If you need to clean up your casting operation and increase your profit margin at the same time, go directly to the areas that generate the most airborne silica dust. The primary source or biggest generator of dust is the vibrating shakeout and the sand carryover, coupled with the amount of sand still attached to the castings after shakeout.
When a green sand mold is dumped onto a conventional vibrating shakeout, a huge volume of fine silica dust is released into the plant environment. Because the amount of open space makes it difficult to capture all the steam laden with silica dust, the foundry becomes dirty.
The amount of effluent relates directly to the sand temperature at shakeout. When the average sand temperature exceeds 100°C, convection currents carry silica dust, which is released into the atmosphere. The higher the sand temperature at shakeout, the convection current will be faster and the volume of steam and dust will be higher.
With a sand-to-metal ratio lower than 6.9:1, the average sand temperature will exceed 100°C. The convection current velocity increases significantly from 100° to 200°C.
The heat-affected zone of the mold is the area where the molten metal comes into direct contact with the sand grains (mold face.) As the sand expands in this zone, internal stresses cause weak grains to fracture. Since the quality of sand varies, the source of supply will impact the fines generated, both mechanically and thermally.
Another important calculation tells us that for every kilo of metal poured into the mold, 15% by weight of new sand should be added to replenish the losses and keep the sand system in balance. Because the amount of cores can vary from job to job, using core sand as new sand additions can be a hit-or-miss method unless it is separated from the green sand, and then metered back in at a given rate.
The second source of airborne silica dust is the sand still attached to the castings after shakeout (in pockets, cavities, and corners.) As the heat from the metal drives out the moisture, the loose sand falls off and contaminates downstream operations (cleaning, finishing, melting.) Workers in these areas are exposed to silica dust from sandy metallics.
The next source of dust is from shot blasting these sandy castings. The sand that goes into the shot blast system not only accelerates wear of the equipment, but also is degraded and added to the waste stream. This increases sand disposal costs. The sandy returns that go to remelt without being shot blasted lower the melting efficiency and increase slag and slag handling and disposal costs.
New innovations in sand casting separation and cleaning have saved foundries hundreds of thousands of dollars annually.
Didion International Inc.’s latest invention is the Mark 5 Series rotary media drum, which performs shakeout, sand conditioning, double sand screening, casting cleaning, and casting cooling in one efficient step, so the foundry stays much cleaner with less airborne silica dust. By combining these processes the foundry can save $40.00 to $80.00 per ton. The patented design has the lowest operating cost per ton in the global metalcasting industry.
Foundries benefit in several ways, including less capital equipment to purchase, less floor space, much lower energy costs, less maintenance time and costs, lower shot consumption, fewer replacement parts, cleaner working conditions, and cleaner returns reduce slag build-up. The New Mark 5 Series streamlines production with cool and clean castings going directly to the finishing department. The return sand is blended and conditioned so it is consistent in temperature and moisture content providing better control at a mixer. The sand stays in the system where it belongs, so the foundry stays cleaner. Dust collection is efficient due to the small open area of the drum. The Didion requires 75% less dust collection than a vibrating shakeout rated at the same capacity. Counterflow air eliminates fugitive dust from escaping, protecting workers from airborne silica dust.
Many foundries shot blast castings twice – first to preclean the castings and returns, then after casting grinding to blend in the grinding marks. Since the Didion also cleans the castings, the first shot blast step can be eliminated. The gates, runners, and sprue often are removed in the Didion, eliminating hard manual labor and the clean returns go directly back to remelt.
Didion is the first and only shakeout system whereby core sand is separated from green sand and discharged independently at separate points. This feature is popular among high-production automotive foundries with heavily cored work that need to meter only 15% core sand back into the green sand system.
The variable-speed machines let the foundry control the action. The media bed protects nonferrous and fragile castings, while cleaning and cooling them. Custom lining configurations are available for aluminum, brass, malleable grey, ductile, and steel castings. Installations include green sand, shell, no-bake, and lost foam systems.
Recent improvements to the Mark 5 include new interlock liners that are thicker and wider for longer life; 50% fewer seams and contoured rifling for delicate castings; self-relieving, tapered separation chambers; split pillow block endtrucks, with quick change bearing inserts; and automatic chain tensioning. System improvements include water quenching directly following the machine to eliminate long cooling conveyors, further saving equipment costs, energy, time, and space. For foundries using casting baskets and stage cooling, water quenching following the rotary media drum can streamline production flow and eliminate the bottleneck in the cleaning room.
More information is available at www.didion.com