View A Video on Blow Molding - A Quick Introduction
Blowmolding products include CD cases, tanks and any consumer item that is hollow and three dimensional in shape. Blow molded products can be used to hold automotive oil, cosmetics, pesticides and herbicides. Blow molded plastics are all thermoplastic resins, and include high and low density polycarbonate, polypropylene, polyvinyl chloride, Barex, butadiene styrene, polystyrene, polyamide, polysulfone and acetal. There are 3 main types of plastic blow molding-injection blow molding, extrusion blow molding and stretch blow molding. These processes are 2 steps and vary mostly during the preform stages. Blow molded products take their shape depending on the shape of their mold cavity. They come in standard sizes and shapes, while some custom blow molding products are specifically made for unusual and unique processes.
The blow molding process begins with melting down the plastic and forming it into a parison, or preform by injection molding. The parison is a tube-like piece of plastic with a hole in one end in which compressed air can pass through. A metal ram pushes the softened and malleable preform, expanding it to the desirable height of the product. Using a blow pin, air pressure is introduced through the inside of the parison, forcing it to balloon out and assume the shape of the mold cavity. The end product is cooled by conduction, cold water running through the mold cavity, or the evaporation of volatile fluids in the container. The purpose of blow molding services is to form a uniform finished product that is smooth, airtight and does not need to be joined together in any way. The whole process takes a few seconds, and blow molding machines can produce up to 20 thousand containers in an hour. Blow molding services use three basic methods for producing blow molded plastic: extrusion, injection and stretch blow molding.
Extrusion blow molding is the simplest type that allows for a wide variety of container shapes, sizes and neck openings, along with handleware of many kinds. It is based on the glass blowing process, and uses 2 mirror molds to create the containers, which causes a thin seam down the middle. Injection blow molding is a hybrid of the injection molding and blow molding services and is suitable for smaller containers but not for handleware of any kind. This process injection molds the plastic preform right before it is blown into a mold. There are two different types of stretch blow molding-injection stretch blow molding (ISBM) and reheat and blow (RHB) molding, both of which are typically used to manufacture P.E.T bottles for juice, water, etc. In the ISBM process, a preform is injection molded and then transferred to another station on the machine, where it is blown. Because ISBM is so costly, it requires long runs for products such as peanut butter jars, water bottles and liquor bottles. In RHB, a preform is purchased from a vendor that has already put it through an injection molding process. It is then reheated in a relatively simple machine in order to prepare it for blowing. RHB is advantageous because the purchaser does not have to invest in expensive injection molding machinery but still has access to a large catalog of existing preforms.
Distinct advantages exist for choosing the blow molding process over other plastic manufacturing processes. It is the most well-suited method for forming hollow plastic parts. It has much lower mold costs than injection molding. Blow molding has a faster cycle time than that of rotational molding. In many cases, one blow molded plastic piece can take the place of many individual parts. However, the scattering of mold thickness is a potential problem, and quality control is important to make sure none of the blow molded products have holes, leaks or unevenly thin walls. Strong, precisely even walls are better created by rotational molding or plastic extruding, but for the thousands of applications which require high volume production over precision, plastic blow molding is an ideal choice.
Blow Molded Plastics Types
- are
formed by blow molding processes.
- are
plastic parts formed by blow molding.
- create
hollow plastic parts.
- is
the process in which a hot parison or preform is placed into a two-part
mold. In the mold,
the parison or preform is inflated with compressed air, which presses
it against the walls, and then is cooled and removed by separating
the mold halves.
- blends
the two methods of blow molding and injection molding and is typically
used in the manufacturing of soda
bottles.
A preform, which is similar to the parison, is either injection
plastic molded
and then transferred to a blow molding machine, or the injection and
blow molders are combined.
- is a two-stage, stretch blow
molding process in which a preform is injection molded and then
transferred to another station where it is blown and ejected from the
machine.
- is
a process that uses inflated thermoplastics in molds.
- are
hollow containers made of plastic.
- is
a stretch blow molding process in which a preform is injection molded
by a vendor and then purchased
and reheated
by another company.
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Blow Molded Plastics Terms
- The lack of haze or cloudiness in a
plastic material.
- A material that at room temperature has the ability
to return to its original shape after being stretched up to twice its
length.
- A high-barrier plastic
resin packaging material.
- Also known as the "E" dimension, it is the
material enclosing the neck opening of containers that accommodate particular
closures.
- Extra plastic attached to a molding along the parting line.
Flash must be removed (i.e. deflashed) before the part can be considered
finished.
- A technique of making inert thermoplastic items
responsive to adhesives, inks and paints. The submersion of the item
in an open flame induces surface oxidation.
- A procedure
that diminishes permeability and increases the chemical resistance of
different plastics and elastomers. The exposure
of polymers to fluorine gas causes molecular structural surface changes.
- The thinnest part of the wall of a bottle.
- The hollow part of a group of molds that forms a
container on bottle machines.
- A vertical line formed at the point where the mold
halves meet, the prominence of which depends on the accuracy with which
the halves are matched.
- Bottles comprised of layers of special plastic
materials that are co-extruded to preserve and use the unique characteristics
of each material to improve the barrier qualities of the container and
increase the product shelf life.
- Refers to the natural translucent appearance of
the plastic material of containers and closures.
- The component of the molding apparatus that produces
the bottle finish.
- The collapse of a portion of a container structure,
due to a loss of interior pressure, which occurs during aging and storage.
- The empty cylinder of plastic melt that emerges from
the extruder die head and expands in the mold space from air pressure,
producing the molded part.
- The mark on a plastic bottle where the two halves
of the mold meet at the point of closure.
- A heat-softened polymer shaped like a thick test tube
with neck threads that is inflated while inside a blow mold.
- A procedure for mechanically altering the weight,
size or wall thickness of the parison as it forms.
- A decrease in dimension of an article after the molding
process, which is caused by the contraction of the plastic material upon
cooling.
- The seam that joins both edges of a blank forming a
body.
- Any treatment technique, including chemical,
electronic and flame treatments, that makes plastic receptive to inks,
lacquers and adhesives.
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