(239a) Integrated Product Design and Control in Manufacturing Processes

The primary objective of modern manufacturing processes is to produce, in a consistent manner, products with specific application-driven attributes. Since such attributes cannot be determined during production, manufacturers are clearly unable to assess the performance of products in their end-use application and have traditionally relied on measurable process and product variables as surrogate indicators of product attributes. Nevertheless, the only true indicator of whether or not a product has met its end-use performance requirements is actual feedback from end-use customers, but, for a variety of reasons, such feedback has, thus far, been completely missing from traditional control schemes. Our goal is to develop, implement and validate a framework for integrating product design with appropriate control strategies that explicitly incorporate customer feedback information to ensure that the products attain the desired end-use properties consistently. The framework integrates the determination of product specifications, consequent manufacturing process design, and required operating conditions with the development and implementation of a control scheme required to achieve specified manufacturing objectives. With an objective involving explicit control of end-use performance attributes, the resulting control scheme consists of three multi-rate feedback loops in which actual customer feedback information is explicitly incorporated.

The procedure is illustrated with an actual process for manufacturing polymer organoclay nanocomposites by melt intercalation, whose primary application is in packaging films. This presentation will focus on our current work which encompasses experimental process operating condition determination and a theoretical stability study of the control system.

The process conditions were determined experimentally using a fractional factorial experimental design to investigate the effects of key processing conditions (such as the organoclay concentration, mixing time, melt temperature, and extruder screw speed [1, 2]) on the mechanical properties of the nanocomposites such as secant modulus, elongation at break, and energy at break. The data analysis revealed that both the organoclay concentration and the interaction between organoclay concentration and melt temperature have significant effect on the secant modulus of the material, while the mixing time, once it exceeds 5 minutes, has no significant effect on the secant modulus of the nanocomposites. From these results we determined specifically that in order to obtain a material with desired high secant modulus, the process should be operated at a melt temperature of ~240°C, a mixing time around 5 minutes, and with organoclay concentration higher than 3wt%.

A theoretical stability study of the control system was conducted via simulation, where we investigated the effect of two distinct sample times, ts₁ for the inner loop (usually of the order of seconds/minutes in practice), and ts₂ for the outer loop (usually of the order of hours in practice), on the multi-rate cascade system's overall stability. (Since the sampling time for the outermost customer feedback loop is of the order of days?which is by far significantly longer than the slowest plant characteristic response time?it is assumed that this third sampling time will have no effect on overall control system stability). Upon assuming relatively simple nominal first-order-plus-time delay dynamics for the process characteristics in each loop, the simulation results indicated, among other things, that ts₁ exerts the dominant effect on overall system stability, while ts₂, beyond a critical value, exerts no effect on stability. These results imply that once the stability of the inner loop (concerned with the control of the frequently measurable process variables) is established for a given sampling time, the companion sampling time for the outer loop should be selected to be at least as long as the critical value necessary for overall system stability.

Reference

[1]. Dennis, H.R., et al., Effect of melt processing conditions on the extent of exfoliation in organoclay-based nanocomposites. Polymer, 2001. 42(23): p. 9513-9522.

[2]. Chavarria, F., et al., Effect of melt processing conditions on the morphology and properties of nylon 6 nanocomposites. Polymer Engineering and Science, 2007. 47(11): p. 1847-1864