Transfer molding ejection

Figure 25 Schematic diagram of an injection/transfer molding machine [9]. (a) Hydraulic separation unit for upper mold plate, (b) Hydraulic separation unit for middle mold plate, (c) Shuttle system with automatic sprue nipple removal, (d) Brushing unit for cleaning middle mold plate, (e) Hydraulic ejector for automatic ejection.

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. 

Figure 6.12 Transfer molding sequence (a) the mold is closed and material is placed in the pot (b) the plunger descends into the pot, causing material to melt and flow through runners into cavities (c) after cure, the press opens, the plunger retracts, and the parts are ejected with cull and runners.

Flow inside a mold cavity in injection, compression, or transfer molding ideally should be such that the cavity is completely filled while the material is still fluid. If such is the case, the curing under sustained cavity heat will proceed uniformly until the part is sufficiently rigid to withstand the rigors of ejection. But such is rarely the case. 

At the end of the cycle, when the mold is opened, the remaining polymer solidifies into a disk shape cull). It is removed as a single piece along with the sprue. The part is removed by lifting the middle section of the mold followed by ejection pin action. In the transfer molding method, it is desirable to keep the sprue and the cull attached, contrary to injection molding. 

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. 

In screw aggregates, the molding compound, as in injection molding, is plasticized with the rotating screw, after which the screw acts as a ram to eject the compound into the compression mold (feeding). In the latest technical developments, one or more twin-screw extruders are used, from which the plasticized material is transferred by means of robot handling technology into the compression mold. 

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