Coatings in aluminum permanent mold casting.

The die coatings used in permanent mold casting of aluminum establish a barrier between the molten aluminum and the mold, influence the solidification rate of the metal, promote venting and filling, aid casting ejection and provide the desired casting surface finish. Because mold coatings play such a critical role in aluminum permanent mold casting, the coating must be properly applied and maintained, or casting quality will be adversely affected even when other casting parameters are in order.

Mixing and application of mold coatings contribute significantly to coating effectiveness and longevity. Coatings should be stored at 55-100F, mixed to proper dilution (depending on the coating and manufacturer) and tested for correct viscosity using a Baume density test or other method. It is important to maintain suspension of the diluted coating by agitation when it is not in use. This should be done in the spray gun prior to application, as well as in the primary mixture container. If not correctly diluted or mixed, the coating may be too thin or thick and will not be applied properly or achieve the correct properties. Coatings should be sprayed onto a warm mold (typically 300-400F), and consistent mold temperature throughout will provide more uniform drying and reliable durability.

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The coating also must be sprayed onto the mold from the correct distance. If the coating is sprayed too close to the mold, it will go on too wet and dry slowly, leading to poor adhesion and flaking. If the coating is sprayed too far from the mold, the mold heat can cause it to dry before it contacts the mold, and the coating will be applied as a powder without adequate durability.

Permanent mold coatings should be applied with quality equipment and sprayed in overlapping passes across the mold surface to assure complete coverage and consistent coating thickness over all mold features. Permanent molds often are coated with several layers, each serving a different purpose, such as a base coat, insulation coat and top coat specifically manufactured for casting release or surface finish.

Though instruments can measure coating thickness on a metal surface, few hold up well in the heat of a permanent mold, so it is difficult to record an accurate measurement during application.

Reap the Benefits

Permanent mold coatings help improve the fluidity and filling of molds during pouring, as the sharp edges of the minerals in the coating help break the surface tension of the advancing metal front (Fig. 1). Additionally, the valleys between the minerals provide some form of venting, removing air from the mold cavity. This can be particularly helpful if the mold has adequate vents for air to be evacuated from the mold cavity. Vents can be placed along the expected mold filling sequence to allow air to escape ahead of the molten metal front. As the coating ages through multiple fills, these benefits lessen as the peaks and valleys become less pronounced due to wear, and the castings may become prone to filling problems.

One of the key features of permanent mold coatings is the ability to help control the solidification of the metal in the mold by providing a refractory barrier that reduces the heat transfer to the mold material. Coatings have varying amounts of insulating filler materials and can be designed specifically for maximum insulation. Coating application is a critical factor in determining the insulation value of the coating, because a porous coating provides air pockets that can reduce heat transfer and increase insulation. Coating porosity results primarily from coating drying rate, so correct dilution, viscosity and application are critical.

Thicker coatings often are used on gating and riser systems and brushed on rather than sprayed. Since the air pockets in a porous coating can be as important for insulation as the refractory minerals, some of the insulating benefits of the thicker coating may be lost if the porosity is significantly reduced. In one study, a 45-micron thick coating, applied at a high drying rate, displayed 69% porosity. The same coating at 95-micron thickness with a slower cooling rate produced 62% porosity.

Another facet of the heat transfer is the formation of an air gap during solidification (Fig. 2). As the metal begins to solidify, the casting pulls away from the mold wall slightly, creating an air gap. Current research is focusing on injecting helium into this gap. With a higher heat transfer rate than air, helium could speed solidification, leading to shorter cycle times and improved productivity.