Plastics are a class of materials with diverse characteristics, low cost per unit volume, and relative ease of conversion into finished goods. In industry practice, value engineering techniques have consistently found that plastic components provide high function per unit cost [1], motivate further materials development [2], and have undergone “explosive” growth [3]. Such commercial growth, however, has permitted inefficiencies in plastics manufacturing that are no longer acceptable in the marketplace. Current issues now threatening plastics manufacturers include continued global competition [4], increases in feedstock and commodity prices [5], and surging environmental awareness [6]. As a result, plastics manufacturing systems need to be well engineered, make optimal use of human and natural resources, and provide competitive yet socially responsible solutions.
Advances in the plastics industry have been fueled by sustained improvements in polymeric materials, product design, and process technology. Yet, requirements for plastics manufacturing processes continue to advance in tandem. This chapter provides an overview of common plastics manufacturing processes. The goal of Section 2.1 is only to briefly describe these processes so that their common characteristics can be recognized. Section 2.2 characterizes these processes with respect to machine and control system design and operation. Section 2.3 then introduces the concept of performance measurement with respect to process control, and provides some important considerations relative to plastics processing.
An actuator is a device that uses power to change a process state in accordance to a provided control signal. As shown in Figure 3‑1, actuators are usually placed between the controller and the process. Many actuators are often used in a single plastics manufacturing system to produce heating, cooling, flow, pressure, rotational motion, linear motion, etc. In most processes, the power delivered by these processes is on the order of kilowatts while the power delivered by the control signal is on the order of milliwatts. Because of this discrepancy, actuators usually require an external power source in parallel with the control signal to fulfill the commanded process changes. In the next section, common specifications for actuators are discussed. Afterwards, the chapter describes the design and operation of common heating and cooling systems used in plastics processing.
Hydraulics and pneumatics are fluid1 power systems widely used for mechanical actuation of plastics manufacturing processes. Hydraulic and pneumatic systems have several advantages compared to electric motors and mechanical linkages. Perhaps the single most significant advantage is that hydraulic and pneumatic actuators provide the highest power density, which means that relatively small actuators can simultaneously provide high actuation velocities and output forces.
Electric motors have become increasingly common in plastics manufacturing processes for a variety of reasons including their improved response time compared to hydraulics and pneumatics, higher operating speeds, silent operation, energy efficiency, and other perceived benefits. For these reasons, electric motors are being increasingly used as the primary movers in plastics processing machinery [115]. Still, electric drives are not uniformly best in all applications since they have lower power density than hydraulics and relatively high costs compared to pneumatics.
Nearly all plastics manufacturing systems rely on sensors to provide feedback about the state of the process so that the controller can update control signals to the machine actuators. The term “instrumentation” more broadly refers to an electrical device that provides measurement capabilities, and so consists of not only process sensors for measuring physical states but also signal conditioners for amplifying, filtering or otherwise converting electrical signals, and even electrical controls such as relays and solenoids in control loops [145]. This chapter focuses on the most common sensors used in plastics processing as well as their underlying sensing principles. First, however, common sensor specifications will be presented to provide a basis for measuring sensor performance and comparing different types of sensors.
Process sensors output an electrical signal in response to the changing process states. The electrical signals are not usually in a form that is directly usable by a controller or operator. As such, signal conditioning may be required to convert, amplify, adjust, filter, or linearize the sensor’s output. This chapter assumes basic knowledge of electrical circuits, and provides some simple circuit designs for illustration and practical use.
Modern plastics machinery use digital microprocessors executing programmed instructions to perform their process monitoring and control functions. This modern computing architecture provides too many advantages to ignore including low cost, customized programmability, data storage, advanced user interfaces, network connectivity, and others [197]. Until this point in the book, however, all of the previously discussed sensors and signal conditioners have provided “analog” signals that vary continuously within a given signal range. These analog signals can not be directly read by a digital microprocessor. As such, all but the simplest control systems rely upon data acquisition systems to acquire analog and digital information, convert and communicate this process information to the process controller, and subsequently output command signals to the machine’s actuators.
Plastics manufacturing processes must control and coordinate many different analog and digital signals in order to plasticate, form, and solidify the polymers into the desired plastic products. Older machinery may have relied upon multiple, loosely coordinated control loops operating independently. However, increased manufacturing productivity requires tighter coordination and supervision of the process states. Fortunately, advances in electronics has greatly facilitated the development of modern plastics machinery with greater consistency and higher throughput while also being easier to use, safer, and providing many advanced capabilities.
As described in the last chapter, controllers in plastics processing machines typically have many digital and analog inputs and outputs. The primary added value of the controller is to determine the appropriate control actions given the state of the sensed inputs. Many if not most of the control actions are related to discrete events. Examples of discrete control actions include turning on a pump when a button is pushed or sounding an alarm when a safety gate is opened. The control laws for such discrete events can be readily implemented in ladder logic or software if-then statements.
Plastics manufacturers must develop and maintain processes to meet cost and quality targets. In theory, plastics manufacturers would establish an optimal process which would then continuously operate without production of defects. In practice, plastics manufacturing processes exhibit significant variation for many reasons. As a closed loop process, the consistency of a process state is constrained by the quality of the feedback signal as well as the response of the controller. Even if the control system was perfect, however, variation can enter the process by the operation of the machinery itself.
The productivity of plastics manufacturing systems is jointly determined by the design and operation of the processing machinery. In many if not most processing applications, the cost or quality of the manufactured products can be reduced without any physical change to the machine design. The suggested optimization methodology is shown in Figure 12-1. The model parameters and uncertainty are provided from the process characterization described in Chapter 11. Given these models, the capability of the process is assessed to estimate the potential yield of the process given specifications on the product quality.
Quality control techniques are vital to ensuring the consistency of incoming received materials, internal manufacturing processes, and outgoing manufactured products. A flow chart of a quality control methodology containing several common quality control methods is shown in Figure 13-1. The materials received by the facility for use in their manufacturing processes typically undergo acceptance sampling. If the materials do not meet specification, the lot should be rejected. As the plastics manufacturing process operates, on-line metrology of the manufactured data provides data regarding key quality attributes. This quality data can be used with a feedback controller to automatically adjust the process settings to regulate the product quality. Many plastics manufacturing systems do not use on-line metrology systems and so these elements are indicated with dashed lines.
Automation is the technique of making a process or system operate automatically without human intervention. There is no doubt that automation has become integral to modern plastics manufacturing [363]. Yet, the automation of manufacturing processes needs to be considered during the product development since it may affect the part design, tooling, sensors, control system, and selection of the process conditions. However, effective automation requires a level of process consistency that many plastics manufacturers never attain. For this reason, the topic of automation is provided at the end of the book to suggest that the design and performance of the plastics manufacturing systems be optimized prior to attempting automation.
