This is the second in our series of excerpts from "Better Be Running! Tools to Drive Design Success" by Dr. Ronald Hollis, President, CEO, and Co-founder of Quickparts.com (Atlanta, GA). Written for business managers, the book focuses on manufacturing processes, tooling choices, and production strategies that can help companies bring products to market faster. To order the book, go to www.betterberunning.com .
Stereolithography (SL) means to print in three dimensions. SL is the first and most popular liquid-based additive fabrication (AF) system that produces plastic parts from cross-sectioned CAD data. Electronic CAD design data is converted to an STL (Standard Tessellation Language) file format. Special software slices the CAD model into thin layers and creates build instructions for the machine. Layer by layer, the Stereolithography Apparatus (SLA) machine replicates a plastic physical model out of photo-curable resin. The resin turns into hard plastic whenever touched by an ultraviolet laser.
SL allows you to create a 3D plastic object from a CAD model in several hours. Prior to this technology conventional prototyping methods could take days or even weeks. Whether you are a design engineer wanting to verify your concept or a manufacturing engineer needing form, fit, and function feedback, SL gives your team a quick, accurate way to convert virtual data into real objects. It allows you to test designs in their physical environment before committing to expensive tooling.
Stereolithography was invented by Charles Hull and made commercially available by 3D Systems, Inc. in 1988. Because 3D Systems was the first to market the SLA machine, many folks frequently misuse the term SLA to generically describe all rapid prototyping techniques and any liquid-based, UV AF process. One example of a process that produces parts similar to SL is made by Objet Geometries, Ltd. Objet machines produce parts using a technique known as PolyJet. This technique jets or sprays a photopolymer resin instead of using a vat of the resin and solidifies this resin with a UV bulb instead of a laser beam. The parts are similar to those made by SL but have much smoother surfaces. Since 1988, over 40 AF systems have entered the worldwide market.
Step by Step with SL
First, an operator loads the STL file from your CAD data into proprietary software, which digitally slices the model into thin layers of approximately 0.005 inch (five thousandths) and produces a removable, stabilizing structure to support the part during the build. Next, the physical process begins with a vat of photo-curable liquid resin and an elevator table in the vat, set just below the surface of the resin.
A computer-controlled optical scanning system directs the focused laser beam so that it solidifies the 2D cross section corresponding to the slice on the surface of the photo-curable liquid resin. The laser's depth of penetration is greater than the desired layer thickness, and is known as overcure. Overcure plays an important role in producing solid SL models and also affects the build time.
After a layer is complete, the elevator table lowers enough to cover the solid polymer with another layer of the liquid resin. A leveling wiper system moves across the surfaces to recoat the next layer of resin on the surface. The laser then traces the next layer. In simple terms, the energy of the laser "flips" the liquid material to a solid material upon contact. As chemistry buffs know, this is called a phase change in the resin. The process continues in successive layers, building the part from the ground up, until the system completes it. The elevator then rises from the vat and the operator removes any excess, uncured liquid polymer from the part.
Finally, the part is placed in a UV oven for curing. The part is then hand-finished to remove the support structure and to smooth the minute "stair-stepping effect" seen from building the part in multiple layers.
It is difficult to predict the cumulative impact of chemical properties and operating parameters on a build, subject to the ever-changing aspects of cross sections and geometries. Therefore, developing easy-to-use pricing algorithms has always been a challenge. The industry-accepted approach is to use geometric information in secondary formulae to predict cost and "guesstimate" the true build time of a part.
Resolution – The Nitty Gritty
Understanding part types as a function of resolution is very important as resolution affects tolerance, surface finish, and cost. Applications fall into three basic types of SL parts as defined by resolution: The standard layer thickness for SL parts is 0.005 to 0.006 inch, the thickness of a sheet of paper. High-resolution parts are grown at 0.002 to 0.004 inch, one-third as thick as a sheet of paper. Machines made by Objet Geometries produce the highest resolution layer thickness of 0.0006 inch, not visible to the naked eye.
The resolution offered by three different process types dramatically impacts the accuracy and feature capability of your parts. Different geometries require a certain level of resolution. The thinner and more fragile part features are, the higher the resolution required. If your part has no highly detailed areas, standard resolution is sufficient.
If you are creating a part that is mostly simple with a few complex features, for example, a housing with buttons, you can build the complex features in high resolution and the housing in standard resolution to realize a cost savings. Typically speaking, the super-high-resolution processes cost twice as much as the high-resolution process, and four times more than standard.
Tolerances and Accuracy – Real World
The accuracy of the part is an important factor in building useful models. When using AF processes, there are typically no drawings or tolerance studies provided to determine whether a part is within tolerance. Therefore, the base dimensions are actual dimensions in the CAD model, which is mathematically perfect. The limiting constraint becomes the ability of the technology being used to produce the part.
In manufacturing, there is no such thing as a perfect part. Skillful engineers know it is necessary to apply tolerance, the permissible limit of variation in a dimension, to the design. What many engineers typically don't know is that SL is not an exact manufacturing process. Even the high-precision coordinate measurement machine (CMM) has tolerances. Its "1-inch" diameter pin is really 1.00005 inch.
Standard SL tolerances are plus/minus 0.005 inch for the first inch, and plus/minus 0.002 inch, inch for inch on most parts and features. Understanding this is critical, especially when mating parts made with the same SL process.
Of special consideration are those tolerances of two or more interfacing parts. Good engineers always specify the largest possible tolerance while maintaining proper functionality. But a clear understanding of how materials and geometries affect SL tolerances is most likely one thing they didn't teach in engineering school. Parts must be designed to distribute or relieve residual stresses. For example, a long bar will react differently than a housing part. While both designs look perfectly flat in CAD, they will both warp slightly, factoring together residual stresses, materials properties, and build orientation.
Informed customers get better pricing and better solutions. To play this game well, you have to understand what orientation really means. The time required to build a part depends on its orientation in the machine vat. Factors include the number of layers required to be processed as well as specific layer-dependent parameters that can affect the time required to complete a layer.
Depending on part geometry, there can be a major cost and time difference in parts built vertically versus horizontally. Vertical builds get better definition and require a longer build time, and therefore cost more. If you don't need perfection on your first draft, you may choose to build horizontally and save 50%.
There is also a tradeoff between the surface finish requirements of a part and its build time. Typically, surface finish is more critical factor because a part with poor surface finish may not be useful to the user, regardless of how long it took to build the part.
In almost every category of life, size does indeed matter. Some of us have learned about machine size issues the hard way. 3D Systems machine names SLA-250 and SLA-500 actually denote their real build platform size. The SLA-250 platform is 250 x 250 millimeters, or approximately 10 x 10 inches. The SLA-500 platform measures 500 x 500 millimeters, which is approximately 20 x 20 inches. However, for the more recent models, SLA-5000 and SLA-7000, meaningful size denotations were dropped and a few zeros were added to make the machines sound "bigger and badder," though they are essentially the same as their predecessors.
The point is to be careful about machine size. So, if you have to produce a 21-inch rod in SL and you are dealing with a fly-by-night SL provider, he may suggest that, due to platform size limitation, he would but the rod in half, build two SL parts, and then rejoin them with adhesive, a more costly solution. While it is true that SL machines offer only choices of platform size (described above), there is a better way to handle this problem. Look at the build platform space in a new way. Instead, turn the 21-inch rod diagonally in the larger vat so that it fits on the hypotenuse of the platform space. Avoid splitting parts whenever possible.
In other words, part size should determine machine selection. If your part has a dimension greater than 20 inches, you have to figure out how the part can best be oriented to make a single piece. Make sure that your service provider has a large-frame machine. If they only have a small machine, your part will be split, which adds cost. If you don't ask them about machine size, they probably will not tell you.
Materials Are a Nightmare
To push ahead of the competition, plastics companies continually release "new and improved" plastics and resins, making shopping very confusing. Basically, it's all gooey gunk. Some of it is toxic, some of it isn't. The performance of materials is geometrically dependent on design; orientation can determine the success of your build.
In materials selection, try to identify the material that supports the function of the prototype, which in turn can support the function of the part in the real world. When shopping for plastics, you need to know the basic material types.
Rigid materials, similar to polystyrene or Acrylonitrile Butadiene Styrene (ABS)-like materials, are used for things like a computer mouse, cell phone, or electronic shroud. For harder parts that require no flexibility, rigid material is tough and can withstand rugged environments.
Durable materials, closest to polypropylene, are used for parts that require a snap fit. Durable materials flex without breaking, but be cautious when building a part that requires flexing. Make sure that your part is oriented properly to support and strengthen the snap feature. A vertical build will add strength to a flexing snap feature, but the horizontal build is innately weaker due to horizontal layering.
Semi-flexible material, like polyethylene, is lightweight and easily deforms. It is used for some bottles and lids. Flexible material, such as elastomeric, is rubbery and used for connecting pieces like gaskets, washers, and boots, which often require a watertight seal. Water sealant can be added to make parts, such as flexible nozzles on liquid dispensers, water-resistant. Special materials are available for high-temperature SL usage.
How Do I Save Money Using SL?
Here are some insider secrets to saving time and money using SL.
- Family Build Concept. A powerful cost-saving secret of using SL is realized using economies of scale, which refers to the decreased per-unit cost as output increases. Engineering managers can save thousands of dollars using family builds. If you produce a single part in the vat, the wait time or operational waste is the same as if you run a family build of 10 different parts. Imagine your single part costs $200 to produce. If you build the family of 10 different parts, the cost is $425 total! Your piece part price decreases from $200 to $42.50 because operational overhead is now distributed among all the parts.
- Build Multiple Parts of Assemblies Together. A good way to capture family build savings is to identity multiple projects needing assemblies of multiple parts, such as cell phones and keyboards. If all of the parts fit on the SLA platform, you can save up to 50%.
- Order Multiple Quantities. Imaging that you need a pen cap made in SL, costing $200. But others in the company, like marketing and the president, also need one. If you order more than one, each additional part will be dramatically lower than the first.
- Using Single Material. Family build practices can extract all overhead from one material. If additional materials are used, overhead will be added to those as well. So, the overhead is the same for either one or more than one part when using the same material during the same build.
- Orient to Fit. Know the difference between building parts vertically versus horizontally. As discussed above, vertical builds take longer, have more definition (based on geometry), and cost more.
- Instant Quoting. Find sources that use instant quoting to literally save your company hundreds of man-hours of labor. Always use instant quoting for pricing because it has no greed factor and no human element. An engineer's time costs a company about $100 per hour, so if spend six hours getting a quote, you are losing money.
Benefits – The Buzz is Real
Whether you are designing a new engine block for Mercedes-Benz or a shoulder replacement for a human being, SL technology is radically compressing development cycles, saving millions of dollars, and opening new doors to innovative solutions.
One dental company heavily employs SL technology to create invisible braces to treat hundreds of thousands of patients. A major automotive company realized a cost savings of 45% by using SL for rapid tooling of small parts. A jet engine blade project, typically taking nine months with standard tooling and machining, tool only one month using SL solutions. Design time for an orthopedic implant was reduced by 15 months. One tooling project that would have cost $1,700,000 using conventional methods cost only $40,000 because it drastically reduced the errors in tooling. Competing architects designing options for the new World Trade Center used SL extensively for highly detailed, full-color miniature models of their visions. The application of SL technology holds unlimited potential and is a challenge to every designer's imagination.
About the Author
Ronald L. Hollis is the President, CEO and Co-founder of Quickparts.com, Inc. He has provided the leadership and execution to build Quickparts into one of the fastest growing companies in the U.S. This was accomplished through innovation and accepting the risks to change the way you buy custom parts with instant online quoting. Dr. Hollis and Quickparts are also the recipient of many awards including Ernst & Young Entrepreneur of the Year finalist; Deloitte Fast 500; Inc 500; and 2004 Catalyst Innovator of the Year. He is a graduate of the MIT Birthing of Giants program and served in leadership positions for YEO and YPO. He earned a BSME, as well as a MSE and Ph.D. with a focus in technology-based business from the University of Alabama. Dr. Hollis is passionate about building businesses that apply technologies to solve problems and drive efficiency.