As the sand temperature rises, so does the convection current velocity which affects the amount of effluent released into the foundry atmosphere.
If you need to clean up your metalcasting operation, and increase your profit margin, the best place to start are the areas that generate the most airborne silica dust. The primary source of dust, or the biggest generator, 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 amount of fine silica dust is released into the workplace. Since it is very difficult to capture all the steam laden with silica dust due to the amount of open area, the foundry environment becomes dirty. The volume of effluent is directly related to the sand temperature at shakeout. When the average sand temperature exceeds 100°C, the convection currents carry silica dust which is released into the foundry atmosphere. The higher the sand temperature at shakeout, the greater the convection current velocity and amount of steam and dust to be captured.
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 (see chart). 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 the sand varies around the globe, the source of supply will have a direct impact upon fines generated from both mechanical and thermal aspects.
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.
Since the number 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 from 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 in the cleaning, finishing, and melting departments. Workers in these areas are exposed to silica dust from sandy metallics.
The next source of dust is shot blasting of these sandy castings. The sand that goes into the shot-blast system not only accelerates equipment wear, but it is also degraded and added to the waste stream. This translates into increasing sand-disposal costs. The sandy returns that go back to remelt without being shot blasted, lowers the melting efficiency and increases slag, slag handling, and slag disposal costs.
The demand to clean up these areas in the foundry, as well as lower operating and labor costs, has led Didion International Inc. to become the world leader in rotary foundry equipment. New innovations in sand casting separation and cleaning have saved foundries hundreds of thousands of dollars annually.
Didion's latest invention is the New 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 industry worldwide.
Foundries benefit on a number of fronts including less capital equipment to purchase, less floor space, much lower energy costs, less maintenance time and costs, lower shot consumption, less replacement parts, cleaner working conditions, and cleaner returns back to remelt reduces 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, environmentally, the foundry stays cleaner. Dust collection is extremely 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 their castings twice – first to preclean the castings and returns, then a second blast after casting grinding to blend in the grinding marks. Since the Mark 5 also cleans the castings, the first shot-blast step can be eliminated. The gates, runners, and sprue are often removed, 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 unique feature is popular among high production automotive foundries with heavy cored work that need to meter only 15% core sand back into the green sand system.
The machines are variable speed so the foundry can control the action. The media bed protects non-ferrous and fragile castings while cleaning and cooling the castings. Custom lining configurations are available for aluminum, brass, malleable gray, ductile, and steel castings. Installations include green sand, shell, no-bake, and lost foam systems.
Recent improvements featured in the Mark 5 Series 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.
The Mark 5 Series rotary media drum combines sand/casting separation, sand conditioning, double sand screening, casting cleaning, and casting cooling so the foundry stays cleaner and makes more money.
Clean, cool castings directly from the rotary media drum into a water quench streamlines production and saves equipment costs, energy, time, and space.For more information, contact Didion International, Inc., www.didion.com.