A unique additive/subtractive process for injection molding components

In last month’s CNC Tech Talk column, I stated that each form of manufacturing – additive or subtractive – has limitations that are easily overcome by the other. This month I will expand on one of the hybrid machining approaches that I introduced last month. Developed by Mantle Inc., it takes advantage of the advantages offered by deposition 3D printing and uses CNC machining to overcome the limitations of deposition 3D printing. I will focus on how this process is particularly applicable to the production of injection mold components – core and cavity inserts – made from P20 and H13 equivalent materials. Still, you will likely discover other feasible applications.

Deposition 3D printing is the most common form of additive manufacturing. With this technology, the material is deposit – usually by extruding it through a nozzle – onto a build surface, layer by layer. There are countless hobbyist and industrial grade fused filament fabrication (FFF) 3D printers that extrude molten plastic or plastic infused with other materials, such as wood, carbon fiber, or metal.

The deposit method of our hybrid machine differs from FFF in at least five ways:

1. This machine extrudes metal-infused paste instead of heated plastic.
2. A removable and reusable build plate contains a pocket filled with a special build surface material. CNC milling flattens the build surface to ensure the first layer adheres properly.
3. After each layer is extruded, a heater dries the material. This solidifies the material and improves its density by removing moisture and, in turn, minimizes shrinkage during the upcoming sintering process. The consistency of the material after drying is that of a pencil, which makes it very easy to machine.
4. Milling is done after each layer to flatten the top of the layer. This ensures good adhesion for the next layer. Precision milling is done every ten layers to smooth sidewalls and cut fine details.
5. Suction is performed automatically during each CNC milling operation to remove machined particles.

After the 3D printing/CNC milling process, the removable build plate is placed in a sintering oven with the printed component still attached. During sintering, the build surface dissolves, ensuring easy component removal and consistent component removal.

Although the process seems complicated, this methodology produces a component that will have all the necessary qualities of a ready-to-mold component. Mold surfaces generally require no post-processing work.

Figure 1. Ready-to-mold components.

Common results:

• Accuracy of 0.004 inches or less in a 4 inch cubic volume
• Hardness of 42 on the Rockwell C scale for H13, 32-Rc for P20-equivalent
• 1 to 3 micron RA surface finish
• The same characteristics as metals (H13 or P20) produced with conventional methods.

The software of this hybrid machine resembles that used for any 3D printer. Once a model is created using 3D modeling software, a stereolithography (STL) file is exported. The STL file is then imported into the software, where parameters are set to specify how the component should be printed and machined. This step usually takes less than an hour.

The main advantage of depositing for mold components

Any form of additive manufacturing that uses a layering approach will enable complex geometries – especially internal geometries – that are impossible to produce with subtractive manufacturing methods. The creation of cooling channels is particularly important for the manufacture of molds. Channels created with a 3D printing process can follow any path. This is particularly important when proper cooling is required; these channels can conform to the shape of the molded component and provide superior cooling and molding performance.

Figure 2. Completed mold components. Photo credit: Westminster Tool.

Although some of these points may be obvious, this process would not produce acceptable results without the aid of CNC machining. Consider these examples:

• Milling to flatten the build surface material ensures proper nozzle height and adhesion for the first coat. Compared to probing the build surface to address flatness and angularity issues (as many FFF 3D printers do), this method ensures perfect squareness/angularity between the bottom of the component and the other surfaces of the component.
• Milling after each coat ensures consistent adhesion for each subsequent coat. This, in turn, provides consistent density in the component (no voids).
• Fine milling after every ten layers ensures smoothness that cannot be achieved by deposition 3D printing alone.

Post-processing work is usually limited to tapping cooling channels and grinding to ensure flatness and squareness. Since sintered components have the same characteristics as traditional materials, they do not require any special consideration – for other post-processes, such as texturing, laser welding or EDM.

If a better surface finish is required on the cast component, polishing is certainly possible, as is the case with any other type of P20 or H13 material. If necessary, it is also possible to heat treat the sintered components to increase the hardness.

Read: Functional injection molds (and more) from 3D printed metal clay

One important limitation: this process does not allow overhangs that require support structures like FFF 3D printing does. Fortunately, at least from the perspective of the production of injection mold inserts and mold components, the required draft angles are in an opposite direction to those of the overhangs. For most mold components, the geometry does not include overhangs.

The advantages of this process are numerous. Users have documented that the Mantle hybrid process routinely removes 60-80% of the time it would take to create injection mold tooling inserts with conformal cooling channels and complex geometries using traditional CNC . Since the toolpaths are generated automatically, the skill levels needed to produce high-quality components are comparable to those of a 3D printer operator, rather than a toolpath specialist. And in cases where prototypes need to be created in a production material that is difficult to mold, P20 and H13 tool steel inserts are often better suited to the task than aluminum tools or 3D printed plastic inserts.