molding dates back to at least 1890, when it was used to produce
celluloid baby rattles. From that time forward, many companies have
tried numerous means to produce blow molded parts in a variety of
materials. The first polyethylene bottle was blown in December of
1942. The rest is history: the U.S. currently produces 30 to 40
billion plastic bottles per year, with the number constantly growing.
For an excellent history of the plastics industry through 1972,
those reading this article may wish to consult Plastics
History U.S.A. by Harry Dubois, published by Cahners Books, Boston,
Mass., ISBN 0-8436-1203-7.
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 and reheat
and blow molding. Extrusion blow molding is perhaps the simplest
type of blow molding, whereby 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. There are various types of shuttle, reciprocating and
wheel style machines for the production of extrusion blown bottles.
Shuttle or reciprocating type machines can be used for small, medium
and high volume production with wheel machines being the most efficient
for huge volume production of certain resins.
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
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.
Many people have heard about stretch blow molding in conjunction
with 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 bottles etc.
Another stretch blow process is commonly called reheat and blow
(RHB). 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 container.
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.,
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.
The decision as to which process will be used is based upon the
desired appearance (clear or not), whether chemical or impact resistant
is desired, and the desired cost/benefit relationship. The ultimate
choice of materials and processes is also based upon the cost of
the tooling involved and the sizes of the production runs. Some
materials lend themselves to certain types of decorating better
than others and some to certain types of decorating to the exclusion
Listed below are representative brands of some types of the machinery
we have discussed above. This list is not all-inclusive and you
will find additional brands by looking through this and other packaging
industry journals and magazines.
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.
thanks to Nissei for the photographs.
Article: The Necessity of Obtaining Closure Samples and Information
Prior to a Plastic Bottle Production Run
Resistant Closure Specifications
Neck Finish Specifications
Article: The Blow Molding
Process Closures & Containers Magazine
Article: The Market Share
Closures & Containers Magazine
Party Packaging Certification Agencies
Guidelines for Closures