Blow Molding Machines

Extrusion blow molding is commonly used to make polyethylene bottles and similar components. In some packaging lines, the bottles are made inline and go direct from molding to filling. The advantage to this is that inventories become dense containers of resin beads rather than pallet on pallet of bottles containing air. A pallet of resin may make a hundred or more pallets of bottles.

Other technologies such as injection molding, inject the plastic between a male and female mold. Most bottles are larger than their neck which prevents the use of an internal mold component. This requires extrusion blow molding.
Extrusion blow molding begins as most plastic processes do with resin beads or pellets. These may come in pallet-sized Gaylord containers or in larger bulk shipments. The pellets must be clean and dry. In some instances, it may be desirable to run them through a dryer between the bulk and the extruder.

Pellets enter the extruder which is a rotating screw designed with progressively finer threads inside a heated “barrel”. The combination of heat from the barrel and the compression of the screw soften and plasticize the resin. The resin changes from individual beads to a homogenous mass.

Resin extruder screw: From Fundamentals of Packaging Technology – Soroka

The screw extrudes the plastic through a vertical nozzle as a hollow tube, called the parison. One a suitable length has been extruded, the mold clamps shut on the parison. Molds are commonly 2 pieces but multi-part molds are sometimes used for especially complex geometries. As the mold clamps shut, it seals the bottom and top of the parison shut. At the top of the parison a hole with the cap threads or other neck finish is formed.
This completes the extrusion part of the process.  Extrusion blow molding continued: Source www.pct.edu
Once the mold has been clamped shut, compressed air is applied through the neck. This expands the parison to conform to the interior of the mold providing the final bottle shape. The mold is water cooled and when the parison makes contact with the mold walls, it cools and stiffens. This allows it to maintain its shape as it leaves the mold although it is still hot to the touch.
The process above describes a parison of uniform thickness and diameter. This will cause differences in wall thickness of the finished bottle. The larger diameter will be thin, the smaller diameters like the neck will the thick. This may be desirable in many instances. In others, it will use excess material or cause weakness in key parts of the bottle.  A programmed parison may be used to vary the thickness as it is extruded. This allows wall thickness to be precisely controlled.

Fundamentals of Packaging Technology – Soroka – As the bottle exits the mold, it will have considerable excess material or “flash”, especially on the bottom or top. The final stage in the process is removal of the flash. This can be done manually by an operator but is more typically done by a machine. One type of machine places the bottle in a female die and a male blade punches the flash free.

Flash removal 1 gallon milk jug: Blow molding may seem like a mysterious process but the reality is that it is very simple and can be done in virtually any plant.