Hydraulic Clamping Units

Figure 3.47 presents a schematic of a hydraulic clamping unit in the open and closed positions. The advantages of the hydraulic system is that a maximum clamping force is attained at any mold closing position and that the system can take different mold sizes without major system adjustments. 

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). 

Several techniques can be used for the clamping unit (1) hydraulic clamps, in which the hydraulic cylinder operates on the movable parts of the mold to open and close it (2) toggle or mechanical clamps, in which the hydraulic cylinder operates through a toggle linkage to open and close the mold and (3) various types of hydraulic mechanical clamps that combine features of (1) and (2). 

Clamping unit Mold Plasticating unit/ Controls, alignment (hydraulics injection unit system, electrical system)... 

To perform the mold movements, the clamping unit needs a drive mechanism. Two fundamentally different systems are used as drives. There is thus a basic distinction between mechanical clamping xmits and hydraulic clamping units. 

The toggle lever clamping unit is now the predominant mechanical system for injection molding machines. It is not a purely mechanical system in the strictest sense, because hydraulics are also used for the movement of the toggle lever in this system. In principle, however, it is a mechanical system. 

For this and other reasons, alternatives to the toggle lever clamping unit are in use. One example of such an alternative is the hydraulic clamping unit. 

Figure 5-6 Hydraulic clamping unit with travel- and clamping cylinder...

Besides the two main types of clamping units (mechanical and hydraulic), there are also combinations of both systems. All systems have advantages and disadvantages. In other words, there is no universal optimum design of clamping unit. The choice of system employed in the injection molding of plastics articles is ultimately determined by the the most cost-effective production method, e.g. single- or multi cavity mold and machine suitabdity. 

It has been shown with the example of the hydraulic clamping unit, how hydraulic modules are employed to execute the mold movements. 

Figure 6-2 Schematic diagram of a clamping unit, with the hydraulics highlighted (left mold closed right mold opened)...

The text focuses on many very important practical aspects of injection molding not (usually) found in treatments of injection molding intended for American engineering students. This includes the design of machine clamping units and hydraulic systems as well as the interaction of the plasticating and mold sections of the machine. Engineering students as well as technicians will find many sections of this book worthy of serious study. 

The improvements made in clamping units provide a great variety of movement and action in the larger BM machines. Small machines still need certain improvements to ensure good flash removal with low deformation of the clamping units. The most important development has been the use of proportional valves in hydraulic systems. With this technology, a machine runs more smoothly and more exactly to permit a wide variety of action in the mold, as well as accurate control of the closing speed (Fig. 4-7). 

In hydromechanical clamping units, toggles are combined with hydraulic cylinders. The toggle is used to open and close the clamp, but the hydraulic piston builds the clamp pressure. Since this requires small hydraulic cylinders, clamp movement is faster and the clamping units are smaller than comparable hydraulic units. However, hydraulic clamping provides better control of the clamp force. 

An all-electric machine has no hydraulic pumps. The toggles in the clamping unit are extended and retracted by a servomotor and a reduction drive gear is used to achieve the required forces. The clamps are much more stable since they have no hydraulic system to generate heat and the servomotors provide extremely accurate movement of the machine components. It is also much cleaner to operate than the other types of molding machines. All-electric machines are the machine of choice for most medical products. 

Clamping units support the mold, hold the mold closed during injection, open and close the mold as rapidly as possible, provide for part ejection, and provide mold close protection. The four types of clamps are hydraulic, hydraulically actuated toggle (mechanical), electrically actuated toggle, and hydromechanical. 

A 2CK) ton HPM injection molder base with toggle clamp. A 220 ton hydraulic clamp unit was also considered. The toggle clamp unit is expected to be less costly by approximately... 

Injection molding machine (Figure 6.1) consists of an injection and plastication unit, mold clamping unit, and hydraulic and control unit. In the injection unit, the polymer is melted by heating and shearing. The... 

In most cases, the compression movement is done by the clamping unit of the injection-molding machine. Modern machines with hydraulic or toggle clamping units are able control the position and the... 

In the injection molding machine shown in Figure 13, the clamp unit is on the left and the injection unit on the right. A mold is shown in position between the platens of the clamp unit with one half of the mold fastened to the fixed platen and the other to the movable platen. When the mold is opened, the movable platen moves away from the fixed platen and the molded part can be removed. After part ejection, the mold is closed in preparation for the next injection cycle. In contrast to extrusion, the screw in the injection imit rotates only during part of molding cycle. When the screw turns, it pumps melt forward. The rearward movement of the screw is controlled by the placement of a limit switch, which stops the hydraulic motor. The maximum length of reciprocation is approximately three screw diameters. The shot size is directly proportional to the amoimt of screw... 

FIGURE 1.141 Clamp unit with a barrel mold the mechanism to change the extrusion slot, using hydraulic actuators with an adjustable mandrel, is visible on the extrusion die... 

The force requirements ensure that the clamp unit must be engineered very robustly, but this conflicts with the need to open and close the mold rapidly to minimize production time. A variety of clamp mechanisms has evolved in the search for a suitable compromise. The two most common types are the direct hydraulic clamp (Fig. 7.2) and the toggle clamp (Fig. 7.3). Other variants include hybrid toggle/hydraulic types, lock-and-block systems, and electromechanical systems. 

Ejects the moulded parts from the tool. The mechanism is activated through the clamping unit. The ejection force may be hydraulic, pneumatic or mechanical. 

A clamping unit triggered by a hydraulic cylinder which is directly connected to the moving platen. The mould is opened and closed by application of direct hydraulic fluid pressure. This also provides the clamping force required to keep the mould closed dm ing injection... 

---