The Injection Molding Machine

The plasticating and injection unit. A plasticating and injection unit is shown in Fig. 3.45. The major tasks of the plasticating unit are to melt the polymer, to accumulate the melt in the screw chamber, to inject the melt into the cavity, and to maintain the holding pressure during cooling. 

The mold cavity. The central point in an injection molding machine is the mold. The mold distributes polymer melt into and throughout the cavities, shapes the part, cools the 

During mold filling, the melt flows through the sprue and is distributed into the cavities by the runners, as seen in Fig. 3.49. 

Key Questions What types of injection molding machines exist  

What is the required size of an injection molding machine  

3 Special Injection Molding Processes Review Questions 

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Processing and Application. Polystyrene and the other thermoplastics would not have been able to enter on their triumphal march if the manufacturers of processing equipment had not been able to provide efficient fabrication methods. A decisive factor was that between 1950 and 1960 the injection molding machine with a screw preplasticizer was developed and extrusion through slit dies to give sheeting or film followed by thermoforming was evolved. 

Injection molding allows the rapid, economical production of small to large parts. It provides close tolerances and the same machine can be used to mold many different articles. Parts can be molded combining the polymer with other polymers and with any number of additives. Further, it can be nm so that various parts can be easily married as part of an entire or combined-parts production assembly of an article. The ability to easily modify the operating conditions of the injection-molding machine is important because of the variety of articles that may be needed, variety of material employed to produce the same (general) article, variety of materials to produce different injection-molded articles, and the variability of supposedly the same polymer material from batch to batch. 

William Willert filed a patent on the in-line, now more commonly known as the reciprocating screw injection molding machine in 1952. In 1953 Reed Prentice Corp. was the first to use Willert s invention, building a 600-ton machine. The patent was issued in 1956. By the end of the decade almost all the injection molding machines being built were of the reciprocating screw type. 

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. 

The SCORIM unit is placed between the nozzle of the injection molding machine and the mold (Figs. 14.11-14.13).The gates (i.e., the sprues and runners) are located so that all potential weld lines are placed between them from the flow point of view. 

Starting at the end of phase A, the mold and the injection molding machine are separated, allowing each polymer to be plasticized according to its specific needs. 

Melt homogeneity in injection molding was further enhanced with the introduction of motionless mixers in the early seventies (37). These devices are placed in the nozzle of the injection molding machine. The mixers enhance the radial uniformity of temperature and composition of the melt. The devices are good for getting better cavity filling and also for saving on color concentrate. 

Thorough cleaning of the injection molding machines is very important. If the parts are made of non-corrosion-resistant metal, cleaning prevents corrosion of the process surfaces. The residual polymer... 

The injection molding machine should be capable to deliver a shot size of 50% or less of the injection stroke. Typically, the machine should be able to inject its rated capacity in less than 2 seconds, except for high viscosity resins or when warping or jetting must be prevented by slower injection rate. Depending on the grade, the cylinder temperature setting should be between 190 and 220°C. 

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). 

Injection molds have two main parts, or platens. Typically the half containing the hollow cavity side is attached to the injection molding machine, and is referred to as the fixed platen. Because the melted plastic enters through this side, it is also called the hot side, or the hot half (Fig. 11.2). 

The section of the mold containing the convex core is usually the piece that moves back and forth to open and close the mold, and is referred to as the moving platen. In some designs, especially in stack molds, the mold cavity is mounted on an intermediate section called the floating platen, rather than directly on the injection molding machine. 

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. 

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