Precision Injection Molding PRECISION PROCESS CONTROL OF INJECTION MOLDING

1 INTRODUCTION The technical requirements of precision injection molded components demand a heightened level of process performance, and with corresponding process monitoring and control technologies. To obtain the desired critical to quality attributes (CTQs) of the molded products, the injection molding process must be consciously designed such that the key process variables (KPVs) are observable and controllable. Failure to understand and control the linkage between KPVs and CTQs may result in undesirable levels of defects during production, unattainable levels of demanded precision, and technical and/or economic infeasibility of the molding application. A fundamental difficulty in control of injection molding is that few of the final molded part properties can be ascertained within the molding cycle. Instrumentation does not yet exist, and may never exist, to yield information about aesthetics, structural integrity, and other part properties prior to opening of the mold and inspection of the part. For instance, a pressure sensor may be placed at the end of flow to detect the arrival of the melt and pressure decay during cooling. Yet such use of the sensor may or may not be able to consistently predict the formation of flash during filling or sufficient part packing so that the molded component(s) meet the desired level of precision. As such, precision injection molded parts generally require integrated product and process design, in which the product development process provides not only the design of the precision component (geometry, material, specifications), but also the validated design of the precision molding process (process parameters, sensors, allowances). This chapter is divided into three sections. First, fundamental principles of process control and performance measures for precision injection molding are reviewed. Second, current process control approaches are discussed with deference to process control of precision injection molding. Third, an application development methodology is presented for precision process control as exemplified with two industry applications.