For years, anatomical models have played important roles in hospitals and healthcare facilities. From educational aids to practice dummies, these models serve many functions for patients, students, and medical professionals. Producing them isn’t always an easy process though, as designers and engineers balance the need for accurate, aesthetic models with the demand for quick deliveries and production. Not to mention that the customized nature of models — they are representations of patients’ unique body parts and organs, after all — means they can be cost-prohibitive to build in low volumes via traditional manufacturing methods. But 3D printing helps overcome those challenges.
PolyJet Multi-Color, a 3D printing method, is one way designers and engineers can ensure that their model designs are built quickly, accurately, and cost-effectively. As the name suggests, PolyJet Multi-Color allows parts to be built in multiple colors with the PolyJet process. This article explores why that’s important. It also presents four key aspects of using PolyJet Multi-Color that designers and engineers should understand before taking on their next project, including: how the PolyJet process works, how and why to use multicolor materials, top considerations for designing, and why the urethane casting process is a viable method for production of low volumes of anatomical models.
PolyJet: The Process
Given that it can quickly and cost-effectively print accurate parts, PolyJet is a fairly common process for building anatomical models like hands, kidneys, hearts, and other body parts and organs. With a default build layering at 0.001 in. — and an ability to jet layers as thin as 0.0006 in., thinner than a human hair — PolyJet permits very fine details, a necessity for realistic models. This process also minimizes post-processing and allows for quicker delivery.
PolyJet is unique in that it enables multiple materials and colors to be used in a single build. This is helpful for identifying specific areas on parts, demonstrations, preoperative planning, and other uses. It can also print transparent materials to show internal features like veins, bones, and muscles, adding another layer of customization for designers and engineers. Glass-like parts that are translucent but color-tinted are another possibility.
Multi-Color: The Materials
Identification is the chief reason medical professionals like incorporating color into anatomical models. Since different areas of complex body parts and organs like the brain and heart can be printed in various colors, clinicians can more easily use the model for its intended function, whether it’s as a pre-operative planning device or a practice dummy.
Along with being able to build parts with multiple colors, designers and engineers can design parts that incorporate textures via graphic images, furthering the visual complexity that can be achieved. For anatomical models, that could mean showing disease or other features requiring complex coloring. The result is a part built quickly with the desired visual aesthetic without the need for additional finishing. Here are a few of the primary materials used for creating these types of models:
ColorPlusClear. This material builds parts in opaque and/or color-tinted transparencies. It has the ability to encase colored geometries in a water-clear, glass-like shell, allowing for fine details to be printed. Compatible with the STL file format, ColorPlusClear comes in a 0.00106 in. resolution.
VeroClear. A transparent, rigid, and accurate material that produces fine feature detail. It has high dimensional stability and produces parts quickly and economically. VeroClear is commonly used for clear medical devices, casings, and components for unique reveals, like a liver with veins. With VeroClear, the delicacy of the small veins is a nonissue because the strength of the model comes from the outer shell. This material is available in two layer thicknesses (0.00118 in. and 0.00063 in.).
TangoPlus. A rubber-like, elastomeric material that has a translucent, amber color. An advantage of TangoPlus is that it has no increased costs from secondary processing. It can be used for printing models such as hearts. While offered with the PolyJet process, this material is not included in the PolyJet Multi-Color offering.
Designing: Top Considerations and Choosing a File Type
Here are four key considerations to keep in mind when designing for the PolyJet Multi-Color process.
Trapped volumes: Building supports in trapped volumes (i.e., cavities that are difficult to reach or are trapped) presents problems. Doing so doesn’t allow finishing tools to spray the supports away because they’re trapped.
Wall thickness: Generally, Stratasys Direct Manufacturing prefers wall thicknesses of 0.060 in. or above, and the company’s stated minimum thickness is 0.012 in. However, these guidelines come down to geometry and specific project requirements. So, for parts that are encased or supported by another material, thicknesses as small 0.001 in. are possible. However, if there is a part that requires two different textures or colors on either side, the minimum thickness of that wall must be 2 mm, because color applied to models is 1 mm deep and would be seen from the other side.
Air gaps between walls: If designers don’t account for air gaps, then the support material may print in this area, producing individual pieces of assembly instead of a single part.
Overlapping walls: For parts built via the stereolithography (STL) file format, each unique material has to be its own file. It then has to be correctly located in CAD so that it’s built in a single piece, rather than with walls on top of each other. If it’s not, then it will produce an error before the build.
Not taking these last two considerations into account can lead to postprocessing, which complicates budgets and schedules.
The files. There are two file formats utilized for multicolor PolyJet parts: STL and virtual reality modeling language (VRML). Which one designers choose depends on the part’s color and transparency needs, as the file formats assign color and texture graphics to parts in unique ways.
STL (.stl). This shell-based format has a couple distinct advantages. For one, it can incorporate transparency, something VRML cannot do. A lot of medical scans default to STLs as well, making it a simpler transition from segmentation export to designing parts.
Here are some specific cases using STL for PolyJet parts. A colored model using a clear material or color-tinted transparency, like medical models that use clear material to show interior anatomical features, will utilize STL. If the 3D model of the part is separated into shells (i.e., different parts combined together into one assembly), designers can choose the colors and level of transparency of each shell. This is also indicated with the PolyJet Color Guide RGD color code and/or checkered pattern, which indicates transparency. Note that with STL, each unique color in the part needs to have its own STL.
For those assemblies that are built as one part, multiple sections of the assembly will be the same color code and submitted as a single STL. For example, someone may design a clear organ with separate yellow, red, and blue tinted areas on the interior. The colored areas are not connected to each other, but since they are part of the assembly, held together by the clear organ, every color will be saved as a single STL. The designer saves time here because individual color codes don’t have to be assigned.
Designers can also use third-party STL manipulation software. In those cases, it’s best to use a Boolean tool that can add, subtract, and intersect space between two objects. If the Boolean tool is not used, there is a potential that overlapping parts could be assigned a random color by the PolyJet technology, and areas with space between walls may generate supports and could separate during handling.
Finally, it’s important to consider the limitations of STL files. A chief restriction is that they do not have the ability to do gradient textures. And while it’s still very possible to incorporate multiple colors, it’s easier to do so via the VRML file format.
VRML (.wrl). The VRML format is ideal for parts designed with opaque colors or a colored graphic texture. VRML is a standard file format for representing 3D interactive graphics that provides the capability to apply graphic texture, also known as ultraviolet (UV) mapping, to a model. UV mapping essentially places all the color in an image file, which is then applied to the model by software. With more color options than STL, VRML has the ability to produce photorealistic colors.
VRML is usually the best choice when designers are looking to include multiple colors in their designs, as the file format makes it inherently easier to do so. Depending on how color is applied, the color could be within the VRML file itself, or the VRML file could be accompanied by one or more texture files (.bmp, .jpg, .tiff, .png). Color can be applied in VRML format to each face of the geometry, to each individual triangle, or with a graphic texture file. Possible uses for this process include anatomical models that show a colored depiction of heat or stress or a prototype when the graphics on the model are complex, such as a heart with multiple colors.
A special consideration for VRML is dealing with geometries that were assembled to form the final part. In those cases, there may be overlapping individual parts. Any colored surfaces that are 2 mm or less from the opposing surface may be seen in this instance. It is a best practice to combine the individual parts in order to remove any interior geometry.
One feature VRML can’t naturally do is transparency because the file type wasn’t created with this in mind. However, there are workarounds. One way is by building a model, via VRML files, with nice photo texture on the surface. If a section has to be clear or color-tinted, then it can be built separately and then post-assembled.
Urethane Casting: The Highlights
Using PolyJet master casts for urethane casting is becoming more and more popular. Widely used for anatomical models, this process is suited for when companies need greater quantities of models because it is generally more cost- and time-effective to produce them this way.
Designers who are planning on using PolyJet with the intent of using it to create master casts for urethane casting should design in a manner that allows PolyJet parts to be used for the casting process. This step allows for an easier transition between manufacturing methods, thus streamlining the production process.
Putting It All Together
Designing anatomical models doesn’t have to be like conducting open-heart surgery. Designers and engineers who understand all key aspects of PolyJet Multi-Color, from the PolyJet process to its design considerations, will be better able to help their teams build quality models both quickly and cost-effectively.
This article was written by Eric Quittem, Product Manager for Fused Deposition Modeling and PolyJet Technologies at Stratasys Direct Manufacturing (Valencia, CA). For more information, Click Here .