The processes involved in Additive manufacturing (AM), including 3D printing, stereolithography, polyjet, and fused filament fabrication, all promise design freedom. This technology has not replaced traditional manufacturing yet. So, many parts will still need the usual manufacturing processes to make in quantity. Understanding the nuances and needs of each type of AM machine will go a long way toward achieving the final, manufacturable outcome you desire.
By Leslie Langnau/Managing Editor
Complexity is free—at least that is what much of the marketing material says about rapid prototyping (RP)/3D printing technologies. To a point, that is accurate. But while you may be able to prototype just about anything you can imagine, it might not be producible, at least not without major revisions.
Whereas there are trusted standards for designing for CNC, for example, AM does not have any yet. Some service bureaus suggest that if standards existed, ones that design engineers trusted, AM would experience greater acceptance.
Insufficient understanding of these limitations is causing a discussion among service bureaus about the need to create a designation that a CAD design is suitable for a specific prototyping technology, such as “designed for 3D printing, “designed for fused deposition,” or “designed for polyjet,” and so on.
Just because you can draw it on your CAD software does not necessarily mean you can build it on whatever RP equipment is available. For example, it is possible to design a table on a CAD program that has a thickness of zero inches. However, you cannot print that. Or, some RP machines are not good at horizontal cylindrical holes, but a powder based RP machine handles such holes just fine.
Another issue: design choices are different depending on whether you are designing for prototype—to check look, feel, and function—or designing for production. “For example,” noted Scott Volk, V.P. Manufacturing, GPI Prototype and Manufacturing Services Inc., if we receive a part with lots of undercuts, we know it is not manufacturable. We have a lot of inventors in the world, but many don’t understand manufacturing, so often their parts can’t be made.”
Agreed Sean Taffert, at Protofactoring, “Some designs can’t be prototyped on RP equipment because they were detailed for manufacturing, and some can’t be manufactured because they were designed for prototyping.”
The use of many undercuts, for example, may require a manufacturer to develop special (and often expensive) tooling for quantity production. So, in addition to a designation of a part’s suitability to a particular RP process, we may need a designation to indicate its manufacturability.
Lost in translation
Part of the issue is the assumption that CAD tools will automatically, accurately, and smoothly move critical design data to each successive tool used in product development. But to protect their products from competition, CAD vendors built their design tools using proprietary algorithms and formulas. When CAD drawings go to the next step, some design data, such as designer intent, do not go with them. And neither STL nor AMF can fill in the data gaps from CAD files.
This issue of full design data occurs with shared or free drawings too. Knowingly or not, the designer of those drawings had a specific RP technology in mind—so it is important to investigate to determine which RP process the drawing suits best.
Points to consider
Additive manufacturing technologies are still relatively new, so there is a gap in knowledge about exactly how they work. Understanding the nuances and needs of each type of AM machine will go a long way toward achieving the final outcome you desire. These are recommendations by many service bureaus.
The first place to start is with the CAD drawing. Ensure that all parameters are fully defined. “Make sure that the surfaces in the original CAD model are “water-tight,” in that only solids are modeled.” said Joe Titlow, Vice President of product management, Z Corp.
Watch your wall thicknesses and knife-edge features. Some machines limit the thickness to 0.030 in. Some part features may need adjustment too—most AM machines have limits on feature size. Scaling will affect these features to the point where they are not printable or buildable. Check with the particular AM machine.
Check for internal voids where support material can get trapped. In some cases, you may need to put in a hole to drain the support material.
Watch tolerances and clearances with mating features. Service bureaus and vendors often recommend a 0.015 in. to 0.020 in. clearance between prototype parts. This clearance will likely change when it comes to the full production stage.
Be sure you save the CAD file to a high-resolution version of the STL file. Make sure you are saving your design to the correct units in the STL file.
A design with internal channels may need to be built on a laser sintering machine, especially if you plan to test the built object. Plastic materials may not provide the engineering properties you need. As mentioned above, thin walls can be a problem for many AM technologies and materials. A service bureau may be able to best advise.
Depending on the machine and the material, the layer-by-layer building process can produce parts that experience delamination. In fused filament fabrication (FFF), a term that means the same thing as fused deposition modeling but is not trademarked. The filament of plastic material may not adhere properly to preceding layers. This lack of adhesion creates a week spot in the part (delamination), which may affect testing.
Another issue with this type of fabrication is that parts may exhibit directional strength—stronger in the X and Y directions, weaker in the Z direction. According to some tests, even if the part is 100% filled with material, differences in directional strength can be nearly 2 to 1.
The layer-by-layer deposition of many AM machines can mean that parts are porous, as the layers may not fully meld together into a solid area. Both the porosity and the properties of the engineered plastic materials should be considered when testing your prototypes.
The extrusion size of the deposited material, along with the layer-by-layer deposition, will also affect how thin you can design walls or other features in your parts. The back and forth weaving needed to build a part can limit geometry, especially for corners.
Another issue to consider is warpage, particularly if parts are large. In laser sintering machines, for example, it’s important to control the heat in the build chamber. As long as the temperature is evenly maintained, these machines build good parts. Within the build chamber, though, is a temperature “sweet spot.” In other areas of the build chamber, it can be harder to maintain temperature across the part, which can result in warp.
Additionally, if you build multiple parts in the chamber, you may not see warpage, but parts may not be exact duplicates of each other due to those temperature variations. Some will have a slightly different size, maybe a different shape. If you choose to use service bureaus to build your part, some can handle this issue with their AM machines. Look for service bureaus that know and maintain their machines very well.
If powder granules don’t melt completely, which can be the case in laser sintering, the result may be parts porous enough to develop holes as they cure.
The grain size of the powder materials and the geometry of the part can contribute to “caking,” which is where the excess material sticks to a part. This excess material can make it difficult to determine the difference between the part and the cake, affecting final dimensions. Extra cleaning steps may be required.
Warped parts can be an issue with stereolithography machines as well. The resins and plastic materials used need UV light to cure. But full curing occurs outside the machine, so care is needed to ensure parts are handled properly when removed from the machine. Depending on the material used, the parts may sag or warp over time, which may be a concern for the end use of the part. Also, take care with large overhangs. If the part will be made for end use, you may want to consider a different material or AM technology.
Some designs may need extra support to prevent warping or sagging until the part is fully cured.
Watch for trapped volumes in part design. As Tim Ruffner, GPI Prototype noted, the area in a cup design, for example, can have a trapped volume, while the area outside the cup has freedom of movement. The material inside the trapped volume will not move at the same rate as the material outside the cup, so the cup wall could fail. Or, you can get void areas or bubbles.
One concern with some polyjet processes is the amount of support material needed. The more complex the geometry, the more expensive it will be to build the part because of the quantity of material needed.
The support material tends to absorb any moisture from the material or the process and swell. If your part has thin areas, the support material could damage them. Large areas of a part may sag within days after build. In these processes, the support material can blend in with the build material, altering its chemistry in a way that makes it unstable over time. Some materials will tear easily.
AM vendors continue to develop new materials that reduce the limitations of the various machines. Both service bureaus and AM vendors can help you determine the best machine and material to use for your part.
GPI Prototyping & Manufacturing Services, Inc.