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Additive Manufacturing

Rapid Prototyping / Additive Manufacturing

By Jack Ziemba, CEO, Aristo Cast

Rapid prototyping (RP) is a process by which a designed three-dimensional article is constructed or fabricated more quickly than would be expected by a conventional process. This technology has been applied for years in investment casting industry, but is now being adopted in other component fabricating industries as well. Additive manufacturing (AM) is another recent term which applies to the creation of a one-off prototype, or eventually multiple articles.

The term "additive" is used to describe the "building up" of an article, rather than for example, the "machining away" of material used to create a finished metallic component.

The advantages of rapid prototyping or additive manufacturing are:

There are several stages in the development of a rapid prototype. The first stage involves a design drawing in one dimension, usually with multiple views. These are then transformed into a three-dimensional model using a computer-aided design (CAD) program. The output of the CAD files are then converted into a STL (stereo lithography) file. The computer files are then "layered" throughout the depth of the proposed article using another computer program. These layers are then stacked to provide the final three-dimensional design.

Finally, the actual prototype is ready to be printed in three dimensions. There are several recognized technologies that may be applied in this step, each using a variety of materials to create the prototype: plastic, wax, resin, powder, powdered metals, ceramic, low temperature metal, etc.

The following table describes several RP/AM technologies that are currently employed in industrial applications: however most will not provide an actual metal part as with investment casting.

RP/AM Methods

Material Choices

Selective laser sintering (SLS)

thermoplastics, metal powders, ceramic powders

Direct Metal Laser Sintering (DMLS)

limited selection of metal alloy

Fused Deposition Modeling (FDM)

thermoplastics and ABS plastic

Stereo lithography (SLA)

photopolymers

Digital Light Processing (DLP)

liquid plastic resin

Fused Filament Fabrication (FFF)

PLA & ABS plastic

Melted and Extrusion Modeling (MEM)

metal wire, plastic

Laminated Object Manufacturing (LOM)

metal wire, plastic filament

Electron Beam Melting (EBM)

titanium alloy powder (most cases)

In the majority of these processes the "printing equipment" traverses a bed of the appropriate material and deposits a binder or focuses a laser to shape each "layer" (cross section of the part) by converting digital signals from the 3D program until the full three-dimensional design is completed. The article can then be finished by coating, sealing, painting, or other means to preserve the surface for further evaluation or service. By far the greatest advantage of investment cast rapid prototyping is that the parts can not only be used for form and fit, but in the majority of cases, function as well. In days gone by the only method to create a prototype was, in most cases, to machine from a solid.

This worked well for form and fit but did not duplicate function from the standpoint that you were dealing with a wrought product not a conventionally cast product. The part that was machined from the solid or printed with a laser would have a different grain structure. This difference, minor as it may be, could give a false positive result in part durability evaluation when compared to the production part. Using the investment cast rapid prototype process allows you to obtain all the attributes of the finished product quickly – without the cost of tooling!

The most common RP/AM processes for creating investment casting patterns are quickcast, Castform and printing the part in pattern wax using a 3D inkjet printers. The latest process to come on the scene for investment casting patterns is the Voxeljet method. This is actually a binder being applied through an inkjet head to a bed of powder that is in essence ground up Plexiglas. Once the pattern is created it is then infiltrated with wax and processed as a conventional wax pattern. The largest advantage that the Voxeljet process has, is the fact that in the pattern removal stage, (whether flash fire or autoclave) the pattern does not expand; rather it shrinks which is very friendly to the investment casting process, as the possibility of shell cracking is virtually eliminated during pattern removal.

In selecting one of the previously mentioned processes the physical size of the patterns and the surface finish and fine detail required must be considered. For patterns requiring extremely fine detail and a very smooth surface finish the ProJet CP 3500 3D printer would be the method of choice as it will produce a pattern that would be truly representative of what was obtainable off a cut cavity injection mold.

The quick cast pattern may be able to meet the Voxeljet in surface finish but cannot go down to the very small wall thicknesses obtainable with Voxeljet. The Voxeljet and Castform patterns with their somewhat grainy surface, cannot meet the ProJet extremely smooth surface finish. The above comments are not a recommendation for any one piece of equipment but just a demonstration that there are differences between each processes capability.

In order to use any of the above mentioned methods for pattern creation certain things must be kept in mind:

  1. Unless you have the luxury of having one of these pieces of equipment in-house you're at the mercy of the service bureau for quality and delivery of extremely valuable part.
  2. Members of your team should be familiar with the types of files that you can digest and/or manipulate in creation of these patterns.
  3. Machines used to create these patterns are very sophisticated and can be somewhat temperamental, when you least expect it!

In the investment casting industry, one or several of these RP/AM technologies may be used to advantage with or without an actual metal component being cast from the prototype. Design engineers can often discern necessary changes to be made in the initial design by studying the now three-dimensional prototype before conversion to the actual metal component. In other instances, the prototype design can be "rapidly invested" e.g., shelled in an accelerated fashion and an actual component cast in metal within twenty-four hours or less. Thus the prototype now in metal can actually be tested and integrated into a product assembly to ascertain actual utilization.

Recently, considerable progress has been made in using the Direct to Metal Laser Sintering (DMLS) 3D printing process in the actual construction of the metal part desired. The main drawback being, that unless the DMLS will be used in production the part is not truly representative of the final product. A process decision has to be made on a job-by-job basis keeping in mind to be truly representative of a final metal part design – no process can come close to that of investment casting.

All patterns or prototypes fabricated with any of the 3D printing equipment require the same careful handling as with conventional pattern manufacture. Advances in shell technology allow fewer/thinner ceramic coats to be applied in less time, and patterns may be removed by conventional flash firing or autoclaving. One thing that must be kept in mind is in the rapid prototyping process there is absolutely no room for error. You don't have the luxury of running another pattern as with production material.

It should be noted that casting design simulation studies, in conjunction with rapid prototyping, greatly accelerates the productivity of a casting supplier to bring a product to market. A project that might take many weeks or months can be brought to completion in just several days. Thus the RP/AM process greatly accelerates the progress that can be made and reduced time between design concept and actual product availability and performance.