Cast In FOAM

Article From: 3/1/2003 Automotive Design & Production, , Contributing Editor

Lost foam casting at GM has moved from being a Saturn exclusive to a more commonplace technology. It is a simple process, but one that depends on subtle adjustments and experience to produce the best results.

Lost foam heads for GM's Vortec 4200 inline six are made up in slices that are glued together to form a foam version of the completed part. The slices allow for the efficient creation of three-dimensional shapes within the part-like air passages, cam bearing shelves, etc.-while minimizing the material used in the process.

Completed foam head clusters are placed on a drying rack after receiving a tinted refractory coating designed to control the evaporation of the foam core during the casting process. From here they are taken to a drying oven where they spend eight hours at 110 degrees F., a temperature high enough to dry, but not distort, the foam pieces.

The foam cores are placed into large, yellow casting flasks that are filled with sand while the flask is vibrated. This process supports the core, eliminates crevices and voids, and ensures the sand is packed as densely as possible around the part.

A large funnel pierces the foam circle jutting up from the sand in the casting flask just before the 1,450 degrees F. molten aluminum is poured. The flasks are carried on "mold cars" that guide the flask through this highly automated process.

GM’s Saginaw Metal Casting Operations (SMCO) currently casts the iron blocks for the 3800 V6 and Gen 3 V8, and the aluminum heads for the Gen 3 V8 and both the block and head for the Vortec inline six. The latter uses the lost foam process, made famous within GM by Saturn, though SMCO and the on-site Casting Development Validation Center (CDVC) have continued to refine it since adding lost foam development to its repertoire in 1995. It’s a relatively simple process, but one full of subtle tweaks.

The lost foam method begins with Styrochem-supplied polystyrene beads (type T170B), which look like white sand granules in their pre-expanded state, and are placed into one of six Styrologic pre-expanders for wet expansion (five of the units operate 24 hours/day) before entering a seven-step pattern making process. The tooling used to make the pattern is pre-heated, which removes excess water, then filled with beads blown into the cavity of the tool. Steam is passed through the tool chest, which heats the tooling and starts the fusion process that links the beads together. An autoclave is used to build high pressure in the steam chest and fuse the beads together. Next, the chest is depressurized, to start the cooling process, water is piped into the chest cavity, and then chilled water is sprayed onto the mold plate. This cools both the tool and the pattern, and allows the pattern to be air ejected at room temperature.

Lost foam patterns are created from slices of the final part, with the slices glued together to create a complete component. In the case of a cylinder head, the slices are joined together in pairs in a form with a central gating assembly (also made of foam) with a special hot-melt glue. Completed assemblies are placed on a rack with several other pattern pairs, then sent to a coating tank. There, the patterns are dunked into a vat filled with a coating mixture that looks like industrial Pepto Bismol, which insulates the foam patterns. It’s necessary to cover 100% of the surface, and drain out the excess liquid before the patterns are sent to a drying oven where they spend eight hours at 110ºº F; a temperature high enough to dry, but not distort, the forms.

GM’s Saginaw facility has three aluminum furnaces, two for blocks and one for heads, and produce 420,000 lb. of molten aluminum per day. This material is sent to one of five FATA casting lines (3 for blocks, 2 for heads), where robots pour the molten aluminum into the carriers. These contain the polystyrene patterns surrounded by compacted sand, and topped with a pouring flask connected to the gating system. The molten aluminum reduces the styrene to gas and liquid, and fills the mold from the bottom up. Says Al Steffe, director, Manufacturing Engineering, Casting at GM’s Saginaw facility: “The trick is getting the aluminum hot enough to fill the mold and melt the foam throughout the form before it solidifies.” Which explains both the insulation step and the x-ray machines at the Casting Development and Validation Center located next door.

Dave Goettsch, a staff engineer at the CDVC, explains what’s happening as the real-time x-ray unit follows molten aluminum into every crevice during a test pour despite the fact that the piece of equipment he stands before has a 450-kV x-ray source, making it one of the most powerful in the world, and capable of illuminating a 160-lb pour. “There are only two other units like this in the world,” says Goettsch. As the aluminum runs through the gates and bubbles up from the bottom of the casting container, it’s possible to see it move through and around the styrene pattern at the rate of about ½-in. per second. Yet it’s impossible for the untrained eye to see when the pattern itself disappears in a cloud of gas and liquid. “The coating on the styrene provides a lot of insulation to both the pattern and the molten aluminum,” says Goettsch. “It keeps enough heat in the aluminum to allow it to flow into the form without solidifying prematurely, and keeps the pattern from disappearing before the metal is solid enough to retain its shape.” The x-ray machine lets the development group determine the proper insulation thickness, pour speed and heat, and can be used to troubleshoot any problems. In addition, a computed tomography system takes film x-rays and creates two-dimensional slices though a part. “With this,” says Goettsch, “we get superior resolution that allows us to see all the walls of a complex casting.”

However, lost foam casting isn’t the only thing the Saginaw facility does. The CDVC has been working diligently on precision sand casting in order to snag production of GM’s Generation 4 small block V8 due in 2005. This engine is lighter, slightly smaller, and more powerful than the existing Gen 3 V8, and demands even more precision in the casting process to create thinner bore walls, stronger bulkheads, and accurately locate the cast-in-place iron cylinder liners. To achieve this, the casting process will include a mechanically induced rapid chilling of the bulkheads to promote swift directional solidification of the aluminum and a fine grain structure, both of which improve dimensional stability. A composite barrel crank core replaces the currentsegmented barrel design, again for tighter and more consistent bore dimensions. The cores are screwed and glued together, the aluminum pumped in rather than poured, and the oil galleries gun drilled and machined.

High performance versions are expected to produce about 450 hp, with 1,500 to 2,000 V8s per day produced at GMs Saginaw Metal Casting Operations for the Romulus engine plant. Most will go into light trucks. It took $75-million in cost reductions at the plant since 1999, and improvements in both uptime and quality for GM’s Saginaw Metal Casting Operations to wrest the new V8 away from competitors in Mexico.