Another major difference between investment and sand casting is the surface finish of the final product. With sand casting, the mold has to be split apart to release the final product. As a result, finished parts have a seam left by the parting lines in the mold. And the sand, which is relatively rough, leaves an equally rough surface on cast parts. In many instances, the parting line and roughness are smoothed over in secondary machining processes, but this requires additional time and labor.
Investment casting, on the other hand, uses ceramic molds that are hard. Finished parts exhibit smooth surfaces and low tolerances, and can have thin walls. After removing the gates and sprues, the cast parts often don’t require any secondary processing.
Investment casting uses liquid slurry to form the molds, so cast parts can have almost any shape, giving engineers extraordinary freedom in their designs to add intricate features and complex shapes. Sand-cast parts usually have to be tapered (with draft angles) or shaped so that they come out of the compacted sand easily and with little friction. It’s also a challenge to sand-cast parts with internal voids or cavities. It requires cores properly shaped and inserted in the mold to form the part’s interior. Several cores may be needed, and it can be time-consuming to form and secure them within each mold.
One advantage of sand casting is that it is relatively simple to change the mold (by modifying the pattern or core boxes, often made of wood) to accommodate design changes. With investment casting, technicians may need to modify or replace the solid metal pattern, or make new molds for turning out wax versions, which can be a more complicated process.
Brad Done is Vice President of Reliance Foundry Co. Ltd., British Columbia, Canada.
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