To achieve consistent quality molded parts with good aesthetics and dimension, a designed experiment which is a systematic study of the effects of variables on the quality of the part should be conducted. These are traditionally used in injection molding to determine the process variables to achieve robustness of the production cycle. It provides tremendous benefits. A large process window is always desired to reduce the possibility of defect. A robust process is one that is set in the center of this window, which ensures that the natural variation in the process will still yield acceptable parts. Six parameters are required to be studied to determine the processing window to produce an injection molded product of good aesthetics and still have good dimensional control. They are Viscosity, Pressure drop across the zones, Cavity balance; Gate sealing, cooling cycle/time • Rheology study or viscosity study: The viscosity remains fairly constant at higher injection speeds. Variations in the injection speeds should have very little effect on the viscosity, ensuring a consistent fill. • Cavity balance : A balanced fill between cavities ensures an equal amount of material packed into the cavities, resulting in cavity-to-cavity consistency. • Pressure-drop studies: A pressure-drop study is done to ensure that the screw has enough pressure to move at the set speed. When more than the available pressure is required, the screw slows down and the cavity fill becomes inconsistent. Such processes are called pressure-limited processes. • Process window study: The process window study is performed to find the limits between which aesthetically acceptable parts can be molded. Typically, the two variables are holding pressure and melt or mold temperature. A center pressure is the most desirable to compensate for natural variations in the holding pressures • Gate seal study: As the plastic fills the mold through the gate, the velocity reduces, the melt cools down, and eventually the gate freezes off. Pressure must be applied to the screw till the gate freezes off to maintain cavity pressure and part weight consistency. A hold time where the part weight is constant is chosen as the set hold time, again to ensure consistency. • Cooling time study: A cooling time study is performed to find out how fast a part can be molded without distortion or any other issues. Since the parts are available, a graph of the important dimensions can be plotted as a function of the cooling time. Typically, for amorphous polymers, a study should be conducted while holding pressure against different melt temperatures, whereas a study of holding pressures with different mold temperatures should be conducted for crystalline polymers. A robust process is one that is set in the center of this window, which ensures that the natural variation in the process will still yield acceptable parts. The technique above ensures aesthetic acceptance of the part. The center of the window would be a perfect process if the parts molded with these conditions had nominal dimensions and if the upper specification limit (USL) and the lower specification limit (LSL) were both contained within this window. A study on the effect of both processing parameters on the dimensions of the part should then be made. The technique of design of experiments (DOE) can be used to perform such a study. Outputs from DOE include dimension contour plots, lines that describe the process conditions to achieve a certain dimension. The DOE studies must be carried out at the four corners of the process window where the parts are aesthetically acceptable. This concept can be extended further and incorporate process capability into the DPW. Control limits can be set based on the variation in the process and the dimensions. This is called the control process window (CPW), which takes into consideration all the naturally caused variations and provides the working windows. Overlaying multiple dimensions on this window makes the actual working window even smaller. Alarm limits are boundaries that are set on an output of the molding process, whereas tolerances are those that are set on the process settings. The concept can be taken to other outputs such as cavity pressure that is the final and most effective way of measuring part quality. Overlaying the various windows generated during the mold qualification process displays the working areas of the molding process. Although mold qualifications using process engineering studies and DOEs are routine, there is an absence of a method of correlating the data from the two techniques. The two techniques each provide important data, but combining them provides a very clear understanding of the capabilities of the mold and the molding process. Injection molding operation should have consistency of molded parts of the required quality. Three types of consistencies that must be achieved are: • Shot to shot • Cavity to cavity • Operation runs to another run The molded parts should be of identical quality in all these variations. Five most important parameters that determine the quality of molded parts are: • Part design • Material • Machine • Mold • Process • Part quality The molding process is one of five factors that influences molded part quality. The other four are part design, plastic material, mold design and construction, and the molding machine. Each of these plays an important role in achieving a consistent molding process. A scientific molding is a technique that optimizes the entire molding process. Wide process windows provide an opportunity to run successful molding operations. Some would argue that the window gets smaller and smaller as the number of dimensions and cavities increases. Unfortunately, that is the reality, though it has been a hidden truth. This is the primary reason most companies have only one approved process sheet. The alarm limits and tolerances are usually set based on past molding experiences. Instead, these must be set based on the experiments performed and the results obtained from them. Understanding the sensitivity of the part quality to these factors gives an insight to setting the process parameter tolerances during molding. Having multiple dimensions and multiple cavities makes the windows smaller and smaller. This further pleads the case for a well-built mold and parts with practically defined tolerances. Concurrent engineering principles and practices become important. A robust process requires less human intervention. The frequency of inspection can be reduced. Upfront planning and a disciplined approach to process development are therefore imperative to the efficiency of the production run and the profitability of the operation. (Extracted from an article by Suhas Kulkarni, PlasticsToday)