Teaching Design for Manufacturing of Plastics Using Computer-Aided Engineering
Chang, Tao C., Journal of Technology Studies
Plastic manufacturing is one of the largest industrial areas in the United States. It accounts for approximately $304 billion in annual shipments and 1.5 million jobs (Society of Plastic Industry, Inc., 2000). Today's business environment is driving manufacturers to bring better products to market faster, with higher quality and lower cost. This is true in the plastics molding and manufacturing industries, as stressed in a 1999 industry trend report prepared by the Plastics Molders & Manufacturing Association of the Society of Manufacturing Engineers. This trend forces original equipment manufacturers, molders, toolmakers, machine manufacturers, and material suppliers to work together and be involved at the earliest stage of product development in today's intensely time-- conscious, competitive environment. In developing a new product, the design stage will typically cost 5% of the total cost breakdown (see Figure 1). However, studies by various companies (Boothroyd, Dewhurst, & Knight, 1994) have shown that design decisions made during new product development directly affect 70% to 80% of the final manufacturing cost (see Figure 2). Therefore, the workforce needs to be attuned to designing with manufacturability in mind to avoid difficult and costly situations in later stages.
Today, technology tools such as computeraided design/manufacturing (CAD/CAM), computer-aided engineering (CAE), computer numerical control (CNC) machining, solid modeling, and stereolithography (SLA) are available to help manufacturers achieve the goal of an ever-decreasing life cycle of a product from concept to market. CAE has been widely used by the plastic injection industry to verify the manufacturability of a design, as evidenced by the number of commercial software packages available today (see Table 1; "Software, CAE," 2001).
Injection Molding and Product Development
Injection molding is a process that softens a plastic material with heat and forces it to flow into a closed mold. Then, the material cools and solidifies, forming a specific product. The manufacturing of quality injection-molded parts depends on the successes of part and mold design, process control, and material selection. A study identified more than 200 different parameters that had a direct or indirect effect on the complicated process (Bryce, 1996).
Traditionally, experienced molding personnel have relied on their knowledge and intuition acquired through long-term experience, rather than the theoretical and analytical approach to determining the process parameters that is used today. The length of the time in finding the right conditions to manufacture quality parts was dependent on the experience of molding personnel. Furthermore, the development of new products and part and mold designs as well as selection of materials and machines also remained a matter of personal judgment. It was considered normal that a mold be returned to the mold maker for modification at least once or twice before it could produce parts meeting the user's specifications. About 20% of the cost of a mold commonly went into redesign and remaking (Bernhardt, Bertacchi, & Moroni, 1984).
The development of computer-aided engineering simulation in the injection molding industry has eliminated various trialand-error practices and greatly streamlined the product development cycle. CAE can be used to check process feasibility, evaluate runner systems, determine optimal process conditions, and estimate the cost of processing a part. Its application can provide the industry with benefits such as resource saving, reduced time to market, and improved quality and productivity. However, one of the causes for reluctance to make use of and realize the whole advantages of CAE is that a significant portion of the industry still lacks the technical skills needed to apply the simulation technology (Berhardt, Bertacchi, & Kassa, 2000). Integration of CAE into higher education should provide trained personnel to reap the benefits of simulation in the injection molding industry. …