An Environmental Analysis of Injection Molding Extended Abstract – Green Manufacturing Processes

for 2006 IEEE 1 An Environmental Analysis of Injection Molding Extended Abstract – Green Manufacturing Processes Alexandre Thiriez and Timothy Gutowski Department of Mechanical Engineering Massachusetts Institute of Technology Rm. 35-234 77 Massachusetts Ave. Cambridge, MA 02139 Plastic components are integral parts in electrical and electronic (E&E) products. 8.5% of the plastic production is dedicated to this market [Fisher et al. 2005]. Although this number might seem small it is larger than the amount of plastic used for the automotive industry (8%) [Fisher et al. 2005]. In E&E products plastic can represent from 3% of the total weight in medical equipment to 33% in small house appliances and 42% in toys [Fisher et al. 2005]. The majority of this plastic used for E&E products is injection molded in order to attain the specific geometric requirements. Injection molding involves melting polymer resin together with additives and then injecting the melt into a mold. Once the resin is solidified, the mold opens and the part is ejected. At first glance, injection molding may appear to be a relatively benign process with respect to the environment due to its low direct emission levels and apparently low energy consumption. However, when calculating the environmental cost of injection molding one must also take into account the ancillary processes and raw materials used in the process. Aside from the raw material production stage which has substantial emissions, the main metric in the whole injection molding process is energy consumption. The large scale of the injection molding industry makes the environmental impacts of this process especially critical. In other words, a small increase in the efficiency of the process could lead to substantial savings for the environment. This paper investigates injection molding from an environmental standpoint, yielding a system-level environmental analysis of the process. It provides a transparent process model that includes all major steps involved in the production of injection molded products and shows the dependency of injection molding on the most important process parameters. This paper presents a summary of our findings along with detail on four major issues: 1. Relationship between energy consumption and throughput. 2. Differences in environmental performance between hydraulic and all-electric machines. 3. Role of secondary/subsidiary process in the energy accounting. 4. Environmental scale of injection molding. Background: With regards to injection molding life cycle inventories (LCI), much effort has gone into studying the production of raw materials (polymers) as well as the product end-of-life aspects, such as disassembly separation and recycling. Amongst the researchers in this area, it is worth mentioning Ian Boustead, who developed a set of “eco-profiles,” or LCI’s, of the most consumed polymers in the plastic industry. He also created life cycle inventories for injection molded PVC and injection molded polypropylene. The former LCI studied two injection molding facilities in France that produce PVC fittings for pipe drainage systems [Boustead Conversion PVC]. The latter LCI studied one facility in the U.K. that produces 12 to 76 g polypropylene components [Boustead Conversion PE]. These studies are product specific, narrowing on one application and one set of processing parameters. In an effort to obtain a range of values typical in injection molding, and thus more breadth of data, this study incorporates measurements from machines processing different products and materials. It also provides a transparent outline of all the sub-processes that make up the injection molding process together with their environmental performance. Other contributors to the field of injection molding include Mattis et al. 1996 and Boothroyd et al. 2002. Mattis et al. used a 3-D solid modeling environment and numerical analysis to explore the influence of mold design, part design, and some process parameters on the process efficiency. Boothroyd et al., whose goals were to develop design-for-manufacturing guidelines, developed a set of empirical equations predicting machine size and processing time of each stage in the injection molding cycle.