Practical tips on gating iron castings.
* There are multiple ways to design a working, error-free gating system.
* By following several steps in pressurized and non-pressurized design, metalcasters can agree on the basic principles of gating.
Have you ever brought a group of metalcasters together to review the gating system on a troublesome casting job? If you have five experienced people reviewing it, you often will receive three or four suggestions on how to fix it. That is the beauty of gating--no single way exists to gate a specific casting. One system can completely differ from another but still work on the same component.
Gating is a science and also an art that will yield defect-free castings with an acceptable yield provided the components are sized correctly. But there isn't a step in a metalcasting process that is more misunderstood than gating. Gating is meant to provide a means for the molten metal to travel from the ladle to the mold cavity. Pressurized and non-pressurized gating systems have different methods of creating a component, but they both follow the same principles to cast the part. Four basic rules apply:
* keep the first molten metal down the spree, out of the casting cavity because it is the worst metal in the mold;
* pour the mold as quickly as possible;
* pour the molten metal as cold as possible. Iron temperatures above 2,600F (1,427C) cause problems with green sand molds and the core materials;
* size the components properly to maintain a proper yield and an even fill (ie. good yields are: ductile iron-55%, gray iron-70%).
Detailed within are tips on how to design efficient pressurized and non-pressurized gating systems for iron castings, focusing on the three main features: down spree, runner and ingates.
Determining the Gating System
There are two types of gating systems: pressurized and non-pressurized (Figs. 1 and 2). These systems can be applied to both vertical and horizontal gating methods. The difference between pressurized and non-pressurized is where the choke is placed. The choke is the smallest cross-sectional area of the system and sets the rate of flow of the molten metal into the casting cavity. It can be placed in the sprue, the runner or the ingate. For pressurized systems, the choke is the ingates. The choke in non-pressurized systems is placed between the bottom of the sprue and the runner. This allows for a smooth passage of molten metal down the runner and through the ingates. Further, the sprue, if tapered correctly, also could be the choke in a non-pressurized system.
The choke placement depends on several factors in an operation. One is how much space is available in the mold because non-pressurized systems will require more room. This is because the distance between ingates is larger, forcing the need for a longer runner to fill all the gates. The intricacy and quality of the parts also are factors. A pressurized system can be slightly more turbulent, which may cause problems in highly intricate parts, but if yield is the main criteria, then a pressurized system provides the desired results because less mass accrued exists.
To calculate the size of the choke in cross-sectional areas, many of the features of the mold, the metal and the casting will need to be considered. The type of gating system used often depends on the metal to be poured. For instance, many ferrous alloys tend to use pressurized systems, while nonferrous metals, particularly aluminum and magnesium, are cast with non-pressurized systems. Metalcasting facilities often have formula sheets that help the gating designers determine how to create the most effective system. These sheets incorporate all the data involved in the casting process (such as alloy, pour rate, choke area, gate thickness, etc.) and can be a useful tool in the gating design.
Another form of choke commonly used by metalcasters are two types of ceramic filters. One is a honeycomb model and the other allows the metal to pass straight through minute passages. These filters help assure the delivery of high-quality metal into the mold cavity and increase yield.
When casting a new component, it helps to examine the past gating systems of similar castings. If those systems performed well as far as scrap and yield, duplicating the gating will be beneficial. Many metalcasting plants run many similar parts, and the experimentation on a gating system already has been performed, so there isn't any reason not to use it. Further, the use of casting process modeling software always can be of help to ensure the designed gating system will produce the desired results.
Whether pressurized or non-pressurized, most gating systems have a sprue, runner and ingates, but the designs will vary. Keeping the first metal down the sprue, out of the casting cavity is key to a good gating system. A correctly sized and shaped sprue will help accomplish this as well as some other factors in the runner and ingates.
Some high-production molding equipment utilizes a sprue that is exactly the opposite of what is wanted. A correctly shaped sprue would be at its largest at the top of the mold and taper down from there. Contrarily, high-production sprues are exactly the opposite because the sprue at the parting line is largest and tapers up to the top of the mold in order to easily draw the mold away from the pattern.
Such a sprue shape can lead to a turbulent pour in which air is drawn into the metal stream. These errors can be avoided by designing cross-sectional areas in the runner immediately adjacent to the sprue. The cross-sectional areas will be smaller than the area of the sprue and allow the pourer to keep the system full. As long as the sprue is full, the chances of keeping the entire system full are much greater.
An important item that often is overlooked is the sprue base. The purpose of the base is to provide a well for the first metal down the sprue and to eliminate any splashing of the metal; the metal entering the runner should be constant and smooth. Rounded bases should be avoided because they will contribute to splashing and a runner system that initially will not fill smoothly.
The best option is a square or rectangular well, which always will keep the area of the well greater than the area of the runner to provide the needed cushion to prevent turbulent metal flow into the runner.
Another method to limit turbulent pours is direct pour, which utilizes a sleeve with a filter placed in an insulating sleeve. This technique allows a straight or reversed tapered sprue with no detrimental effect, and newer practices allow the sleeve to be placed right above the mold cavity, eliminating the need for runners. This process is particularly effective when there is limited room in the flask caused by a large cavity or multiple cavities.
Extend the Runner
When enough room for runners exists, disputes often erupt as to where to place them--in the cope or drag. There is no correct answer to this because either application will work, though with a pressurized system, a cope runner is ideal and with a non-pressurized system, a drag runner likely will work better. The difficulty with having a non-pressurized runner in the cope is that it will not fill completely before the cavity fills. If the process dictates a cope runner and a non-pressurized system (Fig. 3), a 1-3-2 ratio will work. This ratio means that the choke (1) will be the smallest area in the gating system; the runner (3) is the largest area in the gating system and allows the metal to smooth out before entering the cavity; and the area of the ingates (2) will be smaller than the runner but larger than the choke to keep a steady flow of metal entering the cavity. Each metalcasting facility has different ratios for different molds. The ratios for pressurized systems can be significantly larger (14-12-10) with the last number (ingates) always being the smallest because of the choke location.
A different shape runner and a cope runner generally will work better if it is higher than it is wide because it allows the dross to stay out of the ingates. A drag runner doesn't have to be as deep, but width is important. The dross in a drag runner will be concentrated in the first 6-8 in. (15.2-20.3 cm), so ingates must be located farther down the runner. A step-down runner will work better in the drag when there are multiple ingates that should be fed concurrently (Fig. 4). For this type of runner, use an amount equal to the area of one ingate to reduce the space the metal can flow and force it into the ingate. The step should be placed right where the metal should change direction. With a cope runner, it is more difficult to obtain concurrent feeding through multiple ingates unless a pressurized system is used. If a cope runner with a non-pressurized system is desired, a good practice is to position the runner half in the cope and half in the drag. The runner must be extended past the last ingate so the remainder of that first metal will be kept down the runner and out of the cavity.
[FIGURE 4 OMITTED]
To have the molten metal enter the cavity effectively, proper gating is essential. One of the largest errors commonly made is using incorrectly sized ingates. Often, the system will be correctly sized but with ingates that are too thick (tall), rendering the system ineffective. Ingates always should be as wide and as thin as possible.
There are many types of ingates, (lap gates, swirl gates, etc.) but the most common is the wide flat gate. If a formula suggests an ingate area of 1 in. (2.54 cm), then it would be ideal to have the ingates sized 2 in. (5.08 cm) wide and 0.5 in. (1.27 cm) thick. The thickness will cause them to be less effective in producing an inclusion-free part. Square ingates must be avoided in favor of a rectangular shape. Further, large, thick ingates cause additional labor with gate removal and grinding. They also could cause a shrink at the connection with the body of the casting because there is such a high amount of metal going through a smaller area that it superheats the ingate and makes it act like a riser.
When placing ingates, section sizes must be considered. It is better to direct the metal into a thinner section to reduce mis-run defects and shrinkage problems from the heavier sections. The heavier section still will solidify last, so prolonging that solidification by introducing all of the hot metal into that section should be avoided. If the amount of room in the flask limits the options and forces feeding into the heavy section, utilize a hot riser for that area to eliminate shrinkage. This can be done by feeding metal through the riser, into the main cavity. When using a cope runner, ingates should not be placed under the runner. Instead, they should be tangent to allow the first metal to run by the ingate. This will help in reducing turbulence and result in a cleaner casting.
Cordy Champan is the foundry manager and oversees quality assurance operations at Gregg Industries Inc., El Monte, Calif., where he has worked for 10 years.
For More Information
"Experiments in Steel Gating: Three Foundries Report" T. Hays, G. Hartay and C. Ball, MODERN CASTING, March 2003, p. 35.
Visit www.moderncasting.com to view a sample gating formula sheet used by an iron casting facility.
Gregg Industries Inc., El Monte, California
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|Date:||Jul 1, 2005|
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