By Charles Bates, David Burch, and Mike Johnston
The lost foam casting process continues to advance casting technology. One of the most significant recent advances is the use of CAD systems by casting designers that allows early interaction with casting producers, pattern makers, and tool builders. Software now allows casting designers to prepare a part and send it to the foundry for review. Then, the foundry can suggest changes, add draft, optimize it for manufacturability, select locations for gates, add risers if needed, and return the drawing to the designer for review.
When the part has been reviewed and approved, it is sent again to the foundry electronically, where it can be divided into sections for tool production. At this point, the foundry usually makes suggestions for parting lines for individual tool sections, and forwards the drawing to the tool builder. The tool builder may offer suggestions for changes in pulls or parting lines, and then make final decisions and begin tool cutting. Part dimensions can be taken directly from the part drawing, shrinkage can be added, and then the part can be sent to the shop for tool path definition.
|Fig. 1. Offset hangar bracket in its use position. The bracket is bolted to a truck frame and the bumper and leaf springs attached. Fig. 2. Offset bracket rotated for hole review, pull selection, and parting line determination. |
An example part is shown in Figure 1, an offset hangar bracket. The bumper of a truck attaches to the front of the bracket, the frame bolts to the side, and the leaf spring attaches to the bottom bracket. This is the kind of a drawing that a design engineer might provide.
This part and its dimensional requirements can be viewed and immediately interpreted by the foundry and the tool maker by rotating it, as illustrated in Figure 2. This and other parts can be quickly inspected for hole size, location, center-to-center requirements, and other features with relatively inexpensive viewers like ProEngineer, SDRC Unigraphics, SolidView, Catia, and others. Paper drawings made in two dimensions have been replaced by electronic drawings that can be wired anywhere in the world in minutes, reviewed, and sent back. It is a wonderful technology, but an equally wonderful visualization aid for foundrymen, designers, and tool makers.
A few years ago these processes took months to complete, but now take just days. A part drawing recently sent to a foundry was reviewed one afternoon and forwarded overnight to the tool maker to begin cutting the tools. Fixtures for gluing part sections can be begun almost as soon as the various tools for sections of the part have been started.
What is the advantage of this technology? It allows unibody construction to consolidate parts with reduced machining, welding, and bolting of subcomponents. Eliminating welding, and the loss of strength associated with welding, can reduce the finished weight of parts significantly.
|Fig. 3. Photo of a center yoke for an axle development program. |
Another lost foam application is illustrated in Figure 3, a center yoke for an axle development program. This ductile iron casting has several thin flanges radiating from the yoke body in orientations that would make sand-casting core placement relatively difficult.
Where does one start today to design and produce a lost foam casting? A design engineer normally devises a part concept and starts discussions with a foundry. At this point, a good foundry will ask the part designer for a picture or a CAD drawing, not only of the part in question but the associated connecting parts in the area.
Consideration should be given to consolidating surrounding parts into the part of primary interest. Once consolidation has been considered, the drawing can be reviewed by the design engineers and the foundry, and a stress analysis conducted before tooling preparation has begun.
Such an analysis can be depicted graphically, with locations of higher stress highlighted in color variations. Not only can the electronic drawing be fitted to other components, but stress analyses can be conducted to find “hot spots.” The part can be redesigned if necessary to reduce the stress.
A significant advance for lost foam casting is that with visualization and design tools, part designers are working with foundrymen, and foundrymen are working with molders, to get the steam flow and cooling water flow through the tool to make foam patterns quickly and economically. We have learned that the steam chest must be sized to the tool to keep the cycle time down, improve machine speed, and minimize costs. Cooperation is essential to producing such complicated tools efficiently and quickly.
Electronic drawings allow relatively easy communication between the tool maker and the foundry, and between the foundry and the part designers. Parting lines can be placed at mutually agreed locations; water cooling lines can be placed to cool the foam where needed; and steam ports and vents can be placed to facilitate bead fusion, without over-fusion.
With these aids, designers and foundries can be more confident deciding what can or cannot be done. Until recently, the standard approach was something like this: Engineers designed a part and sent it to a foundry; the foundry made some guesses about the tool; the tool makers prepared the tool and shipped it to the pattern molder; molders made some parts and shipped them to the foundry; the foundry took the parts, put them on a tree, coated them, invested them in a flask, poured, and then prayed. CAD tools have made the process much more robust, allowing each person in the design process — foundry tooling, foam production, and foundry process — to interact seamlessly.
|Fig. 4. Cast developmental front axle assembly. |
The transition to good, solid models was the critical step. A few years ago, the best you could hope for were six-sided parts shown on a piece of paper, with one side visible at a time. Interpretation was difficult, required experience and imagination, and could take days or weeks to interpret. And, interpretations were sometimes wrong. Parting lines were difficult to locate and pattern consistency was a problem.
Today, complex assemblies can be produced with minimal steps, reducing costs, and improving quality simultaneously. Figure 4 illustrates two axle castings that replace what once would have required a complex combination of cast, forged, and welded parts to make the assembly.
Another advance has been the willingness of coating manufacturers to work with foundries to develop specialty pattern coatings. These coatings can be formulated to allow gas escape during pouring, reducing the occurrence of laps and folds. This step allows an even broader application of the techniques outlined above, to produce finished parts with more precise and intricate details.