Injection molding machine functions

Figure 11 illustrates the separation of two processes in injection molding machine function plasticizing and forming. Figure 12, based on this schematic separation, demonstrates the injection molding process. 

Injection molding machine Function Mold cooling Robot and handling... 

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). 

The hardness build-up test was carried out on a one-half ounce laboratory injection molding machine. Using a 2.5" x 2.5" x 0.125" plaque mold held at a constant temperature, polymer melt was injected into the mold. The mold was opened at various times after injection, the piece removed, and its Shore A hardness measured exactly five seconds after opening the mold. The development of hardness could be followed as a function of quench time starting with quench times as short as 10 seconds. 

Injection molding is one of the processing techniques for converting thermoplastics, and recently, thermosetting materials, from the pellet or powder form into a variety of useful products. Forks, spoons, computer, television, and radio cabinets, to mention just a few, are some of these products. Simply, injection molding consists of heating the pellet or powder until it melts. The melt is then injected into and held in a cooled mold under pressure rmtil the material solidifies. The mold opens and the product is ejected. The injection molding machine must, therefore, perform essentially three functions ... 

These functions must be performed automatically under conditions that ideally should result in a high quality and cost-effective part. Injection molding machines have two principal components to perform the cyclical steps in the injection molding process. These are the injection unit and the clamp unit (Figure 11.4). We now describe the operation of the various units of the injection molding machine... 

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). 

Note that the feed mechanism of the injection molding machine is activated by the plunger stroke. The function of the torpedo in the heating zone is to spread the polymer melt into thin film in close contact with the heated cylinder walls. The fins, which keep the torpedo centered, also conduct heat from the cylinder walls to the torpedo, although in some machines the torpedo is heated separately. 

The individual structural units of injection molding machines and then-functions will now be described in greater detail. By way of an example, this description will refer to a machine used for molding CDs. 

To understand the functions of individual modules, it is best to trace the path of the polymer through the machine from the raw resin (usually in granular form) to the finished product. The basic principle is demonstrated with the diagrammatic cross section of a CD-producing injection molding machine in Figure 2-7. The complete path of the material from hopper to mold is highlighted. 

To clarify the tasks and functions of an injection mold, it is best to follow the path of the material within the injection molding machine aU the way to the development of the finished article. The material has been conveyed into the injection chamber during the course of the plasticating phase. Along the way it has been melted and homogenized. 

Figure 35-14. Tensile strength in blends of poly (styrene) and high-density poly(ethylene) as a function of the poly(styrene) content of the mixtures. The mixtures were made by mixing the granulates with a torpedo-equipped injection molding machine ( 0—0 ) or with the same machine with premixing in a kneader ( — ). (After C. D. Han.)...

There are two types of machines that can be used for injection molding—one fitted with a plunger and the other a screw type injection unit, the latter being preferred for carbon fiber applications. Section 17.9.2 explains the functions of the individual parts of an injection molding machine and describes the terminology used for this application. 

The minimum volume of plastic that will satisfy the structural, functional, appearance, and moldability requirements of an application is usually the best choice. This is in sharp contrast to machining operations, where one starts with a solid block of material and machines away only until what one needs to make the part function remains. Figure 12-12 illustrates this contrast between a machined valve body and a properly designed, injection molded, equally functional part [2]. 

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. 

Control functions for the molded parts within the mold and the injection molding machine can be provided. 

After assembly, the functional tests on the injection molding machine are performed (Figure 4.27). Here, all relevant features are checked in the process. These include function mold, part demolding, cycle, temperature, parameterization of the settings, flow patterns, injection pressures/hours/courses, and on the articles, wall thickness, optical designs, completeness, dimensional accuracy on gauges, etc. 

An injection molding machine (Fig. 7.1) can be divided into six components plastification/injection section, clamping unit, mold including the runner system, control system, and tempering devices for the mold. The most important part of the process is the mold, which is made of at least two parts to allow ejection (demolding) of the molded part. The injection molding process can be divided into six steps. Table 7.1 presents the main function of each step. 

Reciprocating screw intermittent extrusion blow molding machines use an extruder unit in which the screw is capable of axial as well as rotational movement. The function is virtually identical to that of the injection unit on an injection molding machine, except that the blow molding unit functions at a lower melt... 

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