Microcellular foam process brings performance benefits to moldmakers

Tooling is designed to exploit part, process benefits of Trexel's MuCell process

A novel process for reducing resin and energy consumption and increasing throughput in plastics manufacturing is being widely tested and adopted in molded and extruded sheet and profile applications. If the microcellular foam process licensed by Trexel, Woburn, MA, called MuCell, takes off as expected, it will have major implications for injection moldmakers and extrusion diemakers.

MuCell involves the dissolution of nitrogen or carbon dioxide into the thermoplastic melt in the form of a supercritical fluid. When the material enters the mold or die cavity, the liquefied atmosphere gas in the melt goes through a temperature-pressure drop, which in turn increases the flowability of the resin being used.

Further, the gas released in the mold cavity forms a myriad 9of small (5- to 50- micron dia), well-distributed microsized cells to reduce part density. Six injection molding equipment suppliers are adapting their machines for the MuCell process. They are Engel Machinery, Guelph, ON; Battenfeld GmbH; Arburg, Newington, CT; Ferromatik Milacron, Malterdingen, Germany and Batavia, OH; Husky Injection Molding Systems, Bolton, ON; and JSW Plastics Machinery, Tokyo and Anaheim, CA.

In MuCell injection molding, microcellular foam lowers the melt pressure, which enables the use of molding machines with lower clamp force units. Trexel's process also accelerates mold-fill speeds and reduces resin viscosity. As a result, cycle times are accelerated, more dimensionally-stable parts can be molded.

Tool modification is straightforward

Trexel officials say, in general, existing cold-runner tools largely remain viable with the MuCell process after minor modifications. However, hot-runner MuCell tools require valve gating to ensure the control of temperature and pressure to keep the nitrogen trapped in the melt.

Toolbuilders confirm that Trexel's process often calls for modest modification of existing molds. In the case of a demonstration part, a 24-in-dia cover for a polycarbonate industrial lightning unit, which ran at last month's NPE 2000, on a 1000-ton, two-platen-press converted by Trexel, its tool had no modifications.

In this application, the end advantage is lightweight-ing and cycle time (especially cooling time) reduction. In addition, a part that originally was transparent is enhanced via inherent frosted effects. This resulted from light-diffusion produced by voids generated from the MuCell process.

A second MuCell project demonstrated at NPE showed modest modifications. It involved a MuCell-adapted Milacron 310-ton toggle machine, which ran a four-ft-long, thin, flat high-impact polystyrene builder's measuring device.

"This part is going to push the limits of molding in terms of size, flatness and dimensional stability," says Brian Worthington, R&D director of Hanson Industries, Ludlow, MA. The company, now part of United Plastics Group, Bensenville, IL, built the original tool and now has modified it for MuCell. Hanson licenses a 400-ton Engel press to prototype parts.

The original tool for the part had been mothballed after it produced warpage and dimensional stability defects. The tool then underwent modifications for the microcellular foam process, including the addition of sequential valve gating, minor changes in mold angles, and reformulation of the level of filler in the molding resin.

In the case of existing hot-runner tools, modifications are more extensive. For one thing, valve-gated hot runners are a must; valve gates have to be retrofitted if they are not used in the current tool (a controlled pressure drop is key to properly run microcellular foam parts).

Willi Meyer, Husky project manager, says the valve gate for MuCell applications is of a standard configuration. He reports that many parts being considered for MuCell would have been valve gated regardless, due to a wider processing window and better gate control versus open gates.

Alliances are being forged

One of Trexel's earliest alliances was with a custom molder Mar-Lee Companies, Leominster, MA. To advance microcellular foam technology, Mar-Lee established a technical center in mid 1999 and adapted a 200-ton tiebarless Engel machine to help it understand MuCell.

John Gravelle, Mar-Lee's president, says the technology is "taking off," with more than 25 programs now underway. Projects typically are realizing return-on-investments is under one year, he says.

One of Mar-Lee's major successes in MuCell is a tool for a safety part converted over from standard injection molding. This was a 26-in-long by 4-in-wide cover for a child safety gate made by Safety 1st, an OEM in Canton, MA. Mar-Lee built a family of molds for the gate, with the MuCell tool a two-cavity cold runner mold for the cover.

Surface cosmetics of the light-color part were improved by masking knit lines using mold texturing. The part is lightweight and cycles faster than with standard molding.

A current limit of Trexel's process is surface esthetics, but Mar-Lee is minimizing that through tool modification. "We are having great success with modifying mold finishes to enhance esthetic effects of MuCell molded parts," says Gravelle. In particular, texturing existing mold surfaces is often highly effective in getting "cosmetically acceptable" parts from existing molds, he notes. Generally, texturing works best with lighter colors, including white, off-white and gray. Dark (especially black) and high-gloss parts have proven to be more difficult challenges. Fine-tuning of gating and process conditions can also reduce flow-line effects.

Meanwhile, Mar-Lee and Hanson Industries both recently embarked on programs to build new MuCell-specific molds. Stan Bowker, Mar-Lee's business manager, says work so far indicates "there is plenty that can be done in mold designs to optimize the benefits MuCell offers." Prototypes of new tools have been built, and Mar-Lee is working on designs for hot-runner, stack and multi-material (e.g. coinjection molds).

Coinjecting has been seen as some as a key to overcoming cosmetic effects of microcellular foam molding. The idea would be to create a lightweight foam core, while solid skins would provide Class, high-gloss surfaces. Both Battenfeld and Engel are working on primary equipment adaptations to advance this effort.

Hanson has just completed its first new MuCell-specific tool for a part, which was not disclosed, due to secrecy agreements. But R&D director Worthington pointed out some implications. One is that in some cases, cold-runner systems may be used in place of hot-runners to cut costs. Also, the flow of resins under MuCell is different, e.g. it runs faster in non-standard cold-runner cross sections.

Worthington says materials can be changed to take full advantage of part and tool designs. For example, a new tool is being built for a part specified for extreme high (55%) weight reduction. That can be achieved through MuCell as long as physical properties (which fall as density declines) are beefed up to compensate. He recommends shifting to higher-performance resins, or adding reinforcers. This can be done, says Worthington, as still reduce final part cost.

Finally, Hanson is investigating the idea of designing higher-cavitation MuCell molds. Sources say that converting or building stack molds for MuCell is going to be a tougher challenge. The main difficulty, one observer says, is to stabilize the distribution of the pressurized melt in a stack mold. The engineering of stack molds to achieve that is reported being investigated by leading injection molding equipment suppliers.

Husky's Meyer says a modified sprue bar design is needed to keep melt plastication pressure at required levels and to release pressure for safety purposes. The remainder of the hot-runner system is unaffected, he says

Extrusion die conversions expected to be modest

The earliest licensing of MuCell technology was to sheet and profile extruders. A number of commercial programs are already underway. One advantage for extrusion processors is that little change in dies is required for microcellular foam extrusion, according to Trexel.

For instance, Aluissuisse Composites, Inc., St. Louis, MO, recently announced its use of the process for its Foam-X laminated foam-centered graphic arts boards. Besides performance benefits, Aluisuisse reported productivity increases. Using nitrogen in place of HFC blowing agents is another advantage since the latter are viewed as less environment-friendly.

A second user of MuCell for extrusion, Dumaplast, Maldegem, Belgium, is now using the microcellular foam process to make interior claddings and trim. Some of the thin (0.5 mil) tongue-in-groove profiles (8 ft long and 10-25 cm wide) utilize interior ribs to increase strength. Matt Drozdoff, president of Dumaplast, estimates the market for fluted interior panels, a wood panel replacement, at about US$150 million.

Dumaplast and Trexel have jointly developed new dies to provide correct levels of foaming and balanced flow properties. MuCell allows cross-sections of less than 0.5 mm to be extruded at 15 to 30% lower densities than for conventional panels. Drozdoff says that unlike chemically-foamed profiles, those made from MuCell retain virtually all the needed stiffness and toughness properties at lower densities. Good surface properties are also retained.

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