Most 3D CAD files can be converted easily to STL, but you won’t get the prototype qualities that you expect if you don’t set the correct conversion factors.
Redeye On Demand, Eden Prairie, Minn., fabricated this landing gear from several RP-manufactured parts using FDM machines. The individual parts were outfitted with dove-tail features for fastening together. Aircraft design engineers mounted their electronic and hydraulic motion components, sensors, plumbing, and wiring to verify form, fit and function—quite handily. The Marketing and Sales Departments showed it at trade shows, a very impressive display.
If you haven’t tried making prototypes directly from your CAD files yet, you need to know how to prepare them for the transition. If you have made prototypes, then you know that the output based on your input may not always be what you expect to see. The output can vary from really nice to very bad, or it may not work at all. This is because RP machines accept only fine-tuned STL (Standard Tessellation Language) files. Regardless of the brand of resident 3D CAD you might have (one of the possible 50 to 100 available packages), all files must be converted to STL to work on stereolithography (SLA), selective laser sintering (SLS), and fused deposition (FDM) modeling machines.
Fortunately, most of today’s popular brands of 3D CAD software can accommodate a conversion to STL. But the resident software in each kind of RP machine needs specific attributes set in the 3D CAD’s STL conversion software to ensure that you will get what you see in your 3D CAD file.
STL files are simply a system of small, well-defined triangles that surround the CAD model or the product intended to be manufactured. The more triangles that are produced to define a complex surface for the RP machine, the finer the features will be. When the files have an insufficient number of triangles, what comes out is called a very faceted part. In the worst case, a round hole could become a hexagon or an octagon. This means the settings are not high enough, that is, not enough triangles. The prototyping machine always builds to the STL file, so if we send in a very coarse file, we will get a very faceted product. All the surfaces and every other feature will be pretty coarse. Previewing the CAD file doesn’t help; the CAD file always looks smooth. The problem is in transferring the CAD file to STL where you have to make certain that the settings are sufficient to form enough triangles to obtain a smooth product. Some design engineers don’t realize that when they convert files to STL they must select the resolution manually, which might be as simple as clicking in one of three boxes, such as coarse, fine, or extra fine, while other software products ask questions about chord height or tolerance, deviations, angular tolerance, and a few other attributes.
In a recent contest, college students produced this quite complex vehicle frame using 3D CAD software and a DFM prototyping machine. Compared to welded tubing or a milled component, rapid prototyping ensured that the frame would be not only strong, but also light weight, an essential requirement. (Courtesy RedEye On Demand.)
FDM prototypes can be made in what some might call limited production runs. Typically, an order for prototypes may range from one to 15, but it is not unusual to have a requirement for 100 to 2000 parts for testing as well as marketing and sales department demonstrations. (Courtesy RedEye On Demand.)
But, you can go to the other extreme and select far too much resolution or make the file too fine, and end up with a very large file that is extremely difficult to work with. For example, a large file can lock up the computer or prevent the operator from spinning the image around. So, like most engineering undertakings, the tradeoffs have to be dealt with. While more triangles are desirable for a more accurate model, the files take longer to process. Sometimes, processing takes much longer.
As the designer, you must recognize the tradeoff and go for some compromise. Fortunately, the intended use of the model usually defines the file size. You can get away with making models for form — and sometimes fit — with fewer triangles than if you needed the prototype to test for function, where it might experience a full load or burden. Although materials are becoming more widespread that mimic true production parts such as those made from injection-molded
plastics, their costs are also significant. It is a wise idea to be honest about the prototypes’ intent and select the file size appropriately.
So, making the correct settings will give you the ease of using the file and at the same time, producing an acceptable or the intended part. But beware; your CAD system may not let you view the STL file after you convert it. Don’t count on the CAD file viewer to show you what you might get.
Chuck Brunner, Technical Support Specialist at Redeye On Demand, Division of Stratasys, says that he uses what is called “Magic” software, which is a fixing, rendering software for the STL files. And he can view the STL files in a Stratasys proprietary software called “Insight,” which is buried in the FDM machine. It shows how faceted something really is. Although some software packages do not let you see the STL files, Brunner can look at the model ahead of time and see that it is properly faceted. He also observes that as people learn, initially they may make the mistake of setting up a very rough file and not realize that this is exactly how the part will turn out. To get a better idea of what types of settings are needed for a wide variety of 3D-CAD packages, go to www.redeyeondemand.com.
Information for this article was provided in part by Chuck Brunner, Technical Support Specialist, RedEye On Demand, Division of Stratasys, Eden Prairie, Minn.
Direct digital manufacturing in a nutshell
Direct Digital Manufacturing is not entirely a brand-new technology, of course. It’s just a more recent, jazzy term for a process whose roots are deeply buried in rapid prototyping (RP) and reverse engineering. But whether you call it DDM or rapid prototyping, the process is enjoying more media coverage now that the tools for making prototypes and limited production runs faster are becoming increasingly sophisticated, affordable, and “at home” on the factory floor. Thanks to the RP pioneers, the dream of most design and manufacturing engineers—making hardware from CAD drawings to final product in one step—is coming true. Although we have been able to do this for some time on a limited scale, the ultimate goal is to make the products in relatively high volume, really fast – and that day is rapidly approaching.
Many Rapid Prototyping techniques have evolved over the past 20 years or so, including Stereolithography (SLA), Selective Layered Sintering (SLS), and Fused Deposition Modeling (FDM). One of the earliest forms that had been refined over the years came from the SLA process and seems to be well accepted. Stereolithography can produce extremely robust and relatively large parts with fine features. The process uses a liquid polymer that is cured with an ultraviolet laser to form the prototypes. The parts are flexible, strong, ABS-like, and can withstand high temperatures. And now, new, clear materials are available that let them make a variety of translucent parts which may be ideal for observing the motion of components in an assembly, such as a model of an hydraulic pump.
Parts made from this process are good for most form and fit testing, and with the proper materials, pretty aggressive functional testing. This process is not intended for making more than a few prototypes, though; they are mostly intended for testing and sales and marketing presentations.
SLA machines have the ability to make parts with an accuracy of about ±0.005 in./in., and a feature size of 0.015 to 0.005 in., depending on the setup resolution. The finished parts can be made with excellent strength properties, including the ability to withstand relatively high heat. The big advantage is that they can be delivered to the customer typically in just one to three days.
Selective Laser Sintering
Selective laser sintering (SLS) can use production-type materials to make extremely strong products. This additive process uses powdered material and a CO2 laser, which “sinters” the material in thin layers—similar to the SLA machine. The materials give the prototypes properties that are similar to production-type sintered parts, which can withstand high temperatures.
Like the SLA process, these parts are intended to be made in quantities of ten to fifteen. They may be used for form and fit, field testing, and a little more aggressive functional testing than SLA parts.
Accuracies fall in a range of ±0.010 in./in. with a feature size about 0.020 in. or more. They have
exceptional strength properties and can be made from CAD drawings in just a few days.
Fused Deposition Modeling
One major advantage of using FDM is that it can handle some of the most production-type thermoplastic materials such as many of the brands used in injection molding. Other machines use resins, but thermoplastics yield parts that are very close to the actual injection-molded parts as far as strength and flexibility are concerned. These materials include PCABF blends, ABS plastic, PPSF, and polypropylene. The FDM process uses a wide variety of engineering materials including ABS and poly carbonate to generate 3D models by depositing the thermoplastic material in layers on top of one another.
Just as injection-molded parts are robust, so are FDM parts. For example, when making parts that must snap together, FDM parts are just about as durable as injection-molded plastic. FDM parts can snap together and pull apart 10 to 25 times, which will help you decide if the wall thickness should be thinner to get more flexibility. Parts can be field tested, abused, beaten, loaded, and otherwise tested to destruction to help decide what changes need to be made. The changes can be made in less than two weeks, pretty much the time it takes the engineers to modify the CAD drawings, but only three or so days to actually FDM-fabricate a new part.
The machines are limited to making the size of a single part in the order of 20 x 22 x 24 inches, but these parts can be fastened together with dove-tails to make a much larger part. For example, RedEye made an aircraft landing gear assembly about 13 feet tall by joining several parts. The assembly was used by the design engineers to verify form, fit and function, and the frame on which to install electrical and hydraulic components for functional testing as well.
Although most service bureau customers order quantities under 50 FDM pieces for concept models and functional testing, that number is continuously increasing. It is not entirely unheard of to supply 1000 to 2000 parts of certain kinds of products. This approach may be less expensive for the client than making hard tooling for injection molding, or 2000 parts might be the total production run that gets to the market place. Accuracy for these parts is in the range of 0.005 in./in. with a feature size of about 0.020 in. or greater.