two ways for blow molding.
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Two Ways For Blow Molding

 
      Since the 1973 registered patent to make bottles for carbonated drinks using PET,plastic bottles are extrusion blow mold. After heating and friction heating by the rotating screw, the resin turns to a melt and is extruded through a die in the form of a cylindrical tube (parison). The moud closes on the parison and air is injected through the neck. The cylinder expands and touches the mould surface which cools it down.

    In the injection stretch blow molding process, injection molding machine is used to make the preform. Using this process, the neck finish is almost perfect. Before the preform cools or by reheating, the preform could be stretched by a rod and blown (bi-directional stretching). Other than strengthening the bottle, the wall thickness is reduced so resin usage is more economical than extrusion blow molding.

    In extrusion blow molding, the bottleneck is not smooth, and has to be scraped to remove the flashing. This avoids leakage after a cap is put on. A lot of waste plastic is produced in extrusion blow molding which has to be removed and recycled. The advantages include low investment cost, high production speed, easy to add a handle, the bottle could take many styles and shapes, bottle weight could be adjusted over a big range, extrusion blow moulds are inexpensive and have short delivery time. Hollow bottle handles could be made in the extrusion blow molding process.

    The disadvantages of injection stretch blow molding is difficult to add handle and expensive preform mould, which has a long delivery time. Bottle weight, which is the same as preform weight, is essentially non-adjustable. The way a handle is added to PET bottle is to insert the injection mold handle into the blow mould. The handle has undercuts and is wrapped around by the bottle wall during blow molding.
 
     
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