TXM takes on a tough part

Most molders attempting to add TXM capabilities to their repertoire would start with parts that are bound to succeed, those without demanding tolerances and geometry. Phillips Plastics' Origen Center, however, is not a typical molding operation (see related story in this issue, "Strategies For Breaking Into a New Technology"). For one of its first production parts, the Magnesium Injection Molding business unit at the Origen Center chose to work with OEM Unisys Corp. to mold a highly complex TXM component. 

According to Mike Saunders, a mechanical engineer at Unisys who worked on the project, the OEM's new check processing unit (Figure 1) was built around a machined aluminum plate. More than 50 components were attached to the plate (bottom image in Figure 2). "We went into initial production with the intention of switching to a diecast aluminum or zinc plate and consolidating parts once the design was stabilized," Saunders explains. "For this reason, we chose to machine the plates to lower the cost of making changes." 

Plastic was not an option for this part. A metal base plate fulfilled many requirements—it met marketing's needs for a hefty product weight, dissipated static charges, acted as a heat sink for motors in the unit, provided EMI shielding, and met both structural and flatness requirements. "There are nine shafts that drive checks through the unit, and they needed to be perpendicular to a very flat plate so that checks won't jam," he says. 

But when the time came to switch to a diecast plate, Unisys found that a target flatness requirement of .015 inch could not be met by the diecasting process, which could only hold a flatness of .035 inch. That's when Phillips got the call. 

Holding Tolerances 
Until this point, Phillips' Magnesium Injection Molding unit had been working with existing customers to cultivate lower-volume developmental projects. It purchased the world's first production TXM machine (a 400-tonner) as well as the first Husky production TXM press (a 500-tonner). In late 1999, an existing TXM molder, THX, was also purchased and moved to the Origen Center. 

Phillips' Ed Estergaard, group vp, metals, and Doug Simpson, project engineer, participated in four design review meetings with Unisys. "We were able to give our input on tool design for both manufacturability and flatness requirements," says Estergaard. "Once we reached agreement on the design, we felt confident that we could maintain flatness and keep costs down." In fact, Phillips extended Unisys a unique guarantee: If Phillips couldn't hold flatness to .028 inch, a precision diecasting standard, Unisys would owe Phillips nothing. 

Other tolerances on the part were no less exacting. The track on which checks travel through the unit is molded into the base plate with a flatness of .0025 inch. Elsewhere on the plate, flatness is held to .002 in/in. Location dimensions are also tight because of the many components mounted on the plate (see Figure 3). 

Remember those nine shafts on the plate? Each requires two bearing housings, for a total of 18. Tolerances required for a press-fit into the housing would have been +.0007/-.0 inch with no draft. "This was tighter than we could mold," says Simpson, "so we left a small amount of machining stock. After molding, the part goes to our secondary machining area to bore these 18 holes and tap 37 others. This is actually less work compared to the machining that would have been required with a diecast part." 

Tooling Issues 
During the product development process, Phillips and Unisys also met with moldmaker Tooling Science Inc. (TSI, Maple Grove, MN). TSI has experience building diecast dies, plastic injection molds, and TXM tools. Using an SLA model created by Unisys, the group went over every dimension and tolerance on every section of the part. 

"We decided to keep a nominal wall thickness of .090 to .100 inch for moldability," recalls Simpson. Ribbing was also added to improve material flow. "With TXM," adds Estergaard, "fill times range from 20 to 40 msec because the highly conductive metal cools so quickly. Ensuring material flow is critical as a result." 

Molded-in posts were the most difficult feature on the plate. Each post required supporting ribs, and the posts themselves were only cored out halfway, leaving a half-solid section at the base. "Fortunately, magnesium is not as sensitive to sink as a result of thick and thin sections," says Simpson. "Unisys agreed to add more flare to the ribs, which enhanced material flow to this area." 

Looking at the 15-oz TXM plate (see the top image in Figure 2), it is hard to believe that the tool contains no slides. Phillips wanted to reduce complexity by keeping it an open-and-close mold, so the parting line is designed to step around the part features. Also, sleeve ejectors are used on core pins. 

An original 16-week tool delivery quote was expedited to 12 weeks. While the TXM tool cost is comparatively higher than that of a diecast die, it also produces parts that require fewer secondary operations and therefore costs less to manufacture (Table 1, right). "We decided to put more money into tooling to bring the piece price down," adds Saunders. 

Stellar Results 
By undertaking such a complex project, Estergaard believes Phillips has gained both experience and confidence for future endeavors. "Now that we've tackled the tough stuff, we see greater possibilities for TXM," he says. 

With flatness as a critical factor, Phillips probed more than 100 spots on initial TXM parts to ensure that requirements were being met. According to Saunders, this attention to detail has paid off. Currently at preproduction volumes, initial parts are maintaining the .015 inch goal. "There has been only one plate that strayed to .016 inch flatness," he adds, "and this is still within spec for our purposes." 

Unisys estimates it will sell up to 5000 check processing units in 2001, which translates to a weekly volume of more than 100 parts at Phillips. Unisys is currently assembling the units using TXM plates at its Plymouth, MI plant, and plans to achieve full production quantities in July.