The Basics of Feeding Steel and Ductile Iron Castings.
In casting design, there are two major considerations--the quality of the final component and the yield of the casting. Both of these considerations depend upon the feeding system used to cast the component.
The design of the feeding system starts with the determination of the parting plane. The next step is to identify the gate locations to allow uniform feeding of the casting. Next, an appropriate runner geometry is selected based on where the gates are located, and the sprue location is determined so that it will be as far from the nearest gate as possible. Sizing of the feeding system's elements is done using the geometry of the part and some simple suggestions. Traditionally, these activities have been performed by methods engineers based on their training and experience. As a result, several trial feeding designs can be required in producing a sound casting.
This article presents the similarities and differences in the feeding system design of steel and ductile iron castings by providing suggestions used by foundry experts and guidelines recommended by researchers for better quality castings. These suggestions, which are not intended as industry-wide standards, are merely recommendations to feeding system designers. Also, while the suggestions are listed as they relate to different components of the feeding system, the design of individual components is not independent of the others.
Materials with a short freezing range [liquidus-to-solidus interval less than 122F (50C)] form a skin and solidify parallel to mold walls. This can lead to "centerline shrinkage, which is a collection of shrinkage voids along a line at a relatively thin section of the casting. Since centerline shrinkage is a common failure in steel castings, proper feeding must be provided through a well-designed feeding system to manufacture sound castings. Initially, the following suggestions should be observed in mold design:
* since the gating system is exposed to hotter metal than the part being cast, a higher quality sand should be used in the system regions of the mold;
* the gating system should be kept simple due to the high viscosity of molten steel;
* when a nozzle and stopper system are used to fill the mold, the size of the nozzle should be slightly smaller than the sprue;
* the partial reversal method is used where the mold is turned 30-40[degrees] to place the hot metal on the top of the mold and colder metal (which first entered the mold) at the bottom. Complete reversal of the mold is usually not practical, especially for large castings.
Sprue Suggestions--Sprue wells, which are used in sizes of 1-2 in., are built with a different material than the one used in the mold, such as clay brick, sand with high-proportion silica flour or cement-bonded alumina for higher resistance to erosion. If cold shuts are detected or the fine details of the casting are improperly filled, the sprue cross sectional area should be increased.
Parting Plane Suggestions-Placing the parting plane at the mid-height of the casting provides the advantage of filling the bottom of the mold with colder metal (due to the initial temperature of the sand), which promotes directional solidification, However, high drops of the metal should be avoided.
Riser Suggestions--In designing risers for steel castings, knock-off risers with star-shaped apertures are suggested as well as rectangular vents that allow the escape of the gases during the filling of the mold. Other suggestions include:
* eliminating the gating system for small and shallow steel castings. In this case, the mold can be filled through a riser if the casting is filled using a ladle. Filling from a large bottom-poured ladle cannot be done because of the metal stream's high velocity;
* maintaining the maximum feeding distance for a steel plate of thickness Tat 4.5T. The distance is 4T if there is no edge contribution. For a steel bar of thickness T, the feeding range is about 6 T if there is edge contribution, and 0.5-2T if there is no edge contribution. The addition of chills can increase the feeding distance by 2T for plates and by 1T for bars. The maximum feeding distance may be extended by using a taper or insulating material;
* eliminating blind risers located below an open riser with a heavy section connecting them;
* using the modulus approach to size the risers.
A formula for the calculation of the riser dimensions is:
[[D.sup.2].sub.R][H.sub.a] = 24 [FW.sub.c]/[phi][rho]
where [D.sub.R]: diameter of the riser;
[H.sub.a] : active height of the riser;
F: feed metal requirement (Fig.1);
[W.sub.c]: weight of the casting;
[rho]: density of the metal.
Substituting [H.sub.a] = [D.sub.R] and r= 0.29 lb/[ft.sup.3] results in the following:
The following cubic equation is suggested for the calculation of the riser diameter for steel castings:
[D.sup.3]-4[delta](1+[beta])(a+b/4g)[M.sub.c] [D.sup.2] - 4[beta][V.sub.c]/g[pi]=0
where D: riser diameter;
[delta]: safety factor;
[beta]: fractional total volumetric change on freezing;
g: riser height/diameter ratio;
[M.sub.c]: casting modulus;
[V.sub.c]: casting volume;
a: riser sidewall insulation faction;
b: riser top-cover insulation factor.
Gate Suggestions--A common gating ratio for steel castings is 1:4:4, where the relative cross-sectional areas of sprue is 1; total runners is 4; and total gates is 4. Gates usually are located in the cope and can be curved to streamline the flow toward the casting. For circular parts with spokes connecting the rim to the hub, core gates promote directional solidification. This allows the metal entering the mold cavity to cool while going through the spokes and lets the cool metal fill the rim of the casting away from the riser that will be located on the hub. Keep in mind that horn gates are used for small castings, but they are not economical or easy to mold. Other suggestions to follow include:
* designing round gates instead of square ones of the same cross-sectional area, since round gates minimize friction and result in larger filling rates;
* making sure the diameter of a whirlgate, a slag-cleansing device often used in steel gating practice, is less than the diameter of the sprue, and the cross-sectional area of the whirlgate is greater than the sum of the cross-sectional areas for the gates;
* for large, flat-bottom castings, designing the gated end of the casting to be low to force the metal to run a slight incline;
* designing the cross-sectional area of the gate to be smaller than that of the casting at gate-casting interface;
* designing gates for steel castings to be larger than those used for cast iron castings. If gate sizes are smaller than adequate, cold shuts can be formed where two streams join around a core;
* filling plate castings with multiple gates to minimize erosion of the mold. The cross-sectional area of the individual gates need not be larger than the exit area of the sprue;
* maximizing the number of gates to prevent hot spots;
* flaring the gate toward the casting if a single gate fills the mold;
* in circular parts such as gears or wheels, designing the gating tangentially to the gear to let the metal stream go around the periphery of the casting and prevent the erosion of the core;
* using saxophone-type step gating for deep molds. The gates come off the sprue at several different levels and slope upward. The idea is to fill the different levels of the casting with corresponding gates.
Runner Suggestions--Basic suggestions for runners follow the same common 1:4:4 gating ratio. In addition, the runners, which usually are located in the drag half of the mold, should have a depth that is shallow at the sprue and progressively deeper toward the end. The runner extensions are used to trap slag. Hollow cylindrical castings should be cast with gates and runners inside the casting. This kind of gating delays the solidification of the gates and the runners and reduces the chances of cracks due to contraction of the casting. A disadvantage to this type of gating, however, is the difficulty in cleaning the feeding system after the part is cast.
Ductile Iron Castings
Two types of gating systems are used in ductile iron castings--pressurized and nonpressurized. The difference between the two is the location of the choke, which is the minimum cross-sectional area in the gating system that determines the mold-filling time. For pressurized systems, the choke is located between the runner and the gate, whereas it is located between the sprue and the runner for nonpressurized systems.
Nonpressurized systems are used when a large number of small castings are cast in the same mold with small choke-area requirements. In most other cases, pressurized systems are used. A combination of both nonpressurized and pressurized systems can be used for castings that require a complicated runner system.
Figure 2 can be used to determine a recommended value for the pouring time.
Parting Plane Suggestions--In designing the parting plane for ductile iron castings, minimizing the need for cores and placing heavy sections in the drag is recommended. Place all or most of the casting in the cope for quiet mold filling. The disadvantage of this placement is that the yield will be small for castings that require a short filling time.
Sprue Suggestions--Avoiding the use of parallel sprues and not using them as chokes are a couple of the suggestions in designing sprues for ductile iron castings. Others include:
* locating the sprue symmetrically;
* for nonpressurized systems, estimating the choke cross-sectional area using Fig. 3. For multiple casting molds, the total choke area is the sum of all the choke areas downstream of the sprue. Locating the choke area at the bottom of the sprue for tapered sprues or between the sprue box and the runner also is recommended.
The minimum sprue cross-sectional area can be calculated using the following equation:
[A.sub.sprue] = [A.sub.choke] [square root of]H / h
where [A.sub.sprue]: cross-sectional area of the sprue;
[A.sub.choke]: total choke cross-sectional area; h: height of metal in the pouring basin; H: vertical height of molten metal in the sprue.
For downward tapered sprues, H is measured to the smallest cross-section of the sprue.
Riser Suggestions--Risers should be designed one of three ways: conventionally, as in steel castings; riserless with multiple gates for castings not poured in green sand; or with a single riser and multiple gates. Other suggestions for designing risers for ductile iron castings include:
* using blind risers;
* minimizing the number of risers by feeding multiple sections of the casting with a single riser;
* making the height of the riser larger than its diameter;
* using standard risers to expedite the volume and modulus calculations;
* using breakoff or Washburn cores to reduce the cost of riser removal and cleaning;
* connecting side risers to the casting with a short passage choked at the middle section (riser contact);
* using modulus calculations in riser sizing;
* selecting the risering method (pressure control, directly applied or riserless) using the casting modulus.
Gate Suggestions--For both pressurized and nonpressurized systems, suggestions for gates include:
* if multiple, gating identical castings that are cast in the same mold all in the same manner;
* making sure the minimum gate length is 5 times the gate thickness;
* determining the recommended gate thickness for a given pouring temperature using Fig. 4.
The choke area for pressurized systems obtained from Fig. 3 is the total choke area or the sum of the gate cross-sectional areas. For multiple chokes, each choke area is selected using the weight calculated by dividing the sum of the weight of the castings and risers by the number of chokes. For pressurized systems, make sure:
* the junction between a gate and a runner is such that the bottom surface of the gate is placed on the same plane as the bottom surface of the runner;
* gates come off the runner at a right angle;
* the total area of gate overlap on the runner is 10% more than the choke area;
* gate overlap on the runner is slightly more than the height of the gate;
* two gates are not located on opposite sides of a runner at the same point;
* gates are thin and wide. A height-to-width ratio of 1:4 is reasonable;
* two thin and narrow gates are used instead of one thick and wide gate;
* the minimum gate length is equal to the gate width (the gates can be as long as needed);
* the gates are straight or curved.
For nonpressurized systems, the gate always is located in the cope and its bottom surface should be placed on the top surface of the runner.
Runner Suggestions--Avoiding the use of stepped or curved runners is recommended, and if curved runners are necessary, locate gates as far from the curved sections as possible. Suggestions for pressurized systems include:
* using tall, narrow runners. A height-to-width ratio of 2:1 is reasonable;
* making sure the cross-sectional area of a runner is 3-4 times the sum of the cross-sectional areas of the gates on that runner. The ratio of the runner cross-sectional area to the choke area is between 4:1 and 2:1;
* using tapered blind ends with 20% slope;
* employing a well at the end of the blind end (runner well), which is of particular use when there is not much space in the mold for proper runner extension;
* making sure the minimum distance between the sprue and the first gate on the runner is about 4 times the height of the runner;
* making sure the minimum blind-end extension is about 4 times the runner height;
* making sure the length of the runner extension and the distance between the sprue and the first gate are the same. If this cannot be attained, the latter can be made one-half of the former.
For nonpressurized systems, make sure the runner:
* has a square cross-section at the choke section;
* is always placed in the drag;
* is tapered such that right after the choke, the runner height is 4 times the height at choke, and past the last gate, it is equal to the height at choke.
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|Comment:||The Basics of Feeding Steel and Ductile Iron Castings.|
|Date:||Mar 1, 2000|
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