Casting answers & advice

Q In our aluminum permanent mold shop, we have noticed tool steel molds, which we cut from H-13 billet and do not subsequently heat treat, require break-in time (similar to "seasoning" a cast iron skillet). We have had trouble holding x-ray requirements for approximately 300-500 shots, then the tool seems to "line itself out" and failures are reduced. Why would this occur?

Various methods are used to help improve coating adhesion on new molds. A typical process would be to heat up the tool steel to burn off contaminants, blast the cavity to roughen the surface, and then reheat the mold to 400-500F (204-260C) for mold coating.Mold CoatingThe best case scenario would be to cut the mold cavities to about 90% of the desired shape, stress relieve, and then complete the final machining. An H13 mold that is not heat treated after machining would not be in the optimum condition to maximize tooling life and cracking resistance.Some metalcasting facilities austentize their molds, vacuum N-atmosphere quench in a 5-10 bar furnace, then draw back to the desired hardness, which should be relatively soft for permanent molds (double to triple draw to 38-42 Rc).One concern would be potential dimensional movement or growth of the mold during production. Typically, any permanent mold die made from tool steel already has gone through heat treating and drawing before it is received by the metalcasting facility. The mold also should have been through a stress relief process, which would eliminate any potential dimensional concerns. Finally, the mold should be preheated before use in order to apply the mold coating, so additional break-in time should not be necessary.When cutting tooling in-house from H-13 billet, the material generally is annealed and stress relieved prior to delivery to the facility. All steel molds should be stress relieved before entering production to eliminate growth and movement and the potential for dimensional variance in the castings. A good rule of thumb is to bake at 1,000-1,200F (540-650C) for one hour, plus one hour for each inch of thickness at the mold's thickest point. The typical permanent mold metalcasting procedures call for the mold to be pre-baked for eight to 12 hours at 500600F (260-315C) before starting the coating process in order to remove cutting oil residue and assure proper adhesion. This pre-heating may pro vide some stress relief in mold, but for proper stress relieving, the minimum temperature should be at least 100F (38C) above the anticipated operating temperature of the mold.Your metalcasting facility's issue with difficulties during the break-in period for the new mold may be more closely related to mold coating than mold material. While a new steel mold generally would be subjected to an extended pre-heating process prior to production, the intent of this process is to prepare the mold for mold coating, rather than to stress relieve the mold. Pre-heating the mold will remove residual cutting fluids from the machining process that would prevent mold coating adhesion. But even after the mold is properly cleaned of cutting fluid residue, the texture of a newly-machined mold may not be optimal for coating adhesion. Mold coatings provide solidification control for the cast component, so if the mold coating breaks off, it could change the solidification profile and potentially affect casting quality.Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience. If you need assistance with a technical issue, fax or email your question to: Casting Answers & Advice c/o MODERNCASTING, at B47/824-7848 or moderncasting@afsinc.org.One way some metalcasting facilities manage this is to blast every new mold with an abrasive to provide a texture that will help the coating stick. Even after cleaning and blasting, it is still common for the coating on a new mold not to last as long as normal. Coating life often will improve the second time it is blasted and re-coated.

Recommendations are the opinion of the AFS Technical Dept. based on referenced literature and experience. If you need assistance with a technical issue, fax or email your question to: Casting Answers & Advice c/o MODERNCASTING, at B47/824-7848 or moderncasting@afsinc.org.