Early blow molding had a dilemma; plastic would continue to be extruded even when the mold was closed for blowing. As the industry and technology have evolved, three solutions to this problem have been invented.
The first is an extrusion head that “bobs”, or raises up after the parison has been cut by the mold to ensure there is enough clearance, and then lowers again for the next cycle. The second is a mold that moves out from under the nozzle after it cuts the parison, and blows the mold in an adjacent blow station. In these two configurations, the parison is continuously being extruded, so we call it Continuous Extrusion.
The third method involves accumulating the plastic material and ejecting it from the extrusion head in one burst, which we call discontinuous extrusion. The molten plastic is stored in an accumulator head and ejected using a piston, with both the amount of material accumulated and time spent in the accumulator being dependent on the properties of the container and the material.
For example, very large containers require a higher amount of material, and as that material hangs in its parison form, it can begin to stretch, sag, or even break, which would negatively impact the quality of the final container. In these instances, accumulating the material reduces the time it spends in parison form, and thus gives it no opportunity for unwanted stretching. Operations that extrude heavy materials which require greater pressure often use this solution as well.
W. Müller accumulator heads work on a “first-in first-out” principle, where the material that enters the accumulator first is the material that is extruded first.
1. Ensure the parison length/speed is appropriate for the container's cycle time. If the container is small, you don't need a long parison.
2. If the container is large, ensure the parison is long enough to fill the mold.
3. If necessary, the accumulator will gather all of the plasticized material at once, and extrude it with a higher speed then continuous extrusion, limiting the time the parison spends in suspension.