Positive mold

Positive mold A projecting mold over which the product is formed, usually referred to as a male mold. 

P. Zanzonico, J. Campa, D. Polycarpe-Holman, G. Forster, R. Finn, S. Larson, J. Humm, C. Ling, Animal-specific positioning molds for registration of repeat imaging studies Comparative microPET imaging of F18-labeled fluoro-deoxyglucose and fluoro-misonidazole in rodent tumors, Nucl. Med. Biol. 33 (2006) 65-70. 

Three general types of molds are used for CM. In the positive mold (Figure 14.3a) all the material is trapped in the mold cavity. The pressure applied compresses the material into the smallest possible volume. Any variation in the weight of the charge will result in a variation in part thickness. In multicavity molds, if one cavity has more material than the others, it will receive proportionately greater pressure. Multiple cavities, therefore, can result in density variations between parts if loading is not done with some degree of precision control.1 278 284... 

In matched mold thermoforming (Fig. 10.9), matched sets of negative and positive molds are used to form the part from the softened sheet. Vacuum is generally applied through the negative cavity, as well. The result is excellent dimensional control, and the ability to form very complex shapes. This method is most often used in thermoforming of foams, due to their tendency to deform if not restrained. 

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 5,81 Cross sectional view of a multi-cavity semi-positive mold used for compression molding...

Clearance between two platens of a press in the open position. Mold daylight describes the opening distance of mold halves for part removal. 

Landed force A force with a shoulder that seats on the land in a landed positive mold. 

Positive mold n. A compression mold in which the pressure is applied wholly on the material, and which is designed to prevent the escape of any molding material. 

The positive mold should not be used generally for simple designs or for parts made from free-flowing material because other types of molds are better adapted for these jobs. An important disadvantage in the use of the positive mold results from the plunger rubbing the side of the cavity wall. This scores the cavity wall and may mar the molded piece as it is ejected. 

If the sheet is formed over the male or positive mold and the inner surface of the thermoformed part touches and replicates, the mold is called positive forming (Figure 10.2a). In other way, if the thermoformed part is in the cavities of a female or negative mold and the outer surface of the part replicates, the mold is called negative forming (Figure 10.2b)... 

Fig. 9. Schematic of vacuum draw box prestretching, followed by male or positive mold insertion (18).

Basically, the process is very simple. The sheet is either clamped or framed, or held in place some other way, during the heating process which converts the sheet into a soft material that is easily formed by very low pressure—e.g., by atmospheric pressure against a vacuum or slight air pressure. The mold can be either a female cavity or a positive mold. Based on this very simple procedure, many varieties of sheet-forming techniques have been developed to obtain more accurate wall-thickness distribution and the capability of forming deep-drawn articles with uniform distribution of wall thickness. 

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