Molded plastic, ejection Molds

Jetting Jetting is a condition that results when the mold design has no immediate impediment to flow and the plastics is ejected into a relatively large open volume. This jetted material becomes a weak point on the product and a surface blemish that is difficult to conceal. 

They are then forced through a narrow die to form a hollow tube called a parison. A chilled mold is then clamped around the parison and inflated from the inside by air. The air pressure presses the parison against the mold, and it hardens in the shape of the mold. The mold then opens and ejects the HDPE bottle. The bottle is then trimmed and conveyed to the milk filling station. The waste plastic is ground for reuse. GHG emissions associated with the embodied energy of the packaging machinery may be calculated but typically fall near the 1% cutoff line and can be excluded (Cashman et ah, 2009). 

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. 

Injection molding is a high-volume production technique for turning out thousands of plastic parts per hour. An extruder melts and mixes plastic as above and then forces or injects molten material into a mold that has the shape of the final object. The plastic inside the mold cools, and the mold opens and ejects the part(s). The mold closes, and the cycle repeats. See Figure 8-2. Injection molding enables the mass production of complex shapes... 

Opening the Mold and Ejecting the Solid Product. When the plastic product has... 

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 main processing technique in the automobile industry is injection molding in which pressure is applied to make the polymer flow into a preferred form [3]. Plastics must be cooled below the Tm (melt transition temperature) before removal from the mold [3], The molding process follows a basic injection cycle consisting of mold close, injection carriage forward, injection of material, metering, retraction of carriage, mold open, and part ejection. This cycle has been in place basically since 1946,... 

Hold the melt in the cold mold while it sohdifies and then eject the solid plastic. 

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. 

In blow molding, a tube of molten plastic material, the parison, is extruded over an apparatus called the blow pipe and is then encased in a split mold. Air is injected into this hot section of extruded stock through the blow pipe. The stock is then blown outward, where it foUows the contour of the mold. The part is then cooled, the mold opened, and the molded part ejected. In very heavy sections, carbon dioxide or Uquid nitrogen may be used to hasten the cooling. This process is widely used in molding high- and low-density polyethylene, nylon, polyvinyl chloride (PVC), polypropylene, polystyrene, and polycarbonates. 

The mold is cooled, usually by water, to solidify the plastic. When the plastic object has been cooled sufficiently to maintain its shape, the mold is opened, along the mold parting line, and the object is ejected. The cycle time in injection molding depends on the size of the molded part, as well as on the molding conditions and on the thermal and mechanical characteristics of the plastic being molded. In some case, the cycle time can be as short as two seconds. 

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. 

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