For metal parts, the part cost will always be higher than plastic parts. Depending on the process used, it can be a little higher or a lot higher.
Here are some common processes to create metal parts.
- Sheet metal: When origami works, it is wonderful - sheet metal molds are quick and cheap to make and the lead time is very short. Sheet metal can be a great alterative to injection molded plastic for internal structural parts as it affords much more flexibility during the early stages of the design process when the product may still change or when the forecast for the product is not clear.
- CNC machining: When the part is highly complex in shape, CNC machining may be necessary. This is prohibitively expensive for even relatively small parts, and the return on investment (ROI) to go to a die casting process can often be within a few hundred parts.
- Turning: For shafts and bearing surfaces this is often the only way. There are specialist metal fabrication suppliers who focus only on custom turning shafts - this is the price you will have to pay if you have precision bearing surfaces in a complex piece of machinery.
- Sand casting: This is used to produce small lots of custom metal parts with complex geometry. There is no tooling - sand is used as the tooling material and a sand mold is formed around a positive model of the part (typically a CNC version of the metal part). The positive is removed, the two halves of the sand mold is put together and molten metal is poured to take the shape of the part in the void inside the mold. When the metal sets, the sand mold is broken to get the part back out. This process creates a rough surface finish and does not preserve fine features - sometimes there is a post-machining process to create the tolerances needed for critical surfaces.
- Investment Casting. This is used for cast parts with complex geometry - and is one step above sand casting. A positive model, or pattern, of the part is made using a wax like substance (these days a variety of 3D printers can print high precision parts in a substrate that can burn away cleanly just like wax). It is then put through an "investment process" where a coating is formed around the wax that will become the mold. Then the wax is burned away and the metal is poured into the resulting cavity to assume the shape of the wax pattern. This process also does not keep tight tolerances, and post machining is needed to tighten the tolerance on critical surfaces.
- Die Casting. This process incurs the highest tooling cost (typically 5 figures per mold) and the longest lead time (typically 5 months turn, as opposed to 3 months for injection molding) but results in the lowest part cost. Hardened tool steel dies are created to form molds much like those used in injection molding, then molten metal is poured into the mold to form the part. Post machining is also needed to tighten surfaces with tight tolerances.
There are also other techniques like EDM (for burning a complex shape into a metal blank before finishing it off with a machining run - commonly used to make complex molds), wire EDM and water jet (for creating complex shapes for parts that are effectively flat) and the like.
For certain types of metals sometimes there are secondary processes involved. For example, aluminum parts often go through a hard anodizing process to result in an impervious coating that protects the part from oxidation. Spring steel parts sometimes go through a heat treatment process to improve its material properties.