Prototypes on demand? Peter Arras De Nayer instituut [Hogeschool voor Wetenschap en Kunst] Pressure on time to market urges for new ways of faster prototyping. Key words: Rapid prototyping, rapid tooling, solid freeform fabrication 1. Introduction: As easier and faster ways of designing mechanical parts by means of CAD/CAE tools became common use and thus speeded up design and shortened time to market, new methods of manufacturing were developed to eliminate the bottle-neck of production time. Next too better and more computerized machining, new ways of mechanical part building were elaborated to keep pace with the faster design. Designers would like to have their hands on an immediate 3D-counterpart of their virtual CAD-model. This is where rapid prototyping and rapid tooling techniques comes in. Rapid prototyping (or solid freeform fabrication) techniques focus on instant and easy creation of real-life models, meanly for visual inspection and investigation of assembly problems, if possible with structural strength. Rapid tooling techniques are dedicated to create in a faster way molds and dies for casting and molding purposes. Both techniques translate 3D-CAD-models in physical models by means of state of the art technologies rather than by using conventional machining (milling, turning) or casting. 2. Rapid prototyping and tooling techniques: In order to get a design right it needs to be evaluated for strength, aesthetics, and possibilities for manufacturing and assembly. Even though several aspects can be examined on virtual CAD-models, at some stage a preliminary real life model has to be created. As most money making or loosing on a product is decided on in the designing stage it is important to be able to acquire prototypes even in the early stages of the design. Rapid prototyping builds these models. A first family of techniques uses slices of thin material deposition to build up a model layer by layer. This family includes stereolithography (SLA), selective laser sintering (SLS), laminated object modeling (LOM), 3D-printing and fused deposition modeling (FDM). Second families of techniques are in fact an elaboration of conventional machining and are mainly high-speed cutting tools and EDM (electric discharge machine) tools. As a third family we have several casting techniques as lost-wax casting and silicon molds. Most of the following techniques use a sliced geometry. A CAD-model is exported in an intermediate file de facto an STL-format (StereoLithography) file and this file is put in the process planner which slices the model in machining steps (typically between 25 and 200 microns). These slices of geometry are put through to the fabrication machine, which forms the part. The STL-file describes the outer surfaces of a part by triangular 1
tessellation, where size of triangles used is a direct measure for the possible accuracy of the prototyped part. We now concentrate on some of the most commercialized and most efficient techniques on the market with a comparison on accuracy and cost. 2.1 Stereolithography (SLA) fig. 1: SLA is the oldest rapid prototyping technique and already dates back to the end of the 1980 s. The principle of SLA is the use of a resin which can hardened by means of high energy concentration generated by a laser. This resin is put in a tank in which an elevator is stationed, and above which the laser is located. Positioning the laser light first over the outer boundary and the zigzagging over the internal surface cures a first slice of the model. After lowering the elevator over a distance in between 50 to 200 microns, depending on the desired accuracy, a next layer can be cured. By repeating the process the total model can be materialized. SLA is an accurate technique (tolerances average 50 to 125 microns) but has some serious setbacks: - Due to the use of a liquid it is important to have sustaining structures under overhanging areas and cavities in the model. These structures will have to be removed after totally curing the model. - SLA parts don t have much structural strength because of the weakness of the basic resin. - By slicing the product a series of stair-steps are modeled on the outside of the part, and need to be finished by hand afterwards. Nevertheless SLA has become an economic tool for the creation of prototypes for visualization (if high accuracy is predominant), and for the creation of master patterns for casting and injection molds. Fig. 1 Stereolithography. Fig. 2 selective laser sintering. 2
2.2 Selective laser sintering (SLS) fig. 2: SLS uses the same principle as SLA, but not the same materials. In an SLS-machine a powder is used as material and this powder is selectively sintered (melted together) by means of a laser. As in the SLA-process this sintering is repeated layer by layer until the product is finished. Powders that are used include metals for making molds for small to intermediate batch sizes, waxes to make lost-wax patterns and plastics for the creation of prototypes for visual inspection. Because of the use of powder overhanging structures don t have to be supported with extra material, as the underlying powder sustains it. SLS in meanly used for rapid tooling: the making of wax patterns for lost-wax casting and the making of injection molds for plastics. Problems remaining are retaining equal quality in thin walled sections in the mold and hardness of the sintered material. 2.3 Laminated object modeling (LOM) fig. 3: LOM builds the model by laser-cutting and gluing sheets of paper on a pile until the part is finished. Extracting and trimming of waste material and some finishing by hand and curing could by needed. Impregnating paper after building the model can improve strength and durability of models, especially as master pattern for sand castings. LOM is meanly used for big parts where SLA and SLS machinery is limited in size, and for production of master patterns for sand castings. 2.4 Fused deposition modeling (FDM): Fig. 3 Laminated Object Modeling In an FDM machine, basically a wire of polymer is fused together in a continuous way and thus creating the model. FDM has as main advantage that it is able to create models in ABS with up to 80% of full ABS strength. Other materials include polycarbonate and polyfenylsulfon. 3
Disadvantages of FDM are the limited accuracy (plus-minus 0.2mm) and poor surface quality (Rz=100 micron). So models not only for visual inspection (with limited needs for accurate dimensions) but also for structural investigation can be created. 2.5 3D-printing: The process of 3D printing is much alike SLS: a layer of powder is selectively hardened, not by sintering, but by locally being injected (printed) with binder. After lowering the table and spreading another layer of powder the printing process can be repeated. Powders used are starch/cellulose mixings (part strength 4Mpa) and plaster (strength 10Mpa). After finishing printing the part is usually impregnated with urethane or cyanoacrylate resins to strengthen the part. 3D-printing is probably the cheapest rapid prototyping technique, only for visual inspection of prototypes and assembly checks. Recent machines are available for a cost starting of at about 35000 at Z-Corporation inc. 2.6 High speed cutting: High-speed cutting (milling) is a variation on conventional milling, but with high spindle speeds (up to 70000rpm), high feeds and small tooling, which result in an accurate and smooth cutting of the part. Machining techniques give better accuracy then other rapid prototyping techniques, but are meanly limited in complexity of geometric form. Input is possible also in the STL-format, which makes it easy to manufacture the prototype without the burdens of hundreds-of-parameters of production milling. Use of every material is possible and accuracy obtained is about 5 to 10 microns. Milling and turning machinery for desktop use for lower strength materials as plastics up to aluminum - is readily available for a reasonable price (starting at less then 2000 for a small mill product size 100x100x25mm). Roland inc. offers several small scanning and/or milling machines for reasonable prices. 3. Case study: injection mold for plastics. Research at our institute over the last 2 years aimed at techniques for rapid tooling by means of SLS revealed that an accuracy can be achieved of about 100 microns with a surface roughness of about 4 microns. This was measured in molds for small and intermediate batches (up to 50000 shots). Production time for the molds was as low as 1 weeks where as commercial delivery times for conventional made molds is 1 to 3 months. Further research planned aims at using better material as to achieve molds for bigger batches (up to 100000 shots) and combining SLS techniques and machining techniques to put up complex molds (build up mold-parts for correction and multiple part molds). Following is one of the first tested mold and a good example concerning problems encountered with this new technique. Sintering in the SLS-process used happens only in the machine itself, were as other process uses extra ovens and coatings. The process of sintering resulted in cracks at corners and due to limited accuracy gave excess in plastic material at the edge of the product. After EDM-machining accuracy was better, and less finishing had to be done on the product. 4
Cracks and cavities resulting from the sintering process were fixed by TIG-welding, which resulted in a final plastic product that needed only minor finishing. Other methods of reparation were experimented on for example dividing the mold but this lead to extra separation lines on the product, so this was abandoned. Photo 1: original sintered mold and resulting product. Photo 2: mold after finishing with EDM (electric discharge machining) and product. Photo 3: reparation of cracks by means of TIG-welding and final product. 5
4. Conclusions: Techniques for checking of designs are readily available: virtual CAD-models and physical models made by rapid prototyping techniques give designers all tools to complete the complex design process. These possibilities make sure that in every stage of the design geometric and structural integrity of the design can be tested and assured. Although most techniques described above are fairly costly and maybe too complex to use, there are no more excuses for waiting months on an end for prototypes. - Small desktop machinery provides for modeling of small and fairly easy geometry. - 3D-printing is an alternative for complex geometry at a moderate cost. - SLS parts can cut mold-making times back to a few weeks instead of a few months. - Internet-services provide reasonable cheap ways of having parts rapidly prototyped and delivered (count 3 or 4 weeks for a model). 5. Literature: - Gunnink J.W., 2001, Multi-Axis High speed milling. How to speed up Prototyping and tooling by using STL-Technology. - Peschges K.J., 1999, Stereolithography Fused Deposition Modeling. Technical and economic comparison of increasing dimension and surface quality in rapid prototyping and rapid tooling. Steinbeis-Transferzentrum. Technische Beratung and er Fachhochschule Mannheim. - Wright P. K., 2001, 21st Century Manufacturing. Prentice Hall Contact ing. Peter Arras De Nayer instituut Jan De Nayerlaan 5 B2860 Sint Katelijne Waver Belgium E_mail: par@denayer.wenk.be Contact for research on SLS ing. Andre Voet De Nayer instituut Jan De Nayerlaan 5 B2860 Sint Katelijne Waver Belgium E_mail: avo@denayer.wenk.be 6