Blow Ratio - W > 2D
The exterior design accounts for the actions of the parison within the mold cavity portion of the tooling. The basic exterior dimensions of double wall parts must allow enough material to enter the mold to adequately form the part. Since half of a cylindrical parison is used to form the exterior half of the part and since there is no flow of material along the mold walls, it follows logically that the depth (D) of the cavity should be no more than one-half the length or width (W) of the cavity. A part design utilizing cavity depths that exceed this relationship will be subject to severe thinning. With divided cavities each cavity should meet this W > 2D requirement.

The design of certain complex parts will require changes in the parting-line location in order to stay within this relationship. These steps in the parting line or pinch-off must include provision for repeated opening and closing of the mold halves. A step on a horizontal parting line (90°) that is too vertical will leave insufficient clearance to ensure that a slight misalignment will not result in extensive mold damage. Because of this, 10° draft or greater is generally designed into mold parting line steps. When 10° draft is not possible, the entire mold should be angled in the machine so that the horizontal and vertical sections of the parting line form a positive draft relative to one another. This can only be accomplished on two opposing sides and may be further restricted.