Casting defects are unavoidable due to the complexities of various molds (and the casted parts), as well as the numerous variables involved in producing sound and good castings. If appearance is important to the die caster, the occurrence of stains or discoloration on the casted part can pose problems, if not controlled to manageable levels.
Stains are manifested on the castings usually when the active ingredients in the die lube (release agents) do not disappear completely, or disintegrate, from the casting surface. In ideal scenarios, the “best minimal” amount of die lubricant is sprayed into the die, and upon casting release, the residual heat on the casting should be sufficient to “evaporate-off” any chemical-causing stains.
In real world scenarios however, die-lube over-applications, (too) strong-dilutions of the release agent, the complex die cavity architectures, low die temperatures, and the non-uniform temperatures within the die cavity surfaces result in some residual unevaporated die lube on the certain portions of the castings, leading to stains.
To address and control staining, one may refer to the table below as a guide,
At times, certain active lubricating agents in the die lube may be an aggravating factor to in stain occurrence. To determine which active ingredient may be causing the stain, refer to the table below,
This guide is by no means extensive, but the author hopes to have provided the die-casting practitioner a starting-point on how to troubleshoot stains on casted parts.
See Release Agents for High Pressure Die Casting Part 5 (Housekeeping and Maintenance) here.