Casting Tolerance is the largest dimension range that can be allowed between two casting dimensions. It can be affected by a number of factors, such as wax temperature, injection pressure, mold or shell composition and casting equipment. Although there is a tolerance standard in the investment casting industry, it actually varies from foundry to foundry. XinLiLai casting foundry can produce products with low tolerances with minimal surface finishing or machining required. In addition, our tolerance standard can be adjusted to meet your specific process capabilities. XinLiLai is always prepared to meet your investment castings needs, even if tighter tolerances are needed. Call us to discuss your specific tolerance requirements.
Normal linear tolerances for normal dimensions reflect three sources of variation, prediction of part and shrinkage factors, diemaker and tooling, and process variation. Therefore, the linear tolerances of different casting process are not the same. A broader general linear tolerance between CT7-CT8 is applied by water glass investment casting, while, more precisely, CT5-CT6 is for silica sol investment casting.
Geometric tolerances are normally applied to the following features of investment casting: Flatness, Straightness, Parallelism, Roundness, Hole Tolerance, Curved Holes, Internal Radii and Fillets, and so on.
Flatness is measured as the distance between two parallel planes, within which the entire surface of the part must lie. Volumetric contraction of cooling wax and metal alloys determines the flatness variance of the final investment cast part. Shrinkage always causes some amount of dishing, though this effect will be mitigated to tolerances by applying specialized investment casting techniques. Flatness tolerances vary significantly with different types of metal alloys and the physical structure of the cast products.
Straightness, Roundness, & Positioning (holes or bosses)
The standard tolerance is ±.005 per inch for axial straightness, roundness (bar or tube shapes) and positioning of holes or bosses.
If your design includes parallel prongs supported only at one end, this presents a particular challenge for the completion of precise investment casting. Our engineers will discuss these types of parts with you prior to production.
Parallelism of these prongs is difficult to maintain due to volumetric shrinkage of the supporting structure. Careful design adjustment for volumetric shrinkage is critical to achieving an accurate part. These work pieces benefit from more extensive tooling and finishing.
The concentricity is dependent on measurements taken from the same plane, and does not include out-of-roundness tolerance. If this is the case, the center of a bore to the outside diameter of a part will be concentric within .003" for each 1/2" of wall thickness.
The as-cast angle tolerance is a function of the part design. A minimum tolerance of ±1/2 degree is considered standard.
The standard minimum wall thickness for investment casting lies between .040" and .080". Wall thickness as low as 0.020" has been achieved within a small area. Overall wall thickness is dependent on the alloy used and the size of the surface area.