How to Overcome Blow Molding Design Limitations

Overcoming Design Limitations

The necessity for, and the problems in obtaining, uniform wall thickness in blow molding part causes the molder to consider many special techniques. Some of the concepts to be reviewed in this section are production realities while others have yet to be reduced to practice. Each idea is unique in its own right but each is designed to yield a blow-molded part that has a wall thickness more uniform than that produced by the standard extrusion blowing process. Injection blowing is of course not as restricted in this regard, since the injected parison can be produced with varying wall thickness. This is because material is first injected into a closed mold and thickness is controlled by core and cavity mold dimensions. In using this concept one has only to use a thicker wall for the sections of the part that must undergo the greatest blow-up ratio.

Elliptical parisons for rectangular shapes in blow molding are a practical reality. The extruding of an elliptical shape by using an elliptical pin in the tubing die will place extra material in critical areas needed for high blow-up. The shape or design of the die pin can be tailored to give the material thickness profile desired. This concept of special design can be used on practically any blow-molding machine, regardless of style.

Parison temperature control to yield hot and cold sections in desired areas will produce controlled non-uniform stretch of the tube. The strategic location of cool air jets will set the tube and prevent excess thinning in these areas. Parts such as a narrow-neck，wide-bottom flask will require circular jets to chill the full ference of the parison to prevent out-of-roundness of parts. The cooling force should be directed toward the area that is to have the greatest blow-up ratio. Parison temperature reduction can also be used to produce parts with a heavy side for industrial applications such as housings. This is done either by the use of cooling air or by allowing the back of the parison to chill against the mold before blowing. In this case the heavy wall of the part can be used for attaching or mounting the blown unit to a frame or wall. This concept of blowing can be utilized on most blow-molding machines.

Pulsation of the parison tube is a third means of producing a nonuniform parison thickness for preferred wall thickness control. In this case a special head of the piston type is necessary to yield controlled pulses in the tube. Timing must be such that the thicker ring section of the parison coincides with the area in the part that will undergo maximum blow-up.

Parison sag can be used advantageously in machines of the freefall parison type to yield parts of more uniform wall thickness. The natural sag of the extruded tube will cause it to thin near the top and consequently place more material in the bottom of the mold. This concept has been used advantageously in container production to strengthen the bottoms of parts that must pass drop impact tests.

Offset openings are most readily produced at some blow molded part companies on rising mold type machines. The further refinement of tooling to incorporate a spinning mandrel will also allow the molding of internal threads on the necks of containers, using either an offset or center opening.

Trapped objects such as a small bell inside a blow-molded rattle can be accomplished when blowing over a mandrel. The object to be encapsulated is placed on the mandrel or blow pin and is blown into the part with the air.

Inserts in blow molding can be either of the solid or washer type. Washer inserts are used by placing them around the air mandrel and pulling the plastic over it during the blowing operation. Solid inserts must be locked in the part by means of an undercut. This can be done by mounting the insert in the mold and allowing the plastic to blow partially around the insert head. For applications such as screw-driver handles the blade is mounted in the mold through the pinch off area and the plastic is compressed on the shaft. The blade section in the plastic must have flutes or ears to prevent slippage during use of the part.

Platform blowing is a special technique available in machines such as the Eclipse-Williams-White for blowing large parts from heavy parisons. To prevent excessive parison sag the machine employs a table, which, after rising to meet the parison at the die face, descends with the tube, but at a slightly slower rate. This minimizes necking at the top of the tube and gives more uniform wall thickness of the blown part.