Benchmarking Aluminum Gating Systems for Better Casting Quality.
While the basics of horizontal gating system design have been understood for more than 50 years, planning gating systems must take many other factors into account. The volume requirements for the part being manufactured, process control of metal and sand used to make the casting, the skill of the workforce, and time constraints imposed by the customer all affect gating design.
Molding equipment often dictates the placement and orientation of patterns on the plate to maintain proper mold density and draw sand pockets. Further, it can limit the type, shape and placement of pouring sprues, as well as risering techniques. Many automatic molding machines, and even manual squeeze molding methods, mandate unique requirements for sprue size, shape and location as well as pattern positions on the matchplate. In addition, the use of mold chills to provide directional solidification will affect the productivity of the molding operation.
Melting and pour-off operations also are critical. Beyond having sufficient metal and labor to supply the molding line, an inability to maintain process control of the basic parameters of chemistry, gas and inclusion level, and temperature all affect casting shrinkage (and the need for enlarged risers), misruns, general porosity levels, surface finish, cleaning expenses and mechanical properties.
These factors drive and temper the need to squeeze as many units into the mold as possible in order to maximize productivity. Thus, the number of impressions, the placement of risers and runners, and their dimensions and location with respect to the downsprue, normally define the ultimate profitability of the work. Compromises often must be made in order to achieve the desired quality levels, and this is where the knowledge, art and skill of the production team will influence the cost of manufacture.
A benchmark study initiated by the AFS Aluminum Gating and Risering Committee (2-H) identified and evaluated gating system designs used by aluminum sand foundries in an effort to determine which methods were most effective.
A representative group of 30 commercial aluminum sand foundries were asked to replicate a plastic pattern test shape (Fig. 1) and to mount as many impressions as would be normal in their operation. Quality requirements were furnished with the test shape, and participants were expected to furnish a minimum of three mold systems with gating and risers still attached.
The test casting was designed to permit a minimum of three castings on a 14 x 20-in. matchplate, and a maximum of nine impressions on a 20 x 24-in. automatic plate pattern. It consists of a rectangular box with a heavy section, two remote bosses connected by a feed rib and a thin section with misrun potential.
Foundries were asked to manufacture the samples as though they are "commercial" castings (Level 2-3),with an absence of significant porosity on the two machined surfaces. The machined areas partially covered both a cope and a drag horizontal plane and were intended to reveal any gating-induced trapped air or porosity. Planned quality assessments included X-ray to ASTM standards, a penetrant test on the two machined surfaces and a pressure test on the drilled bosses. The projected volumes on the 319 alloy were for 1500 pieces/month.
Test castings were made to verify the utility of the shape at Reliable Castings, Inc., Sidney, Ohio, and were modified to increase the diameter of the internal bosses and add a connecting rib. Plastic impressions then were made and distributed to the foundries.
As part of the process information, the foundries were asked to:
* report the pouring temperature and time in seconds;
* take two rapid prototyping RPT samples at the time of pour with one solidified under vacuum and the second under atmospheric pressure;
* save a spectrographic disc at the time the castings are poured in order to sample the metal used;
* supply the general sand properties and a selling price.
Measurements were made to define the casting yield, gating and riser dimensions, the use of screens or filters, the number and placement of chills, and any apparent surface defects (Table 1).
Casting yield was measured by weighing the complete gated mold, removing the castings and recording their weight, and developing a ratio of casting weight to poured weight.
Gating ratios for the sprues were developed by measuring the sprue diameter at the base, the cross-sectional area of any runners exiting from the sprue well and the total ingate width at the casting. If screens were used, the open area covered with metal was expressed as an area unit and compared to the sprue diameter. If the sprue area was less than the screen area, the choke point was assumed to be at the base of the sprue, and the ratios were calculated accordingly. In those cases in which the screen was an actual obstruction or choke in the gating system, or if an intermediate downstream choke was used, the first digit in the gating ratio is enclosed in parentheses to highlight the downstream choke.
The RPT samples were measured for specific gravity (S.G.) in both the solidification under reduced-pressure mode and the atmospheric pressure solidification mode. The difference in values between the two solidification modes was assumed to be related to the hydrogen gas content of the melt, and the reduced pressure sample was expressed as a percent porosity of the poured metal.
A visual assessment was made of the machined castings with special attention to visual shrinks and porosity on the cope and drag surfaces. The milled surfaces produced different levels of surface finish and "smear." The large, flat area of the casting was machined with a fly cutter, and the internal pad was milled with a 1.5-in. end mill, producing less smear and possibly even inducing some enlargement of microporosity. On both surfaces 0.04 in. of stock was removed.
Radiographic inspection of the castings was conducted at the XRI Testing facility, Holland, Michigan, using a double film technique to ASTM E-155 standards.
Penetrant testing of the castings was done with a red dye, water-washable system with emphasis on the two machined horizontal casting surfaces. A visual assessment of the number and size of indications was made, and a ranking system using a 1-5 (good-bad) scale was used to rate the result. Table 1 lists the ratings developed with separate assessments made of the cope and drag surfaces. When the castings were molded with the open end in the up" position, the rating number includes a superscript "I" to designate the appropriate surface.
Due to expanded work schedules, only 14 foundries submitted sample castings with 1-8 impressions in time for the study. Two of these operations rigged a single impression pattern, with subsequent plans to make multiple impression equipment, but could not move beyond the initial trial shapes.
Each foundry chose a slightly different way of achieving the commercial quality casting. All of the test castings were compared to determine which gating design features offer the best results, and some of the most telling gating designs are presented in this article.
Foundry "B" used an automatic matchplate molding machine and rigged an eight-impression plate to pour the samples (Fig. 2a-2b). Four impressions were mounted on each side of a common runner/riser bar, which was supplied by a heavily choked runner bar just downstream of the sprue well. A typical reverse-taper sprue was used to introduce metal to the mold, with a glass screen conventionally placed at the bottom of the sprue. Chills were not used.
As a result of the heavy choke, the gating ratio was (1):6:43. The use of the choke is a standard procedure to balance the effect of the reverse-taper sprue in this foundry, and it was effective in producing sound castings. The additional impressions did help the pouring yield, with a 67% level achieved with rectangular risers. On the initial visual inspection, some superficial cope dross was apparent on the sample castings but was removed with the 0.04-in. machining cut on both cope and drag surfaces.
The casting quality was excellent in the evaluation phases, with moderate porosity found on X-ray examination, but relatively few penetrant indications appeared on the machined surfaces. This was somewhat surprising in view of the relatively high porosity level measured in the RPT samples. Gas levels were moderately high at 2.6% porosity, and a "3" on the polished cross section of the RPT sample.
Castings produced by Foundry "D" utilize a proprietary gating system (Fig. 3) featuring an insulated sleeve with a bottom ceramic filter. The combined assembly functions as both a riser and gating system. Round chills were used on both internal bosses.
The system exhibited a pouring yield of 63%. Casting quality was excellent, with no apparent defects found on X-ray and only light porosity found on the penetrant test.
Foundry "E" produced the test casting as a single impression squeezer casting (Fig. 4a-4b) on a 14 x 18-in, plate, with a heavy single riser and multiple chills. The novel features of the casting system are the very small (0.5-in.) downsprue and the use of a metal screen with 50% open area. With the small downsprue, the screen did not function as a choke, producing a gating ratio of 1:1.75:10.1. The pouring rate was not recorded, but was assumed to be on the low side of the rates in the study.
Casting quality was excellent, with no apparent defects found on X-ray and only a few minor penetrant indications on the machined surfaces. Gas level, as measured by the RPT, was low, and it is assumed that the cooling rate of the casting was fairly rapid because of the large chills used. These are believed to contribute to the absence of porosity.
Foundry "G" ran the sample castings on a squeezer line, with three impressions (Fig. 5a-5b) mounted on a 14 x 19-in. plate. The gating system and samples produced a 66% pouring yield.
The rigging includes a 1-in. downsprue with a glass screen at the base and relatively small runners exiting the well. The runner system is external from the risers, and has a "pop-off" cut on a runner extension to the third casting. The foundry used cylindrical risers with a notch and dimple in the top to promote feeding, a system that appears to be fairly effective from the appearance of the risers. Some choke reduction of the downsprue is provided by the screen, but the gating ratio is still among the closest to a pressurized system of those examined in the study. The choked ratio is (1):1.1:2.
The casting quality evaluations showed a Level 3 porosity on X-ray and more penetrant indications. Some cope dross was evident. Metal quality was considered to be a factor in some of the porosity, with 2.6% measured in the combined RPT evaluation. Some large pores also were visible in the polished surface of the RPT sample.
The samples (Fig. 6a-6b) submitted by Foundry "K" were molded on a squeezer line using a 16 x 20-in. plate with four mounted impressions. Of all the foundries, this was the only operation that used a gating system incorporating a ceramic filter. The major elements of the system are shown in the pictures and depict the filter placement and associated enlargement of the runner system. The additional volume of metal in the runner system to accommodate the filter, coupled with possibly oversized risers, contributed to a 53% pouring yield.
In the calculation of the gating ratio, the filter area was assumed to be large enough that it did not function as a choke, and the ratio was accordingly controlled by the downstream runner and ingates. This produced a ratio of 1:1.2:6.5 with the large 1.12-in. downsprue being a major control.
The risers were opened to the cope surface by the molder with a 2.5-in. pipe to promote additional feeding. This additional volume of feed metal probably caused the visual appearance of the lower rectangular risers, which appeared to be as free of shrinkage as the castings.
The quality assessments showed a slightly higher than average level of porosity on X-ray inspection and minor penetrant indications on both the machined cope and drag surfaces.
Foundry "L" produced molds on a 16 x 20-in, squeezer plate with four mounted impressions (Fig. 7a-7b). The gating system features the only group of samples submitted with a top riser on the castings. The riser measures 1.75 x 3.25 x 3.75 in. and is positioned to cover both the inside bosses and pad. Chills also are used on the bosses. The system is somewhat unique in that the ingates are not supported with any direct feed metal at the point of entry.
The downsprue is rectangular and measures 0.75 sq in. at the base to effectively choke the system. A glass screen does not affect the choke before metal flows through a somewhat restricted well and into the runner system. The gating ratio is 1.0:2.22:4.44.
Visual quality of the casting and machined surfaces was good, and the X-rays found only one casting with an elongated pore, suggesting trapped air. The penetrant testing similarly produced a few indications, with a 1 rating on both cope and drag surfaces.
Foundry "N" also uses an automatic molding machine with alternate sprue capabilities, but is using a reverse-taper sprue. The test castings (Fig. 8a-8b) were made on a 20 x 24-in, plate as a four impression pattern.
The 1.18-in.-diameter downsprue exits into a round bottom well and then splits into a double runner system with an almost immediate transition from a drag-to-cope to drag runner. The overlap is controlled to produce a choke effect on both sides with a total area or 0.43 sq in. The foundry personnel refer to this overlap and choke as a "cope dross trap." The choked gating ratio is (1):3:8.6.
Two cylindrical risers are positioned over the drag runner, each feeding a pair of castings. These risers also are dimpled on the top and show the same effective riser piping as those made by Foundries "G" and "J." The ingate to the casting is taken off the bottom of the riser, and a visible reverse shrink was developed on the flat bottom of all risers. The condition is believed to be partially the result of steam pressure from the evaporation of mold moisture.
The visual quality of the castings and machined surfaces was good. The X-ray examination revealed porosity as Level 3, and penetrant testing produced ratings as 2 on both the cope and drag surfaces. The gas level in the melt was low (0.4%) on the combined RPT test, and 2.73 on S.G. of the reduced pressure sample.
Foundry "P" submitted castings produced on a squeezer line, with two impressions (Fig. 9a-9b) on a 14 x 16-in. plate. The castings featured a different form of gating with a metal screen used as a vertical filter element, a much heavier runner and riser system and a large overflow beyond the casting,
The downsprue is a tapered wedge with a 0.44 x 0.75-in, opening into the well. As metal leaves the well it quickly encounters a metal screen with only 50% open area. All sample castings had a large volume of film oxides stacked up at the screen, so this method appears to be a very effective trap. After the screen, the metal flows through a heavy drag runner to an overflow, and then backs into two mold cavities and a heavy open-top riser. The gating ratio is controlled by the area of the sprue choke and has values of 1:4.1:32.8. Three internal chills were used: one on the pad and two on the bosses.
This shop does not pour 319 alloy as a general rule, and accordingly poured the samples with C355. Casting quality was excellent with clean machined surfaces, X-ray at Level 1 porosity, and only a few penetrant indications on both the cope and drag surfaces.
One of the most puzzling aspects of the results is the lack of a correlation between the levels of gas in the melt, as judged by the RPT sampling, and the porosity results found on X-ray. An examination of the actual RPT buttons revealed wide differences in both size of sample, probable vacuum levels used in the testing, and even assumed inclusion levels as judged by the surface roughness of the samples and shape of the pores. A possible cause may have been a difference in performance of the RPT procedure and sample collection methods, which were not defined in the project plan.
Samples from three foundries were judged to have either "no apparent defects" or a Level 1 porosity on the X-ray examinations. These included samples D, E and P. Rating the gas buttons on a scale of 1-10, sample D would be a "5" (2.61 S.G., 4.0%); sample E would have a rating of "2" (2.73 S.G., 0.8%); and sample P would be rated at "2" (2.62 S.G. for C355, 0.5%).
Other considerations (such as pouring technique) not documented in the study could have a pronounced effect. These would include: skimming the ladle to remove surface dross, the use of an adequate pouring basin or cup, keeping the sprue filled and adding metallostatic pressure to the runner system by elevating the pouring ladle above the mold.
The best answer may be that these test samples were to be made as commercial castings and were manufactured to that level of quality. Degassed melts were targeted in most cases, if that was the practice in the foundry and available at the time the castings were poured. With several exceptions, the sample group did meet the accepted definition of commercial castings and accordingly met the objectives of this study.
Without critiquing individual systems, the following practices are recommended:
* pouring basin--An adequate pouring basin is mandatory to keep the sprue completely filled at all times during the pour;
* sprue--A tapered sprue is desirable, with rectangular cross sections preferred. Where this is not possible, aggressive choking of the metal stream as it exits the well appears to offer a significant benefit to casting quality;
* sprue well--Wells have been minimized in some of the gating systems, but it is felt to be beneficial. Designs with flat bottoms and adequate volume are preferred;
* runners--Runner sizes must be adequate to deliver fluid metal to the mold cavities without agitation or jetting. Drag runners seem to be preferred, but cope traps for dross may provide benefit in some cases;
* risers--Cylindrical or ellipsoidal shapes are preferred to improve feeding efficiency and pouring yield. A molded-in top dimple seems to have a positive impact upon feeding. The riser should be positioned slightly away from the casting and have a bottom contour that will ensure continued feeding from the thermal center;
* screens and filters--While they add cost, screens and filters may be required to achieve desired results. With a properly balanced system, their need is minimized;
* chills--Chills can solve problems, but also will add cost to the molding operation. Use modeling to confirm they are needed and that the size and mass of the chills are adequate. Other lower cost options may exist.
A better understanding is required of the relationship between hydrogen levels in the melt, the level and types of inclusions present, mold metal reactions contributing additional levels of hydrogen to metal entering the casting cavity, and the generation of negative pressures in the gating system which can help aspirate mold moisture into the metal stream to add hydrogen to the system. This benchmarking study confirms the need for solutions that will minimize the microporosity observed in the samples.
This paper was adapted from a paper presented at the 1st International AFS Conference on the Gating, Filling and Feeding of Aluminum Castings. Proceedings are available through AFS Publications at 800/537-4237.
Pouring Characteristics and Test Results of Sample Castings Foundry Plate Imp. Yield (%) Pouring Gating S.G. Por. Chills Visual Rate Ratio (%) A 14 x 22" 4 49 1.60 1: 9.9: 17.0 2.71 1.1 2 / 2 B 20 x 24" 8 67 0.92 (1): 6: 43 2.67 2.6 none 1 D 16 x 20" 2 63 1.50 N/A 2.61 4.0 2 / 1 E 14 x 18" 1 36 1: 1.75: 10.1 2.73 0.8 4 / 1 F 14 x 19" 4 69 2.20 (1): 1.4: 8.5 2.72 Nil none 4 G 14 x 19" 3 66 1.60 (1): 1.1: 2.0 2.67 2.6 none 4 H 20 x 24" 4 65 1.55 1: 1.43: 5.33 2.20 18.2 1 / 5 J 20 x 24" (6) (64) 1: 2.4: 11.5 2.69 1.8 none 1 K 16 x 20" 4 53 1.65 1: 1.2: 6.5 2.74 Nil none 2 L 16 x 20" 4 52 1.98 1: 2.2: 4.4 2.70 0.8 2 / 1 M 14 x 18" 4 88 1.10 1: 3.0: 8.5 2.72 0.8 none 4 N 20 x 24" 4 73 1.37 (1): 3.0: 8.6 2.73 0.4 none 1 O 14 x 19" (4) (61) (0.5) 1: 6.0: 36.0 2.72 0.8 Fin 1 P 14 x 16" 2 43 1.40 1: 4.1: 32.8 2.62 0.5 3 / 1 Foundry X-Ray Penetrant Price ($)/1500 A Por. 2-3 C: 1; D: 2 7.95 B Por. 3 C: 1; D: 2 5.79 D NAD C: 3; D: 2 E NAD C: 1; D: 2 F Por. 3, Gas 2 C: 4; D: 2 G Por. 3 C: 3; D: 3 4.86 H Por. 4 C: 5; D: 4 5.47 J Por. 4 C: 2; D: 3 K Por. 4 C: 2; D: 3 6.56 L Elong. 1 C: 1; D: 2 M Sh. 8, Por. 4 C: 5; D: 5 N Por. 3 C: 1; D: 2 6.35 O Sh. 3 C: 2; D: 1 P Por. 1 C: 2; D: 2
|Printer friendly Cite/link Email Feedback|
|Comment:||Benchmarking Aluminum Gating Systems for Better Casting Quality.|
|Author:||Groteke, Daniel E.|
|Date:||Mar 1, 2000|
|Previous Article:||'Expanding the Possibilities' at Rochester Metal Products.|
|Next Article:||Gating Conference Explores the Use of Fluid Flow Modeling.|