Blow molding of thermoplastics

1. Blow Molding
By Sunil Mor

2. Principal
• Blow molding is a manufacturing process that is used to create hollow
plastic parts by inflating a heated plastic tube until it fills a mold and forms
the desired shape.
or
• Blow of air into a plastic tube that is closed on all sides except the point at
which the air enters, the tube will expand and take the shape of
the mold that is around the tube.
or
• Production of a parison (precursor), enclosing of the parison in a closed
female mold, and inflation with air to expand the molten plastic against the
surface of the mold, where it sets up into the finished product.

3. Working
• A hot, softened plastic material tube known as parison is properly
placed inside a two plate cavity mold.​
• When the two part of the mold are closed, it pinches and closes one
end of the parison and encloses a blowing pin at the other end.​
• Now, the parsion is blown by pressuring from within by blowing
compressed air through the blowing pin.
• The hot parison is inflated like a balloon and goes on expanding until
it comes in intimate contact with the relatively cold interior surface of
the cavity mold.

4. Working
• Under pressure, the parison is ultimately assumes the shape of the
cavity of the mold.​
• The mold is allowed to cool and the article formed is removed by
opening the mold.​

5. Types of Blow molding
• There are two basic processes of blow molding:
• Extrusion blow molding and
• Injection blow molding.
• Extrusion processes are by far the more widely used, but injection
blow molding and injection stretch blow molding have captured
significant market segments.

6. Extrusion blow molding
• An extruder uses a rotating screw to force the molten plastic through
a die head that forms the parison around a blow pin. The parison is
extruded vertically between the two open mold halves, so they can
close on the parison and blow pin. Pressurized air flows through the
blow pin to inflate the parison.
• This is the most common type of blow molding and is used to
manufacture large quantities of relatively simple parts.

7. Extrusion blow molding

8. Extrusion blow molding

9. Types of Extrusion blow molding
• Continuous extrusion blow molding
• Intermittent extrusion blow molding

10. Continuous extrusion blow molding
• One of the basic forms of extrusion blow molding is based on
producing a molten tubular parison without interruption.
• In continuous extrusion blow molding the parison is extruded
continuously and the individual parts are cut off by a suitable knife.
• When the tube reaches the proper length, the blow mold is moved
under the die head, where it closes around the parison, pinching one
end closed; the tube is severed by a knife or a hot wire, and the mold
moves to the blow station to clear the way for the next parison.
• Pressurized air is blown into the inside of the parison until it expands
to fill the cavity of the mold, forming the desired product shape.

11. Continuous extrusion blow molding
• The hot plastic product cools and hardens against the mold. Then, the
mold opens, releasing the part.
• Finally, the extra plastic on the ends, called the flash, is removed for
recycling.
• For higher productivity, more than one parison can be extruded from
the die head at a time.
• In the common rising mold type of machine, the blow mold rises from
below to close around the tube; the blow pin enters from the bottom.
• With continuous extrusion the weight of the parison drags and makes
calibrating the wall thickness difficult.

12. Continuous extrusion blow molding

13. Continuous extrusion blow molding

14. Continuous extrusion blow molding

15. Types of continuous extrusion blow molding
• Rotary wheel blow molding systems
• Shuttle machinery

16. Rotary wheel blow molding
• The hollow bodies are blown from the continuously extruded parison
in such a way that they leave the machine as an “endless” chain. The
blow molds attached to the circumference open and close by a flap
motion of usually only one mold half. In the further course of
rotation, the parison is inflated using blowing needles. The bottom
flash of one bottle is connected to the top flash of the next bottle.
• The bottles are deflashed outside the blowing machine and the
sealing surface is usually produced by milling.
• There are systems with 4 to 24 stations, 1 to 2 fold production (two
parisons), 30 mL to approx. 30 liters article volume in the field.

17. Rotary wheel blow molding
• A two-fold, 24-station machine can produce up to 1 million bottles a
day with “neck to neck” molds.

18. Rotary wheel blow molding

19. Rotary wheel blow molding

20. Rotary wheel blow molding

21. Advantages
• Multi-layer coextrusion, with one to seven layers of plastic in the
finished part.
• In some applications, In mold labeling can be integrated with little or
no cycle time penalty.
• Parison programming capability, for optimization of wall thickness.
• Reduced cycle time on light weight containers, compared to shuttle
machinery.
• Ability to achieve very high outputs from a single machine - lowest
"cost per bottle" when compared to other blow molding equipment.

22. Advantages
• Easily implemented view stripe capability.
• Higher production efficiencies than most other extrusion blow
molding equipment types.

23. Disadvantages
• Inability to produce bottles with calibrated neck finishes.
• Downstream trimming required.
• Machines typically dedicated to a narrow range of sizes.
• Product change can be difficult, especially when downstream
trimming changeovers are required.
• High initial capital investment
• Flash between the individual blow molds that must be recycled ƒ
• Due to the specified pitch on the wheel circumference, smaller
bottles produce significantly more flash than larger ones, which is
process related

24. Disadvantages
• An additional finishing step for the neck of the bottles is required
• The neck of the bottles is blown out, a kind of lost head must be cut
off above the bottleneck

25. Shuttle Machinery
• The shuttle is brought under the flow head, where a parison of plastic
material is being constantly extruded. The mold closes and “shuttles”
away from the flow head.
• The mold is then brought to the blow pins, which are then forced
downward into the molds, helping to "calibrate" the necks while air is
forced into the parsion to blow and formed the container. The shuttle
motion allows the parison to be blown and cooled to the side,
without interfering with the parisons, which are continually extruding
from the flow head.

26. Shuttle Machinery
• In a dual-sided shuttle machine, the second shuttle mold is brought
under the flow head while the first set is being blown and cooled,
doubling the machine’s output.
• The machines come in all sizes and configurations from single cavity
to as many as 20-30 and bottle sizes from a few ounces up to a 10
litre jerry can.

27. Shuttle Machinery

28. Advantages
• Low mold cost for low volume output.
• Capable of producing high-quality "calibrated neck" finish with blow
pins.
• In-machine trimming - finished bottles exit the machine.
• Capable of producing handle ware bottles.
• Capable of producing multi-layer bottles (up to 6 layers).
• Relatively inexpensive for small to medium volume production
requirements.
• Most flexible in container size and number of cavities

29. Disadvantages
• Can not process PET material.
• Hydraulics and controls can be complex.
• Requires multiple machines for very high production volumes unless.
• Multiple parisons can be difficult to process consistently.

30. Intermittent extrusion blow molding
• In Intermittent blow molding there are two processes:
• Straight intermittent or Reciprocating Screw extrusion blow molding
• The accumulator head method

31. Reciprocating Screw extrusion blow molding
• Straight intermittent or Reciprocating Screw blow molding is similar to
injection molding whereby the screw turns, then stops and pushes
the melt out.
• Reciprocating Screw blow molding machines are characterized by the
use of a reciprocating screw extruder, as is used in injection molding.
The molds are stationary under the die head and simply open and
close but do not shuttle. As the screw melts the resin, the screw
moves backward, allowing the melted plastic to accumulate in the
end of the barrel. When the screw pushes forward under hydraulic
pressure, the plastic is pushed out of the barrel, extruded through a
flow head and die, to form a plastic parison.

32. Reciprocating Screw extrusion blow molding
• This is the most popular and cost effective method to produce
lightweight dairy, juice and water containers.
• For lightweight containers, cycle times can be very fast with some
under 5 seconds.

33. Reciprocating Screw extrusion blow molding

34. Advantages
• Very efficient container production, with fast cycle times
• Machines are mechanically simple and easy to operate - the clamps
do not shuttle, and the flowheads do not require bobbing
• Moderate initial capital investment.
• Simple machine motions since clamps do not move.
• Capable of 16 cavities small bottles or 8 gallons. Smaller models
available.
• Relative low cost per cavity
• Can easily run homo polymers and very low melt temperatures.

35. Disadvantages
• Limited to monolayer production.
• Integration of parison programming is expensive, as each head utilizes
a separate parison control system.
• Inability to utilize a screen changer.
• Primary materials are HDPE and limited amount of PP.

36. Accumulator head method
• The viscous resin from the extruder is pushed into an accumulator
head, which is comprised of a chamber that collects a set amount of
molten plastic resin and a pair of cylinders that expel the resin in
a shot. The cylinders also control the shape of the plastic as it leaves
the accumulator.
• When the volume of resin pushed into the accumulator reaches the
capacity needed for the part, the molten plastic is forced through an
extrusion die around a core, called a mandrel or pin. This creates a
tube that determines the inner diameter inside of a perimeter ring,
which creates the outer diameter.

37. Accumulator head method
• The dies are moveable so the wall thickness in the tubular parison can
be changed as it is formed or shot from the accumulator head,
allowing the maximum control of finished product detail.
• The accumulator head methods use hydraulic systems to push the
parison out quickly reducing the effect of the weight and allowing
precise control over the wall thickness by adjusting the die gap with a
parison programming device.

38. Accumulator head method

39. Injection blow molding
• The molten plastic is injection molded around a core inside a preform
mold to form the hollow preform. When the preform mold opens,
both the preform and core are transferred to the blow mold and
securely clamped. The core then opens and allows pressurized air to
inflate the preform.
• This is the least commonly used method because of the lower
production rate, but is capable of forming more complicated parts
with higher accuracy. Injection blow molding is often preferred for
small, complex bottles, such as those in medical applications.

40. Injection blow molding

41. Injection blow molding

42. Working of Injection blow molding
• Homogenize plastic melt in exact amounts is injected through an
injection moulding machine nozzle into one or more preform cavities.
Hot liquid is circulated through mould channels around the preform
cavity and the required preform temperature is achieved.
• The mold opens and the core pin carries the preform in semi-viscous
stage to the second stage i.e. the blow mold station.
• Upon closing blow mold, compressed air is introduced through the
core/ blow pin and the preform blows out and contacts the inside
wall surface of the mold.

43. Working of Injection blow molding
• The product cools and then blow mould opens and the product is
ejected.

44. Advantages of Injection blow molding
• Bottle necks of varying shapes, sizes and wall thickness can be
precisely controlled.
• Products are free of flash.
• The wall thicknesses of a hollow body can be predetermined
exactly.
• There is no weld line on the bottom.
• There is no neck on bottom trim.
• It can be used to almost any type of plastics that can be
injection molded.

45. Disadvantages of Injection blow molding
• High change-over costs from one part to another. This is
caused by the large number of molds and core pins required.
• Limitations of parts size which can be made by blow molding.
• Tooling costs are high as compared to extrusion blow molding.

46. Stretch blow molding
• The stretch-blow process can give many resins improved physical and
barrier properties. In biaxial orientation, bottles are stretched
lengthwise by an external gripper, or by internal stretch rod, and then
stretch radially by blow air to form the finished container against the
mold walls.
• Stretch blow molding is the method of producing a plastic container
from a preform or parison that is stretched in both the hoop direction
and the axial direction when the preform is blown into its desired
container shape.

47. Stretch blow molding
• The preform is formed in the same way as injection blow molding.
However, once transferred to the blow mold, it is heated and
stretched downward by the core before being inflated. This stretching
provides greater strength to the plastic.
• Stretch blow molding is typically used to create parts that must
withstand some internal pressure or be very durable, such as soda
bottles.

48. Working of stretch blow molding
• In the stretch blow molding process, the plastic is first molded into a
“preform” using the injection molding process.
• These preforms are produced with the necks of the bottles, including
threads (the “finish”) on one end. These preforms are packaged, and
fed later (after cooling) into a reheat stretch blow molding machine.
• In the SBM process, the preforms are heated above their glass
transition temperature, then blown using high pressure air into
bottles using metal blow molds.
• Usually the preform is stretched with a core rod as part of the
process.

49. Working of stretch blow molding

50. Working of stretch blow molding

51. Types of stretch blow molding
• Stretch blow molding is divided into two different categories:
• Single-stage stretch blow molding and
• Two-stage stretch blow molding.

52. Single-stage stretch blow molding
• Single-stage uses the extruder to inject a parison into a preform mold
where the plastic is rapidly cooled to form the preform. The preform
is then reheated and placed in the bottle mold. The then softened
parison stretches to about twice its original length. Compressed air is
then blown into the stretched parison to expand to the bottles mold.
Once the bottle is cooled the mold is opened and the finished bottle
is emptied from the mold cavity.
• In the single-stage process both preform manufacture and bottle
blowing are performed in the same machine.

53. Single-stage stretch blow molding
• This technique is most effective in specialty applications, such as wide
mouthed jars, where very high production rates are not a
requirement.

54. Two-stage stretch blow molding
• Two-stage stretch blow molding is the same as single-stage, except
the preforms are already made. The single-stage process is usually
done using one machine, where the two-stage process uses preforms
that have already been made and cooled.
• In this process, the machinery 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.

55. Two-stage stretch blow molding
• This process is used for extremely high volume runs of items such as
wide mouth peanut butter jars, narrow mouth water bottles, liquor
bottles etc.
• This allows companies to either make or buy their own preforms.
Because of the relatively high cost of molding and RHB equipment,
this is the best technique for producing high volume items such as
carbonated beverage bottles.
• The stretching of some polymers, such as PET (polyethylene
terephthalate) results in strain hardening of the resin, allowing the
bottles to resist deforming under the pressures formed by carbonated
beverages, which typically approach 60 psi.

56. Two-stage stretch blow molding
• Two stage processing requires an injection line to produce preforms,
and a reheat blow machine to make the finished bottles
• With the two-stage, the process uses extruded or injection molded
preforms that have been cooled, and indexes them through an oven
that reheats them to the proper orientation-blow temperature

57. Advantages of Stretch blow molding
• Low tool and die cost
• Fast production rates
• Ability to mold complex part
• Produces recyclable parts.
• Increase the material’s tensile strength
• Barrier properties
• Clarity
• Reduce weight stretch blow molding produces a container from less
raw material and with improved economics and bottle properties.

58. Disadvantages of Stretch blow molding
• Limited to hollow parts
• Wall thickness is hard to control.

59. Multilayer/Co-extrusion
• Co-extrusion Blow Molding (EBM) is one of the most widely used
techniques in plastic container manufacturing industry to produce
varies kinds of plastics, including HDPE, PVC, PP, PC, and is very
commonly implemented with multi-layer bottles production.
• Co-Extrusion Blow Molding Machine is a machine that implemented
the EBM technique for manufacturing all kinds of plastic containers.
• The plastic co-extrusion process involves extruding two or more
materials through a single die so that materials merge or weld
together into a single structure before cooling.

60. Multilayer/Co-extrusion
• It uses more than two extruders to melt and plasticize the same or
different plastics in different extruders.
• And then compounding, extruding and forming multi-layer concentric
composite parison in the die.
• Its basic process principle is the same as the single-layer blow
moulding technique. Only the moulding equipment uses several
extruders to plasticize a different kind of plastics.

61. Multilayer/Co-extrusion
• There are many reasons to produce hollow articles with a multilayer
wall structure. The most obvious reasons are e. g. to save colored
material or to incorporate plastic waste from industrial and
household waste. As a rule, three-layer systems are used here, with
e. g. 15% colored material in the outer layer, 15% uncolored material
(virgin material) in the inner layer and 70% regrind plastic waste in
the middle layer. Bottles for dairy products usually have a black
colored middle layer which is intended to protect the contents against
UV radiation, while the inner and outer layers are colored white or in
other light colors.

62. Working of Co-extrusion Blow Molding
• A hot tube of plastic material (Parison) dropped from an extruder and
reached the water cooled mold.
• As the mold closed, air is injected from the top or below parts of the
container to blow it up just like a balloon. When the material touches
the mold wall, it is then frozen and maintain its’ rigid shape just as the
mold shape.
• Bottles produced via EBM generates top and bottom flash which
should be further removed and trimmed in the sub-mold. For
situation like handled bottles, then the flash should be removed via
auto de-flashing device or by hands.

63. Working of Co-extrusion Blow Molding
• After the above three process, the containers is now finished. The
bottle should further go through other tests such as leak detection if
necessary based on the usage of the bottle.

64. Working of Co-extrusion Blow Molding

65. Advantages of co-extrusion
• Low initial mold tooling costs.
• Flexibility of tooling; molds can accommodate interchangeable neck
finishes.
• Container sizes can range from less than 30ml to 200 Liter.
• Container shape is not restricted; bottles can be long and flat or have
handles.
• Wide selection of machinery; tooling can be specifically geared to
package volume requirements.
• Each material used maintains its desired characteristic properties
(such as stiffness, impermeability, or environmental-resistance).

66. Advantages of co-extrusion
• Capability of making multi-layer and multi-functional structures that
too in a single pass.
• Reduce the number of steps required in general extrusion process.
• High quality mono-layer extrusion coatings in larger varieties.
• Different polymer selection and property criteria leads to overall
saving.
• Less energy required to power the machine.

67. Disadvantages of co-extrusion
• It requires a more sophisticated extruder and its operator, which
leads to extra maintenance cost.
• Physical properties might vary and some might be unable to be
combined.
• Demands considerable planning as well as forethought in the system
design.
• Sometimes it can be difficult to ensure that different polymers have
similar melt viscosities.

68. References
• Plastic engineering handbook by Joel Frados.​
• Polymer extrusion by hanser publication.​
• Polymer processing by DH Morton Jones.​
• Hand book of plastic technology by W.S. Allen/ P.N. Baker.​
• SPI Plastics Engineering Handbook by Michael L. Berins.​
• SBP Handbook of Plastics.
• Google Photo and google.

69. Thank You