An environmental analysis of injection molding

Abstract

This thesis investigates injection molding from an environmental standpoint, yielding a system-level environmental analysis of the process. There are three main objectives: analyze the energy consumption trends in injection molding machinery, explore the environmental performance of different technological alternatives, and provide a transparent life cycle inventory (LCI) identifying the mayor players in terms of environmental impact. The choice of injection molding machine type (hydraulic, hybrid or all-electric) has a substantial impact on the specific energy consumption (SEC), energy used per kg of processed polymer. The SEC values for hydraulic, hybrid and all-electric machines analyzed are 19.0, 13.2 and 12.6 MJ/kg respectively (including auxiliaries, compounding and the inefficiency of the electric grid). For hydraulic and hybrid machines SEC seems to exhibit a decreasing behavior with increasing throughput. This derives from spreading fixed energy costs over more kilograms of polymer as throughput increases. For all-electric machines SEC is constant with throughput. As viscosity and specific heat capacity increase so does SEC. Finally, SEC varies greatly with part shape. The thinner and the greater the projected area of the part the greater the SEC.

(cont.) When the polymer production stage is included in the analysis, the energy consumption values increase up to 100 MJ/kg. After polymer production, injection molding and extrusion have the greatest environmental impact in the whole LCI. The overall injection molding energy consumption (excluding polymer production) in the U.S. on a yearly basis amounts to 2.06 x 108 GJ. This value is of similar magnitude to the overall U.S. energy consumption for sand casting, and to the entire electricity production of some developed countries.