Melt Oscillation Molds Better Parts

Of all the ways to injection mold better parts, the Scorim process takes the "cocktail-shaker" approach of oscillating the melt within the mold cavity. In this case, though, the aim of sloshing the melt back and forth is not mixing but creating stronger, defect-free parts.

Developed in England nearly 10 years ago, the process is currently available for license from Cinpres Ltd. Until recently, Scorim had been promoted mainly for its ability to improve the mechanical strength of parts that had weld-line or fiber-orientation problems. But Scorim won just a handful of commercial projects. In Europe, they have ranged from eyeglass frames to piston rings and VCR remote-control covers. One of the most recent jobs is a computer-mouse shutter.

Only one American molder, Fawn Industries of Hunt Valley, Md., has used Scorim for a production part--a radio bezel that was susceptible to weld-line problems. Since that job ended, there are no Scorim production jobs in the U.S. right now, according to John Heasman, president of Cinpres North America.

The dearth of commercial applications may not last much longer, thanks to two recent developments that could speed the acceptance of Scorim: Cinpres revamped the auxiliary-equipment package that makes Scorim possible, eliminating the need to modify the molding machine. And the process is now gaining recognition not only for enhancing strength, but for reducing cosmetic defects. That realization opens up a much broader field of potential applications.

Now it mounts on the mold

The old-style Scorim equipment, consisted of two auxiliary melt-oscillation cylinders mounted on the end of the injection barrel, and thus required extensive machine modifications. However, the new Scorim II equipment fits within a relatively compact hot-half assembly attached to, and easily removed from, the machine's fixed platen.

Like its predecessor, Scorim II relies on the coordinated action of two hydraulic pistons to oscillate the melt. On the new system, however, these pistons are installed on the fixed platen rather than on the end of the machine barrel. The cylinders can be oriented either vertically or horizontally, depending on the machine design and space available.

The new system spares the molder the risk and expense of dedicating a modified machine to a single process. It also is suited to a wider range of machines because it doesn't require extra carriage length to accommodate the auxiliary cylinders. "Scorim II is a much more flexible system," says Wayne Copeland, Cinpres-Scorim process engineer. He notes that the new version can be used with many different molds, both standard and Scorim-type. Copeland estimates that the new platen-mounted system costs about $1000 more than the old system but saves roughly $9000 in machine modifications.

Within Scorim II's expanded hot-half assembly, a heated manifold splits the melt flow from the machine nozzle into two streams. The two melt streams then travel through independently balanced hot-runner systems and enter the mold cavity through separate gates, which are typically located at opposite ends of the cavity.

Since molds for Scorim II require an extra sprue, independent hot-runner systems, and compatible gate locations, Scorim II is probably best considered early in the tool-design process. "That's what works best," says John Hahn, engineering v.p. for Mold Makers Inc., which offers a Scorim system for customer trials in its technical center.

What happens in the cavity

According to Copeland, the new and old Scorim systems differ outwardly, but they act on the melt in the same manner: By moving out of phase, the two pistons push each melt stream through the cavity as well as the individual gates and runner systems. The start of the oscillation typically coincides with the beginning of the hold cycle. But there are instances where it could begin with the cavity only 50% full.

Apart from oscillation, Scorim has two subsequent operational modes. Immediately after melt oscillation, Scorim can move the two pistons back and forth in concert, creating compression and decompression effects in the cavity. According to Elliot Grossman, Cinpres-Scorim's technical sales manager, this movement ensures "maximum and uniform packing while the polymer is still in a live state and the material is continuing to freeze in sequential layers toward the center of the part." Finally, the two pistons can be moved to their forward position for some additional compression as the part continues to freeze completely.

Controlling the system is a standard hydraulic drive unit and a programmable controller capable of storing up to 16 oscillation/packing profiles.

Sloshing the melt back and forth through cavity and gate, together with the subsequent Scorim-aided packing, helps eliminate some common problems affecting both reinforced and unreinforced materials.

Take weld lines, for example. There seems to be little doubt that the process helps eliminate--or at least hide--weld lines that occur where flow fronts converge. "There is some argument as to whether the weld lines are eliminated or just not visible," says Ken Kerouac, a Dow Plastics injection molding specialist who has investigated the Scorim process. "Visually it doesn't matter whether the weld lines go away or simply become invisible," explains Kerouac. "Structurally, it is an issue." He points out that even if Scorim simply transforms the weld line into a "long sweeping curve" through the part, the mechanical properties can improve as a result.

In reinforced plastics, Scorim seems to enhance strength through modifying both weld lines and fiber orientation. Pointing out that unfrozen layers of polymer experience shear forces as they slide against previously frozen layers, Grossman suggests that these forces orient the reinforcing fibers in the direction of flow.

Cosmetics count

While Scorim has garnered most attention for enhancing mechanical properties, some observers argue that the process may make a more widespread contribution in the area of part cosmetics. "There's no doubt in my mind that the process improves structural performance compared to conventionally molded parts with weld lines, but I think the biggest bang for the buck will be gained if the process can be used to improve the appearance of unpainted cosmetic surfaces," says Dow's Kerouac.

"We're starting to find that it can solve cosmetic problems," agrees Hahn of Mold Makers Inc. He contends that mechanical strength, while essential, won't entice as many custom molders as the promise of achieving better cosmetics. "Ask a custom molder what his biggest problem is, and he'll say, 'Cosmetics, cosmetics, cosmetics,'" Hahn says. "That's what causes the most rejects."

Scorim's influence on defects like sinks, flow lines, voids, and warpage comes down to packing. According to Grossman, the process keeps the gates filled with molten polymer and thus keeps them open longer than in conventional molding. The open gate, in turn, allows more material to enter the mold at higher pressures, eliminating defects that arise from underpacking.

"As long as the gate doesn't freeze, I'm in control of what's happening in the cavity," says Hahn. He believes the process will prove useful for thick parts that are prone to sinks and voids. He cites polycarbonate light pipes as one promising application.

Kerouac cautions that extra packing capacity may not always be a good thing. "The potential downside is molded-in stresses."

Scorim's limitations

Kerouac also cites a couple of other potential threats to Scorim's usefulness. For one thing, Dow has identified what appear to be some material sensitivities. "The process produces better aesthetics with polystyrene and ABS than with polycarbonate," says Kerouac, though he acknowledges that some of the difference might be due to mold temperature or other set-up variables that could be further optimized.

The other big issue, Kerouac says, is scale-up. "How big a part can you do? We don't know yet." On large parts with lots of wall-surface area, the Scorim apparatus would face the challenge of moving a large volume of material before it freezes, Kerouac notes. "We're still not sure what the equipment can do."

To look further into these material and processing questions, Dow recently took a license for the Scorim II process. All Dow's earlier work (including tests described in a 1998 SPI Structural Plastics Conference paper) had been performed on Scorim I equipment. According to Kerouac, the company plans to continue its early investigations by rating materials' suitability for Scorim processing. Dow will also work to identify the optimum processing parameters for Scorim II.

GE Plastics has been investigating Scorim as well--in particular, as a way to improve the cosmetics of filled ABS parts such as the company's aluminum-filled, metallic-appearance Magix material.

Commercial prospects

Cinpres-Scorim is looking into several areas that hold promise for this process. One is the potential to expand its materials portfolio with applications in liquid-crystal polymers (LCPs), glass-filled nylons, and reinforced thermosets. Also Grossman envisions a Scorim variation with a four-piston head that could create a 0/90° cross-layering effect, what he calls a "poor man's composite."