Blow Ratio - W > D
Since the mold core must fit inside the cavity, there should be no question it meets the W>2D overall size requirement. Within the core there may be another consideration. If a double-wall tray-shaped part has a dividing wall between two compartments, this wall is formed by stretching the plastic into a groove in the mold core. If the designer wants a tall, narrow wall between two deep compartments then the groove in the mold core will be deep and narrow and this configuration will cause processing problems. As the plastic begins stretching into a deep, narrow groove, it begins to thin very rapidly. The plastic quickly reaches the point where it has thinned as much as possible and the internal air simply blows through the wall to the outside of the part. This will continually result in mis-formed rejects.

Because of this thinning, there is one simple yet absolute rule, which governs the design of the ribs or divisions between compartments. The depth (D) of the groove between core sections must not exceed the width (W) of the rib W>D. This rule also applies to any structure or stepped surface. For example, a 1"-tall round post in the center of a tray would have to be 1" or more in diameter.

If the part design requires a mold parting line which steps to various levels for the part to function properly, then the core must have a positive draft on these steps at the pinch-off to match the pinch-off on the cavity element of tooling. Varying pinch-off levels can change the W-D relationships of nearby pockets or ribs. All of the levels within a part must pass the W>D requirement in each direction.