Semipositive mold

The semipositive mold (Figure 14.3c) is by far the most popular. It combines the best features of the positive and the flash molds. Since its design includes a plastic material well of larger diameter, with a tight fitting force above the cavity, the material is trapped fairly positively and the plastic is forced to flow into all corners of the cavity. As the material picks up more heat and becomes fluid, it escapes between the force and cavity sidewalls as flash, allowing the force to seat on the land area. 

Another difference between the processes is the flash formation and its removal. As it is reinforced with fibers, it becomes difficult to remove the flash formed. As compression molding uses semipositive molds, there would be some material loss in the form of flash. 

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 Compression molds, (a) A simple flash mold, (b) A positive mold. Knockout pins could extend through plunger instead of through cavity, (c) Semi-positive mold as it appears in partly closed position before it becomes positive. Material trapped in area b escapes upward, (d) Semipositive mold in closed position. 

The clearance between the force plug and the vertical wall of the cavity in a positive or semipositive mold. Also the ring of excess material that escapes from the cavity into this clearance space. 

The semipositive mold serves a wide variety of applications. It is especially desirable for parts requiring a long draw, for parts that have a heavy section at the bottom of a deep piece, and for parts that must combine heavy and thin sections. This mold tends to produce a better surface finish on medium-impact materials because of their higher density, and also because the greater pressure forces the resin to the surface. Another advantage of the semipositive mold is that it produces a vertical flash line that may be removed easily by finishing on a belt sander. 

FIGURE 1.85 Semipositive Mold with lands (enlarged detail of Figure 1.86)... 

Figure 14.12 Pictures of a sample part with flash made from two different mold closures. (A) Flash from semipositive mold closure. (B) Excess material from flash mold closure.

FIGURE 14.19 Internally landed positive and semipositive molds (both shown in closed position). (Data from Vaill, E. W., Mod. Plast., 40, 767, September 1962.)... 

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

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

Tolerances on finished products 1. Fair to good depends on mold construction and direction of molding 2. Flash = poorest, positive = best, semipositive = intermediate Good close tolerances are easier to hold... 

Land (1) The horizontal bearing surface of a semipositive or flash mold by which excess material escapes. (2) The bearing surface along the top of the flights of an extruder screw. (3) The final shaping surface of an extrusion die, usually parallel to the direction of melt flow. (4, pi) The mating surfaces of any mold, adjacent to the cavity depressions that, when in contact, prevent the escape of material. 

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. 

Land. The portion of a mold which provides the separation or cutoff of the flash from the molded article in the screw of an extruder, the bearing surface along the top of the flights in an extrusion die, the surface parallel to the flow of material in a semipositive or flash mold, the horizontal bearing surface in a two-piece mold, a platform built up to the split line. 

Figure 14.11 Schematic view of semipositive and flash mold closure designs.

Semipositive Horizontal Flash. This is the most suitable design for automatic molds and is most common in the industry (Fig. 10.7). 

Semipositive Vertical Flash. This design (Fig. 10.8) is helpful in cases where a visual flash line scar may be present on the molded parts. The cost of this design is considerable because of the required proper fit between force and cavity. 

Figure 10.7 Semipositive horizontal flash mold design.

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