New molding method thins walls

X-Melt molding technology fills thin parts by compressing the melt in the injection cylinder and allowing it to "explode" into the tool.

Schwertberg, Austria—For all its potential to cut the size and cost of plastic parts, thinwall injection molding suffers from a processing problem that limits its appeal: Parts that meet mechanical requirements don't always fill reliably. And the ones that do fill often require injection speeds of 1,000 mm/sec or more, so fast that only the most hydraulically advanced molding machines can handle them. Engineers at Engel Machinery recently found a way around this processing dilemma with a new molding method that makes thinwall parts even thinner without all the need for speed.

Unlike conventional thinwall molding, in which the screw moves forward to propel the melt into the cavity, Engel's patent-pending Expansion Molding, or "X-Melt," technology uses a more restricted forward motion of the screw to compress the molten plastic within the injection cylinder. Working in concert with the screw, a check-valve bottles up the molten resin until it reaches a pre-set pressure threshold—usually around 200 bar. The valve then pops open, releasing the pressurized melt into the tool. "X-Melt uses the energy stored in the melt to fill the cavity," explains Georg Steinbichler, Engel's director of process research.

Better molding, better designs. According to Steinbichler, X-Melt has several advantages over conventional thinwall molding. The process exhibits only about half the part-to-part variation of conventional thinwall molding, Steinbichler reports. In a hundred-shot molding study of a PC/ABS cell-phone battery cover, Engel engineers found that the X-Melt parts had a weight variation of only 0.003%, compared to 0.007% when the same part was molded on a conventional thinwall machine. And the X-Melt's compressed melt pool exhibits a more uniform temperature distribution than the melt in a traditional injection process. "Normally the melt would vary by 8, 10, or even 20 degrees (C)," he says, noting that these temperature variations can interfere with the tight molding control needed to fill thinwall parts.

Taken together, these two molding benefits may give design engineers more confidence when it comes to thinning out wall sections. Steinbichler estimates that X-Melt, because it varies less and fills more reliably, can consistently produce parts that are 10-20% thinner than conventional thinwall parts—as measured by flow-length-to-wall-thickness ratios.

As for materials selection, Engel has successfully used X-Melt with both amorphous and semi-crystalline engineering thermoplastics.

What the process can't do, for now, is make large parts. X-Melt only works with components of 50 grams or less because the compressed melt pool has to fit within Engel's existing injection cylinders, which were designed for a normal, stepwise injection. Should X-melt take-off, however, this weight limitation could be overcome as Engel tailors injection barrels to the new process, Steinbichler reports.

X-Melt lastly eliminates much of the complex set-up work behind successful thinwall molding. "There's no need to find an injection-speed profile, changeover point, and holding profile," says Steinbichler. Users instead calculate X-Melt's target pressure from the part volume—a figure readily available from a CAD model. "It's a very simple process," he says.

For more information about processing technology from Engel: Enter 535