Tiny cells grow company

A microcellular thermoplastic foam technology invented at the Massachusetts Institute of Technology is putting a small Massachusetts company, Trexel Inc. (Woburn, MA), on the fast track. The innovative process, which gave Trexel birth, uses high-cell nucleation rates within the foaming material to create foams with small, evenly distributed and uniformly sized cells (generally 5-50 micron in diameter). Trexel claims that the foam materials produced by this process, called MuCell^®, have properties and a uniformity superior to conventionally foamed products.

"MuCell uses supercritical fluids (SCFs) of atmospheric gases to create evenly distributed and uniformly sized microscopic cells throughout a polymer," explains David Pierick, Trexel's VP of injection molding. "It's suitable for structural-foam (SF) molding (as well as other injection-molding applications), blow molding, and extrusion; and does not require chemical blowing agents (CBAs), hydrocarbon-based physical blowing agents, nucleating agents, or reactive components."

The MuCell process operates on modified conventional molding machines and uses supercritical fluids (SCFs) of atmospheric gases to promote high-cell nucleation rates during microscopic-cell foam creation.

As the process substantially increases cell-nucleation, a large number of cells are created before any cell growth occurs. As a result, when the blowing-agent diffusion begins to dominate during foam creation, all cell sites begin to grow at the same time and about the same rate. The result: a microcellular foam material characterized by a large number of evenly distributed, uniform microscopic cells.

Atmospheric gases, such as carbon dioxide and nitrogen which are less expensive than other common blowing agents and are unregulated can be used in the process. Equally important, the very high nucleation rates needed to produce microcellular foam can be achieved without the use of standard nucleation agents like talc or chalk.

The technology permits molders to reduce raw-material use, while producing strong, lightweight products never before considered for foamed-polymer applications. According to Pierick, the benefits include: a significant reduction in the weight of components (see table), the ability to foam thin-walled parts while reducing processing temperatures, and the capability to reduce injection pressure and clamp tonnage by 50%. "The process also enables molders to foam materials that cannot be foamed successfully with conventional technologies (such as high-temperature sulfones, polyetherimides (PEIs), liquid-crystalline polymers (LCPs), and thermoplastic elastomers such as Kraton^® and Santoprene^®) and realize a 20-50% weight reduction and a reduction in the Shore A hardness^®," Pierick adds.

Low-cost, complex parts. Initially developed as a way to produce relatively thin extruded sheet and tubes, the MuCell technology now encompasses a broad range of patented molding techniques designed to reduce product costs, improve processability, cut cycle times, and increase the performance of molding machines. For example, the technology enables molders to produce complex 3D objects with solid skins and microcellular foamed cores with improved dimensional tolerances.

MuCell also permits molders to make parts with substantially lower densities than those produced using more conventional molding processes with little stiffness or other mechanical-property penalties. Pierick recalls that in one application, the density of a 0.065-inch (1.65 mm) part was reduced by 25%, with an accompanying reduction in stiffness of just 7%without any part design changes.

Other reported advantages:

Use of SCF blowing agents lowers the viscosity of the material by up to 50%, which allows for a decrease in melt temperatures of as much as 78C (140F), while maintaining flowability.

• A reduction of injection pressure up to 50%.

• Ability to lower clamp tonnage up to 60%, enabling molders to increase the number of cavities in the mold.

• Provides a uniform, controlled internal gas pressure that can eliminate sink marks, while either reducing or eliminating hold time.

• Combination of low viscosity, uniform internal gas pressure, and low molded-in stress results in lower post-mold warpage and a reduction in cooling and cycle times.

Since the molding technology retrofits easily to installed equipment, MuCell has caught the attention of some big-name molding-machine makers. Adapting the process to SF or injection-molding equipment requires the following changes or additions:

An SCF metering system with sufficient capacity to deliver the blowing agent to the screw at the required volume and pressure. Trexel's Equipment Div. supplies configured pump systems for SCF delivery and other proprietary components to licensees.

• A Trexel-designed screw for creating a single-phase solution of the blowing agent and polymers, and possibly a new barrel.

• Minor software and system modifications to create and maintain the uniformity of the single-phase solution throughout the molding cycle.

In addition to retrofit equipment, the molding technology is available on selected new SF and injection-molding machines. Trexel has entered into an OEM license agreement with Uniloy Milacron (Manchester, MI) to equip its machines with the proprietary process. The new machines went on the market in August. Engel Inc. (Schwertberg, Austria) also will supply new and retrofit injection-molding equipment that incorporates the technology.

What's next for Trexel? "The materials and process savings associated with the MuCell molding technology will become even more pronounced during the next 3-5 years as products and tooling are specifically designed for this breakthrough technology," Pierick predicts.

Trexel at a glance

Formed: 1995

President & CEO: David Bernstein. Bernstein served as vice president of sales and support for Teradyne and vice president of Thermedics Detection before joining Trexel in 1985.

Headquarters: 45 Sixth Rd., Woburn, MA 01801

Phone: (781) 932-0202 FAX: (781) 932-3324

Number of employees: 28

Job prospects: Currently looking to hire 12 processing engineers; for more information check out www.trexel.com.