Investment Casting Surface Finish
One of the reasons for choosing investment casting over other casting processes is its superior surface finish. This blog post will look at what it is about the process that results in these finishes, the finishes achievable, and the benefits of a better finish.
Mirroring the Surface
The investment casting process starts with a wax replica of the part to be cast. This is coated with a wet ceramic refractory material, (a heat-resistant ceramic,) that dries and forms a hard shell. The wax is melted out, which leaves a cavity to be filled with molten metal. Once that has solidified the ceramic shell is broken away to reveal the cast part.
The shape and finishes of this part mirror those of the cavity formed by the shell. That in turn depends on the precision and texture of the wax pattern. This means the surface finish of the wax is reproduced on the metal casting, with one important caveat: the ceramic refractory material is composed of particles that add a small degree of roughness to what could otherwise be a very smooth surface.
Surface Finish Achievable by Investment Casting
For cost, speed and strength, the ceramic shell is built up from multiple layers. The first layer applied is the face coat, and this is where particle size determines finish. In subsequent layers the goal is to add strength to the shell.
Finer particles in the ceramic material produce a smoother surface. An additional benefit is that they can better conform to changes in surface geometry, which helps reproduce fine detail.
Skilled investment casting operations can achieve surface finishes of the order of 60 – 200 μin, and maintain 125 μin (3.2 μm Ra) in most locations. As a reference, a good milled surface, one where the machining lines are just visible, has a finish of around 125 μin (3.2 μm Ra). In other words, investment casting can produce a surface as smooth as one that’s been milled. For comparison, a good surface finish from sand casting is 250 μin (6.4 μm Ra).
Benefits of an Investment Cast Surface
The ability to reproduce the fine detail of a wax pattern offers a host of benefits. These can be categorized as those that save money elsewhere in the manufacturing process and those that improve functionality or appearance.
Cost savings result primarily from a reduction in secondary machining. Surfaces used for mounting or sealing will still need some metal removal, but this will be less, and therefore faster, than from other casting processes. (Another benefit is that reduced machining typically shortens the manufacturing lead time.)
In addition, investment cast surfaces generally take paint well with little preparation being needed aside from degreasing. Likewise, parts going for plating need less polishing than those produced by other casting processes.
Functionality and appearance benefits result from being able to reproduce fine detail. Logos and identification markings are examples, and putting these into the casting avoids the cost of other marking processes.
Some investment cast parts are enhanced by creating a specific surface texture or pattern. This again is done by putting the texture onto the surface of the wax. For example, a wavy surface might be used to assist with oil retention.
Discuss Your Requirements for High Quality Cast Parts With Us
Investment casting reduces the need for secondary machining operations and can incorporate fine detail that saves costs elsewhere. This results from its ability to produce a low roughness as-cast surface, which in turn is a function of the wax pattern and ceramic shell production processes.
If you need cast parts that would benefit from a surface finish with the same roughness as that obtainable from milling, talk to Impro. We’ll assess the feasibility of producing them by investment casting and will advise you on the costs, benefits and lead times. Contact us to learn more.