Low-pressure, green sand process produces thin-walled castings.
The need to reduce the weight of automobiles has forced foundries to look toward aluminum casting for lighter weight components. Die and permanent mold casting allows the production of engine and suspension components with thin walls, high dimensional accuracy, good surface finish and excellent mechanical properties, however, the cost associated with these processes is higher than green sand. For aluminum green sand foundries, the opportunity to produce complex castings can be limited by the turbulence (and subsequent absorption of hydrogen and oxygen) caused during high-production sand mold casting and the inclusions and microporosity that result from a loss of heat during mold fill and solidification.
The German firm Heinrich Wagner Sinto (represented in the U.S. by Roberts Sinto, Lansing, Michigan) was faced with this dilemma as it attempted to develop a green sand process for high-production casting of complex, thin-walled aluminum and iron components. The belief was that both the iron and aluminum sand foundries could benefit in terms of casting quality, dimensional tolerances and wall thickness from greater control of metal flow and mold fill.
With these benefits in mind, the firm has developed a process that combines the cost benefits of green sand molding with the mold filling benefits of low-pressure permanent mold casting in a high-production, sand casting format that improves the casting quality of both complex aluminum and iron castings. This Sand Mold Injection Pouring Process allows sand foundries to pour aluminum or iron via counter-gravity, low-pressure methods using a special pouring arrangement.
The pouring technology begins once the green sand mold is made. After compaction within the flask, the green sand molds are conveyed to a rotating device that turns the mold 90 [degrees] onto its side so the parting line is vertical [ILLUSTRATION FOR FIGURE 1 OMITTED]. The mold is then conveyed to the pouring station where a lifting device lowers the mold so its sprue fits tightly over the furnace's white ceramic feed tube. Pressure is applied to the furnace and the molten metal flows counter-gravity (without turbulence and under complete operational control) into the mold cavity.
Once the mold is full, a pneumatic or hydraulic cylinder presses into it from the side to push a portion of the green sand - "a plug" - into the up-sprue of the feeding system to hold the metal within the mold cavity once the furnace and its pressure is disengaged after filling. This allows the mold to be indexed along the line without any loss of molten metal. A ceramic filter placed near the opening of the sprue prevents any sand particles released during this "plugging" from entering the furnace feed tube.
The plugged mold is then conveyed to another rotating device where it is turned an additional 180 [degrees] so that the sprue opening (which has been closed off) is now facing upward. The result of this last rotation is that the up-sprue, which contains the hottest metal in the mold, now serves as the feeding mechanism for the mold as it solidifies. If a traditional mold arrangement is used in this process with the risers remaining on top, the mold's feeding mechanism would receive the first metal and, as a result, also would have the coldest metal. With the up-sprue as the feeder, it receives the last and the hottest metal for more effective feeding.
Although the process was developed with automotive engine components in mind, the process has cast other aluminum parts since reaching lab success in 1996, including steering levers, to a 2-mm wall thickness. On the iron casting side (pouring of iron commenced in March), the focus, at this time, has strictly been on engine components. However, the ability this process has to control the metal flow and mold fill has led to several other benefits, including:
* the ability to repeat and document the filling process for consistent quality;
* cleaner metal entering the mold;
* the reduction of melt turbulence and the elimination of oxide and impurity inclusions;
* the reduction of gating systems, resulting in a cost savings (reportedly a 70%-plus yield on engine blocks);
* improved casting feeding during solidification as the hottest metal in the molds is in the feeder;
* the elimination of ladles and other open transport systems;
* the elimination of dosing equipment, as it is achieved through pressure control;
* improved safety for workers as splashing, fumes and smoke are reduced or eliminated.
This process was demonstrated in June in Duisburg, Germany, for GIFA attendees, and one large automotive foundry (a pilot operation) in Europe currently uses the process to produce aluminum engine blocks.
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|Author:||Lambert, Guy R.|
|Date:||Aug 1, 1999|
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