Open flash mold

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. 

The three common types of mold designs are open flash, fully positive, and semipositive. 

The semipositive mold (Figure 2.2c and d) combines certain features of the open flash and fully positive molds and makes allowance for excess powder and flash [4]. It is also possible to get both horizontal and vertical flash. Semipositive molds are more expensive to manufacture and maintain than the other types, but they are much better from an applications point of view. Satisfactory operation of semipositive molds is obtained by having clearance (0.025/25 mm of diameter) between the plunger (top force) and the cavity. Moreover, the mold is given a 2-3° taper on each side. This allows the flash to flow on and the entrapped gases to escape along with it, thereby producing a clean, blemish-free mold component. 

FIGURE 14.17 Flash mold open, with preform in place, and closed, with flash forced out between mold halves. (Data from Vaill, E. W., Mod. Plast., 40, 767, September 1962.)... 

Figure 19 illustrates one type of compression mold suitable for molding lip type shaft seals with a metal outside diameter (O.D.). Figure 19a shows the compound preform resting on the shelf and the mold beginning to close. The mold continues to close until it gets to the position shown in Fig. 19b. Finally, the mold opens as shown in Fig. 19c. It can be seen that this molding technique produces a molded lip seal with the flash already removed. In general, when using a vacuum chamber on the press, to ensure that all the air and/or gases are evacuated from the mold after closure, one or...

A considerable amount of machining is done with thermosets because, with the exception of a couple of methods, the mechanical removal of molding flash is necessary in thermoset processes. Drilling holes and cutting openings is also commonplace in some processes because it is difficult, bordering on impossible, to mold them in for many designs. 

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