Hungry for Defect-free Castings? Warm Up to the Best Practices of Mold Coatings!
Foundries caught in a rut of repeated casting defects and scrap may be well served with a mental retreat to the kitchen for Breakfast 101––not for comfort food, but for answers.
Permanent mold casting (aka processes that reuse steel molds), ranging from gravity or static mold to tilt pouring to low-pressure casting, and irrespective of alloy poured, is like cooking your eggs in a fry pan. Just as butter, cooking spray, or bacon grease go a long way toward getting those eggs to slide out onto your plate, so it is with mold coatings, which facilitate the smooth release of a casting from the mold.
But the truth is, coatings today do a great deal more than lube the surface between the mold and molten metal, thanks to a range of properties that solve serious problems on the shop floor. Besides the function of release, coatings offer texture, insulation, and durability, although frequently the foundry must choose the primary characteristic they need and potentially forfeit others, unless their coating supplier can custom blend a coating to provide multiple attributes.
Four common casting defects that occur in permanent mold processes can be resolved through the application of mold coatings, explained The Hill and Griffith Vice President and COO Mike Lawry, who presented on this topic at the AFS 2022 Aluminum Casting Conference in June. First are misruns, which cause creases in the metal. This can be solved with a rougher coating and one with more insulation. Second, coatings can eliminate pinhole porosity, where hydrogen gas collects in the metal and leaves small indentations on the casting. Third, when cold shut occurs, gaps are formed on the casting—a rougher coating or one with additional insulation properties resolves this issue, which can be a major problem especially on thin-walled castings. Last, when internal shrink is present, metal has pulled back to create voids inside the casting. For this defect, Lawry recommends using a coating with less insulation, applying a thinner coat, and trimming away some of the coating where it’s too thick but leaving enough to protect the mold.
Protecting the mold and preventing defects, mold coatings have the added benefit of providing castings with the desired surface finish and cooling properties. Coatings are comprised of water; binder (sodium or potassium silicate); refractories including vermiculite, alumina, zircon, iron oxide, calcium carbonite, titanium dioxide, barium sulfate, mica, talc, clays, and graphite; and suspension aids both organic and inorganic. To achieve a smooth finish, foundries use graphite, titanium dioxide, and mica coatings, for example, as top coats or as a stand-alone in low-draft, high-drag areas. For a textured surface, metalcasters use top coat or stand-alone coatings made from alumina, abrasives, and zircon.
Priorities and preferences for coating characteristics vary widely from foundry to foundry based on what parts they’re producing. In today’s trend of increasing casting sizes while reducing casting weight, in the auto industry, for instance, a shift from iron to aluminum and even from aluminum to magnesium is occurring, and with this trend, permanent mold shops are multiplying, according to Lawry. For higher weight parts, the properties most sought in mold coatings are insulation and texture.
“If you’ve got a very thick casting that you know is going to take a while to solidify, maybe you won’t need as much insulation on that,” he said. “But when you get into a thin-walled, large casting, it wants to cool quicker and you need a larger amount of insulation.
“One of our clients made a casting for a Department of Defense five-ft.-long missile,” Lawry added. “They poured it only in the middle and that metal had to run all the way out to the ends of the thinner casting––everything was solidifying before the metal could reach the end. So that’s where the insulation comes into play to such a large degree.”
As demand drives more permanent mold operations at a time when the population of skilled foundry operators is on the decline, teaching and following correct procedures in the preparation and application of coatings has never been more essential.
Before a single drop of molten metal is poured, important steps must be taken with permanent molds and their coatings to ensure optimal outcomes.
Coatings themselves require regular examination and testing. For example, personnel must check coating viscosity (thickness), testing for percentage of solids mixed throughout the solution––solids in the coating are suspended in water and tend to quickly sink to the bottom. A Baume gauge is commonly employed to measure for percent of solids, but Lawry suggested the use of a Zahn cup, carried over from the paint industry, achieves rapid, repeatable results. Viscosity is measured by how long it takes the product to flow through an opening at the bottom of the cup, he explained. Solids testing should be performed at least once a shift and any time a new pail of coating is mixed.
And mixing cannot be overlooked. Because the solids in coatings tend to fall out, continuous agitation keeps the mold coating well blended. Foundries are advised to keep no more than two weeks’ worth of coating solution in their mixing tank, and any unused coating from an application sprayer should be returned to the mixer. Keeping more than that will result in problems and wasted coating.
“Quite often, if you go to a shop and look in their mixer, half of it will have dried material on the sides because they put in a month’s or two months’ worth in there, and it just built up and solidified,” Lawry said. “What you want is a mixer with a spigot on the bottom so the operators can easily go over and fill their spray canister—it really helps if those canisters have ball bearings or even a couple of metal nuts inside to keep the product mixed when they shake it occasionally. If you don’t do that, you’re just going to be spraying out straight water. Think of the ball bearing in a paint can––you must shake it prior to use.
“And again, at the end of the shift, pouring that unused product back into the mixing vessel is very beneficial.”
Before mixing a coating product, workers have to dilute it, and it can be a balancing act to achieve the delicate dilution ratio of the binding agent in the coating and the water. It’s easy to over-dilute to the point where the coating loses its adhesion properties, Lawry said, which is why he has two pieces of advice:
- If you’re doing this in-house, he strongly favors thinner (more water) over thicker.
- But the better method is to buy pre-mixed, ready-to-use coatings and take the guess work out of it. “Yes, it’s definitely going to cause you to do more passes as it’s being applied, but this is what we consider a best practice,” said Lawry.
Inconsistencies occur between different operators who apply mold coatings, which can take a serious toll on casting outcomes.
“Maybe it’s that third shift guy––he likes it his way compared to the first and second shift guys,” said Lawry. “None of them mix it the same, and they all end up with different results. The quality manager doesn’t understand why we have a 97% success rate on third shift and only a 50% on the first shift. So often it is the operators modifying the coatings themselves or how it’s applied.
“When you get the coating premixed, ready to use, yes, you’re buying water, but you have eliminated the problem of operators changing the product. And eliminating that variable is such a great quality measure.”
Mold coatings will do their job best when foundry operators do theirs, and the first prep task to get right is making sure the mold/die is thoroughly clean. Failure to do so can cause a slight ridge in the mold that may change the dimensional aspects of the finished casting, so be certain to remove all the old coating, as well as any soldered aluminum and any lubricant or dirt that may have contaminated the die surface.
The most important variable to control is temperature of the mold, which should be heated to 450F, plus or minus 50 degrees, according to Lawry. He recommends this preheating by oven or furnace rather than by Bunsen burner or a torch, which can leave soot on the die’s surface. As the mold cools, spray it with a water mist to oxidize the surface, which will help the coating to stick better, he said.
For best results in applying the coating to the mold, use an airless sprayer like one used for painting a car, and set the gun pressure to 36–48 psi. Spray evenly, back and forth for even coverage—Lawry said multiple thinner coats will last longer than a heavy, thick coat. Tip: Start moving the spray gun before pulling the trigger and continue moving the gun till you let go of the trigger.
Coatings contain tiny mineral flakes, so again, as previously discussed, operators should put ball bearings in the bottom of the spray container and shake it frequenting while coating the mold. Operators should clean out the spray gun at the end of every shift.
To ensure correct thickness of applied mold coatings, foundries take periodic thickness measurements at multiple areas of the mold using either an inexpensive posipen tool or a more sophisticated ultrasonic thickness gauge.
The latter, available on Amazon for about $150, provides a ballpark micron reading based on magnetic pull. The more sensitive ultrasonic unit, priced in the $2,500 range, will deliver a more accurate readout. At the end of the day, said Lawry, testing is better than not testing, regardless of preferred instrument.
Hill and Griffith provides mold coating training to foundries, and Lawry underscores that metalcasting tribal knowledge can disappear overnight when experienced operators retire, so the best practices in mold coatings must be taught and re-taught to newer members of the foundry shop floor.
“When you lose those experienced individuals, it’s always a setback,” said Lawry, “and the schools don’t always teach these things. We don’t want foundries to have to reinvent the wheel or figure this all out on their own. There are some simple best practices, but not everyone understands them—we want the AFS community to be aware of what to look out for.”