Mold types Multicavity

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... 

Multicavity mold (multiple-cavity mold, multiple-impression mold) n. A mold having several to hundreds of cavities so that many parts may be molded with each shot. In many cases, the parts are identical, but that need not be so. In a type of multicavity mold known as a family mold, some of the cavities may be identical while others are different, or they may all be different. 

A multicavity mold is a mold with two or more mold impressions. This type of mold produces more than one molding per molding cycle. A multiple-gate mold is a mold that has many small gates into the side of restricted flow areas where fragile mold sections necessitate a minimum and distributed material flow. 

Runner layouts should be designed to deliver the plastic melt at the same time and at the same temperature, pressure, and velocity to each cavity of a multicavity mold. Such a layout is known as a balanced runner (Fig. 7.15). A balanced runner will usually consume more material than an unbalanced type, but this disadvantage is outweighed by the improvement in the uniformity and quality of the moldings. Balance in a multi-cavity mold with dissimilar cavities (known as a family mold) can be achieved by careful variation of runner diameter in order to produce equal pressure drops in each flow path. Such balancing can only be achieved efficiently by the use of computer flow simu-... 

Recent studies have contributed robustly to the realization that shear induced melt variations developed in runner sections will affect the manner in which a mold and part forming cavity will fill. These effects occur in, but are not limited to, multicavity molds. The filling patterns of single cavity molds are also affected by flow variation caused by shearing differences. These effects not only influence the way a single cavity mold will fill but how the polymers are oriented and consequently how the part will shrink and warp (1). The varying manner in which neat materials and filled materials shrink and how shear imbalances affect these two different types of materials is the focus of this study and subsequent write up. 

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