Hybrid Casting Process Produces Thin-Wall Ferrous Components.

By dipping cured molds and cores in a solution and firing them for pouring, this process produces steel and iron castings without gas and metallurgical defects.

For foundries across the globe, the demands of automakers often shape the direction of future casting production. With more than 4 million tons of U.S. automotive castings already shipped in 1999, regulations that affect the manufacturing of cars in turn affect casting requirements and the process used to produce them.

For example, the proposed Corporate Average Fuel Economy (CAFE) standards dictate the removal of weight from automobiles to increase fuel economy. In addition, future EPA regulations will mandate further engine emissions reductions in automobiles, which will force automakers to raise the exhaust temperature of the engine to achieve the necessary catalytic reaction temperature in the emission system. The weight reduction and temperature increase could place gray and ductile iron engine components (such as exhaust manifolds) in a precarious situation in which their weight, durability, corrosion resistance and ability to survive sustained exposure to more demanding parameters are in question.

The foundry industry's response to these demands has been to search out alternative higher-alloyed materials, such as stainless steel, to be cast as thin-wall and lightweight components. One critical element to this response is the foundries' integration of the molding and casting technology needed to produce these castings without incurring great expense or a loss of production. One such technology development has been the Convert Process from Cadic Corp., Kawasaki, Japan, which has focused on the production of exhaust manifolds in stainless steel with section sizes as thin as 1.9 mm. The process now is being licensed in North America.

A Hybrid Casting Process

For a foundry to produce thin-section castings that meet automaker specifications with conventional molding and coremaking methods, a number of obstacles must be overcome. There are metallurgical-and process-related restrictions such as dimensional accuracy, pouring practice, gate and riser placement, the generation of off-gas during the pour, and the chilling characteristics of the mold sand that must be factored into any production method. As a result, section size reduction becomes increasingly difficult due to the process limitations of any single type of conventional molding.

The answer for Cadic was the combination of production methods from various molding techniques. By combining the productivity of high-production coldbox, nobake and shell molding with the technique of firing the mold before pouring (as in investment casting), the new process eliminates off-gas production from the sand molds and cores and the associated gas-related defects. In addition, since the molds are preheated as part of the production practice, metal can be gravity-poured without a loss in temperature to the molding media. This results in superior metallurgical properties for the solidified castings. This combination of existing foundry practices results in thin-wall castings that meet high metallurgical quality standards.

In production, molds and cores are produced using the coldbox, nobake or shell process (Fig. 1). The sand mold and core components then are immersed in a proprietary inorganic binder solution. These treated molds are then fired at 1472-1832F (800-l000C) for 1 hr, during which the original mold and core binder burns off and is replaced by the new inorganic binder, which penetrates the gaps within the mold and core solid particles by a capillary action. After firing, the new binder becomes crystallized. The "conversion" from an organic binder (coldbox, nobake or shell) mold or core to an inorganic media eliminates the common metal reactions associated with carbon, oxygen and nitrogen.

The firing temperature, time and strength are precisely controlled in the process. The molds must be heated as quickly as possible to replace the old binder with the new one. In addition, the firing temperature can impact the amount of mold deformation if not properly controlled. At lower firing temperatures, mold deformation may occur in the form of bending, while the molds and cores may break at excessively high temperatures.

Following conversion to the new binder, the molds are poured immediately at elevated temperatures. The molten metal poured at 2912-3092F (1600-1700C) further cures the mold and strengthens it prior to solidification. This feature allows extremely thin-wall castings to be produced since the metal being poured does not lose its temperature at the same rate as castings produced under conventional room temperature molding and pouring practices. In addition, the preheated mold results in: excellent fluidity; no gas generation from the core; fewer feeders; the prevention of hot tears in steel casting; and the prevention of chilling in thin-wall iron casting.

After pouring, the mold undergoes standard cooling and shakeout procedures. A goal in the development of this process was to utilize existing molding and coremaking technology for high-production casting. For optimum flexibility, the process can be adapted for robotics and computer-controlled synchronous manufacturing.

Cores for Investment, Die and Permanent Mold Casting

The process also can be used to produce complex cores for investment, die and permanent mold casting. As the complexity of cast design increases, so do the binder requirements. As a result, binders are used in more than 90% of all cores produced (due to their collapsibility and ease of recycling). Unfortunately, the gases generated from the binder have limited the use of bonded sand.

One solution is the use of the new binder that will turn the organic binder core to an inorganic one. In production, the core is immersed in the new binder. For investment casting, after being dried, the core is not fired, but placed in the wax injection mold. After injection, the wax patterns with cores are assembled into investment trees. The core is then fired (and the binder is turned inorganic) with the rest of the ceramic shell during preheating before pouring. For die and permanent mold casting, the impregnated cores are fired after being treated with the solution to convert the binder to inorganic before the core is placed in the mold.

At this point, the process is focused on thin wall ferrous automotive castings, but opportunities exist in all metals. The processes benefits of a preheated mold without a loss of metal temperature during pour, the elimination of gas and back pressure during pouring, the improved yield from the gates and risers, and the elimination of emissions during pouring could open doors to both ferrous and nonferrous thin wall casting opportunities in various markets.