Tech Trends
Sep 1st, 2007                                Print this article

Group offers new take on chemical foaming

By Stephen Moore

Chemical foaming of injection molded parts is not new, but a new group has entered the space with a technique it says promises not only weight savings, but also lower clamping forces and faster cycle times.

Hyperfoam realizes superior surface finish through application of counterpressure.

According to its developers, the Hyperfoam process can reduce part weight by 10-30%, clamping-force requirements by 50-60%, and power consumption by about 30%. Lower cycle times and part dimensional stability are also cited.

The technology was unveiled by a global group of companies at the recent Chinaplas show in Guangzhou. The development team is comprised of Chinese injection machine supplier Welltec Machinery (Hong Kong), consultant Caropreso Associates (Chester, MA), static mixer expert Sulzer Chemtech (Winterthur, Switzerland), shut-off valve manufacturer Herzog (Wolfertswil, Switzerland), chemical blowing-agent producer Reedy International Corp. (Keyport, NJ), and CVI Modern Technology Development (Hong Kong), responsible for system integration.

The key to process success is high-speed, high-pressure injection—the Hyperfoam machine is outfitted with a nitrogen accumulator and boasts an injection speed of 600 mm/sec. Carbon dioxide gas generated by the endothermic chemical blowing agent (CBA) is incorporated into the polymer melt in a supercritical state and when this supercritical fluid is injected into the mold cavity via a static mixer at very high injection speed, instantaneous cell nucleation takes place, forming a honeycomb-like structural foam upon cooling. The resin and foaming agent are mixed by the injection screw and homogenized by the static mixer prior to injection.

“A fast injection speed is required to ensure uniform bubble distribution,” says Michael Caropreso of Caropreso Associates. “A slow injection speed would see higher part density near the gate and lower densities at the extremities.” The cavity is typically filled in 0.3 second.

Caropreso Associates offers a counterpressure option for Hyperfoam that improves surface finish. This option applies a pressure in the sealed mold that is higher than the gas pressure in the part, thus preventing bubble breakout. Once a skin is formed, the pressure is released. Typical weight savings are 10–15% with counterpressure and 30% without.

The standard injection press offered by Welltec features a 280-tonne clamping unit and a 450-tonne injection unit. The supplier initially targets sales of 30 units by the end of this year. Robert Yan, product manager at CVI Modern Technology Development, says resin savings alone can pay for the system’s expense in one to two years, adding, “The bigger the shot size, the more savings, which is why we are initially offering a medium-size machine.” The developers say that the Hyperfoam process is not resin-dependent, and that it can handle plastics such as HIPS, PC, PA and PP.

Physical processes

Hyperfoam will ring bells of familiarity with any processor as yet familiar with the MuCell process of physically introducing supercritical gas (commonly nitrogen, but occasionally carbon dioxide) to foam a part. MuCell is the trade name of the microcellular injection molding technology developed and licensed by Trexel (Woburn, MA). Sulzer Chemtech and Trexel have tangled in court a number of times, most recently with Trexel winning a defense of its patent in Europe earlier this year.

Commenting on the development, Trexel President & CEO David P. Bernstein says, “Hyperfoam can probably find a market with relatively thick polyolefin parts and possibly some polystyrene-based materials, but it may struggle with thin-walled parts and higher temperature materials.” Elaborating on this statement, Bernstein says CBAs have a negative effect on high temperature materials due to the compatibility of the carriers employed. He adds, “CO2 is also not an optimal blowing agent for thin-walled parts in that it creates larger cells than nitrogen and also involves the use of larger amounts of gas which can create additional splay on foamed surfaces.”

Also, “consistent microcellular foaming of precision thin-walled parts depends very much on precise control of dosing levels,” says Bernstein. “The use of CBAs implies that shot-to-shot dosing levels will be different.” MuCell also has extensive patent coverage of thin-walled microcellular parts, defined in general as less than 3 mm with average cell sizes in the 100 micron or lower range. End products are covered by Trexel patents in Europe, North America, Taiwan, Australia and pending in many other major markets.

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