Extrusion blow molding and injection blow molding
In extrusion blow molding, a head extrudes a tube of molten plastic called a parison. The mold closes on it, pinches it at the ends and air is blown in so the material copies the cavity. It is the usual process for polyethylene containers, containers with handles, small tanks and hollow technical parts such as ducts and reservoirs.
In injection blow molding, a preform is injected first and then blown inside the mold. It is used mainly for PET bottles and containers where the neck needs high precision. This guide focuses on extrusion blow molding, although many venting and cooling considerations apply to both.
The pinch-off
The pinch-off is the area where the two mold halves close on the parison, seal it and cut off the excess material. It is the part of the mold that takes the most mechanical work and one of the most influential on container quality: a poor seal leaves weak seams at the bottom, which is where leaks usually appear.
The pinch-off profile should be defined according to the material, the parison thickness and the process. Because it wears, it is best designed as a replaceable insert in a material harder than the mold body, so it can be renewed without remaking the whole cavity.
A replaceable pinch-off insert turns unavoidable wear into a scheduled maintenance task instead of a major cavity repair.
Venting and surface detail
When the parison inflates, the air between the plastic and the mold wall has to escape. If it is trapped, the material does not copy the cavity: you get hazy areas, poorly defined details, marks on engravings and lettering, or an uneven finish.
Venting is solved with the parting line, fine slots, porous inserts or a light blast of the surface, depending on the material and the finish required. Areas with relief, logos or deep corners need it most.
Cooling and cycle time
In most blow molding processes, cooling takes up most of the cycle. That is why molds are usually made of aluminum, which removes heat quickly, with cooling circuits close to the cavity. The thickest areas, such as the base and the neck, need more cooling capacity than the walls.
Uneven cooling produces warped containers or irregular shrinkage. The circuit design must balance cycle time with dimensional stability and leave access to clean the channels, because scale reduces efficiency over time.
Compatibility with the blow molding machine
The mold is designed for a specific machine. You need to know the platens, the stroke and clamping force, the head position, the blowing system and the water connections. With that data the outer size of the mold, its mounting and the position of the cavity relative to the parison are defined.
The number of cavities the machine accepts and how the flash is trimmed after blowing also matter. Reviewing these points before designing avoids molds that work on paper but do not mount or close properly on the plant floor.
Checklist before requesting a quote
☐ 3D model or drawing
☐ Volume and weight
☐ Material
☐ Neck, handles and engravings
☐ Platens and stroke
☐ Clamping force
☐ Head and number of cavities
☐ Water connections
☐ Pinch-off inserts
☐ Venting strategy
☐ Cooling circuits
☐ Surface finish
☐ Annual volume
☐ Target cycle
☐ Functional tolerances
☐ Container regulatory requirements
Common questions.
Why are blow molds usually made of aluminum?
Because blow molding works at low pressures and cooling dominates the cycle. Aluminum removes heat quickly and is easy to machine. In the areas with the most wear, such as the pinch-off, inserts of a harder material are used.
What causes leaks at the bottom of the container?
Often, a worn pinch-off or a profile that does not suit the material and fails to weld the seam properly. Parison temperature and closing speed also play a part, so mold and process should be reviewed together.
Can existing blow molds be reproduced?
Yes. The cavity can be rebuilt from the current mold, a container or its 3D model, improving points such as cooling or the pinch-off inserts along the way.
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