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MuCell™ Molding Technology: Process Advancements and Commercialization |
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David Pierick, Vice President - MuCell Molding, Trexel, Inc. The MuCell™ molding technology is a proprietary manufacturing process designed to reduce costs by lowering cycle times, part weights and the requirements needed to mold difficult parts. The MuCell Molding Technology uses supercritical fluids (SCFs) of atmospheric gases to create low viscosity polymer melts and subsequently lower injection pressures and clamp tonnages. In addition, the MuCell process enables molders to foam materials that cannot be foamed successfully with conventional foaming technologies, such as high-temperature sulfones, polyetherimides (PEIs), and thermoplastic elastomers such as Kraton and Santoprene (20% weight reduction and a reduction in the Shore A hardness). Invented at the Massachusetts Institute of Technology (MIT) Mechanical Engineering Department, the MuCell Molding Technology is licensed by Trexel, Inc., Woburn, MA, the world leader in the development and commercialization of the MuCell Molding Technology. MuCell Molding Technology is available as a retrofit to installed injection molding equipment and as an option on selected new injection molding machines (only on licensed OEM's). All MuCell-capable machines--new and retrofitted--are also capable of conventional, non-foaming operation. The MuCell™ Molding Technology The MuCell™ Molding Technology uses low-cost, environmentally friendly supercritical fluids (SCFs) of atmospheric gases (CO2 and N2) as physical blowing agents to produce a low viscosity polymer melt. This low viscosity melt is subsequently used advantageously by the molder to reduce cycle times, improve dimensional stability, produce microcellular foam and to reduce weight of most injection molded parts. Initially developed as a method of producing relatively thin extruded sheet and tubes, the MuCell Molding Technology now encompasses a broad range of patented molding techniques that can reduce product costs, improve processability and increase the performance of molding machines. Processing Improvements Many processing enhancements are possible with the new MuCell Molding Technology, resulting in attractive cost savings for molders. For example:
Product Improvements The advantages of microcellular foam material result from the uniformly sized and evenly distributed microscopic cells. With the MuCell Molding Technology, molders can foam complex, three-dimensional objects with solid skins and microcellular foamed cores (skin/core cross sections) with excellent dimensional tolerances. The MuCell process permits molders to produce parts with substantially lower densities than with conventional foam molding technologies. The controlled weight reductions characteristic of the MuCell process can be quite impressive, as illustrated later in this paper in the applications section. The MuCell™ Process Expands Molding Capabilities Generally molders use foamed materials and weight-reducing processes (such as gas-assist) for two primary purposes: to reduce product costs and to improve product quality. Cost reductions and product quality improvements are achieved in a number of ways, including reducing raw material, improving processing (e.g., reducing fill pressure and reducing cycle time), and enhancing product characteristics (e.g., eliminating sink marks). The application and effectiveness of conventional foaming, gas-assist, and other technologies, however, are often constrained by cost limitations, control complications, product-design challenges, and other factors. Trexel's successful adaptation of the MuCell™ technology for injection molding gives molders expanded capabilities. With the MuCell process, product designers can specify lightweight, polymer-saving molded foam materials for products for which conventionally foamed materials would be impossible and for which other lightweighting techniques (such as gas-assist) might be ineffective or not cost effective. MuCell Molding Technology and Conventional Foaming Only a very small percent of injection molded products are foamed today, perhaps less than 5%. In addition, injection molded products that are foamed are generally limited to 0-5% weight reductions. In comparison, the MuCell microcellular molding technology can achieve density reductions of up to 60%. The use and expansion of conventional thermoplastic foams is limited primarily by the large size of the cells that characterize materials produced by standard foaming techniques and the lack of uniformity of those cells. Large cells necessitate relatively thick part cross sections to ensure that the cells are contained within the material, which prevents thin-wall foaming. (Thin cross sections are often flawed by breaks and holes.) In addition, large, non-uniform cells introduce brittleness, which decreases mechanical properties (e.g., strength, toughness, and fatigue). In contrast, the microcellular foam material produced with the MuCell Molding Technology does not contain large voids. For instance, while thin wall foaming below 0.100 in (2.54 mm) is seldom successful with conventional foaming processes, the microscopic, uniformly sized cells produced by the MuCell microcellular foam process enable molders to foam materials with cross sections as thin as 0.020 in (0.5 mm). With the MuCell Molding Technology, molders can produce high-volume, thin-wall parts without significant loss of mechanical properties. Traditional foam processes also can produce material inconsistencies due to the inherent difficulty of controlling the levels, uniformity, and release of blowing agents (such as in the use of CBAs). The precisely controlled-MuCell process, however, improves consistency and reduces variability. The MuCell microcellular foam process is less sensitive to material composition than conventional foaming processes, which enables molders to use more cost-effective resin grades. Since the blowing agent is not "ignited" to liberate the gas in the MuCell Molding Technology, all materials can be foamed with the same atmospheric-gas blowing agents, without limitations caused by high processing temperatures. For example, high-temperature materials such as polysulfone and polyehterimide-which are difficult to foam with conventional techniques-are suitable for the MuCell process. (With the MuCell Molding Technology, weight reductions of up to 25% on parts with wall thicknesses of 0.100 in/2.54 mm have been achieved with these high-temperature materials.) The MuCell Molding Technology and Gas-Assist Although gas assist is an effective technique for both reducing product weight and improving product functionality, the technology is limited by bubble placement, bubble size, and part complexity, which generally restricts the use of gas assist to the production of thick parts (unless special part designs are used). With the MuCell Molding Technology, the entire part is foamed with microscopic cells, without requiring the injection of holes or voids, as in gas-assist. Because the MuCell molding process is a precisely controlled density-reduction process, weight reduction is achieved without the need to address either gas bubble placement or size. In addition, control of closed-loop gas voids is eliminated since the cross-section structure is homogeneous. The MuCell process is not limited by part complexity. Because the MuCell microcellular foamed structure is homogeneous, it is not necessary to design methods of injecting gases. (Few if any modifications are required to existing molds to apply the MuCell Molding Technology.) With the MuCell Molding Technology, the weight reduction is uniform and consistent and occurs throughout the part. As noted above, the MuCell process is suitable for thin walling. While gas-assist generally is not effective for parts thinner than 0.150 in (3.8 mm), the MuCell technology can be applied to complex, three-dimensional parts, with sections as thin as 0.020 in (0.5 mm), and once again no major mold modifications are required. Advantages attributed to gas-assist processes--such as a reduction in fill, pack, and hold pressure and a resultant reduction in clamp tonnage--are also realized by the MuCell process, but for different reasons. In gas-assist, the injected gas provides some of the fill, pack, and hold pressure (thereby reducing the overall pressure); in MuCell processing, the overall pressure is reduced because of the low melt viscosity and internal gas pressure. Applications of the MuCell™ Molding Technology The MuCell Molding Technology is suitable for a broad range of thermoplastic polymers. Because the MuCell process uses an inert gas it is less sensitive to material composition than conventional foaming processes and more-cost-effective resin grades often can be applied. Trexel has applied the MuCell™ microcellular foam process to numerous applications on commercial injection molding equipment in Trexel's production-scale test laboratory. To date, the high-temperature engineering resin polyphenylsuflone (trade name Radel), nylon, polycarbonate (PC), polycarbonate/acrylonitrile butadiene styrene (PC/ABS), polyethylene (PE), Acetal, polypropylene (PP), and thermoplastic elastomers (TPEs), such as Santoprene and Kraton, have been foamed. Currently, extruders and molders around the world are developing and introducing new and innovative microcellular foam products for a variety of industries, including the automotive, medical device, consumer goods, and electronics industries. Applications for injection molding microcellular foam material include a wide range of automotive, food service, and electrical markets. Air Distribution Manifold An air distribution manifold was produced on a 900 ton Engel machine during the Interplas show at Birmingham, England (October 4-7, 1999). Both a cycle time and material weight reduction were demonstrated. Although these results are impressive, it is believed that with some mold modifications, the MuCell benefits would be even greater.
Engine intake manifold gasket A 33% glass filled nylon air intake manifold gasket was molded with a tremendous reduction in clamp tonnage, cycle time and various weight reductions.
  Automotive Mirror Bracket Support
Nylon 6/6 Cable Ties Application studies on the use of nylon 6/6 in injection molding applications have produced the following results:
Cable ties foamed from nylon 6/6 are shown in Figure 2. Figure 3 shows a scanning electron microscope (SEM) view of the material cross section.   PC/ABS In-mold Decoration - Automotive A- Pillar MuCell Molding Technology is also used to reduce injection pressures and melt temperatures to improve the quality of in-mold decorated parts. MuCell material was injected onto a fabric.
Areas for Potential Savings when using MuCell Typical types of cost reductions due to MuCell The above applications demonstrate some new and unique ways MuCell Molding Technology has reduced the overall manufacturing costs. By creating a single phase solution between the polymer and super critical fluid injection pressures can be reduced, clamp tonnages cut by 30 to 70 %, cycle time reduced by 30% and part weight can be reduced. Obviously, these improvements can be translated into cost reductions by reducing material and by reducing cycle time, however other benefits are not as easily apparent. Purchasing smaller machines can reduce initial capital investment, especially if a tiebarless machine is purchased. For example, it has been proven on commercial machines that it is now possible to run molds normally run in 500 ton presses in a 200 ton tiebarless press. Clamp tonnage was reduced to such an extent that the only limitations were whether the mold would actually fit between the machine base rails and whether the shot size is adequate. Additionally, a typical 500 ton machine consumes 21 to 25 kw/hr, whereas a 200 ton machine consumes 10 to 12 kw/hr. This is a 50% reduction in energy costs. See Figure 4 for a breakdown of the difference in capital cost and energy expenditure for a 500 ton mold run in a 200 ton tiebarless machine using MuCell.
Figure 4. Additional cost savings due to reduced capital expenditure and energy consumption. Analysis of cost reductions due to MuCell Figure 5 demonstrates from actual applications how MuCell has reduced costs. Obviously, cycle time and material reduction has been the greatest benefit for the applications run to date, but other significant cost reductions include colorant reduction, energy consumption reduction and tooling costs.  On each commercial trial run with MuCell, the customer has performed a Return on Investment analysis. Figure 6 is a summary on some of the parts run along with their savings / year and payback of the initial investment in terms of time. As you can see, the savings per year is based only on cycle time or material savings. Ancillary savings such as reduced energy consumption or reduced capital cost is not considered.  Implementing the MuCell™ Molding Technology The MuCell™ microcellular foam process follows four basic steps:
Equipment The MuCell™ molding technology can be retrofitted easily to installed equipment with low-cost hardware and software modifications. Adapting the MuCell process to injection molding requires the following changes or additions:
Trexel has entered into commercial license agreements with Engel, Inc., Milacron, Arburg, Ferromatik, to supply new injection molding equipment capable of utilizing Trexel's proprietary MuCell™ microcellular foam process. Retrofit packages for installed equipment are also available on selected equipment. Additionally, EPCO has also taken licenses and is capable of retrofitting installed equipment. As noted above, all MuCell-capable machines--new and retrofitted--are also capable of conventional, non-foaming processing. Conclusion The MuCell™ molding technology permits molders to produce new and innovative products while reducing product costs. The MuCell process can be used to mold complex, thin-walled microcellular foam parts with dramatic weight reductions. Product improvements include the elimination of sink marks, lower post-mold warpage, and skin-core cross sections. Process improvements include reduced processing temperature, reduced injection pressure, reduced clamp tonnage, and reduced cycle time. The MuCell Molding Technology uses supercritical fluids (SCFs) of atmospheric gases as physical blowing agents--not CBAs, hydrocarbon-based physical blowing agents, nucleating agents, or reactive components. The MuCell process, which can be retrofitted to existing equipment, is available in selected new injection molding machines; all MuCell-capable machines also can be used for conventional, non-foaming processing. 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. Patents and patents pending in Asia, Europe, and North America cover Trexel's MuCell Molding Technology. Return to Top | Return to Technical Papers & Presentations |
