The Injection Cylinder

Modem designs fitted with non-retum valves through which the plasticised rubber compound passes into the injection cylinder portion are capable of giving precise shot volume. The non-retum valve is activated by the injection pressure. The injection ram/non-retum valve unit ensures complete purging of the injection chamber. 

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. 

Equation (7) for the accepted assumptions concerning the character of distribution of the material temperature is accurately fulfflled whenever the total time that the material is present in the injection cylinder (plastification time) and in the mold corresponds to the time of the viscous-plastic state of the material in any given temperature-time regime, i.e. when... 

The two wings have the same injection volume so that a single hot runner distributes the melt from the injection cylinder between the two mold cavities. The prior injection unit and the finished part are produced at the same time, whereby... 

When the die is filled, the plunger is returned to the up position, the molten metal in the nozzle flows back into the goose-neck, and the injection cylinder refills through the fining port. 

With the dies locked in the closed position, a measured quantity of molten metal is fed from the holding furnace into the injection cylinder. This operation can be automated but is usually done by hand, using a ladle of a size appropriate to the shot size required for the casting. The plunger is pushed forward, usually by means of a hydraulic cylinder, forcing the molten metal into the die cavity, where it quickly solidifies. Pressure on the metal varies between 350 bar and... 

An electrical control device that senses changes in voltage or a potential difference by comparison to a standard voltage and can transmit a signal to a control switch. Technique of premelting injection molding powders in a separate chamber and then transferring the melt to the injection cylinder. The device used for preplastication is known as a preplasticizer. 

A hardened steel insert in an injection mold that contains the tapered sprue hole and has a suitable seat for the nozzle of the injection cylinder. Sometimes called an adapter. 

It is also possible to evaporate solvent from the sample applied onto the needle in the injecting cylinder 8 without descending the needle of the injector into the chamber 9 (Fig. 2). This procedure is possible in the case of solvents characterized with low heat of vaporization. In this case, auxiliary gas flows through the inlet 30 over the ball valve... 

Both parts of the auxiliary gas stream unite in the injecting cylinder 8, from which, through the quartz tube 3, they are carried away to the outside of the injector, along with the solvent vapors, through the outlet 37 and the six-way valve 24. 

When the injector container is lifted, its side openings 4 (Fig. 5) remain in the injecting cylinder 12 within the lower housing. The cylinder has glass windows through which application of sample with a micro-syringe onto the filling of the container can be watched. To make this... 

Many molds, particularly those used for engineering thermoplastics, run at relatively high temperatures, for example, 80 C (176° F). If the mold is not insulated then, heat losses to the atmosphere, and to the machine, can easily equal those lost by the injection cylinder. So, insulate the mold from the platens and, if possible, insulate the outside surfaces of the mold. If a hot runner mold is being considered then, try and reduce heat exchanges between the hot runner parts and the cold, component-forming parts. 

The rate of decomposition of plastics, is dependent on both temperature and time. For example, a plastic may be degraded by a short exposure to a high temperature or, by a longer exposure to a lower temperature. How long the plastic is in the injection cylinder is therefore important. With some materials, for example ABS, the injection moldings may appear satisfactory on ejection but, on storage, colored areas (yellow/brown) may develop if the residence time is too long. 

The actual residence time may be determined experimentally by measuring the time taken for colored plastic to pass through the injection cylinder. May be calculated very roughly by the following formula ... 

Do not purge with another material as this could cause haziness. Turn off the heaters, withdraw the injection unit and slowly purge clean until the injection cylinder is empty, then leave the screw in the forward position... 

Nozzle na-z9l [dimin. of nose] (1683) n. In injection or transfer molding, the orifice-containing fitment at the delivery end of the injection cylinder or transfer chamber that contacts the mold s sprue bushing and conducts the softened resin into the mold. The nozzle is shaped to form a seal... 

The maximum injection pressure is the maximum available pressure for injection. In hydraulic machines, the injection cylinders are rated for a maximum hydraulic pressure, typically 14 to 21 MPa (2000 to 3000 Ib/in ). Pressure on the melt increases because the screw diameter is usually smaller than the diameter of the injection cylinder. Thus, injection pressure Pinj is given by... 

Compared to extrusion, injection molding is presented as a cyclical process. A complete cycle consists of the following operations, starting with plastic granulate material dosage in the injection cylinder, followed by fusion of this material until it gets injection consistency. After the fusion, the molten plastic material is injected into the closed mold and this injected material is cooled until its solidification. Finally, the product is extracted with the opened mold. ... 

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