Gating design via computer fluid flow modeling.
A major factor in the successful pouring of castings is the design and execution of the gating system.
Turbulence during the pouring process causes re-oxidation as the melt's dissolved elements (which have a high affinity for oxygen) contact air and form nonmetallic oxides. These oxides cluster in the melt as nonmetallic macro-inclusions and become embedded in the casting after solidification.
Inclusions that float to the surface of the casting and are detected by visual inspection result in costly rework or scrap for the foundry, but subsurface inclusions that are discovered after machining cause greater worry in additional machining costs and manufacturing delays. Subsurface defects at highly stressed areas or at pressure boundaries within the casting that go undiscovered can cause leakage or component failure.
To mitigate the effects of re-oxidation, clean metal programs have been in place in many foundries since the mid-1980s. Useful innovations, such as metal filtration, disposable ladle liners, direct pouring through risers with filters and metal treatment in ladles, have been incorporated with varying degrees of success.
Another new development has been software for computational analysis of metal flow in the mold. These programs are capable of illustrating flow conditions quickly, inexpensively and in great detail. In addition, these simulations provide an unlimited number of iterations with varying geometry and pouring conditions, allowing designers to determine the validity of a design without expensive shop trials and casting inspections.
However, the foundry industry hasn't reached the potential of these programs as current users tend to simulate the filling of individual castings, thus generating information that affects only one part. The realization of the simulation software's potential will be based on the development of fluid flow modeling systems that affect the bulk of a foundry's castings.
During 1995-97, Goulds Pumps, Inc., a 130-employee steel foundry in Ashland, Pennsylvania, applied this theory to its development of pouring systems. It evaluated some of the accepted theories of pouring systems as well as the designs it had developed for its castings by testing them through the Procast computer simulation software from UES, Dayton, Ohio.
Modeling Accepted Beliefs
The foundry's initial fluid flow models determined that some of the accepted flow behavior assumptions in gating system design may be incorrect. Figures 1 and 2 demonstrate this point. Figure 1 illustrates a comparison of a typical flow line illustration found in gating literature and a 3-D fluid flow simulation using an equivalent sprue design. The simulation model (r) demonstrates the actual turbulence, void zones and sprue overflow that occurs in a pouring cup, which is absent in the gating literature version. Figure 2 illustrates the comparison between the assumed flow lines in a sprue well and the simulation of the process. The expected calm melt, as illustrated in gating literature (1), isn't reflected in the simulation.
Similar discrepancies between accepted design rules and fluid flow modeling observations can be found when evaluating other components of the gating system such as sprue wells, runner bars, runner extensions, ingates, 90 [degrees] bends and "T" junctions. Computer simulations have the capability of capturing the action of a flowing melt every 1/20 of a second. The only comparable technique available to foundries is X-ray, but it can be performed only on a limited basis and at a high cost.
Pouring Cup Simulation
A critical area of gating system design, especially with the parting line system, is the sprue cup combination. Many of the casting defects associated with air aspiration and liquid metal turbulence are influenced by the design and execution of this part of the gating system. Nevertheless, this area is one of the most overlooked in the design because the rules available in technical literature are too general and often lead to poor results.
For example, choked tapered sprues are a proverbial recommendation of gating manuals. The bottom section of the sprue, called the choke, is calculated to deliver a constant flow rate into the mold cavity, and the top section is calculated to represent the shape of a vertically free falling stream. The pouring cup or basin is an enlargement of the sprue top to permit a wide enough target for the pourer. In theory, these designs are intended to maintain a filled sprue and constant metal height throughout the pour, thus delivering a constant flow rate with minimal turbulence and air aspiration.
In practice, these designs do not work, and few foundries use them. They are not suitable for bottom pouring when a constantly changing flow rate exists. For lip or teapot pouring, variables such as the ladle stream cross-section area and stream impingement angles must be factors in the geometrical design of the sprue cup system.
Another factor of choked tapered sprues is their lack of practicality at commercial foundries that contend with wide variations in cope heights and pour weights. The use of calculated chokes with tapered sprues requires a different size sprue in each mold, making the entire process unmanageable. This approach also precludes the use of standardized preformed sprue cups that provide shape consistency, production economy and the application of high-quality resistant materials.
For these reasons, many foundries purchase cylindrical sprue cups made of shell sand or mineral fiber materials. They are inexpensive, simple to use and can be cut to a desired height without altering the outlet diameter. However, they are a source of air aspiration, turbulence and associated casting defects. In the long run, they are very costly.
For a foundry to use engineered, preformed sprue cups effectively, it must standardize its processes to a few sprue shapes and sizes that are used across a wide range of pour weights and cope heights. Using computer simulation, a foundry can achieve this standardization and optimize the pouring conditions. Goulds Pumps tested a variety of sprue cup designs in hopes of eliminating turbulence and air aspiration. Following is a an excerpt of its computer simulation experiments with an analysis of each design.
As illustrated in Fig. 3, the CAD solid model to test the sprue cup variations was very simple. The finite element mesh of the test casting contained 45,000 elements. The fluid flow analysis setup requires the input of various physical properties of molten metal and the mold, interface coefficients and boundary conditions. The pouring velocity was approximated using actual pouring time data.
A greater challenge was to model the impingement angle of the metal stream from a top pour ladle. It was accomplished by videotaping several pours and taking measurements on the video screen. The cross-section diameter and angle of the stream were incorporated into the CAD model and the simulation preprocessor.
In the first test example, as shown in Fig. 4, the metal flowed into a standard 1.5-in. inside diameter cylindrical sprue. Shown at 0.5 sec of elapsed time on the left, a large portion of the sprue remains void along its length, and a spiral (corkscrew) motion begins to take place. These conditions indicate massive air aspiration and turbulence.
The second stage of the example in Fig. 4 illustrates this simulation after 1 sec. The view clearly demonstrates that the sprue is at least 50% void from top to bottom. The desired full sprue conditions were never achieved during the pour.
In the second test example, as shown in Fig. 5, the metal flowed into a tapered sprue (4 in. x 4 in.) with a tapered transition that was designed using widely accepted gating calculations. The tapered transition eliminated the spillback associated with the flat bottom of squared pouring cups. Nevertheless, a vortex begins to form at 0.5 sec. This condition takes place at the transition between the squared cup and the round sprue. Rounded sections invariably cause vortexes in sprues, and the anti-swirl flats on sprue funnels are not effective in stopping vortex formation once the metal reaches a rounded section.
The second stage of this pour in Fig. 5 shows that the vortex has taken full form at only 1 sec. into the pour. Unfilled areas also are visible along the sprue. Although the lower sections of the sprue will eventually fill, the vortex will remain throughout.
In the third test example, as shown in Fig. 6, a squared sprue section configuration (4 in. x 5 in.) represents a major improvement over previous designs. With the choke offset toward the front, vortex effects are absent and full sprue conditions are attained from the onset of the pour. However, as shown in the second stage of Fig. 6, despite the good initial pouring conditions, the sprue cup was not able to keep up with an increased metal delivery of 19.5 lb/sec at a more pronounced angle of 50 [degrees]. The result is a spillback of metal over the top back of the cup. In addition, a momentum effect caused by the design of the sprue cup connection compounds the overflow problem.
The fourth test example, as shown in Fig. 7, is a modification of the one shown in Fig. 6. The sprue cup measures 4 x 5 in., and the choke is 0.81 x 0.81 in. The same choke cross-section was used while the front face of the sprue was enlarged. Initial conditions, as shown in the first stage, are excellent. The sprue quietly begins to fill from the very outset of the pour.
In the second stage of Fig. 7, a steady metal level has been achieved with full sprue conditions after 1.8 sec, despite the severe stream conditions (19.5 lb/sec at a 50 [degrees] angle). These results are what is required to keep re-oxidation defects at a minimum. The simulation showed these conditions throughout the entire pour without an overflow. Actual foundry trials confirmed these results. When a comparison is made through the simulation between the design that had spillback and the effective design, the differences in metal flow are clearly demonstrated.
As a result of the fluid flow modeling, Goulds Pumps converted 200 of its patterns to accept the 0.81-in. squared choke sprue cup [ILLUSTRATION FOR FIGURE 8 OMITTED]. It replaced a 1.5-in. diameter cylindrical sprue used for castings up to 400 lb. The quality indicators for the steel foundry have been at an all time high since the switch.
A limited investigation was performed on a 16 in. diameter, 1 in. thick disc stainless steel casting that was previously poured with a filter system. dye check on the cope face after machining showed that the castings poured with the new cup and no filter had the same or marginally better inclusion-free quality as the castings produced with the old cup and a filter.
Beyond testing, the main indication of success was the feedback from the pouring crew. Pourers stated that mold pouring was smooth with the new sprue design as they were able to maintain a filled sprue cup from the outset. They also reported that the raising metal front in open risers is consistently clean. This feedback was sufficient enough to pursue development of another pouring cup for castings between 400-900 lb.
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|Date:||Jun 1, 1998|
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