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Additive forming processes, which make parts by adding material, are well understood as rapid prototyping techniques. Stereolithography, for example, is widely used to make physical objects by building up successive layers of light-curing resin. Recently, similar additive processes capable of forming metal parts have gotten considerable attention. These techniques are especially interesting to manufacturers of metal tooling and molds, which are much like prototypes in that they involve complex shapes produced as single items or in very small quantities. The ability to produce these objects in tool-quality metal creates important new options for mold shops.
One recently introduced process builds molds from the inside out, layer on layer, to form internal features and geometries not possible with conventional metalworking techniques. This process is being made available to mold shops as a service called MoldFusion that is offered by a technology partnership between D-M-E Company (Madison Heights, Michigan) and Extrude Hone Corporation (Irwin, Pennsylvania). MoldFusion adapts stereolithography techniques to build shapes by fusing powder tool steel layer by layer in a binder matrix. The process creates near net shape 3D structures from 3D CAD models.
Developers stress that MoldFusion is not a replacement for conventional tooling, but a solution for applications that are not practical or possible using conventional metalworking techniques. For example, mold makers can take advantage of this process to more effectively implement conformal cooling of mold inserts. It allows designers to create internal water lines and water jacket features that conform more closely to the shape of mold geometries. Typically, these conforming geometries could not be produced with conventional machining methods. Molds with conformal cooling can achieve cycle times 20 to 35 percent shorter compared to conventionally cooled mold inserts.
Customers send CAD data to D-M-E, where it is reviewed for application effectiveness before being electronically communicated to the ProMetal Division of Extrude Hone. The ProMetal 3D printing process forms successive layers by depositing powder tool steel in a binder matrix to match successive 2D slices of the CAD model. Molds are built up layer by layer to near net shape. Upon completion, the “green” part is loaded into a sintering furnace that fuses the tool steel powder into a form that has 60 percent density, while burning off the binder. In a second furnace cycle, this porous structure is infiltrated with molten bronze via capillary action to reach full density.
The near net shape part is then ready for final machining by the customer. According to developers, all common finishing processes can be applied, including turning, milling, grinding, electrical discharge machining and polishing. Coatings to enhance wear and chemical resistance, such as nickel and chrome plating, can also be applied.
The process can create mold components up to 12 inches (305 mm) in length and width and 10 inches (250 mm) in height.
Structural mass reduction is another mold design strategy that MoldFusion lends itself to. This design strategy substitutes truss or honeycomb structures in place of masses of solid metal. The truss or honeycomb sections can be built into mold components to improve thermal isolation and reduce tool weight.
MoldFusion is also available as a rapid tooling technology for prototype and short-run mold applications.
In general, the ability to obtain near-net shape mold components allows the mold shop to reduce or eliminate low-value rough machining time on high complexity jobs. This, in turn, allows the mold shop to concentrate on more effective mold designs and high-value machining processes.