Mold cavity unbalanced

Unbalanced cavity layout in multiple cavity mold Nonuniform cooling not properly appKed Poor or no venting... 

Unbalanced cavity pressure buildup, mold distortion, dimensional variation/poor shrinkage control, stresses, flash, etc. 

Once the melt arrives at the sensor, a temperature rise takes place that can be detected with the aid of intelligent electronics and can be used for various open and closed-loop controls. The basic idea is that a melt with the same viscosity must be at the same time at the same place. If the viscosity of the melt changes, the sensor position is reached sooner or later, so temperature increases occur sooner or later. In this manner, the variation of the viscosity can be indirectly monitored. Figure 5.19 shows the time profile of eight cavity temperature curves, where each signal was measured in a separate cavity (8-cavity mold). It can be clearly seen that the filling time of the first cavity filled ends about 1 second before the last cavity filled, which corresponds to a completely unbalanced filling process of a multi-cavity moid. 

The problem of differently filled or unbalanced cavities in multi-cavity hot runner molds as shown in Figure 5.24 is well known in practice. 

Unbalanced cavity in multi-cavity mold Adjust runner or gate size... 

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-... 

ISO 3167 specifically deals with the production of multipurpose test specimens. It requires the use of a balanced, two-cavity mold with a defined gate design to prepare test specimens used to measure properties of plastics materials. Two identical cavities with large gates and a balanced runner system promotes filling and packing consistently and nniform orientation, minimizes shear effects, and produces uniform specimen. Figure 20-2 illustrates differences between old style, multicavity, unbalanced family mold and ISO test specimens (6). 

Although there are 4 cavities in one mold, only one cavity ( 1 in Fig. 5) was taken for the measurement in order to get consistent data. The rim thickness was the average of the five samples. The experiment was carried out by short shot first and showed that the runner system has unbalanced melt flow (Fig. 5). Some portions are filled whereas some are only partially filled. The injection molding process was done by the conventional method first, and then the microcellular process was introduced as the foaming molding method. Fig. 6 shows the rim thickness of the roller by conventional molding. The thickness variation is more than 0.10 mm, and points 4 5 have the least thickness. 

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