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” process.) Molten metal is poured into the cavity and after solidification the shell is broken apart to release the casting parts.
Breaking the shell avoids any need for draft angles to let the cast part release. However, there is still the question of ejecting the wax pattern from its mold.
Various types of wax are used for investment casting patterns but all tend to exhibit significant shrinkage. This results in the wax pulling away from the walls of the mold as it solidifies. Ejection is therefore usually very easy, so no draft angles are needed.
Caveats to The “No Draft Angles” Rule
There are situations where the investment casting tooling designer may add draft angles.
- If shrinkage is insufficient for good release
- In molding of cores
Wax cools faster further from where it enters the mold and where the ratio of surface area to volume is higher. This influences where shrinkage occurs: regions slower to solidify will shrink more. (Mold heating may be used to manage solidification rates.)
If this shrinkage limits how the pattern releases from the mold the designer has some options. They could reorient the pattern in the mold or they may need to add a small amount of draft to some surfaces.
Cores are used in investment casting to form internal features like ports, passages and cavities. Cores may be produced either from soluble wax or a porous ceramic. Soluble wax cores are molded in the same way as the principal wax patterns and typically have enough shrinkage to not need draft.
Ceramic cores, used because they will remain in the pattern and be integrated into the ceramic shell, are also produced by molding. Depending on geometry, these may need draft on some surfaces for easy release.
Talk to Your Foundry Early in The Design Process
The absence of draft angles is one of the advantages of investment casting over other casting processes. It can save on machining, simplify part fixturing for machining, and avoid constraints on part function.
For most geometries no draft angles are needed. However, there are situations where some draft makes the part easier to cast. Experienced investment casting specialists are best qualified to advise on such exceptions: the prudent designer will initiate a discussion before part design advances to a point where changes are costly. Contact us to start that process.