BLOW MOLDED PRODUCT DESIGN

Numerous standard rules apply to the blow-molded parts making and are valuable in preventing serious problems and complaints in the manufacture or use of the item. The unique features of the blow-molding process eliminate the need of costly assembly of parts such as found in injection molding, but special care must be exercised in part design. The selection of raw materials offers many advantages in properties and design, since the extrusion process is capable of handling high molecular weight polymers. This is not always true in plastic process equipment that relies on the injection of a molten material into a closed mold. The injection process, by necessity，uses thermoplastic materials of lower molecular weight and consequently reduced toughness and stress-crack resistance.

Blow Molded Part Design

Uniform wall thickness in blow molding is an extremely difficult goal to accomplish. One of the first rules to accept in part design is the use of minimum blow-up ratio to minimize nonuniform walls. A maximum ratio of 5:1 should be adhered to，and if possible all work should be within a ratio of 3:1. Blow-up ratio is defined as the relation of maximum part diameter to the diameter of the extruded parison. In applications such as containers, the parison size must be no larger than the neck size of the container to prevent neck pinch-off scars； but few other applications have this limitation. The use of an improper ratio of parison to part can result in excess material usage to satisfy minimum wall thickness specifications. Part design that demands excessive blow-up will also result in slow production cycles because of excessive cooling times of the heavy area, in addition to nonuniform walls.

Applications that do not require optimum stress-crack resistance may be better suited to injection blowing than to extrusion blowing to obtain reduced weight through more uniform wall thickness. Part design that utilizes a shape such as a pyramid or cone will usually result in an item of less weight by injection blowing than by extrusion blowing.

Details of design, such as angles, edges and comers, are of significance in blow molding. Sharp angles and sharp edges must be avoided and fillets and rounds should be employed in all corners and ribs. The use of a generous radius in all these design areas will prevent excessive thinning of the plastic and prevent molded-in stresses. In applications such as detergent containers it is important to employ generous chime radius to prevent thinning. The extremes of a heavy bottom pinch-off section and a thin bottom comer can result in stress cracking of the container.