Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 12

This book introduces the subject of process analysis, instrumentation and control for modern manufacturing in the plastics industry. Process analysis is the starting point since plastics processing is different from processing of metals, ceramics, and other materials. Plastics materials show an unique behavior in terms of heat transfer, fluid flow, viscoelastic behavior, and a dependence on the previous time, temperature and shear history which determines how the material responds during processing and its end use.

Many of the manufacturing processes are continuous or cyclical in nature. The systems are flow systems in which the process variables, such as time, temperature, position, melt and hydraulic pressure, must be controlled to achieve a satisfactory product, which is typically specified by critical dimensions and physical properties which vary with the processing conditions. Instrumentation has to be selected so that it survives the harsh manufacturing environment of high pressures, temperatures and shear rates and yet it has to have a fast response to measure the process dynamics. Many times the measurements have to be in a non-contact mode so as not to disturb the melt or the finished product. Plastics resins are reactive systems. The resins will degrade if the process conditions are not controlled. Analysis of the process allows one to strategize how to minimize degradation and optimize end use properties.

Linear systems in which there exists a one-to-one relationship between the input variable and the output response are the easiest to analyze and control. Plastics on the other hand show a nonlinear dependence on part/product cooling which varies with the square of the part thickness, laminar flow which varies with the cube of the wall thickness and mechanical strength/stiffness which varies with the cube of wall thickness. Also, wall thickness influences the crystallization, shrinkage, morphology and critical dimensions of the product.

In order to make corrections to the process, actuators, also known as final control elements, must introduce energy to the system. This hardware is in the form of servo valves, solenoid valves, servo motors, heaters, and blowers. The sizing, response time, ruggedness and linearity must be considered. All the above hardware has to be assembled into a system and programmed with a suitable algorithm to carry out automatic control. The control configuration and the algorithm are dictated by the system itself. Common control modes are feedback setpoint control which is common in extrusion, servo control which is common in injection molding and blow molding cyclical processes, and combinations and variations thereof.