Melting mold operation controls

The dynamics of the molding process are determined through control of different but related machine elements such as motors, heaters, servovalves, etc. These machine elements are typically controlled via a hierarchical closed loop control architecture as shown in Fig. At the innermost level, only the machine elements are regulated by real time comparison of the desired machine set points with the machine feedback, such that the difference (or error) is used to correct the process. At the second level, state variables such as melt temperature and melt pressure are controlled to track prespecified profiles and provide more precise control of the state of the melt. At the outermost level, the machine inputs are adjusted by the machine operator to improve the quality of the part through specification of better set points given feedback of part quality. 

Ultimately, the reciprocating screw system was developed. Adaptation of such units led to improved rates of softening or melting of the plastic, closer tolerances on short size (amount of plastic to mold), better control of temperatures and more reliable overall performance. They are also highly effective for vented operation. 

An iajection mol ding machine is operated by hydrauHc power and equipped with an electric motor and hydrauHc pump. The maximum hydrauHc oil pressure is ca 14 MPa (2000 psi). A hydrauHc cylinder opens and closes the mold and holds the mold closed duriag iajection another cylinder forces the screw forward, thereby injecting the melt iato the mold. A separate hydrauHc motor turns the screw to plasticate, homogenize, and pressurize the melt. Control of these movements is a combined function of the hydrauHc and electrical systems (35—37). 

A major difference between extrusion and IM is that the extruder processes plastics at a lower pressure and operates continuously. Its pressure usually ranges from 1.4 to 10.4 MPa (200 to 1,500 psi) and could go to 34.5 or 69 MPa (5,000 or possibly 10,000 psi). In IM, pressures go from 14 to 210 MPa (2,000 to 30,000 psi). However, the most important difference is that the IM melt is not continuous it experiences repeatable abrupt changes when the melt is forced into a mold cavity. With these significant differences, it is actually easier to theorize about the extrusion melt behavior as many more controls are required in IM. 

TS melts in the plasticator and cure to a harden state in the mold that operates at a higher temperature than the plasticator (Chapter 1). For the best controlled machines used for molding TSs the heated plasticator usually includes a water jacket to ensure that the melt temperature profile is under control.3... 

The term IM is an oversimplified description of a quite complicated process that is controllable within specified limits. Melted or plasticized plastic material is injected by force into a mold cavity (Figure 4.1). The mold may consist of a single cavity or a number of similar or dissimilar cavities, each connected to flow channels or runners which direct the flow of the melted plastic to the individual cavities (Chapter 17). The process is one of the most economical methods for mass production of simple to complex products. Three basic operations exist. They are the only operations in which the mechanical and thermal inputs of the injection equipment must be coordinated with the fundamental behavior properties of the plastic being processed. These three operations also are the prime determinants of the productivity of the process since manufacturing speed will depend on how fast we can heat the plastic to molding temperature, how fast we can inject it, and how long it takes to cool (or solidify) the product in the mold. 

The operation of the injection and clamp units and other components of the injection molding machine (opening and closing of the mold and melting and injection of the polymer material) requires power, which is supplied by an electric motor. The orderly delivery of this power depends on auxiliary systems the hydraulic and control systems. The hydraulic system, the muscle for most maehines, transmits and controls the power from the electric motor to the various parts of the maehine. Maehine functions are regulated by a careful control of the flow, direction, and pressure of the hydraulic fluid. The elements of the hydraulic system for most injection molding machines are essentially the same fluid reservoir, pumps, valves, cylinders, hydraulic motors, and lines (Figure 11.8). 

Runners are the ehannels through which the polymer melt is fed into the mold cavities from the eyhnder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and uniformly. Control of the size of the runners provides a means of controlling the flow resistance and balaneing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold flame. Consequently, the ejected part is accompanied by the runner system, which must be removed and, in the ease of thermoplastics, reground for reuse. The use of the hot runner mold whereby the runner ehannels are heated to keep the polymer in the molten state, eliminates this need for plastic runner separation and avoids possible generation of scrap material. With proper machine operation, a hot rurmer mold requires a smaller amoimt of melt per shot than an equivalent cold runner mold, leading to redueed injeetion time and faster cycles. 

A mold must be considered as an important part of IM. It is a controllable complex device providing different functions or capabilities to permit molding the desired product (Table 3). If not properly operated, handled, and maintained, it will not be an efficient operating device. Under pressure, hot melt moves rapidly through the mold. Basically, temperature-controlled water (with ethylene glycol if the water has to operate below its freezing point) circulates in the mold to remove heat from TPs with TSs electrical heaters that are usually used within the mold provide the additional heat required to solidify the plastic melt in the cavity. Air is released horn the cavity to eliminate melt burning or voids in the products. 

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