2. Extrusion
Extrusion means forcing a molten polymer by means of
pressure through a shaped die.
Most common manufacturing of plastic resin
Combines pigments, additives and resin
High heat, high pressure molten mixture
Create warm plastic for possible further “finishing”
operation such as pelletizing, calendaring, or molding
The basic machine, known as the extruder, consists of
a screw which rotates inside a cylindrical barrel.
The extrusion molding is capable to manufacture
continuous products.
The extruder consists basically of:
a) a hopper
b) a barrel
c) a screw
6. Operation Principle of Extrusion
Five (5) stages :
1) The extruder: plasticated & forced out through the die
2) The die:The hot molten of soft plastics takes shape.
3) Forming:The hot material is further shaped.
4) Post-forming:The material is cut or further shaped.
5) Secondary processing
7. EXTRUSION
Shaping
through die
Final Product (pipe,
profile)
Preform for other molding
processes
Blow molding (bottles),
Thermoforming
(appliance liners)
Compression molding
(seals)
Secondary operation
Fiber spinning (fibers)
Cast film (overhead
transparencies,
Blown film (grocery
bags)
EXTRUSION BASED PROCESS
8. Extrusion Process
Feed section
◦ Compact to a solid mass
◦ Pre heat
Compression or plastication section
◦ Melting progresses, degassing occurs
Metering section
◦ Internal heating from viscous flow
◦ Pressure is developed to extrude the material through
the die
9.
10. Screw Extruder
Functions of screw:
a) Convey
b) Mix
c) Plasticating (melting)
d) Metering
e) Venting
Types of screw:
a) Single screw
b) Twin Screw
12. The Basic Machine (Single screw
extruder) used for :
•Profile extrusion – a direct manufacture of product
from the extruder
•The products are continuous lengths whose cross
sectional profile is determined by die shape
13. Single Screw Extruder
Single screw extruder (SSE) depends on
drag flow (viscosity and frictional
properties) for melt conveying and
output of polymers.
Therefore, it is not suitable for
extruding heat-sensitive polymers,
materials that exhibit strong tendency
to degrade, and materials that slip
excessively on the barrel surface (but
stick to the screw).
14. This is because the viscous heat
generated within the polymer can
cause excessive heating and may
thermally degrade the polymer.
So, a different melt conveying
mechanism is needed for these types
of materials.
It is for these reasons that the twin
screw extruders (TSE) are developed.
Twin-screw extruders do not depend on
friction (drag flow) for output, as we
shall see later.
17. Twin Screw Extruder
• Both screws are parallel and side by side inside the
barrel.
• These extruders are suitable for extrusion of difficult to
extrude polymers and for materials that require greater
mixing.
The main application of twin-screw extruder is profile
extrusion of PVC for window frames and pipes
production.
PVC is a polymer known for its thermal instability and
unusual frictional properties - it requires elaborate heat
stabilizing and lubricating systems as additives before it
can be processed.
As such, PVC is a difficult polymer to run in a single
screw extruder.
18. Advantages of Extrusion
Low cost tooling and short lead
times
Low cost parts
Ability to create very complex
cross-section products
Work with brittle materials
(materials only encounters
compressive and shear stresses)
Excellent surface finish
19. Main Problem in Extrusion
a) DIE SWELL
When polymer exits the die of the
extruder it will swell, this action is
referred to as die swell. (Exit flow is
larger than die opening).
Die swell occurs because the sudden
release of pressure causes the
polymer chains to relax.
21. Main Problem in Extrusion
B) MELT FRACTURE
Melt flow instability through a die during
plastics molding, leading to helicular,
rippled surface irregularities on the finished
product.
Two (2) driving forces:
a) Slip and Stick- Melt sticks to wall then breaks
free causing pulsation in pressure.
b) Skin Rupture: Die swell causes pressure
build up in melt at exist, then with sudden
cooling the surface breaks.
25. Process of blow molding originated from blowing of
glass then adapted to polymers
Evidence of Egyptians and Babylonians using blow
molding for plastics
Enoch Ferngren and William Koptike are the people
credited for the process of blow molding
Sold the first blow molding machine to Hartford
Empire Company in 1937
“In the United States, soft drink industry contribute to
the amount of plastic containers from zero in 1977 to
ten Billion in 1999.”
25
26. Polymer process that creates hollow plastic parts
This process begins with melting down the plastic that
is to be blow molded in to a parison
Hot air blown into the parison forces it against the
mold walls where it is then cooled
Mainly limited to processing of thermoplastics
Most plastic bottles or hollow parts are processed this
way
26
27. Parison
In blow molding,
the hollow tube of
plastic melt
extruded from the
die head, and
expanded within the
mold cavity by air
pressure to
produce the molded
part.
27
28. Polypropylene (PP)
Good impact strength (poor at cold temperatures),
Good chemical resistance,
High abrasion resistance,
High melt strength.
Polyphenylene Oxide (PPO)
Good flame retardancy,
Good chemical resistance,
Good impact resistance, and
Retains mechanical properties in high heat environments.
PolyethyleneTerephthalate (PET)
Used in injection blow molding, produces clear
amorphous preforms. Since there is less orientation, the
impact strength is reduced.
Polymer characteristics relevant for blow
molding
28
29. High-density polyethylene (HDPE)
High impact strength
Low temperature toughness,
Excellent resistance to chemicals,
Good electrical insulating properties,
Poor ultraviolet resistance.
Melting Point 266°F 130°C
Tensile Strength 4550psi
Acrylonitrile-butadiene-styrene (ABS)
ABS is a hard, tough material;
Good impact resistance;
Good electrical insulation properties;
Versatile additive, filler, and reinforcing agents acceptance
Polycarbonate (PC)
Excellent resistance to heat,
Hard, tough material,
Good impact resistance,
Excellent transparency.
29
30. 30
1. PET or PETE – Questionable – depending on the quality, repeated use may cause the
leaching of DEHA, a know human carcinogen.
2. HDPE – OK – not known to leach unwanted chemicals.
3. PVC or V – Bad – Strong evidence to show the leaching of DEHA, a known human
carcinogen.
4. LDPE – OK – not known to leach unwanted chemicals although not as widely
recycled as #1 or #2.
5. PP – OK – not known to leach unwanted chemicals although not as widely recycled
as #1 or #2.
6. PS – Bad – suspected to possibly leach harmful carcinogens
7. Assorted but usually polycarbonate – Bad – may contain leaching BPA.
31. 3Types of Blow Molding Process
b) Injection Blow Molding
a) Extrusion Blow Molding
c) Stretch Blow Molding
31
32. Begins with a parison/ pre-form and is then
placed into an enclosed cooled metal mold
Then air is blown into this mold to shape
the material to the desired shape
Once it is cooled the part is ejected from
the mold
Can be made continuously or
intermittently
32
33. The production cycle consists of the
following steps:
The parison is extruded vertically in downward direction
between two mold halves.
When the parison reaches the required length the two
mold halves close resulting in pinching the top of parison
end and sealing the blow pin in the bottom of the parison
end.
Parison is inflated by air blown through the blow pin,
taking a shape conforming that of the mold cavity.The
parison is then cut on the top.
The mold cools down, its halves open, and the final part is
removed.
33
38. • Polymer melt is supplied to mold halves from injection
molding machine
• Pre-form injection molded over a mandrel and transferred
to the blowing die
• Air is injected under pressure through the mandrel blowing
the polymer against the mold walls where it cools and
freezes as with extrusion blow molding
38
39. In Injection Blow Molding method a parison is produced
by injecting a polymer into a hot injection mold around a
blow tube or core rod.
Then the blow tube together with the parison is
removed from the injection mold and transferred to a
blow mold.
Following operations are similar to those in the
extrusion blowing molding.
Injection Blow Molding is more accurate and controllable
process as compared to the Extrusion Blow Molding.
It allows producing more complicated products from a
wider range of polymer materials.
However production rate of Injection Blow Molding
method is lower than that of Extrusion Blow Molding.
39
41. Injection-Blow Molding (IBM)
In je c tio n
m o ld
B lo w m o ld
o p e n c lo s e d
P r e fo r m e x p a n d e d
to b lo w m o ld
P r e fo r m c a r r ie d
o v e r o n c o r e p in
A ir
41
43. Shear & temperature dependent
viscosity
Temperature-dependent tensile
strength on the pin
Tensile elongation during inflation
Crystallization kinetics on the core pin
Crystallization kinetics during blowing
and cooling
43
44. Injection molded preformed top
Heaters raise the pre-form above the
glass transition temperature
Core rod stretches polymer to the
bottom of the mold
High pressure air expands the polymer to
the walls of the mold
44
45. Stretch Blow Molding is similar to Injection Blow Molding.
Stretch Blow Molding involves injection molding of a
parison, which is then stretched in the downward
direction by means of the blow tube.
The extended parison is then inflated in a blow mold.
In this method biaxial molecular orientation is produced.
The specific molecular orientation provides higher
mechanical strength, rigidity and transparency of the
material.
Material, commonly used in this method is Polyethylene
Terephtalate (PET).
Stretch Blow Molding is used for manufacturing
containers for carbonated beverages.
45
48. Single stage
◦ Pre-form and blow molding process
happen in the same machine
◦ Convent for small batch production
Two stage
◦ Pre-form is made in one machine
◦ Blow molding occurs in another
◦ Faster production for large batches
48
50. – Thermoplastics
• PET
• PC
• HDPE
• LDPE
• PP
• ABS
• some PVC
– Limited levels of reinforcement for composite
materials
– Thickness should be as uniform as possible to
avoid distortion
50
51. Blow Molding
Materials
◦ Good stretchability and a high MW are preferred
◦ HMWPE is the most widely used for high volume packaging
◦ PP used in processes that promote orientation
◦ PVC is used for bottles in Europe (homopolymer can be crystal clear)
◦ PET is primarily used for injection blow molding. Preforms injected into
cold mold to an amorphous state and then reheated to 100C for blowing
◦ Nitrile, SAN, PVDC, PPO, PC, and PA
Products
◦ Packaging, bottles for drinks, containers for cosmetics and toiletries,
automotive containers and bumpers.
◦ Coextrusion products for chemical resistance and structural
51
53. Comparison
Extrusion and Injection Blow Molding
Extrusion blow molding is characterized by:
Best suited for bottles over 1/2 pound (200 g) and shorter
runs.
Machine costs re comparable to injection blow molding.
Tooling costs are 50% to 75% less than injection blow
molding.
Generates 20% to 30% scrap due to sprue and head
trimming.
Requires additional equipment to grind scrap and reuse.
Total cycle time is shorter due to less parison transfer time.
Wider choice of resins possible due to resins with higher
viscosities can be used.
Final part design flexibility can be greater with the use of
asymmetrical openings. 53
54. Comparison
Extrusion and Injection Blow Molding
Injection blow molding is characterized by:
◦ Scrap free.
◦ Better suited for long runs and smaller bottles.
◦ Higher accuracy in the final part.
◦ Uniform wall thickness.
◦ No seam lines or pinch marks
◦ Transparencies are best because crystallization can be
better controlled and the blowing can be more stress free.
◦ Improved mechanical properties from improved parison
design and from stretch blowing.
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