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Draft Angles in Investment Casting

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Draft Angles in Investment Casting

Draft angles on cast parts limit the designer’s freedom and complicate secondary machining operations. One of the advantages of investment casting over other casting processes is that draft angles can be practically zero.

This blog post explains what draft angles are, why they are applied, and how, with a few exceptions, the investment casting process manages without them. It will give part designers a more detailed understanding of the issues involved and the reasons for considering this near net shape manufacturing technology.

Draft, and Why It’s Needed

A draft angle is a shallow gradient applied to what would otherwise be a square surface. Draft angles are used to let the casting pattern and the cast part come out of the mold easily. They are put on the vertical surfaces of the mold where they widen out going from the bottom of the mold cavity to the top.

In sand casting a draft angle reduces the tendency of sand particles to come free as the pattern is lifted out. Typical draft angles are 1.5°. Die casting also uses draft angles, albeit smaller than those used in sand casting, to help eject the cast part.

How Investment Casting Minimizes Draft Angles

Investment casting is a complex multi-step process. First, a pattern replicating the part to be made is molded in wax. This is done in a type of injection molding process using an aluminum mold tool typically.

After the pattern is ejected from the mold, wax runners and risers are added and the assembly is coated in a ceramic slurry. This is dried to form a hard shell, after which the wax is melted out, (hence this being called the “lost wax

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