Molten Spray Moldmaking

Because additive manufacturing processes are not affected by geometric detail, they ought to be ideal for making molds and dies. The complex features of these parts that slow down machining do not have to affect the speed of a process that builds parts layer by layer. However, size is a problem. Equipment for building additive metal parts is typically much more expensive than comparably sized CNC machine tools. Cost, among other factors, prevents additive equipment from scaling up to the size of a large mold or die.

 

An exception is an additive tooling process developed within a laboratory near Cleveland. Belcan Corporation’s Solon, Ohio-based Specialty Equipment Engineering Division designed and built a machine for proving out an additive metal manufacturing method called RSP, or Rapid Solidification Process. This process, which has the potential to be easily scalable, uses a molten metal spray to create solid, complex, precision metal components. A company partly owned by Belcan and operating within the Solon facility, RSP Tooling, has worked to refine and industrialize this technology.

 

The process was licensed from the U.S. government’s Idaho National Laboratory, which initially saw it as a way to make steel billet. RSP Tooling was founded on the idea of using it to make molds and dies instead. James Knirsch, RSP Tooling’s president, says an end user with an RSP system will be able to make a large core or cavity in less than a day, and at less cost than conventional mold machining. In addition, the mold tooling produced this way would have longer life than what a machined mold can provide.

 

The process is now proven, and the RSP machine in Belcan’s facility has made working dies. Scaling up the process to larger dies is the main development work that still needs to occur, because the cost advantage of this process becomes pronounced only as the molds and dies get bigger.

 

 

The core of RSP technology is a method for spraying metal so the material solidifies rapidly (hence the name). Spray that lands as liquid in a 10-micron layer solidifies so fast that the 10-micron layer landing atop it can solidify just as quickly, and so on. The equipment used today can build a metal part at a rate of about 360 pounds per hour.

 

Most metals can be applied this way. When tool steel is sprayed, Mr. Knirsch says the resulting hardness is better than that of a machined part after heat treating. He notes that heat treating’s properties improve according to how quickly the part cools. RSP provides the equivalent of heat treating with instantaneous cooling.

 

 

The RSP moldmaking method begins with a model of the tool. Typically this is plastic, created through machining or 3D printing. A model for a large mold might be created in sections. This model is then used to create a ceramic “positive” of the molded form (“positive” because the mold is negative). The ceramic positive becomes the platform on which the metal tool is built.

 

The geometry of the part produced in this way conforms precisely to the ceramic piece. Because the spray is atomized, the surface finish matches that of the underlying form. One time, to achieve a mold with a realistic leather texture, RSP Tooling put an actual leather cover atop the master. The mold replicated the leather in fine detail. On another occasion, RSP was applied over a polished piece of quartz glass. The resulting sprayed metal surface had a mirror finish with no need for polishing. This faithfulness to the finish and details of the master means that a mold created through RSP requires machining only for pin and cooling holes, and for features that cannot be produced on the master because they involve undercuts.

 

Build time is one advantage of making a mold this way. All the RSP steps combined, for the model, master and spraying, take less time than making the same tool through machining. Mr. Kirsch says another advantage is cost—but only if the mold is big enough. Because of the high initial cost, an RSP system will not be cost-effective for small tooling. As the mold size increases, though, little about the process changes. The cost of an RSP machine increases little with bigger travels. The spray cycle time does not increase much, either. In machining, equipment cost and process cost are both directly proportional to the size of the part, but not in RSP. As a result, the true strength and value of RSP will ultimately be determined by the size of the mold or die that an RSP system can be engineered to create.