The die casting process has many failure modes which must be monitored and prevented. Which failure modes are an issue are dependent on the product’s needs and end use.
Typical problems for die casting products include, but are not limited to:
- Porosity opened up after machining creating leak paths
- Dimensional problems in the casting due to shifts, wear or moves in moving components in the die
- Appearence issues due to fill problems or die condition
- Trimming problems leaving excess material on the casting
- Leaking castings due to some combination of other issues
- Missed or improper secondary operations, such as painting or assembly
Many different techiques exist to detect and prevent each of these failure modes
Porosity can be detected using X-ray techniques. This is often effective in determining total porosity amounts, but determining specific location of small voids may be difficult. Destructive machining is also an effective tool to evaluate porosity, but can be costly and may give inaccurate predictions based on sample sizes.
The best methods for controlling porosity are via control of the die casting process itself to prevent occurrence.
Dimensional problems can be detected with either in line or off line gaging. Frequency of inspection is set up based on severity of potential problems and process stability. Poke yoke methods are usually effective in preventing dimensional problems from being passed on, when their implementation is feasible.
Dimensional problems in castings are usually the result of tooling issues. Corrective actions to reduce or eliminate these problems are sometimes expensive, but are usually solid long term fixes to prevent recurrence.
Appearance issues can be detected visually, which is a cheap, but often unreliable method for detection. Vision systems haven’t been used extensively yet for this type of problem due to its variable nature.
As with most failure modes, the best method for preventing a problem from getting to the customer is to prevent them from occurring in the first place. To that end, process controls in place to maintain a robust process are key. Beyond that, tooling maintenance programs with agreements with customers are the best way to maintain tooling condition.
Trim problems can have multiple causes, depending on the method and equipment used to remove flash from die castings. Tooling related issues can usually be prevented with a proper maintenance routine and a good trim die design.
Detecting trim related issues is often difficult as it is usually a visual inspection. For high severity problems, in line gaging or poke yoke devices can be used.
Castings usually leak for one of the above problems. Either porosity or surface issues in the parts create leak paths from the inside to the outside of a casting. Preventing the existence of these defects is an effort in process control.
Detecting leaking castings is usually done with an air decay leak tester. The advantage of air decay testing is its relatively quick test cycle. For castings that have zero leak requirements, air decay testing may not be appropriate.
Process Failure Modes and Effects Analysis (PFMEA) is an effective tool for determining where a process needs control and inspection steps. Excess or unnecessary quality steps is a waste of resources while an uncontrolled process is a customer problem in waiting. An experienced die caster will balance these needs in setting up the part production process.
Mark Fischer, Engineering and Quality Manager