From Plastic to Metal

Once a plastic model is built, several casting methods can deliver your part. The most common process used is investment casting, but some plastics are rigid enough to use as a pattern for a sand mold, and using them as a pattern for plaster molds also is common.

Plastic prototypes can be used in much the same way a wax pattern is used in traditional investment casting. Just like wax, the pattern is submerged in a slurry repeatedly and encased in a solid ceramic shell. When the desired shell thickness is achieved, the plastic is fired out of the mold, and molten metal can be poured into the cavity left behind. This process is most valuable with parts that have highly complex geometries and require smooth surface finishes.

Plastics tend to leave some amount of ash behind when they are fired out of ceramic molds. Depending on the material used, that ash content may be limited, or it may be substantial. Each process, because of the materials they rely on, generates different amounts of ash. The ash can produce inclusion defects in the finished cast component that you receive.

The material used in a prototype investment casting also can present a structural problem for the ceramic mold. Many of the plastics used to create prototypes expand when heated, which can cause mold breakage. However, in the last five years, many SLA generators have switched from a solid plastic to a material that has a honeycomb-like pattern in the prototype. The lattice structure allows the part to collapse when the mold is fired, eliminating breakage.

Many investment casting facilities are also quite comfortable using SLS patterns, according to Miller. Because some of the prototyping machines build parts in styrene infiltrated with wax, the finished product can look and feel very similar to traditional wax investment patterns, and they can be stuck right onto a gating tree, which metalcasting facilities use when producing an investment casting. Those materials also do not expand within a ceramic pattern and have low ash content, Miller said.

If a sand cast product will suit your needs, the three devices produce parts that can be used as patterns, but each will exhibit different behavioral characteristics. While FDM parts are very durable and hold up well to repeated use, the materials can be so porous as to allow some sand grains to force through the plastic, resulting in defects. SLS parts have the advantage of emulating the surface finish of a sand mold. A grainy surface results from sintering that interlocks with sand grains. Only recently have there been SLA parts developed with enough rigidity to be used as a pattern for sand molds.  The superior surface finish of those parts makes them an attractive choice for the creation of any pattern, but they can lose their structural integrity quickly, sometimes on their first use.

Plastic prototypes also can be used in the rubber plaster mold (RPM) process for creating a rapid casting. RPM takes a plastic part and coats it in rubber. The rubber coating is then cut in half and removed from the model, creating negative images of the desired part. Plaster molds then can be created using the rubber as a pattern.

Particularly in the RPM process, designers have to decide whether the part they need quickly should have superior surface finish and accuracy, or whether those attributes can be sacrificed for the sake of speed and cost.

In the end, “the better the model, the better the casting,” said Mike Kaiser, Prototype Casting Inc., Denver, Colo. METAL