| |
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.
|
|
