Q: We’ve tried several different ways to eliminate metal-penetration defects. What else can we do to improve casting quality and process profitability?
A: Fusion (or “burn-in”) defect and penetration (or “burn-on”) defects are quite common. These are found on automotive castings where hot spots are present, like delicate areas of water jackets or galleries, but are even more prevalent on medium and large hand-molded castings. All lead to a significant amount of waste, but they present an opportunity for process-time improvement and cost reductions.
These defects may be environment-related, machine -related, process-related (e.g., compaction and pouring conditions), metal-related, sand-related (e.g., type, grain size, and distribution), and/or people-related.
Critical influencers of these defects are the sand, metal, and the design of the casting. The sand type is important and its grain size and grain-size distribution have a major influence of final quality. Fine sand typically has no penetration or fusion, whereas coarse sand can create very severe defects. Shown below is the critical influence of sand grain size and metalostatic height on penetration.
Sand purity is another issue: Low melting and fine impurities create problems. The type of sand, its morphology, and grain-size distribution influence compaction and the number of voids in the molded sand substrate. A broad distribution usually generates fewer voids than a narrow distribution. Unclassified or even unwashed sands tend to create more penetration defects. Round or sub-angular sands are preferred to angular sands.
Casting conditions matter, too. Lower pouring temperatures improve results. For gray iron, every 5° to 10°C increase in pouring temperature can make penetration and fusion more severe.
The effects of pouring height are emphasized in the graph above. To reduce pouring height and kinetic energy, choose a bottom gating system. The viscosity of the melt is another factor: Higher amounts sulphur or phosphorous in the melt, which reduce viscosity, provoke more penetration defects.
Last, the design of the casting influences defect formation. Domes, edges, or corners with small radii on the sand core or mold create hot spots in the casting and are prone to generate sand fusion and metal penetration.
We want fewer voids in the sand, which are more prone to burn-in and burn-on. Contrary to this, we want those voids in order to achieve good gas-permeability. Therefore, we have developed a refractory coating that combines that fine grain size with high wetting force, whereby all the material soaks into the subsurface of the sand substrate to fill up the voids. On the other hand, this coating is applied only at critical areas where burn-in and burn-on are expected or observed. Using this principle it has been proven that these casting defects can be eliminated in the hot spot but still provide sufficient gas-permeability of the entire core or mold.
These “impregnation” coatings are available in water- and solvent-based formulations. The refractories are usually zircon-silicate, for the convenience of general application.
TP COAT™ A66 (solvent-based) or SOLITEC IM 901 (water-based) coatings can be used for poorly compacted sand areas on cores or molds. The solid-rich, highly refractory coatings with strong impregnation soaks deep into the substrate of the molding material. Binder bridges are covered and porous spaces in the first three to five grain layers are filled. Penetration is reduced/eliminated and a notable improvement in the casting surface can be achieved. Cleaning and grinding work per cast part can be reduced significantly through pre-treatment of the affected areas.
Core and mold coatings make up only around 1% of the overall cost of a cast part. Selecting or using the wrong coating can increase the cost of cleaning or losses due to rejects, which can amount to 10-15% of casting costs.
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