How to obtain even wall thickness in extrusion blow molding (it ain't easy)

摘要

In the extrusion blow molding (EBM) process, a hollow tube of molten material (called a parison) is extruded out of a head and through a male and female part, commonly referred to as pin and bushing or mandrel and die. It might sound simple, but a whole host of factors must come together to let this parison inflate into a part of even wall thickness in both the axial and hoop direction. The melt coming from the extrusion screw must be homogeneous; the heat in the head must be even; the parison must be straight and have even wall thickness; the parison wall thickness must be programmed to reflect the container geometry; all parisons in a multi-cavity system must have the same length; if the container is oblong, the parison wall thickness should reflect that; and venting must be sufficient to avoid any air entrapment.