Opening the gating for aluminum: sand molded aluminum gating systems have traditionally been based on tenants of iron casting, but recent research has focused on making them specifically suited to the more turbulent metal

Modern Casting, April, 2007 by Daniel E. Groteke

The American Foundry Society (AFS) Aluminum Gating and Risering Committee (2H) is continuing a program that has been underway for a number of years to understand and improve the gating systems used in sand casting facilities. The most recent effort has been concentrated upon resolving the problems associated with the reverse taper sprues used in automatic molding machines.

Such sprues have regularly shown adverse affects on the stream of entering metal. This design forces the metal to break away from the sides of the sprue wall during the pouring operation, which causes an aspiration of air, and mold gases enter the metal stream (Fig. 1). The gases can become entrapped and generate oxides that usually show up on the cope surface of castings.

[FIGURE 1 OMITTED]

Evaluation of the traditional, "expert" gating systems--those proposed by Prof. John Campbell, Univ. of Birmingham, Birmingham, U.K., and the Battelle system used by the AFS Cast Metals Institute--have shown that wide, thin runners appear to be partially effective in introducing quiet molten aluminum to the mold cavity. Further reductions in runner thickness were even more effective, but the shape would not be generally feasible in most commercial molds.

The result of those evaluations led to an initial, private effort to utilize a thin plate as a choke in the gating system, which ultimately was tried directly under the sprue. Benefits include a reduction in the aspiration of air and mold gases in the metal being poured and a more uniform stream of metal in the runner system to benefit misrun and filling problems.

Results on some initial test castings looked very favorable, but the concept needed to be explored in more detail. This led to the current, multiphase 2H Committee project, partially funded by AFS research funds, designed to improve the new radial choke concept with modeling, demonstrate and quantify the level of improvement, and verify the benefits with production castings in volunteer metalcasting facilities.

The program has advanced to the final stage, and the results to date are summarized in this article.

Radial Choke Modeling

Modeling was conducted on a mold assembly (Fig. 2) that underwent a series of changes, including an adjustment to the shape of the sprue well, the addition of a choke plate in various thicknesses and the elevation of the plate with respect to the runner bar. The final result also was compared to a commonly used commercial well under the sprue with the application of various glass screens on the parting plane.

[FIGURE 2 OMITTED]

The testing demonstrated that it was possible to choke the initial pour effectively with the use of a thin plate immediately under the tapered sprue, or a radial choke (Fig. 3). The metal then flowed through a round well and into a normal commercial runner system, filling the castings with only a slight increase in pouring time. The thin plate achieved the objective of filling the reverse tapered sprue very quickly and keeping it full throughout the pour. Variations in web thickness offered the necessary control of the pouring rate. The modeling further demonstrated that the use of glass screens was less effective than a choke and did not permit the sprue to fill completely until late in the pour.

[FIGURE 3 OMITTED]

The most recent iteration of the sprue and well has shown, in actual practice, that the 0.09-in. (2.3-mm) web thickness, when coupled with a tapered sprue of 1-in. (2.5-cm) diameter, will result in a pouring rate of approximately 2 lb./second (0.91 kg/second) with a 6-in. (15.2-cm) cope.

Evaluating Choke Performance

To compare the radial choke with past gating systems, the same mold materials, molding practices and metal conditions were used. The gating on the pattern was modified to permit a quick change from the initial gating to that using the new radial choke.

In the casting trial, six molds using each gating system were produced consecutively on an automatic molding machine. All molds were hand poured with degassed 319 alloy at temperatures ranging from 1,300 to 1,350F (704 to 732C).

The 16-lb. (7.3-kg) weight poured in 8.5 seconds through conventional gating and 15.5 seconds through the radial choke system, while using a web thickness of 0.065 in. (1.65 mm). All 12 molds poured without difficulty and filled completely.

The evaluations consisted initially of a visual examination of the shot blasted castings. All were considered commercially viable, and samples that were x-rayed showed no significant defects. Half of the castings were then given a skin cut of 0.035 in. (0.89 mm) from the cope surface, followed by another visual inspection. There was a notable improvement in the castings made with the radial choke.

In a final attempt to quantify the level of improvement, the machined castings were given a red dye penetrant inspection and read after an extended time interval. Significant indications were considered to be those in excess of 0.06 and 0.125 in. (1.5 and 3.2 mm) in diameter.

The results show an approximate 70% reduction in the number and size of indications when made with the radial choke system (Table 1).