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.

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).

Metalcaster evaluation of the radial choke system also has produced positive results (see sidebar). However, while production at higher pouring rates has been explored only minimally, it does appear as though there are upper limits to the rates that can be utilized. Furthermore, the types of defects resulting from aspirated air and mold gases are very similar to those found with contaminated metal. Use of the radial choke will minimize the additional generation of dross and trapped air found with reverse tapered sprues but will not resolve problems with contamination that already exists in the metal.

For More Information

"Optimization of Aluminum Gating Systems Used with Automatic Molding," S. Sikorski and D.E. Groteke, 2005 AFS International Conference on High Integrity Light Metal Castings.

Metalcasters Put Radial Choke to the Test

The final phase of the development of the radial choke system involved extensive in-plant trials. Plants participating in the initial benchmarking study used the new choke on both new and existing patterns in current production. Castings prone to trapped air or cope dross indications were preferred.

Littlestown Foundry Inc., Littlestown, Pa.

Littlestown Foundry started with a version of the choke that predated its optimization. The metalcaster has modified a total of seven patterns; six more will follow. All molding occurs on a horizontal automatic molding machine with automatic pouring of degassed metal filtered at the pouring pump.

One conversion consists of eight impressions mn from a common runner bar. The castings are filled through a web thickness at the choke of 0.09 in. (2.3-mm), and the 8-lb. (3.6-kg) weight is poured in five seconds. A number of production runs have been made with no casting losses for cope dross, 'trapped air or misruns.

A more recent conversion is a pattern that produces four thin-walled castings, which are subsequently fully machined. The shank wall thickness is very thin, requiring pouring the mold with two sprues to avoid misruns. Only one production run has been made thus far, but all machined castings have been free of dross, oxides and leakage on pressure test. Prior practices at Littlestown have used multiple glass screens in their gating systems. The screens have been eliminated with the radial choke system. Eliminating the operation of screen setting has increased molding productivity and saved the cost of the screens. Molding rates on non-cored work have been increased by an average of 25 molds per hour with the gating change.

AC Foundry Corp., Battle Creek, Mich.

AC Foundry has completed a conversion to the new choke system on a casting with a history of trapped air and cope dross on a flat surface above an area where metal cascaded into the mold cavity. The full sprue shows four impressions poured through a center sprue. The molds are made on an automatic molding machine and poured manually using degassed 356 alloy. The web thickness is approximately 0.06 in. (1.5 mm) and yields a 16-second pouring time for 22 lb. (10 kg) of alloy (1.38 lb./ second [0.625 kg/second]). There have been no losses for cope dross, trapped air or misruns on the first production run.

Reliable Castings Corp., Sidney, Ohio 

Reliable castings Corp. has evaluated the new choke system only for an obsolete casting. The pattern was modified in a manner that allowed consecutive production of molds made with both the new and old gating systems.

Six molds with each system are poured manually with 319 alloy. The conventional gating pours at 1.88 lbs./second (0.85 kg/second); the radial choke system pours at 1.24 lbs./second (0.56 kg/second). Both use a 0.875-in. (2.22-cm) diameter downsprue. The radial choke web thickness is approximately 0.07 in. (1.78 mm) and has yielded one misrun casting. Future casts will be made with a slightly thicker web. Reliable's normal glass screen is not used on the molds made with the new choke, which would provide a savings of $0.07/ mold. Facility management notes an improvement in surface finish and visual quality and feels there would be additional savings possible in the cleaning room.

An AFS Aluminum Gating and Risering Committee (2H) Project By Daniel E. Groteke, Q. C. Designs Inc., St. Joseph, Michigan 

Daniel Groteke is a longtime member of the AFS Aluminum Gating and Risering (2H) Committee and president of Q.C. Designs Inc., St. Joseph, Mich., an engineering service company. He may be reached by email at [e-mail].

Radial Choke Gating Shows Fewer Defects

TABLE 1 reveals an approximate 70% reduction in the number and size of
machined and penetrant indications when the castings were made with the
radial choke system.

Mold/
Impression   Machining Indications               Penetrant Indications
                                                 +0.06 in.   +0.125 in.

Original
  Gating
7 - 1        Clear                               0           0
7 - 4        Clear                               6           1
8 - 1        One pore away from gate             10          4
8 - 4        Four specks in front of gate        8           10
9 - 1        Oxide films in front of gate        2           0
9 - 4        Two specks/pores in front of gate
10 - 1       Oxide films in front of gate        16          18
10 - 4       One pore in front of gate           10          8
12 - 1       Three pores in front of gate        8           3
12 - 4       Clear                               9           2
TOTAL                                            86          51

Radial
  Choke
1 - 1        Clear                               9           1
1 - 4        Round pore in front of gate         4           2
2 - 1        Clear                               2           0
2 - 4        Clear                               0           0
3 - 1        Clear                               5           3
3 - 4        Clear                               4           2
4 - 1        Clear                               1           0
4 - 4        One pore away from gate             2           1
6 - 1        Clear                               0           0
6 - 4        Clear                               5           1
TOTAL                                            32          10