Adding Value Through Machining After Molding

The job isn’t always done when parts emerge from the press. At least, that’s the case at Reliance Engineering, which occasionally sends injection-molded plastic parts back to the toolroom for additional machining. In some cases, parts require this additional work. In others, secondary machining helps reduce the initial tooling investment for a job. Regardless, the shop’s capabilities in this area have led customers—and potential customers—to look at the company “a little differently,” says Ron Caron, general manager at Reliance’s parent company, Built-Rite Tool & Die. “There’s no question that it’s helped us get jobs we wouldn’t have been able to get before.”  

Investing in secondary plastic machining capability makes particular sense in the context of the company’s business model. Based in Lancaster, Mass., Built-Rite Tool & Die and Reliance Engineering are two divisions of an integrated company, with the former specializing in mold manufacturing, and the latter specializing in thermoplastic injection and thermoset transfer and compression molding. A third division, LSR Engineering, specializes in liquid silicone rubber injection molding. Although each division has its own building, management team and customers, Reliance and Built-Rite have always thrived by working together to provide comprehensive services, Caron says. 

However, providing such services has become more difficult in recent years because parts are designed with increasingly tighter tolerances that are difficult to achieve through the molding process alone, Caron says. As an example, he cites a medical-industry job that prompted Reliance to turn this difficulty into opportunity by installing a Haas Mini Mill, the company’s first piece of equipment dedicated solely to secondary plastic machining. Shown in the picture above, this 7-by-11-inch part has several features that require positional tolerances of 0.002 inch—just tight enough to create uncertainty as to whether natural shrinking of the molded plastic material might throw the part out of specification. Machining certain critical features on the Mini Mill not only ensured accuracy, but also eliminated the need to squeeze that work into the existing production schedule on the mold shop floor. 

That job also revealed another advantage of secondary machining. Given that the molded plastic part needed additional work anyway, the shop opted to machine a feature located outside the mold’s line of draw rather than incorporate a side action. Although ensuring accuracy was the primary factor in winning the job, the ability to use a simpler, less-expensive mold certainly helped open the door to additional work from that customer and others, Caron says. 

Secondary machining of parts that don’t necessarily need it has since become a relatively common tooling cost-reduction strategy for lower-volume production molds. In fact, the company dedicated a portion of a 7,500-square-foot facility expansion to these operations in 2012. In addition to the Mini Mill, that space houses a Haas VF4 VMC and two Omni GT JR CNC lathes, among other equipment. Notably, this space is located not at Reliance Engineering, but at Built-Rite’s moldmaking facility. This enables the secondary machining operation to take advantage of the capabilities and expertise already in place at Built-Rite rather than duplicating resources. At the same time, keeping secondary machining segregated within the Built-Rite facility eliminates the extra effort of cleaning machinery prior to switching back to core and cavity work. 

Caron adds that expanded plastics machining benefits not only the company as a whole, but each individual division. “Reliance Engineering can offer a value-add by contracting the machining to Built-Rite, while Built-Rite can offer the service to other custom molders,” he explains. He adds that their status as separate divisions alleviates concerns about the Reliance molding division seeing work that Built-Rite is doing for other custom molders. “Overall, this has been a no-brainer.”