Stereolithography—Staying relevant in the 21st Century
Turning 30 invites a time of reflection. The landscape of additive manufacturing has changed dramatically since the 1988 commercialization of stereolithography (SL) as the first viable 3 D printing technology. In fact, the terms “3D printing” and “additive manufacturing” only entered the popular vernacular in recent years.
The scope of additive manufacturing has changed dramatically in recent years. Technologies encompassing thermoset and thermoplastic materials, as well as metal, have proliferated. The 2017 Wohlers Report lists 96 different AM equipment suppliers across a broad range of technologies. Advances in equipment, software, and material have been substantial since the early days of SL, concurrent with increasing computer power and growth in the 3D CAD installed base.. Competition within the early technology supply base has increased as patents expire and new players come to the international market. So, what’s in store for this young technological grandfather?
Stereolithography (SL) like all additive manufacturing processes provides a means to translate 3D computer models into a physical shape without machining. Material performance characteristics combine with 3 D printing methodology to shape application capabilities. Today, the inherent characteristics of 355 nm laser based SL technology characteristics leverage ever-expanding material capabilities to facilitate maturation into one of the widest used and highest utility AM processes. The term SLA, a registered trademark of 3D Systems, is often used by some to encompass a variety of 3D printing processes that fall within the ASTM grouping of AM processes as “Vat Polymerization.”
Stereolithography in this discussion will be focused on “industrial SL “, as the original technology has evolved and is differentiated from all other vat polymerization processes by:
- Platform sizes ranging from 250 mm (9.5 in.) square to over 800 mm (31.5 in.) square.
- Ultraviolet laser (355 nanometer wavelength) light source
- Materials specifically formulated for 355 nm UV including clear, pigmented and composite systems
- Imaging from above (build platform travels downward)
Photo Caption: Metal Plated High Accuracy Sample Part Demonstrating Resolution and Precision of 355 nm Stereolithography
The use of a laser to instantly cure a photopolymer with a UV laser with a nominal spot size less than 0.2mm provides one of the highest combinations of accuracy and resolution of any AM process, especially considering the range of part sizes the process is capable of. Today’s 355nm SL materials can produce parts that have excellent dimensional consistency and surface aesthetics ranging from transparent to a variety of colors resembling typical injection molded parts. These materials have overcome robustness and aging issues encountered in earlier generations, allowing for parts manufacture with a broad range of mechanical properties allowing functional applications in prototyping, patterns, and beyond.
Photo Caption: Aerosport is successfully producing highly accurate parts that need very little finishing at the end of the process in the UnionTech™ RS Pro600, a recent entry to international markets.
Additive Manufacturing (AM) processes utilizing thermoplastic materials are often cited for robust mechanical properties. Current generation SL materials can be selected to significantly overlap the performance range of many of the commonly used thermoplastics in other AM processes while retaining all the accuracy and aesthetic benefits of the SL process.
Stereolithography is often typecast as a prototyping process sometimes based on an outdated understanding of material capabilities. The attributes of 355nm SL equipment in combination with the latest generation of photopolymers enables applications that extend prototyping capabilities as well end uses. Significant opportunities in patterns for secondary forming operations ranging from large scale mass customization (dental aligners), low volume urethane part production, tooling for low volume injection molding, and metal clad composites are now being actively pursued. These applications are practical examples of how innovation can be attained via integration of SL with other conventional processes.
Innovation via Technology Integration
The 3D printing process is often positioned as a disruptive technology but it is better thought of as an enabling technology.
In the late 1990’s, the founders of Align Technology imagined a different business model for correcting the alignment of teeth with a series of retainers. Today, this application is possibly the highest volume application example of mass customization. Converting the CAD images of individual patients to patterns used to thermoform the final aligners enabled what most would call a disruptive business model.
SL patterns for a secondary thermoforming process remains the dominant technology of this application today, based on the rapid processing times on large format machines optimized for this single application of mass customization.
Investment casting, one of the oldest known metal forming processes, has used the SL process for over twenty years. The ability to manufacture hollow smooth walled patterns for use in a foundry process that coats the pattern with ceramic, then fully burns out the pattern in preparation for molten metals to be cast in the hollow form. While molded wax patterns dominant most high-volume applications, SL eliminates tooling costs for lower volume casting but also facilitates sizes and part features not readily obtained in a molded pattern process. The latest generation of SL photopolymers for this application have excellent dimensional consistency and contain no heavy metals found commonly in 355nm photopolymers. This combination ensures accurate patterns as well as minimal ash after burnout that can cause casting defects.
Photo Caption: Investment Casting Pattern Manufactured With Somos® Element
Similar hollow part methodologies used for investment casting can be applied to large parts, creating “lightweight” parts with tailored mechanical properties and reduced weight (less material cost). Materialise, a global AM software company based in Belgium, has developed multiple software options for hollowing and reinforcing lightweight structures. This development has facilitated the cost-effective manufacture point-of purchase displays, architectural model, and other art applications.
The ability to manufacture full density highly-accurate patterns also facilitates another well- established molding process known as urethane molding, RTV or silicone molding. After careful secondary finishing, the pattern is embedded in a silicone rubber casing that becomes a 2-part mold for the casting of urethanes. Polyurethane materials can be formulated to achieve properties consistent with levels of performance from injection molded thermoplastics. The silicone tool can be used for low volume series when either multiple prototypes or low volume production is required. Many service bureaus have developed specialized methodology for supporting regular low volume part production capabilities that can both shorten supply chains as well as allow for iterative improvements as a low volume design ages.
Creation of injection molding tooling has been an area of development interest since the earliest days of SL. The principal impediments to this potentially high-volume application for prototype and bridge parts include strength and temperature resistance of the 3D printed tool, predictability of tool life (durability), compatibility with a large range of injection molding materials including glass filled systems (abrasion resistance) . Also, high speed CNC machining of soft metal (typically aluminum) tools , a parallel technology , provides the potential of short lead times and predictable tool life. In just the last 2 years, the convergence of SL part build accuracy, material capability (heavily silica filled photopolymers and business models that combine SL tool building expertise with injection molding know-how has led to increased use of SL tooling. Successful integrators of SL 3D printing and injection molding typically recognize that the goal is an injection molded prototype. Injection molders lacking an internal CNC machining operation can readily print mold sets for prototype production in actual end-use designated thermoplastics. This avoids the difficulties that can arrive with a pass-the –baton methodology that marked early efforts to equip service providers with injection molding tooling know-how.
Photo Caption Dr. Sean Wise of RePliForm (left) and colleague Rick Dunlap are holding new electroplated stereolithography parts including a copper plated wave guide and nickel coated flexible mesh at the 2017 AMUG (Additive Manufacturing Users Group) conference in 2017.
Like investment casting, electroplating of a substrate material to improve physical or mechanical properties is well known. Dr. Sean Wise of RePliForm, Inc has actively optimized electroplating techniques for 3D printed parts using copper and structural nickel since the year 2000. All 3D printed materials can be electroplated to improve strength, wear resistance, EMI/RFI shielding, flammability resistance, and aesthetics. Photopolymer based printers; however, offer smooth, non-porous surfaces that plate readily with basic parts preparation. 355 production SL machines and state of the art materials combine to provide the largest range of part size and substrate options. The same highly filled silica photopolymers used for injection molding tools can create extremely thin (0.010 to 0.040 in) substrates for a Nickel/copper/SL composite with mechanical properties approaching die cast nickel. This level of mechanical performance creates a significant bridge of opportunity between the gap of polymer AM and direct AM metal. Aside from mechanical properties, the design flexibility of SL combined with a copper coating can create a cost-effective wave guide or “antennae configuration.” There are other parts currently in production where the structural nickel creates a renewable wear surface.
Photo Caption: The stereolithography process is successfully being used to create tooling for short runs.
Turning 30 does bring changes. 355 nm SL, inclusive of equipment, materials , and software ;has matured into one of the most widely used 3 D printing technologies not only for prototyping but for use in end-use production processes. The trajectory of current developments and utilization in a broad range of direct and indirect manufacturing techniques, as well new prototyping capabilities bodes well for a long life.