Design Considerations

Most of the same design considerations for conventional investment wax patterns apply to QuickCast build style patterns also. These considerations include sharp corners, wall thickness, holes, undercuts, draft, dimensional accuracy, etc.

Although sharp corners are achievable in the investment casting process, generous radii are preferred whenever possible. Shell investment casting involves pouring molten metal into a fired ceramic shell, creating stress concentrations at sharp corners. Solidification is accelerated at external corners and retarded at internal corners, causing residual stress buildup leading to the formation of cracks in the shell. Ample fillets and radii facilitate the flow of the molten metal into the shell. This will result in better quality and more dimensionally stable parts. Input from the foundry engineer will identify problem locations and provide suggestions for change. Corners and fillets should have minimum radii of 0.040 in. (1.0 mm). If your CAD system is unable to perform this function adequately, it may be accomplished manually with the foundry by adding wax fillets.

The minimum thickness of casting walls is determined by the fluidity of the metal to be cast. Another factor is the length of the section involved. If the wall is long, a heavier wall may be required. The ratio of thickness to height and location of the adjacent heavy sections are a factor. The minimum wall thickness of cast parts is generally 0.025 to 0.060 in. (0.600 - 1.5 mm) but foundry recommendations should be adopted. QuickCast build style walls thinner than 0.040 in. (1.0 mm) are difficult to drain. For best results in designing and spacing thin cooling fins  or heat sinks, communicate with your foundry. Resin expansion, metal  flow and foundry gating are factors in wall thickness design.

Small holes are susceptible to shell failure during firing. As an example,  by omitting holes for screws and fasteners, you increase the part yield and these can be easily machined later. If holes cannot be eliminated, the length to diameter ratio should be not greater than L/D=1 for blind holes and L/D=2 for through holes. If the holes are smaller than .125 in. (3.0 mm) the foundry may require alternate techniques to shelling, such as coring the holes.

Undercuts are more producible on QuickCast style patterns than conventional wax patterns and draft is not required, because the need for tooling is eliminated.

There are two major factors that influence dimensional accuracy of QuickCast style pattern investment castings:

The first factor is the shrink rate which you would need to obtain from the foundry. This is the shrinkage rate of the metal as it changes from a molten to a solid state. Although shrinkage is predictable, there is some variation in the amount of shrinkage on individual features, which result in the loss of dimensional accuracy. You may find that additions of machine stock may be necessary in insure the machining aspect of the part and working with the foundry will help you define these areas as well.

The second factor is the QuickCast style pattern itself. Remember that whatever your pattern is dimensionally, the casting process will reproduce minus the foundry shrink rate.

A thorough dimensional check of key characteristics on the patterns in advance has the potential of saving you a substantial amount of money and time by helping to avoid problems with the fit, form and function of the castings at a later stage. See Incorporating Metal Shrink Rate section of this guide.

Typical linear tolerances published in investment casting handbooks and adopted by foundries are shown below. These tolerances are repeatable under production conditions. Tighter (premium) tolerances can be cast, but require added operations at additional cost. You must communicate any special requirements to the foundry.