Shot Lube Stain

Shot lube stains are the discolored surface stains on die-casting parts resulting from the mold release solutions or lubricating oils. The spot mostly appears dark or grey with or without remaining visible unaidedly. Plunger lubricants or die oils are heavily used in the process of withdrawal.

This defect shouldn’t be treated or considered similar to the blackening problems in casting alloys. Shot lube stain is more like a constituent or subset of blackening. Though many different reasons contribute to blackening, lubricants trigger this particular issue.

This article briefly explains the sources, effects, and measures to prevent shot lube stains in castings during production.

Sources of Shot Lube Stains in Casting

As mentioned, lubricants or oils are the primary reason for stained castings. But these essential ingredients can trigger the issue in either of the following ways –

• Excessive Amount –Intentionally or accidentally applying too much lubricant is highly unlikely in modern casting production. However, damaged mold sections may require additional oils to fill the gaps, leading to stains.
• Extreme Concentration –Some particular oils are heavily concentrated, often requiring the addition of water or supportive oils. Direct application of such highly concentrated lubricants can easily trigger stains on surfaces.
• Faulty Processing –Compromised, defective, or incorrect setup adjustment can inject more or less lubricants into the molded alloys/metals. Slim amounts barely cause stains, but excessive amounts can initiate the defect.
• Improper Lubricant –Different alloys need distinctively exclusive lubricating oils for the casting. Changed compositions also call in adjustments in applicable amounts. Any parameter less than perfect will end up staining the surface.
• Defective Lubricant –Even the lubricant itself can host impurities, alteration, and defects. Using such oils can barely keep up with the desired lubrication. Instead, more defects start hitting the die casting along with black spots.

Die-casting stains primarily affect the overall aesthetics. Even if the defect takes close observation or seems negligible, favorable conditions may worsen the issue. And parts intended to serve architectural purpose becomes worthless over time.

Preventing Lube Stains in Die Casting

The most appropriate approach to skip shot lube stains consists of the following actions –

1. Proper Lubricant Amount –Applying the chosen lubricant accordingly is the best countermeasure to avoid stained castings. It’s necessary to get rid of any irregularities or damages in the mold/die to limit potentially excessive demand.
2. Type of Lubricant Required –Choosing the most suitable lubricant can help the molten metal/alloy to shape properly. Any liquid alteration, change, or difference will require a review of the lubrication type, amount, and even concentration.
3. Perfect Die Casting Setup –Stains are the least troublesome issues from improper setup. Still, paying attention to the design, parameters, and effectiveness of the involved machinery should enable accurate injection of lubricants.
4. No Contamination in Die –Any impurity or contaminants in the liquid stream can somewhat affect the lubricant’s efficiency. Avoiding all sorts of contamination is always desirable in the casting process, regardless of the alloys/metals.
5. Lubricant Validity/Purity –Untenable, old, or uncertain lubricants should be avoided for top-quality die casts. Even the permissible or certain ones that are considerably older in practice can come up with issues due to modern machining.

Die-casting lubricants hold a significant responsibility in the manufacturing process. And its imperfect, erroneous, and inadmissible use leads to visible-invisible stains with potential chances of future growth.