Pfinztal, Germany —Injection-molding machines do more than make parts at Fraunhofer ICT, the German research institute. Instead, the machines have become full-fledged assembly lines, thanks to a new in-mold assembly and fastening method that produces plastic parts and installed screws during the same molding cycle.
Fraunhofer's method takes place on a conventional multi-shot molding machine. The initial shot or shots create one or more plastic host parts, while the last shot produces pre-loaded screws within the host. One variant of the method uses a stack mold to create multiple plastic host parts. Another joins the molded host to a metal or plastic part placed in the mold before the first shot. Fraunhofer engineers have evaluated applications ranging from tiny mobile phone enclosures to large automotive door modules whose exterior panels, carriers, and fasteners are molded together in a stack mold.
A molding machine might seem like a terribly expensive screwdriver, but in-mold fastener production and installation cuts the costly assembly steps that normally follow the molding process, according to Lars Ziegler, a technology manager at Fraunhofer USA (Plymouth, MI). "Our method combines three production and assembly steps into one," he says, comparing the single-cycle method with separate steps for molding, fastener production, and assembly.
Fraunhofer's in-mold assembly not only makes plastic parts but also makes threaded fasteners during the same injection-molding cycle.
Multi-component molding machines have been used to make plastic assemblies with movable parts for nearly 20 years. "But earlier assemblies had parts that were rotatable, not detachable," Ziegler says, adding that Fraunhofer's in-mold assemblies unscrew with conventional hand-tools.
Materials selection and other design tricks, rather than specialized molding hardware, let Faunhofer's molded-in screws unscrew. As Ziegler explains, in-mold assembly exploits shrinkage differences between the host and fastener materials. Axial shrinkage of the screw material provides the tension to hold the molded-in fastener in place. At the same time, a mold core inside the screw limits the radial shrinkage that would otherwise cause it to loosen.
Ziegler reports that in-mold assembly works with many neat and reinforced plastics—including PP, nylon, and polyesters. Some pairings work better than others. "We try to use amorphous resins for the screws, to minimize shrinkage effects," Ziegler says. And from a production standpoint, fast-cooling materials make the most sense for the host components, which have to cool before the fasteners can be injected.
Fraunhofer engineers have yet to quantify the pullout strength for the various threadforms and material combinations suitable for in-mold production, but Ziegler argues that the molded-in screws exhibit at least the holding power of separately molded threaded fasteners. "The molded-in type should be even better," he says, explaining that careful materials selection and process control allow partial welds of varying efficiencies between the screw and host materials. "We're able to make the screw a bit sticky," he says. "But it will still come out."
For more information about in-mold assembly from Fraunhofer: Enter 537