by Chuck Alexander, Director of Product Management, Additive Manufacturing, Stratasys Direct Manufacturing
An examination of where the 3D printing technologies deviate from each other and how to use a simple seven-step strategy to narrow down which process is right for your project.
3D printing is not a one-size-fits-all manufacturing process. While there are currently seven distinctive 3D printing technologies with dozens of subsequent process variations, the most widely used 3D printing processes are material deposition (Fused Deposition Modeling), material jetting (PolyJet), vat photopolymerization (Stereolithography) and powder bed fusion of both metals and plastics (Laser Sintering and Direct Metal Laser Sintering). Each offer unique opportunities and solutions. However, they are not all equally suited to tackle every manufacturing challenge.
Design without restrictions
With conventional production methods, the intended end-manufacturing process usually defines how a design evolves. For example, if a new design will ultimately be used in investment casting, it will require a wax tool and mold with design planning of drafts, bosses and seams, which reduces certain complexities to ensure a good mold. If a design will ultimately be CNC machined, it must follow machine angle, volume and geometry restrictions. But 3D printing diverges from traditional design steps.
With 3D printing, the designer or engineer begins with their ideal design, which is then reviewed through the various 3D printing technologies. When we call 3D printing a revolutionary technology, we really mean it’s a liberating technology—it liberates designs from manufacturing limitations. While there are many tricks that optimize a design for a specific technology—and therefore take full advantage of 3D printing’s capabilities—3D printing fundamentally caters to design.
That doesn’t mean 3D printing can magically build all designs; there is a learning curve, and what is possible with machining may not be possible with 3D printing and vice versa. However, if an experienced designer/engineer enters 3D printing with an open mind and a design that can’t be produced through conventional means, then they could see remarkable results.
Given that we’ve just unshackled a design from conventional manufacturing chains, how do you wade through the pool of 3D printing processes and materials?
One suggestion is to answer these seven questions:
1. What is the application of your part?
2. How does your part need to function within this application?
3. What’s the environment your part needs to survive within?
4. How durable does your part need to be for its functions—does this product require a long strenuous lifespan or is it a prototype or other consumable?<
5. What aesthetic requirements does your part need to meet?
6. What are the economics of your part? Will this turn into large volume production, or will this be a custom one-off, two-off production run?
7. Which of these factors is the most critical for your part to achieve its intended purposes?
Let’s take a look at two examples.
Manufacturing tailored to design
Because of 3D printing’s inherent versatility, there isn’t always just one solution. For example, Fused Deposition Modeling (FDM) and Laser Sintering (LS) may both be great fits for your design. The questions listed above will help give you a clearer answer to narrow your choices. While your design may feasibly be built using two or more 3D printing processes, one particular process may have an advantage that could inevitably lower costs.
Example: Model for company showroom
This first example is an imaginary—but common—design challenge. It demonstrates the steps to finding the best 3D printing technology.
• Application: Model for lobby room at a company—model must clearly show important product features
• Function: Non-functional display model
• Environment/Stability: Non-strenuous environment—average room temperature 78°F with overhead fluorescent lighting
• Durability: 2 to 5 year shelf life
• Aesthetic: Transparent part which showcases inner features to visiting customers
• Economics: One-off part only
• Priorities: Aesthetics; this is a showcase model which must be eye-catching and indicative of the kinds of products our imaginary customer builds
Upon receiving these preliminary answers, typical 3D printing service provider project engineers will begin defining technology options for the customer.
First up is the priority. The designer has indicated that the most important aspect of his product is detailed aesthetics. This factor—aesthetic surfaces for a non-functional transparent part—mean you can immediately eliminate three choices. LS and Direct Metal Laser Sintering (DMLS) do not offer transparent materials, and they require medium to extensive labor to achieve aesthetically smooth surfaces. FDM does offer a transparent-like material, but it builds in thicker layer lines, which require hand and machine labor to smooth enough to qualify as aesthetically pleasing. Additionally, LS, DMLS and FDM technologies use engineering-grade material, which is not a quality the designer needs for this application. Therefore, the two best options are PolyJet and Stereolithography (SL), which both print in thin layers and offer transparent materials. The service provider could feasibly offer up both these 3D printing processes as viable solutions.
The next step is to uncover a bit more about the prototype in terms of size/volume, tolerances (when critical) and design features to determine which process is best for the application.
PolyJet and SL are similar technologies in that they both print photocure resins with excellent feature resolutions. PolyJet offers the fastest printing speed for small parts, while SL is cost-effective for larger parts. Because PolyJet prints in such thin layers—16 µm—its speed is affected by larger objects.
The sample model design measures roughly 8×13×14 in.; therefore, it would likely have to be built in segments and bonded if we chose PolyJet; segmenting the object will add labor and manufacturing time. Let’s also imagine the customer wants the part to have a glossy coat, color-matched to his company’s brand. Painting a 3D-printed part throws thin layer lines into higher contrast. To keep a smooth surface and add paint, some sanding will be required. We know from experience that SL offers a more easily sanded surface. So while PolyJet is a viable solution, the part will build faster and require less post-processing when built with SL.
The above is a fairly simple example with a somewhat clear-cut answer. We’ll now walk through a much more difficult engineering challenge: manufacturing satellite antenna arrays for NASA’s Jet Propulsion Laboratory (JPL).
Real-world example: COSMIC-2 antenna array
Our team at Stratasys Direct Manufacturing was involved with this project. A project like this required a medium to high level of previous 3D printing experience and involved a good amount of testing. NASA JPL began the COSMIC-2 project knowing it would be a smaller project compared to the COSMIC-1. They also needed to minimize manufacturing costs and time.
Because their low volume production run would be below 50 parts, NASA JPL ruled out machining and instead turned to 3D printing. This venture had stringent requirements and critical mechanical properties.
1. Application: External satellite antenna arrays
2. Function: Support radio antenna wires and transmit and receive radio waves from earth<
3. Environment/Stability: Outer space with its extreme temperatures and pressures. Additionally, it needs to withstand the vibration from being launched into space and the movements of the satellite itself.
4. Durability: These are going into space for decades, so they have to last for generations.
5. Aesthetics: While these parts aren’t entering any beauty pageants, they have to be compatible with S13G white paint, which protects parts from radiation.
6. Economics: This is a low volume production run that doesn’t justify tooling or machining and is dependent on speed and part consolidation to cut down on manufacturing time and costs.
7. Priorities: Durability in an extreme environment
Based on these answers we can rule out two options right away. PolyJet and SL materials are susceptible to UV degradation and aren’t viable options given the durability and stability requirements. FDM, LS and DMLS offer durable, high-performance materials.
These antennas can’t have a conductive material surrounding them, so that rules out DMLS—our sole metal 3D printing method. With DMLS ruled out, we must compare LS and FDM, as both of these processes offer tough materials. LS offers FR-106, a FAR 25.853 material rated for 15- and 60-sec vertical burn tests that pass smoke and toxicity requirements. FDM offers ULTEM 9085, a material that meets FAR 25.853 and UL94V-0 standards. Both these materials have great strength-to-weight ratios and smoke and toxicity ratings; therefore, we have to study the geometry to determine which process is more ideal for this antenna array.
FDM is great at angled geometries and large flat surfaces, while LS is excellent for organic and curving shapes, but less ideal for large flat surfaces. LS parts are 3D printed in a chamber heated to just below the melting point of the plastic; LS designs with large flat surfaces are therefore more susceptible to warp. Additionally, FDM is capable of tighter as-built tolerances at ± 0.010 or ± 0.0015 in./in., whichever is greater, while LS standard tolerances hit ± 0.020 or ± 0.003 in./in., whichever is greater. These antenna arrays have sharp angles and flat surfaces, so while LS is a viable option, FDM is the more suitable choice as it requires less post-process work to flatten or sand LS parts into tolerance.
Based on knowledge of JPL’s requirements, the design team was able to choose the process that made the most sense from a cost, complexity and material perspective. As mentioned earlier, these are unique parts in that the intended application had never been done before with 3D printing, so NASA JPL had to take FDM samples and test them according to NASA class B/B1 flight hardware requirements. These tests included:
• susceptibility to UV radiation and atomic oxygen
• outgassing (CVCM index was measured to be 0%)
• thermal properties tests—in particular, compatibility with aluminum panels (aluminum has a slightly different coefficient of thermal expansion than non-glass-filled ULTEM)
• vibration/acoustic loads standard to the launch rocket<
• compatibility with S13G white paint and associated primer
We were confident that FDM would take well to the paint and primer based on our experience of finishing it to hundreds of surface requirements over the past decade.
The chart gives a simplified overview of common 3D printing processes and areas in which they excel.
Post-processing considerations
All 3D printing processes afford a great level of complexity, detail, accuracy and material diversity. With the right expertise and surface finishes (such as sanding, painting and polishing), a part susceptible to heat and light, like SL, can achieve better environmental stability. The right surface finish for a part built from LS can alter rough surfaces to a high gloss aesthetic look. One drawback of this post-processing is that it lengthens production time and can increase costs.
A few application tips worth noting about the selection process. PolyJet and SL are commonly used as master patterns for cold mold or low-temperature molding techniques. Stratasys uses both these processes in their urethane casting techniques. Outside of those uses, SL is a go-to standard for one-off investment cast patterns due to its custom formulated resin and specialized build process for investment cast patterns.
Figuring out the right technology for your application doesn’t have to be a guess. At the same time, it’s important to be open to new technologies as your project evolves. Experimenting with new processes, trying new materials and starting to understand the pros and cons of each process will ultimately open doors to better manufacturing. You may be surprised where 3D printing takes you.
Stratasys Direct Manufacturing
www.stratasysdirect.com
