The main task of parison heads (blow heads or even just heads) is to divert the compact melt flow as supplied by the extruder into a vertical downwards flow movement and to form melt tube with a uniform circumferential wall thickness distribution. These heads can be distinguished in different ways.

On the one hand, heads can be distinguished according to how a tube is formed from a compact melt stream, analogous to the extrusion dies used in pipe extrusion. Parison heads are also distinguished according to how the melt tube is ejected.

One way to form a tube is using a spider head, also called a central feed head. Here the core part, the so – called strainer or torpedo, is held in position by little arms, the spider legs. However, since every leg is a flow obstruction, leading to weakening knit lines of weld lines, strainers with offset spider legs are often used. Here the offset knit lines do not go through the entire wall thickness.

Advantages of Spider Heads

• Due to central feed, spider heads lead to a very even wall thickness distribution over the circumference
• Distribution behavior is largely independent of material and operating point, which means that different materials with (within limit) different viscosity and thus different flow behavior can be processes. Likewise, different temperature, pressure or throughput settings have no great influence on the circumferential melt distribution in the parison.
• Low flow resistance
• Quick color change
• Low pressure loss

Disadvantages of Spider Heads

• Knit lines in the wall and bar markings on the surface
• Comparatively high manufacturing costs
• Complex wall thickness control
• Bores for support air, meant to prevent the parison from collapsing or to pre-blow and thus pre-stretch a parison before closing the mold, have to go through the spider legs.
• Multilayer heads are not feasible as a pure spider head solution. Here, combination solutions with mandrel or spiral distribution are required.
• For strength reasons, the maximum possible diameter of spider heads is limited, as the slender spider legs cannot hold heavy strainers.