New Filling/Feeding Process Produces Vertically-Parted Aluminum Green Sand Castings.
As the demand for aluminum castings in automotive applications continues to grow, high-production aluminum casting methods must be developed that promote quiescent filling of the molds and result in quality components. Today, permanent mold and diecasting are favored for high-production aluminum, however, the cost advantages of high-production green sand cannot be ignored. As a result, a process has been developed to take advantage of the benefits provided by green sand molding and permanent mold casting.
This article examines the development of high-production, flaskless vertically-parted green sand molding for aluminum. By bottom-filling molten aluminum into the mold cavity via the adoption of low-pressure pouring and then promoting directional solidification with active feeding, this casting process produces aluminum castings at a rate of up to 360 molds/hr.
Low-Pressure Bottom Filling
Traditionally, high-production vertically-parted green sand casting primarily has been used for iron. For aluminum casting, however, high-production gravity pouring would result in excess melt turbulence, entrained oxides and inclusions, and poor casting quality. To promote quiescent filling of the sand molds, a solution had to be developed to allow total operator control over pouring. By following the lead of permanent mold aluminum casting, the solution was to develop a bottom feeding system utilizing low pressure to move the melt.
In this new process, a closed system sets the melt under pressure, providing the operator with direct control over the melt's flow rate and the ability to elicit a quick response to the pressure to begin filling. The goal in the filling of high-production aluminum castings is a melt velocity of no higher than 0.5 m/sec throughout the fill. In addition, the melt flow rate, when it enters the runner system of a high-production vertically-parted molding system, must be increased to fill the casting cavities. If the metal flow is too rapid in the runner system, it will erode the mold walls and cause inclusions. If the mold cavity fills too slow, it will result in improper casting solidification. The control provided by low-pressure filling ensures the melt rates necessary to maintain low turbulence and produce quality castings.
In operation (Fig. 1), the low-pressure system bottom-fills the mold cavity. Then, upon completion of filling, a sand core is pushed into the gating system to prevent the melt from running back into the furnace when the pressure is released to prepare for the next mold. The question that now arises is where to place the risers in high-production vertically-parted sand molds to ensure directional solidification in bottom-gated castings.
Tests were conducted on the top and bottom feeding of 15-mm bottom-gated plates. As shown in Fig. 2, the plate with a top riser displayed a large heat center, while the plate with a bottom riser demonstrated directional solidification. Another option explored was the side riser, however, the yield for this approach proved to be too low to be cost-effective for high production.
Directional solidification was achieved with a bottom-gated, bottom-riser approach, and this provided the foundation for casting feeding. However, there was another addition to the feeding process to increase the effectiveness of mold filling--active feeding.
In active feeding (Fig. 1), air pressure forces the feeding of the casting. This technique allows the process to bottom-gate and bottom-feed the castings, which provides the highest yield, controlled feeding pressure and directional solidification.
To force the feeding, an air pressure of 80-150 mbar is directed at the riser. Higher pressure creates unwanted penetration into the mold. This active feeding method uses pressure 3 times greater than that used in gravity casting. In addition, active feeding will provide its highest pressure toward the end of solidification while gravity feeding provides its lowest feeding pressure toward the end of solidification. Due to the semi-solid state of the riser at the end of solidification, the higher pressure reduces porosity.
In vertically-parted green sand aluminum casting, the active feeding system operates automatically. A tube is inserted into the mold just above the riser by a robot. After the low-pressure system fills the mold and the gating system is closed off by the core, the pressurized tube is pushed into the riser and pressure is applied. In production, active feeding is best suited for larger castings with fewer risers. However, the method of gating the castings also determines the use of active feeding. In addition, a combination of active feeding and gravity feeding can be employed to optimize solidification.
In application, this high-production casting process has been used mainly to manufacture automotive components. As shown in Fig. 3, four 1-kg aluminum brake caliper castings are produced per mold with low pressure and active feeding of the risers. As shown in Fig. 4, one control arm per mold can be produced with a combination of active and gravity feeding with risers to ensure proper solidification with the component's localized heat centers. Also shown in Fig. 4, wheel rims have been produced with active feeding in a two-on arrangement.
This article was adapted from a paper from the 1st International AFS Conference on the Gating, Filling and Feeding of Aluminum Castings.
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|Author:||Rasmussen, Niels W.|
|Date:||Apr 1, 2000|
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