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REPRINTED FROM EUROPEAN PLASTIC NEWS - September 1998
Microcellular Moulding
Engel is the first OEM equipment supplier to enter into such an agreement with Trexel. It has already provided a 150-tonne machine to Trexel as part of the agreement, and is currently assembling a 500-tonne machine, which is due for delivery to Trexel at the end of this month. Trexel's MuCell process is a proprietary manufacturing process for producing microcellular foamed plastics. This process is licensed by the company to and users (EPN, July/August 1997, page 21). The technology was initially developed by Massachusetts Institute of Technology in conjunction with Trexel. Using supercritical fluids (SCFs) based on atmospheric gases rather than on chemical blowing agents, this novel technology crates evenly distributed and uniformly sized microscopic cells throughout a polymer. Suitable for extrusion, injection moulding (including structural foam processing) and blow moulding, the MuCell microcellular foam process can be used to produce many thermoplastic products. Extrusion was the first process that Trexel focused on and the company has been working on these applications for the past three years. As a result, several polystyrene and polypropylene-based products will be introduced to the market in Europe this year by Trexel licensees. Significant additional work is underway for PVC profiles, wire and cable insulation and blow moulded HDPE bottles. Applying the MuCell microcellular foam process to injection moulding results in a weight reduction of up to 40%. In addition, moulders can achieve significant processing improvements, including lower clamp pressures, lower hold pressures, lower hold times and reductions in cycle times. Importantly, injection moulded products produced with the MuCell microcellular foam process generally retain key mechanical properties after foaming. All thermoplastics are considered suitable for foaming with the MuCell process. And the company has used the technology successfully with both amorphous and crystalline resins. These include: ABS, PA, TPE, PC, PC/ABS blends, PE, PP, PPO/PPE and PS. Dan Szczurko, vice president business development, comments that the great advantage of Trexel's MuCell microcellular foam process is that it enables moulders to use foam in product design where it previously was not suitable. The subsequent advantages include precisely controlled density reduction (to 40%) which in turn leads to reduced product costs. Injection moulded products have excellent compressive strength and resilience. Product cross-sections with skins and foamed cores are achievable, while the elimination of sink marks is an added advantage. The appearance of the product is similar to those made of solid material because of the extremely small cell sizes involved in the foam and thin-walled components. In terms of the machine development, the companies are keen to stress that new Engel machines can be supplied with the modifications necessary to run the MuCell process, or that existing machines can be retrofitted. The modifications include the addition of a gas injection system to introduce the super critical gases, a modified barrel with holes in it for the gas injection system to feed through, and a modified screw which mixes the SCF and the polymer together properly to give a single-phase solution. Szczurko says that in both new and retrofit situations, the MuCell option can be either enabled or disabled so the machine can operate as an ordinary injection moulding machine when not processing MuCell products. Szczurko claims that the return on investment of the extra equipment would invariably take less than a year because the process results in less material usage for each manufactured item than if it were made in solid resin. He suggests three months for nylon and longer for cheaper materials such as PP. He agrees that the process does still have areas that could be further researched. For example, the process can be run using existing moulds but as the resin expands in the mould and the finished product expands slightly as the mould opens, work on mould design and gating would be an advantage. Because of the skin/core cross-section phenomenon, products moulded with the MuCell Process often exhibit a higher retention of stiffness than would normally be expected with this degree of material reduction. The injection moulding process improvements related to the MuCell process can include: lower processing temperature (up to 35oC reduction with most materials); reduced cycle times; reduced fill pressure (up to 50%); better mould filling; reduced packing pressure; reduced holding pressure; reduced processing costs; and up to 50% reduction in clamp tonnage. Trexel illustrates the last point by commenting that it produced nylon wire ties with the MuCell microcellular foam process and these were moulded with 30% less clamp tonnage than solid material. This long, then product filled completely with no holding time. In addition to reducing product weight, the MuCell process does not use hydrocarbon-based blowing agents and does not require nucleating agents or reactive components. Trexel asserts that products produced with the MuCell process are 100% recyclable. Looking at the commercialization of the process, Trexel is currently involved in development projects in a wide array of industries, including the automotive, medical devices, consumer goods and electronics industries. The recent Trexel/Engel collaboration is the first OEM agreement , which makes the MuCell process available on standard commercial equipment to injection moulders for worldwide commercialization. In particular, because of the lower clamp tonnage characteristic of the MuCell process, it is expected to be particularly attractive to moulders who use Engel tiebarless machines. The MuCell process will be available on new equipment and will also be offered as a retrofit to existing machinery. Trexel will license each MuCell moulding machine with a standard commercial license. In addition to the general injection moulding industry, Trexel is also developing the MuCell technology for structural foam applications. Historically, moulders producing structural foam have been able to reduce product density by 5-10% by injecting nitrogen gas and chemical foaming agents into the melt. Higher weight reductions have been commercially possible but difficult to control due to inconsistent cell size and cell distribution. In structural foam applications the MuCell microcellular foam process - which produces a homogeneous dispersion of microscopic cells - permits moulders to achieve a controlled, predictable weight reduction of an additional 5-15%, without using chemical blowing agents. The process also generally improves the compressive strength, tensile strength, impact resistance, toughness, permanent set, cold-temperature performance and fatigue life of structural foam products. As with any other injection moulding applications of the MuCell process, significant processing advantages also can result from the use of this technology. For example, the lower viscosity of the melt results in lower clamp tonnage, improves mould fillings, and improves mould flow - particularly advantageous for such products as industrial pallets. Return to Top | Return to Magazine Articles 98-99 |
