Injection
Blow Molding Basics
Injection blow molding is a
manufacturing process that combines the accuracy of injection molding with the
minimal waste and rapid processing of blow molding. Injection blow molding can
be used with plastics and is generally used to make hollow shapes such as
bottles. The manufacturing process is more expensive than other blow molding
techniques, so larger containers are generally fabricated using extrusion blow
molding. The final products of injection blow molding have no seams, and can be
glass clear.
Injection Blow Molding Process
The
injection blow molding process has three phases: Injection of the material into
the preform mold; blowing of the preform, or parison, into the product mold;
and ejection of the product from the machine. A commonly used machine
configuration is a three-stage blow molding machine, which rotates through 120
inches for each phase of the process. In the injection phase, molten plastic or
glass is injected into the preform mold over a core pin, which forms the interior
of the parison and finishes the interior of the neck. At this point, the neck
of the container is fully formed. The parison is removed from the mold and
inserted into the blow mold. The parison is inflated inside the blow mold by
pressurized air. After a cooling period, the product is ejected from the
machine, tested for leaks, and moved to another station for labeling or
finishing.
Designing for the
Injection Blow Molding Process
Designing
products to be fabricated using injection blow molding includes selecting a
thermoplastic resin for the product application, designing the mold geometry to
take into account material shrinkage during cooling, calculating the right
amount of material to be injected and choosing the right pressure to inflate
the parison in the mold. Tolerances in the core rod and preform mold design are
critical so that the core rod sits exactly in the center of the mold and the
resulting parison has constant wall thickness prior to blowing.
Products Fabricated Using
the Injection Blow Molding Process
Injection blow molding is used to create hollow products such as
bottles, and is generally only used for small containers such as medicine
bottles or single-serve beverage containers. The injection phase of the process
can create a neck with very fine tolerances, with designs such as screw threads
or threads for childproof caps. Unlike standard injection molding, undercuts
can be incorporated into an injection blow molded design for some products,
such as mascara bottles.
