You could be mistaken for thinking that stereolithography has matured. However, Paul Stevens reports that not only are the latest innovations creating significant new opportunities, but there is also an entirely new technology that has the potential to cut production times and costs.
Stereolithography was a truly amazing technology when it was first commercialised. The ability to create relatively accurate and robust parts rapidly from CAD data made a significant difference to the product development process. And as well as being used for appearance appraisal, stereolithography models could be used for producing multiple replicated parts by means of investment casting, vacuum casting and other processes to enable functional testing to be performed or for the direct creation of low-volume production parts. Indeed, a limited amount of functional testing was possible with the models as manufactured by the stereolithography apparatus (SLA).
In less than 10 years stereolithography has evolved such that accuracy and surface finish are better, production is faster and newer resins enable tougher, more robust parts to be created. Nevertheless, the focus of attention more recently has often been on other technologies, such as 3D printing and direct laser sintering of plastic and metal parts. Despite this development of complementary or alternative rapid prototyping and manufacturing technologies, stereolithography remains a valuable process and the most recent developments warrant further investigation.
3D Systems Corporation is one of the leading developers of stereolithography machines and materials. Towards the end of 2008 the company launched the iPro 8000 SLA Center, a mid-range, high-productivity system that offers all the performance attributes and benefits of the previous iPro SLA Precision Centers but in a smaller format and with a more compact footprint (Fig. 1). Layer thickness is 0.05 to 0.15mm and the maximum build envelope is up to 650 x 750 x 550mm, though different sized interchangeable resin delivery modules (RDMs) mean that the volume of resin in the RDM can be more closely matched to the size of the part(s) being built.
The iPro 8000 SLA Center is designed to build small and large parts quickly and economically with excellent surface smoothness, feature resolution, edge definition and tolerances. It is claimed that parts produced on this SLA system have the same accuracy as parts produced by CNC machining. Based on 3D Systems' patented SLA technology, the iPro 8000 SLA Center features a new-generation, proprietary Proscan digital scanning system that is equipped with a new Steadypower Imager for reliable output and repeatability. In combination with 3D Systems' portfolio of Accura materials, this next-generation stereolithography machine is said to provide excellent part-building capabilities, improved quality control and faster, cleaner changeover between modelling materials.
Resins
3D Systems' range of Accura resins gives users a broad choice of material properties for rapid prototyping and rapid manufacturing applications:
- Accura 25 Plastic simulates the performance, properties and aesthetics of moulded polypropylene.
- Accura 55 Plastic simulates the performance, appearance and feel of moulded ABS.
- Accura 60 Plastic simulates the properties and appearance of clear polycarbonate.
- Accura Xtreme Plastic delivers extreme performance and durability, simulating the look and feel of moulded ABS and coping with temperatures of up to 60°C.
- Accura 48HTR Plastic delivers outstanding rigidity and dimensional stability in temperatures of up to 130°C.
- Accura Bluestone Material is an engineered nanocomposite plastic that is claimed to deliver unparalleled stiffness in demanding jig, fixture and wind-tunnel applications.
- Accura Clearvue Plastic offers excellent transparency, outstanding durability and enhanced yields and manufacturability.
At the same time that the iPro 8000 SLA Center was launched, 3D Systems introduced the iPro 8000 MP SLA Precision Center for manufacturing small, precision custom parts primarily for medical and dental applications. Equipped for automated, pushbutton data processing of hundreds of files at a time, the iPro 8000 MP SLA Precision Center is suitable for the rapid mass-customisation of parts such as orthodontics, implant drill guides, dental restorations, hearing aid shells and ear moulds.
Another system announced by 3D Systems towards the end of 2008 was the iPro 9000 XL SLA Center for the production of extra-large, high-definition parts for prototyping and end-use applications (Fig. 2). With the ability to build parts up to 1524mm long, the iPro 9000 XL SLA Precision Center is suitable for applications requiring large, single-piece parts. Its 1500 x 750 x 550mm build envelope is claimed to be the largest commercially available for stereolithography. Building large monolithic parts without any seams or joins results in improved aesthetics, better durability and lower cost.
As with the other two machines described above, the iPro 9000 XL SLA Center uses the Proscan digital scanning system and builds parts from Accura materials.
Lee Dockstader, vice president and general manager of stereolithography products at 3D Systems, says: "It is exciting to think about all of the industries that can benefit from the extra-large, uni-part manufacturing capability of the new iPro 9000 XL SLA Center and all of the new applications that this manufacturing capability has made possible. With this system, we can build an entire car bumper in one piece, specifically addressing our loyal automotive customers' desire to build bigger, better and faster. Paired with our growing portfolio of Accura Pro plastics, the iPro 9000 XL SLA Center is a versatile system for a variety of automotive, aerospace, construction and investment casting applications."
3D Systems' Clearvue Plastic, mentioned above, is the most recent addition to the Accura family. It produces virtually colourless parts for a wide range of applications, including automotive headlamp lenses, hydraulic and pneumatic functional models, and form, fit and function parts and prototypes (Fig. 3). It also enables designers and engineers to visualise internal structures and simulate lenses and other optical components. According to 3D Systems, Clearvue offers outstanding flexibility and durability, with a modulus range that bridges two of the most widely used thermoplastics - ABS and polypropylene. Such are the material's properties that it can even be used to produce parts for testing snap-fit assemblies.
For biomedical or skin-contact applications, DSM Somos now has USP (United States Pharmacopeia) Class VI approval for its Watershed XC 11122 resin, which is a clear, water-resistant material that is already widely used in the medical device industry. Developing advanced biocompatible stereolithography resins for the medical industry is becoming increasingly important; according to Wohler's Report 2008, the medical/dental industry is the third-largest user of additive manufacturing after consumer products/electronics and automotive, but it is growing at a faster rate than either of the others.
Originally introduced by DSM Somos in 2007, Watershed XC soon found favour in the medical industry for prototyping clear medical device housings and fluid flow analysis models due to its functional properties and high dimensional stability (Fig. 4). USP Class VI approval creates new opportunities for prototypes to be used in clinical trials.
Software developments
Of course, whatever material is selected for a stereolithography application, the way it is used can make a significant difference to the part's performance and cost. And this is where software can have an impact. DSM Somos has developed a software application that enables lightweight prototypes to be created with enhanced structural integrity.
Based on patented technology from the Milwaukee School of Engineering (MSOE), Tetrashell hollow-build software uses MSOE's Tetralattice technology to facilitate the manufacture of hollow stereolithography parts with variable skin thicknesses, supported by a Tetralattice support structure (Fig.5). Potential application areas include investment casting patterns, reduced-density metal-clad composite structures, and lightweight large, thick-sectioned parts.
DSM Somos believes that Tetrashell enables parts to be built that would not have been cost-effective to create as solid parts - yet without compromising the parts' structural integrity. Brian Bauman, a product development manager with DSM Somos, comments: "The Tetrashell software will allow stereolithography part builders to widely vary wall thickness, as well as to adjust support parameters - and consequently opens doors previously closed to stereolithography because of cost or weight considerations. Due to the overall reduction in mass, the Tetrashell hollow-build style will create material savings for customers with bulky parts, while at the same time increasing part accuracy."
While the developments from 3D Systems and DSM Somos could be viewed as incremental innovations - albeit important ones - Huntsman Advanced Materials has unveiled its Araldite Digitalis rapid prototyping machine that uses an entirely new technology in conjunction with radiation-curing resins.
Described as fundamentally different to other technologies, the Araldite Digitalis is neither based on lasers nor on light-reflecting MEMS (micro-electro mechanical system) as used in 3D printers. At the heart of the Araldite Digitalis is an MLS (MicroLightSwitch) exposure system that operates via a computer-controlled micro-mechanical shutter system that selectively exposes a larger surface area of resin in a single step. This type of exposure, in contrast to a laser that exposes one point at a time, is said to enable much faster and more accurate manufacturing of simple or complex parts.
The Araldite Digitalis apparatus consists of three key elements: the operating console, the vat enclosure with a recoater, and the exposure system. UV (ultraviolet) light from UV lamps is distributed through fibre optics onto the exposure bar where MLS units distribute UV light pixels on to the resin surface. The computer-controlled shutter mechanisms steer the exposure of the UV light to avoid scattering, and micro lenses refocus the UV light after leaving the MLS. Importantly the illuminating angle is 90 degrees, which guarantees that uniformly high accuracy is always achieved.
With its innovative technologies and potential to reduce production times and costs, the Araldite Digitalis won the Euromold-Award 2008 Gold Prize in December 2008. The Euromold awards honour the three best products and services out of the broad spectrum of exhibits at Euromold.
