Closed molding

An iajection mol ding machine is operated by hydrauHc power and equipped with an electric motor and hydrauHc pump. The maximum hydrauHc oil pressure is ca 14 MPa (2000 psi). A hydrauHc cylinder opens and closes the mold and holds the mold closed duriag iajection another cylinder forces the screw forward, thereby injecting the melt iato the mold. A separate hydrauHc motor turns the screw to plasticate, homogenize, and pressurize the melt. Control of these movements is a combined function of the hydrauHc and electrical systems (35—37). 

Fig. 13. Injection molding machine A, hydiaulic motoi foi turning the screw B, hydraulic cylinder and piston allowing the screw to reciprocate about three diameters C, hopper D, injection cylinder (a single-screw extmder) E, no22le F, fixed platen G, tie rods H, mold I, movable platen , hydraulic cylinder and piston used to move the movable platen and supply the force needed to keep the mold closed and K, machine base.

To make a decorated plate, the mold is opened shortly after the main charge of molding compound has been pressed iato shape, the decorative foil is laid ia the mold on top of the partially cured plate, ptinted side down, and the mold closed again to complete the curing process. The melamiae-treated foil is thus fused to the molded plate and, as with the decorative lamiaate, the overlay becomes transparent so that the ptinted design shows through yet is protected by the film of cured resia. 

Excess flash—Indicates the compound flow was excessive during the mold closing process (shelf loaded compression molding) [3]. 

Figure 23 Four-cavity injection mold (a) mold closed, (b) mold open.

Return ejector pins to retraced position as mold closes for next cycle... 

As the size of the molded product becomes larger it is more difficult to control uniformity and to maintain a sufficient clamping force to keep the mold closed during filling. RIM overcomes these problems by largely carrying out the polymerization reaction in the mold. The most widely used RIM materials are PU and PU-reinforced elastomeric materials. Most of the automotive interior panels (such as dashboards) are produced using RIM. 

FIGURE 18.11 Steps employed in simple extrusion blow molding of a test tube. From left to right mold closed softened material introduced air or other gas injected forcing the softened thermoplastic against the walls of the mold and, after suitable cooling, the mold is opened giving the molded plastic test tube. 

Figure 7.77 Schematic illustration of compression molding process with mold open (left) and mold closed (right). From Z. Tadmor and C. G. Gogos, Principles of Polymer Processing. Copyright 1979 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

The choice of clamping system is very much dependent on the rate of production. Conventional threaded bolts are the simplest choice, but they may also be the most cost efficient for low volume production with cycle times in the range of hours. On the other hand, for high-volume production mold closing as well as mold opening must be achieved much quicker, and the choice may be a hydraulically driven mold carrier. 

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. 

Figure 3.47 presents a schematic of a hydraulic clamping unit in the open and closed positions. The advantages of the hydraulic system is that a maximum clamping force is attained at any mold closing position and that the system can take different mold sizes without major system adjustments. 

Fig. 14.15 Schematic representation of the blow molding process, (a) The extruder head with the blowing pin and open mold (b) the extrusion of the parison (c) the mold closed with the parison pinched in the bottom and sealed at the top (d) the inflated parison forming a bottle.

Blow molding — The process of forming hollow articles by expanding a hot plastic element against the internal surfaces of a mold. In its simplest form the process comprises extruding a tube (parison) downward between opened halves of a mold, closing the mold, and injecting air to expand the tube, which is pinched on the bottom. 

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... 

GmbH). A center mold plate separates the two processes and is supported by guide arms on the debars. During processing a robot loads a metal blank into the hydroform section and moves the shaped blank from the previous cycle to the IM side. The molds closing action drives the hydro-forming process, and the already shaped blank is overmolded with plastics. 

The process is used for high volume BM of very small containers such as pharmaceutical vials and whiskey bottles. A multi-cavity mold is used with an extruded parison whose circumference approaches twice the total width of the closely spaced cavities. Before the mold closes, the parison is stretched and semi-flattened laterally so that it extends across the full width of the cavities. The process is usually combined with blow/fill/seal techniques. 

The body mold closes on the parison that is blown normally by a neck calibrating blow pin. Immediately, with the mold still closed, the liquid contents are injected through the pin. The pin is then withdrawn and the neck is formed and sealed under vacuum by the neck-forming members. Both mold parts then open to eject a filled and sealed container. Small containers may be formed entirely by vacuum rather than blowing. 

FIGURE 14-49 Schematic diagram of insert molding part inserted (left), mold closes and plastic injected... 

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,... 

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