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Two main components of extruder are Barrel and Screw
Die is not a component of extruder; it is a special tool that is fabricated for the particular
profile to be produced
Internal diameter of barrel typically ranges from 25 to 150 mm and L/D ratio ranges from
10 to 30.
Higher L/D is used for thermoplastics and lower for elastomers
Extruder rotates around 60 rpm
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The screw performs three functions and is divided into sections:
Feed Section: Stock is moved from the hopper port and is preheated.
Compression section: Polymer is transformed into liquid consistency, air entrapped amongst the pallets is extracted from the melt, and the material is
compressed
Metering section: Melt is homogenized and sufficient pressure is developed
to pump it through die opening.
Function of Breaker Plate
Filter containment and hard lumps
from the melt.
Build pressure in the metering
section
Straighten the flow of polymer and
remove its memory of circular
motion imposed by screw.
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Typical pressure gradient in an extruder Extruder and die characteristics
Side view cross-section ofdie for coating electrical
wire by extrusion
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Die configuration and extruded products
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Melt fracture
(a) Sharkskin (b) Bambooing
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In this process polymer is heated to a highly plastic state and forced to flow under
high pressure into a mould cavity, where it solidifies.
The process produces discrete components that are always net shape.
Complex and intricate shapes are possible to produce, however the challenge is to
design mould so that the part can be ejected successfully
Process is economical in large production as the cost of mould is very high.
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Mould is closed Melt is injected cavity
Screw is retracted Mould opens and part is ejected
Typical Moulding Cycle
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Details of two plate mould for thermoplastic Injection
Moulding (a) Closed (b) Open mould
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Three plate mould (a) Open (b) Close
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Two alternative type of Injection Moulding machines
Screw preplasticizer Plunger type (Older machine)
Shrinkage Considerations
High thermal expansion, which is up to 10% for typical thermoplastics
Contraction of crystalline polymer is more than than amorphous
Shrinkage is expressed as reduction in linear size (mm/mm) when cooled
from moulding temperature to room temperature.
Fillers in the plastic tend to reduce shrinkage
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Defects in Injection MouldingShort Shots: solidified before completely filling the cavity. The defect can
be corrected by increasing temp. and/or pressure.
Flashing: the polymer melt is squeezed into the parting surface between mold
plates; it can also occur around ejection pins. The defect is usually caused by
(1) vents and clearances in the mold that are too large; (2) Injection pressure
too high compared to clamping force; (3) melt temperature too high; (4)
excessive shot size.
Sink marks and voids: Occur in thick molded section. When the outer skin
solidifies and the and inner core remains mushy. Later when it solidifies a
depression is seen on the outer surface due to shrinkage. A void is also caused
by the same phenomenon. These defects can be addresses by increasing packing pressure following injection. A better solution is to design a part
having uniform section.
Weld lines: It is formed when the melt comes from two sides and forms
weld. The properties of weld line are inferior.
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Compression Molding
(1)Charge is loaded (2) and (3) Charge is compressed and cured and
(4) part is ejected and removed
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Port transfer molding
Plunger transfer molding
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Blow Molding
(1) Extrusion of parison (2) Parison is pinched at the top and sealed at the
bottom around a metal bow 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 and
(4) mold is opened and part is removed
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Injection Blow Molding
Stretch Blow
Molding
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Rotational Molding or Rotomolding
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Vacuum Thermoforming 1. A flat sheet issoftened by
heating.
2. The softened
sheet is kept over
a concave cavity
3. A vacuum draws
the sheet into
cavity
4. The plastic
hardens on
contact with thecold mold surface
and the part is
removed and then
trimmed
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Pressure Thermoforming
Vacuum Thermoforming with positive mold
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Pre-stretching the
sheet prior todraping and
vacuuming it
MechanicalThermoforming
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