Stereolithography + cast ceramic = tooling in record time.

When it comes to building a large injection mold fast, Pitney Bowes, Inc. bows to no one. In a recent project, the Stamford, Conn.-based business-machine maker went from part concept to prototype tooling in just seven weeks, thanks to its effective use of CAD/CAM/CAE, stereolithography, and a novel ceramic tooling material.

This rapid-tooling program was implemented for a large mailing-machine base component molded of glass-reinforced thermoset polyester bulk molding compound (BMC). The project was discussed recently at the Molding '95 conference in New Orleans. Measuring 20 x 27 x 4 in., the part replaced three metal components on earlier mailing machines. Despite the part's complexity - an unusual geometry. chock full of ribs and bosses - Pitney Bowes still managed to build a 10,000-lb, ceramic-and-steel mold in less than one-third of the typical 22-week lead time.

PARTNERING FOR SPEED

The company formed an alliance with three other companies, each specializing in a particular portion of the rapid-tooling process. "Rapid tooling requires many technological resources. Nobody had enough knowledge in every area," explains Pushpavadan Nagarsheth, Pitney Bowes business-unit fellow for plastics and process technology. The rapid-tooling team consisted of Dr. Paul Jacobs, research director at 3D Systems, a stereolithography supplier in Valencia, Calif.; Drew Santin, president of prototype molder Santin Engineering Inc. of W. Peabody, Mass.; and Sean Wise, director of R&D at Cemcom Corp., a ceramic material supplier in Baltimore. "This was a marriage made in heaven. The real message here is the ability to work with people who have a specific expertise in each part of the process," says Jacobs.

The development process began with Pitney Bowes and Santin Engineering creating a 3-D CAD model of the part using Pro/Engineer software from Parametric Technology Corp., Cambridge, Mass. "Working in solids modeling was a huge help," says Santin, especially since thermoset injection molding demanded some design concessions before the part even left the computer screen. "The process requirements drove many aspects of the design," he explains. For example, draft angles had to be increased 1-2 [degrees] to steer clear of a potential release problem. Rib and boss shapes also had to change to accommodate restrictions on the geometry the ceramic material could cover. The 3-D model let Pitney Bowes perform a finite-element analysis on the tentative part design to ensure that it would meet the application's strict stiffness requirements.

SLA MODEL TO CERAMIC TOOL

Once the computer model had been fine-tuned, Pitney Bowes produced prototype parts on a stereolithography apparatus (SLA) from 3D Systems.

Because the parts exceeded the work envelope of the SLA, Jacobs recommended joining two smaller parts into a larger prototype, using something like a woodworker's mortise-and-tenon joint. One of the models from this process went into form-fit-and-function testing. A more refined version of the model - one incorporating the final draft allowances - became the tooling master.

Cemcom then poured its proprietary ceramic material over the stereo-lithography master to come up with a core and cavity set.

The ceramic tool ultimately required several steel inserts to withstand tensile stresses in some regions of the part. And an insert had to be installed directly across from the gate to keep the glass-filled BMC from eroding the ceramic.

Heat transfer presented another potential problem. Ceramics have only about one-fifteenth the thermal conductivity of steel, and the BMC material molds at 350 F. "Heat-transfer issues were solved during the mold layout," Wise recalls. The ejector system, for example, sits at the base of the core, letting the ejectors remove excess heat (beyond the amount needed by the thermoset to initiate curing). Also, a system of copper rods and steel blocks runs through the mold to aid in heat transfer.

A six-plate mold-base system with double-sided ejectors from D-M-E Corp., Madison Heights, Mich., completed the tooling picture. "It was a big investment," says Santin. "But 70% of the components on the prototype tool went into the production tool for a substantial cost savings." In fact, the final mold cost only about one-third as much as a conventional tooling project, according to Nagarsheth. The finished production tool ultimately went on to Glastic Corp., the Cleveland-based thermoset molder that produces the mailing-machine parts.