Edges & Corners are external features such as you would see on any cube such as a child’s Alphabet Block. Structurally Edges & Corners have little impact on the strength of a casting. However, Edges and Corners are still an important consideration of casting design.
Investment Castings will naturally exhibit a .008” – .012” radius along an edge even when tooled “sharp”. I often say that foundries “don’t cast razor blades” (but then even razor blades are not truly “sharp” either). The freezing of the wave front and back pressure from air in the cavity prevent the liquid metal from achieving a truly “sharp” edge. Casting Designers should provide for this condition on their designs with an allowance of at least a .015” R Max along cast edges & corners.
Foundries, however, generally prefer not to have “sharp” edges and corners on cast parts. In Investment Casting a sharp edge can create a weak point in the ceramic shell mold. (If you’ve ever tried to paint a sharp corner you’ll understand that it can be very difficult to build thickness along a sharp edge.) A vulnerable “thin” area in the ceramic shell that may cause the shell to crack from the stress of the De-Wax operation or fail as the mass of the hot metal is being cast. O’Fallon Casting recommends that a radius roughly equivalent to the wall thickness be allowed on all external Edges & Corners.
OFC also recommends that dimensions for Corner & Edge radii have an R-Max tolerance applied. An R-Max tolerance provides flexibility to the pattern mold designer to strategically omit the Edge Radii along parting planes in the mold. Omitting an edge Radii allows the parting plane in the pattern mold be located at the top of features which simplifies the design of the mold and makes it less expensive to design and build. Cosmetically, in this manner, the parting lines on the casting will blend with the “Sharp” edge. (Edges along non-parting surfaces in the pattern mold will be cut with the full radii.) An R-Max allows for a Sharp Edge at parting planes that will reduce the cost of the Pattern Mold and also improve the cosmetic appearance of a casting.
If for the function of the casting it is necessary for edges and corners have a radius this can certainly be accommodated. A design includes a ± tolerance (rather than an R-Max) for the Edge Radii the parting planes then need to be placed adjacent to the root of the Edge Radius rather than at the top of features. This requirement will increase both the design and construction time of a pattern mold and so also its expense. Also a casting produced from a tool cut with an edge radius at the parting surfaces will exhibit a residual parting line adjacent the root of the edge radius and that will likely grow heavier as the pattern mold wears.
Edge and Corner radii are an important consideration for casting design. Even when tooled sharp, castings will display a slight radius that should be noted on a drawing as .015 R Max. Foundries prefer that casting designs permit an Edge Radius equivalent to the wall thickness and recommend that they be designated with an R-Max tolerance so that the radius may be omitted from parting surfaces in the pattern mold.