In order to comply with emerging federal workplace-silica standards, foundries must focus on the primary sources of airborne silica dust, specifically the shakeout and sand carryover processes.

Cleaner Air for Safer Workplaces and Lower Costs

June 16, 2016
Choosing the right shakeout equipment can reduce the volume of airborne dust — and reduce operation and labor costs. Understand vibratory effects Dealing with the heat Rotary advantages

The emergence of federal regulations that call for lower tolerance levels of silica exposure has created new engineering and practice challenges for foundries. In order to begin the process of compliance, foundries first must focus on the areas that are the primary source of airborne silica dust, specifically the shakeout and sand carryover processes. Choosing the right shakeout equipment can reduce the volume of airborne dust by effectively containing the sand as it is separated from the castings.

When a green sand mold enters a vibratory shakeout, the heat dissipating from the casting means that a large volume of fine silica dust is released and carried into the plant atmosphere by way of steam. The process makes it difficult to capture all the steam and the dust it carries. The amount that is released is directly related to the sand temperature at shakeout: When the average sand temperature exceeds 100°C, convection currents carry silica dust into the foundry atmosphere.

As the temperature of the sand at shakeout rises, so does the velocity of the convection current, and total volume of steam and dust that is released also rises. With a sand-to-metal ratio lower than 6.9:1, the average sand temperature will exceed 100°C. As the temperature rises from 100° to 200°C the convection current velocity will increase significantly.

Clean castings being discharged as airborne dust is contained within the rotary drum.

The heat-affected zone of a mold is the area where the molten metal comes into direct contact with the sand grains (mold face). As the sand in this area expands, weaker grains of sand will fracture from stress, increasing the potential for silica dust. Because different sands respond differently in such situations, a particular foundry’s potential for silica dust generation will be influenced directly by mechanical and thermal factors involved in the shakeout process.

Another consistent fact is that for every kilo of metal poured into a mold, 15% by weight of new sand should be added to replenish the losses and keep the sand system in balance. Since the amount of cores may vary from job to job, using core sand as new sand additions is a hit-or-miss method, unless it is separated from the green sand and then metered back in at a rate determined by the necessary volume to achieve 15% of new sand.

A second source of airborne silica dust is the sand that remains attached to the castings in cavities or surface recesses. Heat emanating from the cast part will dissipate moisture as steam, leaving loose sand to fall away. Silica dust continues to be produced as the casting proceeds through subsequent stages of production. For example, new dust generation occurs when “sandy” castings are shot-blasted. While the health risks to workers are the primary concern, sand that remains after shakeout and then enters into the shot-blast system contributes to wear on the equipment. At the same time, sandy returns lower the melting efficiency and create increases in the volume of slag, the need for slag handling, and slag-disposal costs.

Sprue exits the rotary drum, degated, cleaned and ready for remelt.

Each of these factors returns the discussion to the matter of capturing sand at the point of mold shakeout. DIDION innovations in sand-casting separation and cleaning have helped foundries identify and solve the problem at the source. By selecting an effective shakeout system they are able to make their operations cleaner, and to lower operating and labor costs. These results have delivered hundreds of thousands of dollars in annual savings to foundries, and offer avoidance of liabilities under the emerging regulatory standards.

The DIDION® Mark 5 Series rotary media drum is a single-step, patented design that performs shakeout, sand conditioning, double sand screening, casting cleaning, and casting cooling in a single efficient step. DIDION projects that combining these processes cuts operating costs $40-80 per ton — reportedly the lowest operating cost-per-ton of any comparable system — but the results also include cleaner workspaces as the overall volume of airborne silica dust is reduced.

Foundries benefit in a number of ways, the developer notes, including: less capital equipment to purchase; less floor space occupied; significantly lower energy and maintenance costs; lower shot consumption; fewer replacement parts; cleaner working conditions; and cleaner returns for increased melting efficiency up to 15%.

DIDION rotary media drums streamline production, as cool and clean castings go directly to finishing. Return sand is blended and conditioned, so it is consistent in temperature and moisture content providing better control at a mixer. Sand stays in the system where it belongs, so the foundry stays cleaner.

Dust collection is highly efficient thanks to the drum’s small, open area. It requires 75% less dust collection than a vibrating shakeout rated at the same capacity. Counter-flow air captures fugitive dust within the drum, protecting workers from airborne silica.

Many foundries shot-blast their castings twice; once to pre-clean the castings and returns, and again (after grinding) to blend in any marks on the casting surface. Because the rotary media drum thoroughly cleans the castings, the first shot blast step can be eliminated. The DIDION process often removes gates, runners, and sprue, so there’s no hard manual labor involved and clean returns go directly to remelt.

The DIDION Mark 5 is the first and only shakeout system to separate core sand from green sand, discharging each one at separate points. This unique feature is popular among high-volume foundries with heavily cored castings that meter the core sand back into the green sand system. The media bed protects nonferrous and fragile castings while cleaning and cooling. Custom lining configurations are available for aluminum, brass, malleable, gray and ductile iron, and steel castings.

In short, DIDION rotary foundry equipment allow the foundries to lower operating and labor costs while providing a cleaner and safer workplace in the wake of new emerging regulations.

Mark M. Didion is product manager for DIDION International. Contact him at 636-278-8700 or [email protected]. Or, visit