Mold types Semipositive

Figure 5.82 Example of mold types (a) positive compression mold, (b) flash compression mold, and (c) semipositive compression mold...

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. 

The same type of resin-treated material as described above can be shaped into a variety of products by modification of a compression molding technique. Such molded laminates have excellent properties similar to high-pressure laminates. Usually a semipositive type of mold is favored with ample clearance for flash around the mold area. Molding pressures range from 250 to 2000 psi with temperatures of about 150-200°C. The cycle depends on the nature of the formulation and its thickness. 

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