Roobol on Painting
Cleaning Magnesium Die Castings
What is the best method of cleaning any residual silicone containing (polysiloxane) mold release off magnesium die castings? The die castings must be absolutely clean, as they are later encapsulated with polyurethane, using the reaction injection molding process. Any advice would be appreciated.That will depend on the size and number of castings to be cleaned. You can clean up to a few hundred pieces by hand, but for thousands of parts an automated process is more suitable. Are you concerned about the appearance, or the adhesion between the plastic and metal? Because they will be "potted" inside a urethane shell, the adhesion is likely not going to be a major issue; and in fact most of the alkylated methyl polysiloxane mold release products are designed to be paintable release agents (or in this case have good adhesion to urethane plastic). If your company does the molding, switch immediately to a paintable mold release if the currently used one is not paintable. Otherwise, get whoever molds them to change release agents.
If the urethane is not clear, then appearance factors are moot. But if you absolutely insist on getting the castings scrupulously clean, several options are available to you. The least costly will be aqueous detergent cleaning if this release can be removed this way. If not, contact the mold release supplier to identify a solvent to use for cleaning, either by solvent spraying and letting parts drain (better), or by solvent spray or dip followed by manual wiping with clean towels (not quite as good). For non-fragile parts that are not too large (weighing approximately two to three pounds or less) a vibratory cleaning can remove tenacious material, but may need to be followed by a solvent-cleaning step.
Follow-up from this questioner: We are concerned about adhesion of the polyurethane to the casting. That is the ONLY concern. The mold release supplier says that the silicone used is a "paintable" silicone. However, it does seem to present a barrier to adhesion in any case. Is a mild alkaline cleaner (approximate pH 9 ) a good first step for cleaning these parts? The current process uses such a step followed by tap water rinse, then iron phosphate, then a sealer, (with tap water rinses between the latter two stages).
How are you checking adhesion? Alkaline cleaning plus the phosphating steps could be enough to remove the mold release and permit good adhesion, but if you are already doing that you should know if it works or not. Try using your current clean/phosphate on several parts and on several others sand them with fine grade abrasive paper followed by a thorough cleaning using a rinse and wipe with MEK-soaked cloths. Test each group to see which pretreatment method provides better adhesion.
Airless Spraying Or Manual Application?
I've been put in charge of getting the entire inside of our plant painted although I'm certainly no expert on paints. The three painting contractors I have had in to bid on the job all say that spraying costs less than brushing or rolling paint on. I can understand that because it is easier and faster. But I'm worried about the spray going everywhere and making a mess that will be even more costly to clean up afterwards. What do think I should have done? The painters will do either one but the bids are a lot higher if they cannot use spraying on most areas.You failed to mention if your plant has sensitive machinery might be affected by overspray particles. Unless you have such equipment, I think you are overly worried about spray drift. For high bays, a "dry-fall" paint can be readily vacuumed up. It is true that with spraying the fine paint particles will generate a lot of "dust" that has to be cleaned, but I'd still have them use it. Remember that they will use airless spray, not air spray to apply the paint. The former is far less messy that the latter method. If you've no knowledge about airless spraying, ask one of the contractors to let you visit at a jobsite where they are airless spraying so you may observe it first-hand. You might find it far less untidy than you imagined.
Looking for a reprint of this article?
From high-res PDFs to custom plaques, order your copy today!
