Blow molding is a specific manufacturing process by which hollow plastic parts are formed and can be joined (greased). It is widely used for making BlowMolded Furniture. In general, there are two main types of blow molding: extrusion blowing and injection blow molding.
The extrusion blow molding process starts with melting the
plastic and transforming it into a molten and hollow plastic, which is known in
the industrial environment as parison or which is nothing more than a piece of
plastic similar to a tube or hose with a hole in it. an end through which
compressed air can pass.
The formation of the parison is very important
for the development of the product, the plastic hose is extruded in the trafila
and in the male (bushing) of the head of the blower, the manufacture of both
sets is dimensioned respecting how the formation of the parison will take
place, so that the blown body has a thickness and diameter according to the
design
Production starts as follows: the parison is
then “grabbed” in the blow mold and air is blown into it. The
strong pressure of the compressed air pushes the plastic into the cavities of
the mold for plastic blowing, and through the cooling system of cold water that
passes inside the mold, the plastic cools and hardens, transforming into a
product, and then the mold opens and the part is extracted through the blowing
pin. Figure (a) demonstratively illustrates the blowing cycle of extrusion
molding.
Extrusion blow molding is usually done with polymers such as high or low
density polyethylene (HDPE/LDPE), as well as polypropylene (PP).
There are two types of conventional extrusion molding blow molding machines,
the continuous blower or the accumulator head.
Machines
with continuous extrusion: In this type of equipment, the extruder screw works
uninterruptedly, the mold then closes when the programmed length is reached. Normally
the plastic blow mold moves to the die which is cut after closing the mold and
then moves to the blowing position. This type of machine is used to
manufacture smaller bottles and gallons of up to a maximum of 5 liters.
Machines with accumulator head: In
this type of equipment, the plasticized material is stored inside the
accumulator head that works as a reservoir. In this equipment concept, the
mold does not need to be moved to another position during the blowing process,
since the material that will form the trafil of the next cycle is being
accumulated in the head, being released at the programmed moment. This
last type of machine is widely used for parts larger than 5 liters, such as
cans, drums, etc.
In the case of injection
blow molding, the formation of the vial or bottle takes place
through the previously injected plastic part, which is called a preform , the preform. is an injected tube with already formed
thread, and of pre-established thickness for better formation of the bottle or
flask, in which the material of this preform is often PET, or even
polypropylene (PP)
A process consists of the preform
subjected to heat, causing the tube to soften, then the mold closes and a high
pressure compressed air is blown into the tube, causing the preform to stretch,
enveloping the entire cavity of the mold, and then the mold opens to extract
the part.
Blow molds are widely used in the manufacture of
bottles for cosmetics, food bottles, pots and
blow moldedfurniture. The main materials used in
the manufacture of blow mold cavities are 7075 aluminum and stainless steel.
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