Capturing the Parison
In order to successfully design a blow-molded product, one must understand the interactions between the parison and the mold. The most basic forms for blow molding are the tube, sphere or football shapes. Since the parison is nearly this shape to begin with, there is no trouble obtaining excellent material distribution for these shapes. It is not necessary to capture the parison at all points along the perimeter of the mold cavities, or "pinch-off", since the expanding bag inside the mold can move in all directions with equal ease and distribution. Cavity width (W) to depth (D) limits are determined by the elastic properties of the material.

If the part shape is flattened to a disc- or panel-type part, the parison must be captured by the entire pinch-off. If the parison does not extend to all areas of the pinch-off, it can extend only a short distance on one axis and a longer distance on the others, causing very uneven material distribution.

Once the complexity of the part progresses to a double-wall tray shape with side walls, the parison must not only be captured at all points along the pinch off, but must also meet the material thickness needs for a variety of molding conditions. Many of the design criteria used to make a complex tray will be the same for designing a complex industrial part, functional part, or carrying case. The inner and outer walls of a tray are formed simultaneously and integrally, but interior and exterior designs are essentially independent.