One of the reasons for having parts made by investment casting is that the process is capable of maintaining tight tolerances. This minimizes secondary machining and enables reproduction of fine. What’s left unasked though is, what tolerances can investment casting maintain?

This question is answered by investment casting accuracy grades. If you’re specifying or sourcing investment cast parts an understanding of these grades will help you select a capable metal casting foundry.

The Accuracy of Investment Casting

Casting is the process of creating metal shapes by pouring molten metal into a mold and letting it solidify. Molds can be either permanent, meaning they’re reused, or expendable, in which case they are made, used once, and broken apart.

Investment casting is an expendable process where molds are made by coating a wax pattern in a ceramic shell. The wax is melted out to leave a cavity. Cores are placed in the cavity to create voids in the cast part. Metal is poured in and left to solidify, after which the shell and cores are broken apart. Parts are cut away from the metal distribution channels and sent for finishing.

This process is more accurate than the other well-known expendable mold process – sand casting – but less so than permanent mold casting. However, the tooling is less expensive and lead times are shorter.

Reasons for Inaccuracies in Casting

The heat of casting causes a lot of mold expansion and contraction. In addition, with few exceptions, the cast metal shrinks significantly as it solidifies. This results in a degree of part-to-part variability. Sand casting suffers additional variability due to movement of the sand, which doesn’t happen in investment casting.

Other sources of variability are pattern wear and variation in pattern assembly and core placement.

Grading Casting Process Accuracy

ISO 8062 “Dimensional and geometrical tolerances for molded parts” provides a common language for comparing casting accuracy. For foundries, Part 3 of this standard is of greatest relevance.

The standard provides an allowable tolerance per unit length. That is, longer linear lengths have correspondingly larger tolerances.

Three tolerance bands are defined – Coarse, Intermediate and Small (CT, IT and ST) – and each is broken into a series of levels. Within the CT band, the band most commonly used by foundries, there are 16 tolerance levels, ranging from CT1 to CT16. These progress from tightest (CT1) to loosest (CT16).

Investment Casting Accuracy Grades

Investment casting foundries usually commit to maintaining an accuracy grade between CT4 and CT8. To quantify this, at CT4 the tolerance on a dimension of 40 to 63 mm is 0.36 mm. At CT8 the tolerance on the same dimension is 1.4 mm.

For comparison, sand casting typically achieves between CT 8 and 9.

The reason for the number of accuracy grades in investment casting comes down to the ceramic used. Silica sol is capable of CT4 but is also the most expensive. Less costly sodium silicate is usually specified for accuracy grades of CT 7 to 8.

Part design and the alloy used also play in part in the accuracy achievable. Discuss your requirements with an investment casting specialist to determine what’s practical.

Reasons to Consider Casting Accuracy Grade

If seeking to minimize machining and ensure a high level of part-to-part consistency, it helps to specify the casting grade required. Conversely, when evaluating foundries, evidence of accuracy grades achieved helps with making comparisons.

Another reason for using these ISO grades is that they provide a common language when discussing or setting expectations for casting accuracy. A foundry that is unfamiliar with the standard should probably not be trusted with your business!

Ask us About Investment Casting

Impro has made substantial investments in investment casting technology in both Asia, Europe, and Mexico. If you’re looking for a source of high quality castings, we can help. Talk to us about your volume, quality and price expectations and you may be pleasantly surprised.