Book Kulkarni 10-07-08 - hanserpublications.com

1 Robust ProcessDevelopment and Scientifi c MoldingTheory and PracticeSuhas KulkarniISBNs978-1-56990-501-21-56990-50 1-0 HANSERH anser Publishers, Munich Hanser Publications, CincinnatiSample Chapter 8: Design of Experiments for injection Parameters in injection molding 1458 Design of Experiments for injection MoldingPlanned experiments have been around for centuries. In the 17th century a doctor planned some experiments to find a cure for an ailment of that day. He engaged various patients and various combinations of medicines to find the right cure. Planned experiments were used in agriculture to find the right combination of factors, such as soil type and fertil-izer, to produce the highest yield of crops.

2 Since the time involved in waiting for the results of these agricultural experiments was one complete season, which in some cases was one complete year, the technique of a planned experiment was very helpful. As planned exper-iments became popular as time saving and efficient techniques, people with a background in mathematics and statistics became involved and developed them further. Each came up with techniques that helped in analyzing different types of data in different scenarios. The people who have been most associated with the development of Design of Experiments are G. Taguchi and Ronald Fischer.

3 The field of designed experimentation or DOE is vast and considered a specialty of its own. Within the context of this book we will focus on Factorial Experiments . Factorial experiments work very well for injection molding . The use of any procedure (not just a DOE procedure) must be backed by a complete understanding of the underlying principles, which helps not only in the understanding of the analysis but also more importantly in the interpretation of the analysis. Factorial experiments, their analy-sis, and their interpretation are easy to comprehend and do not require a very strong math-ematical background.

4 Parameters in injection molding Applying and using DOE in injection molding is relatively simple compared to its use in other manufacturing or production processes, because here most responses to process changes are linear. For example, there is a direct relationship between part dimension and holding pres-sure. If the dimensions of a sample part molded at two holding pressures are known, it is safe to predict that the part dimension produced when molded at the average of these two pres-sures will lie at the average of the two dimensions, as shown in Fig. The same relationship exists between other responses and processing parameters.

5 In injection molding , all plastic material variables are related to speed, pressure, time, and temperature. The linear response can be explained with the help of specific volume versus temperature graph discussed earlier. In Fig. it is shown with the molding area corresponding to the injection , pack and hold phase of the molding cycle. In this area the curves are linear regard-less whether the material is amorphous or crystalline. A similar graph for a PBT-PC blend generated at different pressures is shown in Fig Such a graph is also called a PVT graph. 8 Design of Experiments for injection Molding146 Shrinkage is the change in volume as the plastic is being cooled.

6 Based on the linear rela-tionship between the volume and the temperature it is safe to assume that the part dimen-sions also respond linearly to process parameters such as temperatures and pressures. Faster mold fill rates will result in lower heat loss in the melt before it reaches the end of fill. There-fore, the plastic characteristics are in the top right hand side quadrant of the PVT graph, but are still in the linear area. Increasing or decreasing the fill speeds will result in a proportional change in dimension. During the cooling time, the melt is now out of the shaded molding Figure Relationship between holding pressure and part dimensionFigure Specific volume versus temperature graph showing the area corresponding to the injection , pack and hold Parameters in injection molding 147area shown in Fig The melt begins to solidify and the volume begins to decrease follow-ing the plot shown in Fig The plot is linear until the plastic reaches its glass transition temperature (Tg) where the curve exhibits an inflection point.

7 If the part is ejected above its Tg, the relationship between the specific volume and the temperture is linear. If it is ejected below the Tg, there will be some non-linearity. For efficient injection molding , parts must always be ejected at a material-specific ejection temperature that is always above the Tg. If the parts are being ejected below the Tg, the molding is not efficient and additional time is added to the cycle unnecessarily. The parts must always be ejected above or close to the Tg, keeping the cooling curve in the linear region of the PVT graph. The plastic will continue to shrink and therefore a post-mold shrinkage study must be done on the parts.

8 If the molding process was robust and consistent, the post-mold shrinkage will also be consistent, produc-ing consistent parts. The application of the above concept is discussed in the following exam-ple explaining the concept of DOE. A note on the proportional changes mentioned above is required here. These relationships can be directly or inversely proportional to each other. For example, packing pressure can increase the length of a part, but in some cases, such as an internal diameter of a part, there can be a decrease in the diameter of the part with increase in the packing pressure.

9 Predic-tion is not easy and experimentation is of Experiments: DefinitionThe simplest description of Design of Experiment (DOE) is a planned study. For example, studying the effect of holding pressure on the length of the part is a designed experiment. The length of the part at a low holding pressure and a high holding pressure is measured and Figure PVT relationship for PBT-PC blend (Courtesy: Sabic Innovative plastics )8 Design of Experiments for injection Molding148is then plotted as a function of the holding pressure, see Fig This is the most basic DOE that can be considering two parameters, holding pressure and melt temperature, and their influ-ence on the length we need to perform four experiments and determine the length at the fol-lowing holding pressure / melt temperature combinations.

10 Low low, low high, high low, and high high of the, see Fig. we add one more parameter to this, for example mold temperature, we end up with eight necessary experiments. We are repeating the above four experiments at a low and high value of the mold temperature, resulting in eight experiments, as shown in Fig. As the number of parameters to be studied increases, the number of experiments Effect of holding pressure and melt temperature on part length Figure Study of effect of holding pressure on part Terminology Terminology Factor Any input to the process is a factor. Therefore, all processing parameters that are input to the molding machines are factors.