features will leave excess material on the casting.  If this excess material causes problems for the assembly, then added cost will be incurred in removing the material.

Summary

The vast majority of casting problems are designed into the part before a casting is even made.  Taking into account potential casting problems during the design phase will drastically reduce costs and customer problems for the product line.  Buyers of die castings should take this into account early in new program development.