Pinch relief

Alignment dowel (2) pinch insert (3) head piece (4) pinch relief ... 

If filtration is too slow, the pressure may be momentarily increased by pinching off the relief valve. It is wise to keep a spare filtering tube in case of clogging. 

Reboiler temperature pinches. As column pressure increases from normal operating pressure to the relieving pressure, column temperature rises. This may reduce the temperature difference (or create a temperature "pinch ) in the reboiler. The pinching that can be relied on to occur can be credited toward the relief requirement. Each situation must be anal3rzed separately. Some common situations are ... 

The knife-edge-cutter width of the pinch-off depends on the resin used, the wall thickness of the blown part, the size of the relief angle, the closing speed, and the time when blowing starts. As a general guide for small parts... 

Relief angle In an injection or blow mold, the relief angle is the angle between the narrow pinch-off land and the cutaway portion adjacent to the pinch-off land. 

Fig. 28. Poor weld line (weak) pinch-off had knife edge relief angle was either too large or too small.

In bottle blow molds, there will be a hardened steel insert with a land of 0.076-0.13 mm (0.003-0.005 in.), a relief angle of 20° with a total depth of 0.76 mm (0.030 in.) measured from the inside bottom of the blow mold, and then a 45° cut to the bottom of the relief section in the pinch-off area. Normally, the total of this relief section will be 90% of the parison wall thickness to be pinched (see Fig. 31). This design will also minimize residual flash. It is best to design the pinch land at 0.25-0.4 mm (0.010-0.015 in.) and have metal to remove, if the pinch is not adequate. 

Poor pinch-off relief angle was either too large or too small. 

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