Clamping pressure injection-molding machine

Closing the Mold. One half of the mold is attached firmly to the melting/feeding end of the injection molding machine. The other half slides horizontally to open or close the mold. The moving half is driven mechanically or hydraulically. When it closes the mold, it is held firmly by clamping action to withstand the injection pressure. 

In creating a process window for the combination of material, machine and heater used in this study, the first consideration was the moldability, which was dictated by the maximum injection pressure and the maximum clamp force. No changes in melt and mold temperature, injection velocity, or gate size could remedy this situation. Thus, an injection molding machine with higher injection speeds, pressures and clamp force was required to mold thin-walled syndiotactic polystyrene. 

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). 

The size of injection molding machines is usually specified in terms of injection capacity and clamp force. These factors are not directly related. Other important variables are the L D (length to diameter) ratio, barrel size, plasticizing rate, injection rate, and injection pressure. Major components of injection molding machines include the injection unit, the clamp unit, and the machine base which contains the power and control units. 

The main component of an injection-molding machine are (1) the injection unit which melts the molding material and forces it into the mold (2) the clamping unit which opens the mold and closes it under pressure (3) the mold used and (4) the machine controls. 

A typical arrangement for a direct screw-transfer injection-molding machine for thermosets is shown in Figure 2.16. The machine has two sections mounted on a common base. One section constitutes the plasticizing and injection unit, which includes the feed hopper, the heated barrel that en closes the screw, the hydraulic cylinder which pushes the screw forward to inject the plasticized material into the mold, and a motor to rotate the screw. The other section clamps and holds the mold halves together under pressure during the injection of the hot plastic melt into the mold. 

The purpose of the injection unit is to heat and melt the polymer, inject the melt into the cavity, and apply pressure during the cooling phase. The most common type of injection molding machine is the reciprocating screw. In this type of machine, the screw rotates to plasticize the polymer, moving backward to deposit a volume of polymer melt ahead of the screw (shot). Once the correct shot size has been built, the screw then moves forward to inject the melt into the mold. Injection molding is a discontinuous process, and the clamping unit allows for the mold to open and close for part removal and to provide pressure as the cavity is filled. This is depicted in Fig. 1.19. 

---