Injection molding machine hydraulic operation

An injection molding machine is operated by hydraulic power and equipped with an dectric motor and hydraulic pump. The maximum hydraulic oil pressure is ca 14 MPa (2000 psi). A hydraulic cylinder opens and closes the mold and holds the mold dosed during injection another cylinder forces the screw forward, thereby injecting the mdt into the mold. A separate hydraulic motor turns the screw to plasticate, homogenize, and pressurize the mdt. Control of these movements is a combined function of the hydraulic and electrical systems (35—37). 

Safety device All machines are equipped (or should be equipped) with applicable electrical, hydraulic, and/or mechanical safety devices. Some of them, such as injection molding machines, have all three modes for safety operations. 

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). 

Movable platen n. The large back plate of an injection molding machine to which the back half of the mold is secured during operation. This platen is moved either by a hydraulic ram or a toggle mechanism. 

The only problem with this process are the shut-off valves in the mold, which are operated by a hydraulic system. Modern injection-molding machines supply an internal control for these valves but it is also possible to use an external hydraulic unit. The opening and closing of the valves can be controlled in dependence of time (e.g. starting from the beginning of injection) or of the screw position. Usually the reproducibility is higher when the screw position is used for control purposes. 

Plastics are no different in this respect than other materials. If steel, aluminum, and ceramics were to be made into a different complex shapes and no prior history on their behavior for that processing shape existed, a period of trial and error would be required to ensure their meeting the required measurements. If relevant processing information or experience did exist, it would be possible for these metallic (or plastic) products to meet the requirements with the first product produced. Experience on new steel shapes always took trial and error time that included different shaped high pressure hydraulic steel cylinders that failed in service when used in a new injection molding hydraulically operating machine (author s experience). 

Automatic press n. Hydraulic press for compression molding or an injection machine which operates continuously, being controlled mechanically, electrically, hydraulically, or by a combination of any of these methods. 

The process of injection molding plastic is really very simple. I ll describe the operation as it would happen in our machine. And you can study the drawings in figures 3 through 5 to help you understand the process a little better. Keep in mind that not all machines are alike. But they all have the same purpose and that is to melt plastic and force it into a mold to form a specific part or item. Not all machines are hand operated. Some are air driven while others may use a hydraulic ram. On the larger commercial machines softened plastic is... 

Since injection molding clamping functions are fast and can be dangerous to the operator, certain safety features are required. An interlocked sliding gate at the front of the machine protects the operator. The interlocks are electrical, hydraulic, and mechanical. 

Melting (plasticating) the plastic is accomplished in a plasticator (screw in barrel as described in Chapter 3). This melt is forced into a clamped mold cavity. The liquid, molten plastic from the injection cylinder of the injection machine is transferred through various flow channels into the cavities of a mold where it is finally shaped into the desired object by the confines of the mold cavity. What makes this apparently simple operation complex is the limitations of the hydraulic or electrical circuitry used in the actuation of the injection plunger and the complicated flow paths involved in the filling of the mold (Chapter 17). Finally opening the mold to eject the plastic after keeping the material confined under pressure as the heat in the melt is removed to solidify the plastic into the shape desired. 

To mold micron-scale precision parts with shot weights of only 0.0022 g, electrical operating IMMs are being used. Those who desire precise shot size (dosing) can buy electric injection machines with servomotors capable of very accurate screw positioning. With hydraulic IMMs, molders can install a valve gate that shuts when the correct amount of plastic has been injected into the mold. 

The machine is generally made of (a) a hydraulic system, (b) plasticating and injection system, (c) mold system, and (d) a clamping system. The hydraulic system delivers the power for the operation of the equipment, particularly to open and clamp down the heavy mold halves. The injection system consists... 

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