Combining stereo lithography apparatus (SLA) with a CNC router makes it possible to produce injection molding prototypes of a much higher quality than can be produced on SLA alone. SLA is an outstanding method of producing geometrically complex prototypes. The problem is that it's not capable of producing fine details with the crispness associated with CNC machining. Moving the part to a conventional CNC machining center for touchup would be very expensive. So, ARRK uses a low-cost, easy-to-operate CNC router to produce fine details on SLA parts resulting in an unparalleled level of quality at an affordable cost.

The prototyping process is essential in any industrial setting. Prototypes can be both visual and functional or act as a visual medium from which marketing strategies can be developed and product modifications can be made, prior to investing thousands of dollars in a production tool. ARRK Creative Network is the largest model maker and prototype manufacturer in the world. Over 650 professionals take advantage of the latest technology to provide outstanding quality, precision and service. The company's services include stereo lithography, CNC machining, fabrication and casting. ARRK has factories in San Diego, California; Troy, Michigan; Thailand, Japan, and England, with sales offices throughout the world.

While SLA has become an extremely popular prototype manufacturing tool, it cannot produce highly accurate models sometimes desired by industry. As a result, prototypes produced by this method are often rejected for styling reasons. CNC routers can dramatically improve the quality of prototypes produced with stereo lithography. Modifications to the SLA model can be made by machining the SLA material itself or machining small pieces of ABS which can then be bonded into the original model. These can be small logos or icons as well as any overall modifications to the parts including the addition of ribs, bosses, through-holes, etc. A typical example of this practice in action is producing the bezel for an electronic component using stereo lithography and using a CNC router to produce details such as LED readouts. It should be noted that the option of cutting directly into the stereo lithography part is preferable when it can be accomplished because it eliminates the need to mount, flush and sand the machined insert.

This technique can also be used to make modifications to SLA parts in much less time than would be required to produce a new part. In one case, ARRK made an SLA part from a PRO/Engineer file. When the customer checked the fit of the prototype, they decided to make several modifications and needed the prototype returned the next day. This didn't leave enough time to produce another SLA part. Therefore, ARRK bonded additional material onto the prototype and machined it on the router to the new specifications. The cost was only 15% of what it would have cost to produce a new SLA prototype.

The key to the success of this process is the recent availability of inexpensive, but very precise CNC routers. ARRK has two three axis gantry routers from Techno, New Hyde Park, New York. These routers have a positioning accuracy of ±.1 mm (.004 inches) in 300 mm and a repeatability of +0.01 mm. The accuracy of the router is the result of several features inherent to the table. Anti-backlash ball screws permit play-free motion that makes it possible to produce circles accurate to the 0.0005 inch machine resolution. These ball screws also make it possible to produce parts as accurate as the machine resolution. The ball screws have excellent power transmission due to the rolling ball contact between the nut and screws. This type of contact also ensures low friction, low wear and long life.

The Techno CNC router is able to finish SLA parts and many other injection molding prototypes from scratch at half the cost of doing the job on a conventional CNC machining center. The router has a lower hourly rate because it requires a small fraction of the investment of a machining center. Material costs are dramatically reduced up to 50% by using a router for modifications and for the creation of entire prototype models. The models can be constructed by machining each wall and fabricating the model together rather than machining the part of out of solid. This method provides the same precision that can be achieved on a full size CNC machining center.

A prototype produced by ARRK.

ARRK typically receives geometrical data from customers as either a native CAD file or an IGES or DXF format neutral file. The company reads this file into one of several CAD systems that they use including AutoCAD, Pro/Engineer and SDRC Ideas. To produce an insert covering a detailed area of a stereo lithography part, a section containing the detailed area must be removed from the SLA geometry and transferred to a CNC routing program. When the geometry is finished, the designers produce an IGES or DXF file and import it into EZ/CAM, their CNC machining software. This package produces a file that is read by the MAC-100 controller used by the router.

Besides their use in finishing SLA components, the routers also provide a unique method of producing box-like prototypes such as bezels for electronic systems. Bezels are normally produced on a CNC machining center from a solid block of material. This is a very time-consuming process on an expensive machine that typically requires a high degree of operator attention. Machining parts from a solid block also is expensive from a material standpoint.

The original model of the housing is exploded into six separate walls. On a router, these walls are produced from sheet material and bonded together to form a cube. Material cost is typically reduced by 50% and machining time is cut by about 33%. Finally, the hourly cost of the CNC router is typically half the cost of the machining center. As a result, the cost of producing the prototype is cut by about 50% without any reduction in quality.

Producing bezels piece by piece also provides the opportunity to enhance the finish of the prototype. ARRK recently produced a business telephone bezel using this approach. Making the bezel prototype in pieces made it possible to polish the top panel so it was transparent and produce a second panel that fits underneath it containing the buttons. This second panel was silk-screened, adding a level of realism to the prototype that would have been impossible to achieve with a solid block.

All in all, the use of CNC routers provides major advantages for producers of injection molding prototypes.