Semipositive

It is important to note that the effective density (r) must be everywhere semipositive definite so that all effective quantities are then properly defined. Within the WDA, the explicit enforcement of the sum rule [see Eq. (127)]... 

Similar to the requirement of the semipositivity on the WDA effective density pWDA WAD pSADA must be everywhere semipositive definite as well... 

However, this cp (r)-formulation is more general than the Y/p(r)-formulation, beeause (p(r) behaves truly like an orbital, with positive and negative regions, while /p(r) is everywhere semipositive. It is also interesting to notiee that Eq. (177) closely resembles Eqs. (47), (137), (147), (162), and (163). 

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 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 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. 

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... 

Theorems on the existence of periodic semipositive solutions of differenti-... 

In order to have a unique identification, a constraint on the stochastic force is imposed so the symmetric matrix in Eqn (15.102) to be semipositive definite ... 

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. 

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. 

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