Molded plastic, ejection cavities

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. 

The feed system is an unwanted by-product of the molding process, so a further requirement is to keep the mass of the feed system at a minimum to reduce the amount of plastic used. This last consideration is a major point of difference between cold and hot runner systems. The cold runner feed system is maintained at the same temperature as the rest of the mold. In other words, it is cold with respect to the melt temperature. The cold runner solidifies along with the molding and is ejected with it as a waste product in every cycle. The hot runner system is maintained at melt temperature as a separate thermal system within the cool mold. Plastic material within the hot runner system remains as a melt throughout the cycle, and is eventually used on the next cycle. Consequently, there is little or no feed system waste with a hot runner system. Effectively, a hot runner system moves the melt between the machine plasticizing system and the mold to a point at or near the cavity(s).3 32> 326-332,490... 

The ejector pins should have an adequate cross-sectional area to minimize the possibility of distorting or puncturing the molded plastic at the time of ejection from cavities, as most of the thermoset compounds are slightly soft during that time. 

The "catalytic" molded plastic part is then inserted, either manually or automatically, into either (a) a second mold (tool) base or (b) a second cavity within the same mold (tool) base. The second molding cycle injects a non-catalytic (or "non-plateable") resin which overmolds (encapsulates) the non-raised background areas of the first shot plastic part. This second shot cycle completes the molding process. The finished (ejected) plastic substrate, as shown in Figure 6, contains predefined patterned areas of exposed "first shot catalytic" resin and areas of "second shot-non catalytic" resin. 

The hydraulic pressure or mechanical pressure is applied to make sure all of the cavities within the mold are filled. Plastics are allowed to cool within the mold. The mold is opened by separating the two halves of the mold. The product is ejected from the mold with ejecting pin. The runners and sprue are trimmed off and recycled. 

The sequence of events during the injection molding of a plastic part, as shown in Fig. 3.40, is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front of the screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected. 

Runners are the ehannels through which the polymer melt is fed into the mold cavities from the eyhnder nozzle. In a multicavity mold, it is necessary to fill all the mold cavities simultaneously and uniformly. Control of the size of the runners provides a means of controlling the flow resistance and balaneing the flow into the mold cavities. In most multicavity molds, the runners form part of the mold flame. Consequently, the ejected part is accompanied by the runner system, which must be removed and, in the ease of thermoplastics, reground for reuse. The use of the hot runner mold whereby the runner ehannels are heated to keep the polymer in the molten state, eliminates this need for plastic runner separation and avoids possible generation of scrap material. With proper machine operation, a hot rurmer mold requires a smaller amoimt of melt per shot than an equivalent cold runner mold, leading to redueed injeetion time and faster cycles. 

Under transfer molding, the mold is first closed. The plastic material is then conveyed into the mold cavity under pressure from an auxiliary chamber. The molding compound is placed in the hot auxiliary chamber and subsequently forced in a plastic state through an orifice into the mold cavities by pressure. The molded part and the residue (cuU) are ejected upon opening the mold after the part has hardened. Under transfer molding, there is no flash to trim only the runner needs to be removed. 

Compression molding is the most common method hy which thermosetting plastics are molded [ 1-3]. In this method the plastic, in the form of powder, pellet, or disc, is dried by heating and then further heated to near the curing temperature this heated charge is loaded directly into the mold cavity. The temperature of the mold cavity is held at 150°C—200°C, depending on the material. The mold is then partially closed, and the plastic, which is liquefied by the heat and the exerted pressure, flows into the recess of the mold. At this stage the mold is fuUy closed, and the flow and cure of the plastic are complete. Finally, the mold is opened, and the completely cured molded part is ejected. 

The mold for thermoplastics receives the molten plastic in its cavity and cools it to solidity to the point of ejection. The most is provided with cooling channels. The mold temperature is controlled by regulating the temperature of the cooling fluid and its rate of flow through the channels. Proper cooling or coolant circulation is essential for uniform repetitive mold cycling. 

IMM with an oil hydrauUc system provides the power to turn the screw to plasticate the plastic, inject the melt into the mold cavity, close the mold clamp, hold clamp tonnage, release the clamp, and eject the molded part. A number of hydraulic components are required to provide this power, including motors, pumps, directional valves, fittings, tubings, and oil reservoirs/tanks. See drive-system control hydraulic fluid influenced by heat, injection molding machine hydromechanical clamp See clamping, hydromechanical. 

The theory of stripping is quite simple. As the mold opens, and after the cavity has moved away from the core side, the ejection starts, caused by the stripper moving forward. In doing so, the plastic product is pushed over the hump in the core (Fig.4.52) this causes the plastic to expand so that the portion that is inside the groove in the core can slip out of the groove. 

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