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Blow Molding 1

Blow Molding
The principal methods used to process
thermoplastic materials into finished or
semifinished products are, in order of importance:
screw extrusion, injection moulding, blow
moulding and calendaring
Blow moulding and calendering are often postextrusion operations.
Hollow articles, such as bottles and other
containers, can be manufactured by the blowmoulding process.
The most common procedure is to extrude
vertically downwards, from a crosshead die, a
hollow thick walled tube or pari son of melt.
This tube is surrounded by a split mould of the
appropriate shape, one end being clamped around
a spigot incorporating an inlet for compressed air,
which is used to blow the parison into contact with
the water-cooled mould.
When solidified, the moulding is removed and its
ends are trimmed of excess material.
While blowing and cooling are being carried out, the
mould unit, of which there may be several associated
with one extruder, is removed from beneath the
extrusion crosshead to allow a new parison to form.
Figure 2.8 shows a simplified view of this process. In
figure 2.8a, the two halves of the mould are about to
close on the parison, thereby severing it at the top and
squeezing it into contact with the spigot at the bottom.
Figure 2.8b shows the parison blown into contact with
the mould.
Both the elastic properties of the melt and the effect of
gravity on the suspended parison affect its thickness
distribution, and only relatively modest dimensional
accuracy can be achieved.
Also, the amount by which the diameter of a parison can
be increased by blowing is limited. In addition to
extrusion blow moulding, there is also an injection blowmoulding process, in which the material to be blown is
first injection moulded.
Although better dimensional accuracy than in extrusion
blow moulding can be achieved, the process is more
Blow Molding
Modified extrusion and injection-molding process
Hollow thin-walled parts of various sizes
High production rates
Low cost for making beverage and food
General Process
The process can be simplified into 3 steps
1) Melting the resin
2) Forming a preform tube
3) Blowing the preform tube into the shape desired
Steps 1 and 2 can be done either in an extruder or in an injection molding machine.
There are two types of blow molding
Extrusion Blow Molding
Injection Blow Molding
Injection Molding gives rise to a third process called stretch blown molding
There are fundamental differences in the these two types of
blow molding.
Extrusion Blow Molding
Extrusion blow molding:
(1) and (2) parison is pinches at the top and sealed at the bottom around a metal blow pin as the
two halves of the mold come together
(3) the tube is inflated so that it takes the shape of the mold cavity
(4) mold is opened to remove the solidified part.
The continuous nature of the
extrusion process is a problem
with extrusion blow molding.
Six ways to solve the problem of interference:
Intermittent Extrusion Blow Molding
Continuous Extrusion Blow Molding
Accumulator Head (Ram Extension)
Rising Mold System
Parison Transfer System
Multiple-Mold System
Figure: Parison transfer system for continuous extrusion blow
Solving Interference Problems
Intermittent Extrusion Blow Molding
Stop the extrusion of the parison while the part
is cooling in the mold.
Continuous Extrusion Blow Molding
An accumulator receives the material from the
extruder and the creates the parison as needed.
Accumulator Head or Ram Extrusion
The extruder output flows into a reservoir that is
external to the extruder itself. A ram pushes the
resin out of the reservoir and through the die
that is mounted at the end of the accumulator to
create the parison.
Solving Interference Problems
Rising Mold System
When the mold closes on the parison and air
injection has occurred, the mold is moved off to
the side. The pinching off the mold creates a
break point in the parison so that when the mold
moves aside, the parison is not disturbed.
Parison Transfer System
The parison is cut from the die by a knife and
transferred by a mechanical arm to a mold
where it is blown, cooled, and ejected.
Multiple-Mold System
Several Molds are mounted to a rotating wheel.
While one mold is closing the capture the
parison, the mold ahead is in a position for the
blowing of the part, and other parts are closed
for cooling, and further up a mold opens to eject
the part
Plastics Materials and Processing
Injection Blow Molding
Injection blow molding:
Parison is injection molded around a blowing rod
Injection mold is opened and parison is transferred to a blow mold
Soft polymer is inflated to conform to a blow mold
Blow mold is opened and blown product is removed.
Extrusion vs. Injection Blow Molding
• Bottles over ½ pound
• Tooling costs are 50% to 75 % less
• Generates 20 to 30% scrap
Additional machinery needed to grind scrap
• Total cycle time is shorter
• Wider choice of resins
• Greater flexibility in part design
Long runs and smaller bottles
• No trim scrap
• Higher accuracy in the final part
• Uniform wall thickness
• No seam lines or pinch marks
• Better transparencies
• Improved mechanical properties
Stretch Blow Molding
A: Stretch-Blown Pin; B: Air Entrance; C: Mold Vents; D: Preform;
E: Stretch Rod Extended; F: Cooling Channels
Stretch blow molding produces a part
with biaxial molecular alignment. In the
process a preform, or parison, elongated
mechanically in the mold and than
expanded radially in a blowing process.
A desirable resulting molecular
orientation yields a material with
increased strength. This means that
products that are strength-based designs
can be produced using less material
than if they were to be produced using
simpler blow molding techniques.
Materials Considerations
One critical requirement is that the
polymer must have good melt
The common blow molded plastics
• Polyolefins
– Polypropylene (PP)
PET bottles are excellent barrier materials and
are widely used for soft drinks
The polyolefins are easy to process and have the
advantage of good electrical properties, moisture
resistance, and low cost.
Sensitive to oils and can have stress crack
These resins are usually blow
molded in the range of 320o –
410oF. Mold temperatures should
range from 50o – 100oF.
Polyvinyl Chloride (PVC)
Very desirable properties of clarity, smooth
surface low glass and aroma permeation,
high stiffness with low wall thickness, high
chemical resistance, and easy labeling
and printing
Very temperature sensitive, therefore not
normally injection blown because the
process requires two heat cycles
Typical melt temperature range for PVC is
375o – 420oF.
Polyethylene terephthalate
Easily recycled, very lightweight, it makes
a good gas and fair moisture barrier. It is
strong and impact-resistant. It is naturally
colorless and transparent.
Have to be careful to avoid a temperature
that is two high or acetaldehyde forms.
Temperature range for PET to blow mold
is 200o – 212oF. Mold temperatures should
be in the range of 35o – 40oF
Conclusion – Wrap Up
This process lends itself to
any designs involving hollow
shapes. Equipment
availability is good in most
geographical locations. Can
save tooling dollars over
injection molding.
Most commodity grade and
engineering grade resins
may be blow molded. The
viscosity of the plastic must
be high to keep the parison
from stretching too much
prior to mold closure.
All types of bottles, toys, air
ducts for automobiles,
chemical & gasoline tanks,
household goods. Can be
obtained from these blow
molding processes.
Cycle times are slower than
injection molding. Piece
prices are higher than
injection molding.
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