Composite SL Materials Paving the Way For Advancements in Rapid Tooling
Stereolithography has long held
promise for lower cost, limited-run tooling, as well as for "rapid"
tooling due to the relative accuracy and speed of the process.
Nevertheless, numerous university studies and industrial trials have
highlighted major gaps that prevented the widespread adoption of SL for
widespread tooling use. Specifically, poor elevated temperature
mechanical properties and related wear issues limited tool life.
In 2002, when Alphaform AG (Feldkirchen, Germany) introduced DSM Somos' ProtoComposite SL resin technology to the Formula 1 industry for building high-performance test parts, interest in SL for tooling was rekindled. Parts built from this new class of materials demonstrated a step change level of stiffness, strength, dimensional consistency, and elevated temperature performance as compared to similar parts built of conventional unfilled SL resins.
Despite these improvements, skepticism remained, relative to tool robustness and reliability. In response, DSM has worked cooperatively with a variety of service bureaus, original equipment manufacturers, and tooling manufacturers to better understand the full potential of Proto-Composite SL materials and display their property advantages over standard SL resins for rapid tooling applications.
Composite SL Resins and Rapid Tooling
While ProtoComposite technology appears be widely applicable to various types of tooling, including rapid information modeling (RIM), injection molding, vacuum forming, and laminate construction, primary emphasis has initially been placed on its potential for rapid tooling for injection molding.Here, high-speed CNC machining of aluminum currently dominates the industry due to fast turnaround times and tool durability-yet composite ceramic-filled resins (namely Somos ProtoTool and Nanoform materials) now also show potential advantages for reduced manufacturing time, as well as other benefits like enhanced latitude in design geometries.
Thanks to new capabilities in elevated temperature performance, high modulus, improved dimensional consistency and advantageous thermal expansion characteristics, SL tooling can increasingly be viewed as a complimentary approach to machined tools with selection of the optimal approach based on the specifics of the situation and design.
Composite SL Resin Performance Versus Unreinforced SL Materials
Composite SL resin performance is a function of the properties of the matrix resin, filler/reinforcement as well as the matrix-reinforcement interfacial bond (influences stress transfer). Reinforcement type, shape, size, and concentration in the formulation all modify the basic resin properties.
Dimensional Consistency
Physical properties of Somos ProtoComposites that affect dimensional consistency-such as moisture absorption, coefficient of linear thermal expansion, and cure-related shrinkage-are significantly improved as compared to properties of previous, unreinforced SL materials. Current materials demonstrate cure related shrinkage of less than 0.05 percent facilitating very accurate, detailed SL part creation. Reduced moisture absorption ensures dimensional consistency over time.
Mechanical Performance At Molding Temperatures
Durability of a tool through multiple shots depends greatly on the
mechanical integrity of the tool material at molding temperatures. Two
ready measures of elevated temperature performance are heat deflection
temperature (HDT) and tensile properties at elevated temperature.
HDT is a standardized test that provides a comparative measure of resistance to flexural deformation under specific test conditions. Composite materials such as ProtoTool 20L and Nanoform exhibit heat deflection temperatures ranging from 100 percent to 500 percent higher than those of neat SL resins, depending on the standardized load condition-low versus high.
Modulus
Composite SL resins exhibit markedly higher strength and modulus (stiffness) than unfilled resins. This helps to maintain the mold's dimensions and integrity while injecting thermoplastics. The flexural modulus for standard SL resins ranges between 145 and 435 KSI; however, ProtoTool 20L and Nanoform 15120, due to their high ceramic reinforcement content, demonstrate modulus performance of approximately 1,350 KSI and 645 KSI respectively. This increase in stiffness directly impacts tooling feature durability.Typical of all SL resins, modulus decreases as a function of elevated temperature. The higher stiffness of Proto-Composite resins translates to increased stiffness at molding temperatures. Proto-Tool 20L, for example, still maintains tensile modulus equivalent to a neat SL resin at 200 degrees Fahrenheit.
The SL Tooling Learning Curve
While the physical and mechanical attributes of new composite SL resins have rekindled interest in rapid tooling applications, tool design and processing optimization remain critical differentiators relative to meeting expectations. At the 2004 Frankfurt Euromold expo, over 1,700 parts of a simple "poker chip" design with finely detailed lettering were injection molded in ABS, using a ProtoTool mold. Other trials of larger, more complex geometries have produced successful part runs ranging from handfuls to hundreds of parts in a variety of thermoplastics. Each experience produces beneficial information that is helping define the proper scope of application for composite resins in tooling applications. Management of expectations as development continues will be critical for the successful posi-tioning of this technology.
