Rapid prototyping (RP) is a critical link in today’s fast-paced design cycle and is a cost-effective way to shorten product development from concept to production. The RP process builds concept models to verify new designs for form, fit, and function, which often includes aggressive testing. Such prototype model testing considerably shortens time to market and eliminates costly design errors. Both the manufacturers of RP equipment and the materials suppliers are constantly upgrading the capabilities of their products to give engineers more robust models and new ways to deliver products on time and on budget.
Of the two major ingredients of the RP process—machines and materials—the materials have been perhaps the most challenging area for engineers and chemists to develop. A major material limitation lies in the strength of the prototype parts to withstand real-world loads, as it would in actual service. Clearly, this is an area that requires constant research and improvement.
Stereolithography (SLA), Selective Laser Sintering (SLS), Fused Deposition Modeling (FDM), and 3D Printing currently are the four major rapid prototyping technologies. SLA, the most widely used process, employs a laser beam to solidify layers of liquid resin. The SLS process also uses a laser to sinter layers of powdered material. In contrast, FDM machines build models by extruding layers of production-type material on top of one another. The 3D printers “print” a water-based adhesive delivered by the print head on layers of mixed plaster and resin, which bonds them in accordance with the CAD cross-section pattern. All four technologies are considered additive, since the prototype is built by adding material, as opposed to subtractive processes, such as machining where the material is removed.
Compared to the SLS and FDM processes, the SLA has the fastest turn-around time and produces parts with highest accuracy and the smoothest surface finish. On the other hand, when a prototype needs to have production-part-like structural durability, the SLS and FDM processes have an advantage since they use materials with about 80% the strength of actual production parts.
Because stereolithography is an extremely accurate, stable, and repeatable technology SLA parts are well suited for form and fit verification. However, engineers often were hesitant to conduct high impact functional tests since SLA parts were rather fragile. But this limitation is becoming much less of a factor.
The advancements in modern chemistry and formulation processes allow the development of new resins known in the industry as mimics. These SLA materials closely mimic the properties of various engineering plastics. For example, if the production part will be made of ABS or polypropylene, engineers can choose prototypes made from the ABS-like or polypropylene-like SLA resins. The prototype will have mechanical properties closely matching the properties of the production injection-molded parts and can be used for functional tests, including high pressure and high temperature. So, the complete form, fit, and function verification can be done using one SLA prototype.
Modern SLA machines can process a complete portfolio of stereolithography materials to address various needs of engineers. Service bureaus greatly increase their efficiency from the use of newer machines that come with rapid delivery modules, which allow quick changeover of materials. The shortened set-up time reduces the cost and overall turn-around time.
The Prototype Model
The type of the stereolithography machine mostly determines the feature resolution and the maximum size of the part. The selection of the SLA resin however is the deciding factor when reviewing options based on how the prototype will be finished, used, handled, and tested. SLA resins available today imitate the physical properties of engineering thermoplastics such as Acrylonitrile Butadiene Styrene (ABS), polycarbonate, and polypropylene. For example, an ABS-like resin will produce as close a match to the production ABS part as possible within the SLA technology.
Often translucent or clear parts are highly desirable during the concept development, functional verification, and approval phases of the project. Previously, machining and polishing were traditional methods of fabricating optically clear prototypes from polycarbonate and acrylic. Later, translucent resins became available for rapid prototyping. But early parts were formed semi-clear or became cloudy quickly. They had a greenish or bluish tint. The completely clear SLA resins became available only in the last three years.
If exposed to light for a long time, parts made from clear SLA resins experience slight clarity degradation. For the majority of parts this degradation poses no problem, because the parts are usually discarded after design verification or concept presentation. The loss of clarity can be minimized by coating or sealing parts if the optical property has to remain constant for longer periods of time.
Optically clear parts open new possibilities for designers. Some parts, such as lenses, see-through shields, retail packaging, and medical devices are made of clear plastic in production, and clear SLA parts allow building prototypes that look exactly like the real product. They can be used for marketing presentations, focus groups, shows, and concept reviews.
Building architectural models of clear materials greatly simplifies the architect’s job. Until the transparent enclosures let machine designers see how parts of various mechanisms moved inside an enclosure, they had to make holes in order to peek inside.
Abe Reichental, the President and CEO of 3D Systems, gives an example of one company that had a transmission housing made of clear SLA resin. They populated it with the gear train and gaskets, filled it with transmission fluid, and ran the prototype to study the fluid dynamics in various modes. Another 3D customer uses clear rapid prototypes to test engine blocks. They ran the crank shaft with an electric motor and conducted various tests to study the motion and fluid dynamics inside the case. Now engineers had a full view of the entire engine.
Another example of flow analysis where clear SLA parts allows visual study is in pumps. A common pump design problem is called cavitation, the generation of a void or the formation of bubbles in the liquid. Pump housings made of clear material gives engineers an unobstructed view inside the pump so that even the smallest areas of cavitation can be spotted.
Building light-transmitting parts places additional requirements on the clarity and finish of the models. Clear SLA parts are successfully used to build prototypes of automotive instrumentation displays, lenses, head lights, and tail lights. Light guides, pipes, pointers, and diffusers are just some examples where machining acrylic or polycarbonate gave way to clear SLA process.
Clear materials are typically geared toward a specific rapid prototyping technology. For example, clear resins available for use in stereolithography systems are not compatible with the SLS or FDM equipment.
Several translucent ABS-like SLA resins are currently available, and several clear resins were recently added to the engineer’s RP toolbox. Previously, clear resins usually involved durability tradeoffs. Newer, clear resins no longer have these trade-offs. Just five years ago translucent materials were rather fragile and could not be used for high impact, high pressure, or high-temperature tests. Newer, much tougher resins were developed to address this problem.
New translucent and clear SL resins such as RenShape® SL 7870 made by Huntsman, Accura® 60 made by 3D Systems, and WaterShed® XC, as well as WaterClear®Ultra produced by Somos are only a few examples.
Carl Holt, Commercial Sales Manager for NAFTA for Stereolithography of Huntsman Advanced Materials, explains that his company specializes in the formulation of ready-to-use SLA resins and offers approximately 30 SLA resins for various applications. Huntsman clear materials are high-clarity photopolymers; they are neither a polycarbonate nor an acrylic.
Huntsman’s RenShape SL7870 is a high clarity resin that also has extremely high impact strength. Another benefit of this material is that it does not age as rapidly over time as many other earlier clear resins. Huntsman also produces a UV-blocking coating (primer) to protect the clear parts from sun damage.
Clear resin manufacturers typically publish standard sets of ASTM information on their Web sites. Since new materials are constantly added to the portfolios (although not all service bureaus include them in their line), it is always a good idea to check with the resin manufacturers to find out what new materials became available recently and which service bureaus are capable of processing them.
The process for building clear parts involves the same steps as when using other SLA resins, except some additional steps are required at the end. A 3D CAD solid model is required to generate an SLA prototype. All 3D modeling software packages are capable of exporting a file known as an “.STL” file (which stands for Standard Tessellation Language). For standard clear finish the support structures are removed, while the support areas and the exterior surfaces are wet sanded. Sanding the part reduces clarity, so a clear coat is then applied to create a smooth, clear surface. The higher-grade clear finish requires deeper wet sanding to remove the build layer staircasing and create smooth surfaces. The clear coat is also applied and can take 12 hours to completely dry.
The cost of an SLA part depends on part height, volume, total part surface area, and layer resolution. The cost of resin selected for the part and required finish are also important factors. In the case of clear parts, clear resins are relatively expensive, but the largest cost factors are the extra finishing and drying required to produce clear parts without blemishes.
How the RP Machine Works
In a typical process for making RP models, the CAD file is loaded into the SLA machine’s computer where a proprietary software program “slices” the 3D model into layers spaced the distance equal to the thickness of the resin that will be cured during the processing of one layer (for example 0.005” thick).
The liquid-clear material is placed into a vat at the bottom of the stereolithography machine and a platform, upon which the part will be built, is positioned just below the liquid resin surface. The platform depth equals to the thickness of the layer. The machine guides a laser along the surface of the resin to expose the liquid according to the pattern in the slice. The laser beam cures the liquid resin that it lights upon. Once the first layer is completed, the platform is lowered so the next layer can be built on top of the first layer. The process repeats until all layers are built. After that the part is taken out and placed under UV light for final curing.
A bridge-type support structure is built at the start of the SLA process to keep the bottom of the part off the platform and to support the bottom curved surfaces of the part. Without these thin columns, the initial layers, only few thousands of an inch thick, would sag and prevent the next layer from being built. The support structure is removed after the part is completed and cured.
Several finishing levels are typically offered by SLA service bureaus. The most basic finishing level includes just removing the support structure. The next, standard level adds light sanding of support areas and exterior surfaces and sand blasting. For parts where surface flatness is important, the part is sanded smooth to remove the build layer stairstepping caused by layering. When the SLA part is intended for mold making or to be painted, the interior walls are also sanded and the part receives a coat of primer.