The MuCell Injection Molding Process involves the highly controlled use of gas in its supercritical state (SCF) to create millions of micron-sized voids in thin wall molded parts to make a foamed part. With the correct equipment configuration, mold design and processing conditions, these microcellular voids are relatively uniform in size and distribution. The voids are created or nucleated as a result of homogeneous nucleation that occurs when a single-phase solution of polymer and gas (commonly nitrogen, but occasionally carbon dioxide) passes through the injection gate into the mold. The MuCell Microcellular Foam injection molding technology is a complete process and equipment technology which facilitates extremely high quality and greatly reduces production costs. The technology is targeted at precision and engineered plastic components with maximum wall thicknesses of less than 3 mm.
The single-phase solution is created through the operation of a conventional injection molding machine which has been modified to allow the creation. The key modifications to the system involve the use of a precision SCF delivery system to deliver SCF to special injectors based on mass flow metering principles. The SCF is then injected into the barrel where it is mixed with the polymer via a specially designed screw. A shut-off nozzle maintains the single phase solution while the injection molding screw maintains sufficient back pressure at all times to prevent premature foaming or the loss of pressure which would allow the single phase solution to return to the two phase solution.
Basically, MuCell Injection Molding Process involves four steps:
Firstly, a supercritical fluid (SCF) from an atmospheric gas (CO2 or N2) is injected directly into the polymer through injectors installed in the barrel to form a single-phase solution. The SCF delivery system's screw and injectors are designed specifically to facilitate the rapid dissolution of the SCF in the polymer. The next step includes nucleation where a large number of nucleation sites (orders of magnitude greater than with conventional foaming processes) are formed throughout the polymer during the molding process. A substantial and rapid pressure drop is necessary to create the large number of uniform sites. Thereafter, cell growth is controlled by processing conditions including precise control of pressure and temperature by machine control hardware and software. The last stage involves shaping followed by injection into the mold. Part shape is controlled by the mold design.
The process generally offers a 50-75% improvement in key quality measures such as flatness, roundness and warpage, also eliminating all sink marks. These improvements result from the fact that relatively uniform stress patterns are created in the molded part rather than non uniform stress characteristic of solid molding. As a direct result of the uniform stress and shrinkage associated with the MuCell Process (which occurs because the pack and hold phase of the molding cycle is eliminated), the parts that are produced tend to comply far more closely with the mold shape and, presumably, the dimensional specifications of the part itself. This means that when using the process, fewer mold iterations are needed to produce a compliant part, saving time and cost. The quality advantages of the MuCell Process are complemented by certain direct economic advantages, including the ability to produce 20-33% more parts per hour on a given molded machine, and the ability to mold parts on lower tonnage machines as a result of the viscosity reduction and the elimination of the packing requirement that accompanies the use of supercritical gas.
The striking features of microcellular foam (MCF) are fine cell size, high cell density, inorganic blowing agent and no nucleating agent. Since the cell is very fine, without careful attention, MCF may be taken as a plastic material rather than a cellular product. When cell is reduced to micron range without using nucleating agent, decreased convection in the cell and less open cell make a uniform structured product with better insulation characteristics.
The process has been implemented successfully with a broad range of injection molded thermoplastic polymers � including acetal, liquid crystalline polymers (LCPs), polyamide, PA6 and PA6/6, polycarbonate (PC), polycarbonate /acrylonitrile butadiene styrene (PC/ABS), polyethylene (PE), polytherimides (PEIs), polypropylene (PP), high-temperature sulfones - as well as TPEs and TPUs. In addition, the Mucell process allows to foam materials that cannot be foamed successfully with conventional foaming technologies.
Industries such as automotive, medical devices, consumer goods, electronics, food services and industrial and consumer structural parts are currently benefiting from MuCell's remarkable advances in microcellular injection molding. The applications that have been selected take advantage of the controlled weight savings and significant reductions in cycle time. Many products have been converted from larger machines to smaller MuCell equipped machines utilizing the drastic reductions in clamp tonnage. MuCell-capable injection molding machines and structural foam machines are also capable of conventional, non-foaming operation. Molders can gain a competitive advantage with the MuCell process by utilizing its processing benefits like endothermic reaction, lower viscosity and lower melt and mold temperature. These benefits typically result in significant reductions in cycle time, material consumption and injection pressure and clamp tonnage. This microcellular foam process is unique because it can be applied to thin-wall parts and to materials that cannot be foamed successfully with any other foaming technologies.
The company has developed its Series III SCF System which features the world's first all-electric discontinuous dosing system for microcellular foaming. The system uses a unique and patented pulse dosing technology to provide accurate and repeatable discontinuous dosing for MuCell ® microcellular foam production. A key design feature of the Series III MuCell ® system is that gas compression is accomplished with an electric drive motor instead of air driven pumps. According to Trexel, this will help to reduce the required maintenance activities by 50% compared to the Series II MuCell ® system. It is designed to take a supply of nitrogen from a high pressure cylinder and provide a precise dose of nitrogen at a chosen set point. The nitrogen supply is fed to a gas piston . An electric drive motor is attached to the gas piston, compressing the nitrogen in the gas piston to a predetermined pressure. This pressure becomes the supply pressure for the hardware required for a pulse dosing system. The company offers the Series III system as a stand-alone package for those customers who do not want an integrated system. However, it will also sell the Series III MuCell ® system designed for seamless integration by injection molding machine manufacturers into their existing and new small machine designs. New opportunities for injection molders originate from a breakthrough molding technology , offering capabilities never before seen with other molding processes. It opens new markets for innovative product design, optimized processing and reduced part costs.
Recently, Unilever 500 gram Rama margarine tub bagged the 2008 Deutscher Verpackungs Preis (German Packaging Award ) and a WorldStar Packaging Award. The innovative tub design was developed by leading European rigid packaging company Veriplast Solutions using its super-light injection molding technology (SLIM), which combines the MuCell® microcellular foaming process along with Veriplast's Extra Slim Label, an innovative down-gauged in-mold label. SLIM® technology utilizes the MuCell® process, which involves the use of precisely metered quantities of atmospheric gases (nitrogen or carbon dioxide) in the injection molding process to create millions of nearly invisible microcells in the end product. The gas nucleates cells during injection and allows the thin wall cavities to be filled with reduced pressure and with reduced clamp tonnage. This permits additional thin walling. The microcells then replace their equivalent volume of plastic, resulting in a cumulative reduction of upto 10% in packaging weight without any perceptible difference in the final tub quality. Veriplast combines this with its own Extra Slim Label technology, which is significantly thinner than the market standard and provides an additional environmental benefit by reducing the CO2 footprint by 30% compared to standard labels.
The MuCell technology is based on a patent awarded to Dr. Nam P. Suh et al. at the MIT (Massachusetts Institute of Technology). Trexel, Inc. based in Boston , USA is the exclusive licensor of the proprietary MuCell technology. On the basis of the fundamental patent of MIT, Trexel Inc. invested tens of millions of dollars on research and development, successfully established and commercialized the technology related to extrusion molding, injection molding and blow molding. More than 2000 patents applied by Trexel Inc. form barriers that prevent competitors seeking for the development and commercialization of similar products from entering the market. Trexel has licensed most of the leading injection molding equipment suppliers to produce and sell fully integrated MuCell Machines. Leading ones include Engel, KraussMaffei, Arburg, JSW, Mitsubishi Heavy Industries Ltd., etc.