Plastic Molding & Forming
Blow Molding is a highly
developed molding technology developed back
in the late 1800's to produce celluloid
baby rattles. It is best suited for basically
hollow parts (such as plastic bottles) with
uniform wall thicknesses, where the outside
shape is a major consideration.
The first polyethylene bottle
was manufactured using the blow molding
process in December of 1942. This was the
real beginning of a huge industry which
currently produces 30 to 40 billion plastic
bottles per year in the U.S. alone.
A thermoplastic resin is
heated to a molten state
- It is then extruded through
a die head to form a hollow tube called
- The parison is dropped
between two mold halves, which close around
The parison is inflated.
The plastic solidifies
as it is cooled inside the mold.
The mold opens and the
finished component is removed.
There are basically four
types of blow molding used in the production
of plastic bottles, jugs and jars. These
four types are:
- Extrusion blow molding
- Injection blow molding
- Stretch blow molding
- Reheat and blow molding.
Extrusion blow molding
is perhaps the simplest type of blow molding.
A hot tube of plastic material is dropped
from an extruder and captured in a water
cooled mold. Once the molds are closed,
air is injected through the top or the neck
of the container; just as if one were blowing
up a balloon. When the hot plastic material
is blown up and touches the walls of the
mold the material "freezes" and
the container now maintains its rigid shape.
Injection blow molding
is part injection molding and part blow
molding. With injection blow molding, the
hot plastic material is first injected into
a cavity where it encircles the blow stem,
which is used to create the neck and establish
the gram weight. The injected material is
then carried to the next station on the
machine, where it is blown up into the finished
container as in the extrusion blow molding
process above. Injection blow molding is
generally suitable for smaller containers
and absolutely no handleware.
Extrusion blow molding allows
for a wide variety of container shapes,
sizes and neck openings, as well as the
production of handleware. Extrusion blown
containers can also have their gram weights
adjusted through an extremely wide range,
whereas injection blown containers usually
have a set gram weight which cannot be changed
unless a whole new set of blow stems are
built. Extrusion blow molds are generally
much less expensive than injection blow
molds and can be produced in a much shorter
period of time.
Stretch blow molding
is perhaps best known for producing P.E.T.
bottles commonly used for water, juice and
a variety of other products. There are two
processes for stretch blow molded P.E.T.
containers. In one process, the machinery
involved injection molds a preform, which
is then transferred within the machine to
another station where it is blown and then
ejected from the machine. This type of machinery
is generally called injection stretch blow
molding (ISBM) and usually requires large
runs to justify the very large expense for
the injection molds to create the preform
and then the blow molds to finish the blowing
of the container. This process is used for
extremely high volume (multi-million) runs
of items such as wide mouth peanut butter
jars, narrow mouth water bottles, liquor
The reheat and blow molding
process (RHB) is a type of stretch blow
process. In this process, a preform is injection
molded by an outside vendor. There are a
number of companies who produce these "stock"
preforms on a commercial basis. Factories
buy the preforms and put them into a relatively
simple machine which reheats it so that
it can be blown. The value of this process
is primarily that the blowing company does
not have to purchase the injection molding
equipment to blow a particular container,
so long as a preform is available from a
stock preform manufacturer. This process
also allows access to a large catalog of
existing preforms. Therefore, the major
expense is now for the blow molds, which
are much less expensive than the injection
molds required for preforms.
There are, however, some
drawbacks to this process. If you are unable
to find a stock preform which will blow
the container you want, you must either
purchase injection molds and have your own
private mold preforms injection molded,
or you will have to forego this process.
For either type of stretch blow molding,
handleware is not a possibility at this
stage of development. The stretch blow molding
process does offer the ability to produce
fairly lightweight containers with very
high impact resistance and, in some cases,
superior chemical resistance.
Whether using the injection
stretch blow molding process or the reheat
and blow process, an important part of the
process is the mechanical stretching of
the preform during the molding process.
The preform is stretched with a "stretch
rod." This stretching helps to increase
the impact resistance of the container and
also helps to produce a very thin walled
The extrusion blow molding
process allows for the production of bottles
in a wide variety of materials, including
but not limited to: HDPE, LDPE, PP, PVC,
BAREX®, P.E.T., K Resin, P.E.T.G., and
Polycarbonate. As noted above, a wide variety
of shapes (including handleware), sizes
and necks are available. Injection blow
molding allows for the production of bottles
in a variety of materials, including but
not limited to: HDPE, LDPE, PP, PVC, BAREX®,
P.E.T., and Polycarbonate.
Besides the P.E.T. noted
above for stretch blow molding, a number
of other materials have been stretch blown,
including polypropylene. As time goes on
and technology moves forward, more materials
will lend themselves to stretch blow molding
as their molecular structures are altered
to suit this process.
Molding Machine Manufacturers -
For shuttle extrusion type
machines Bekum, Battenfeld/Fischer, and
Hayssen are probably the best known in the
United States. For injection blow molding
machines JOMAR is a well known brand. For
stretch blow and reheat and blow type machines
there are Sidel, Nissei and other machines
produced by Johnson Controls and others.
For wheel machines you might wish to contact
Johnson Controls or Wilmington Machinery.