Traditionally 3D printers are used to print parts directly — most of the time this is all they are used for. However, because a 3D printer will print whatever you send to it, parts that are difficult and expensive to CNC machine typically cost much less to make using 3D printing.
Making a composite part casing mold, for instance, is expensive to CNC machine because it has a negative space; many tool paths and an array of cutters are needed to machine the complex negative-space geometry. Composite parts with complex geometry require a two-piece mold. Machining the mold out of aluminum or steel can cost thousands of dollars and be prohibitive to do in a prototyping capacity. But 3D printing a two-part mold usually costs much less and can be made in days as opposed to weeks for a CNC machined mold.
A common material used in theromoplastic-based 3D printers is ABS plastic. To be able to work for a composite mold, extra finishing is needed. When a complex surface is created on a 3D printer, it will leave visible build lines; it’s best to sand these out of the mold for both aesthetic reasons and to aid part release. Once the mold is sanded smooth, several layers of mold release wax need to be rubbed into the mold by hand. As a final step to ensure the composite part releases cleanly from the mold and doesn’t damage it, a water-based layer of mold release film agent is sprayed onto the mold’s inner surfaces.
To create the part, layers are cut from sheets of bidirectional composite fabric each layer is cut from bi-directional sheets of composite fabric, such as carbon fiber or Kevlar, and placed in the mold. The sheets can be whetted out with resin by hand before they are placed in the mold. To calculate how many layers of fabric are needed, plan about 0.010” for each layer. Another technique is to place all the layers in the mold, pull a vacuum, and suck the resin into the mold (resin transfer). This technique works well if the mold is made using a PolyJet or SLA machine. The resin transfer technique would probably not work as well for a mold made on an FDM machine because the parts tend to be porous and would not hold a vacuum as well. If the cavity were treated with a layer of resin (gelcoat), it would then make the mold cavity airtight and able to hold a vacuum.
In terms of both speed and cost, making a mold using 3D printing can’t be beat for complex composite parts. There are exceptions: the technique probably would not work well if pre-preg composite cloth were used. The FDM mold wouldn’t be able to handle the autoclaving temperatures, which are sometimes as high as 650 degrees Fahrenheit. Another consideration is size of the part. Most 3D printers have a build size of 12” X 12” x 12”. If the mold is larger than this envelope, it would need to be made in multiple pieces and solvent bonded together. A practical size limit exists where machining a mold out of aluminum would actually cost less.
At Pongratz Engineering we often are pushing the limit of what can be done with prototyping technology. Contact us with any prototyping and product design questions and needs. www.pongratz-engineering.com or 612-384-0122.
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