Shaping Processes For Plastics

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Shaping Processes For Plastics
1. Introduction

• Plastics is one of the polymers

• Polymers plastics and rubbers

• Plastics materials have general two types
• thermoplastic polymers, or thermoplastics (TP)
  thermosetting polymer, or thermosets (TS)

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Shaping Processes For Plastics
Thermoplastics (TP) repeated heating and cooling
is possible
Thermosets (TS) no repeated heating and cooling
is possible
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Shaping Processes For Plastics
2. Plastics versus Metal
- On a volumetric basis, usage of plastics
exceeds metals

• Plastics can be easily formed by molding into
  intricate part geometries

• No further processing required -gt net shape
  processing

• Low density relative to metals
• Good strength-to-weight ratios

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2. Plastics versus Metal (Cont.)

• High corrosion resistance

• Low electrical and thermal conductivity

• Melting point of plastics is several hundreds.

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Shaping Processes For Plastics
3. Plastic shaping processes
- continuous extruded products with constant
cross section other than sheets, films, and
filaments

• continuous sheets and films

• continuous filaments (fibers)

• Molded parts

- Hollow molded parts with thin wall

• Casting, foamed products

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Shaping Processes For Plastics
4. Properties and mechanics
Plastics materials heated up to melt -gt flow
Molten state -gt polymer melt
Viscosity? Relates SHEAR STRESS experienced
during fluid flow to RATE OF SHEAR
Most shaping involves fluid flow thro small
opening High flow rate? high rate of shear? high
shear stress
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Shaping Processes For Plastics
Viscosity relation _at_ CONSTANT TEMPERATURE
Viscosity
Pseudoplastic fluid
Newtonian fluid
Shear rate
This phenomenon Pseudoplasticity
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Shaping Processes For Plastics
The relationship between shear stress and shear
rate
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Shaping Processes For Plastics
Ka constant called the viscosity
coefficient nflow behavior index
When n 1, the above is applied to Newtonian
fluid, and in this case K becomes ?.
Polymer melt, n less than 1
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Shaping Processes For Plastics
Viscosity of polymer melt also function of
temperature
Temperature increases ? Viscosity decreases
Like many other liquid
Shear rate increases ? Viscosity decreases
Viscoelasticity
Polymer melts an ability of recovering
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Shaping Processes For Plastics
Die swell due to the viscoelasticity
- Extruded material remembers its former shape
and attempts to return to it after leaving die
orifice
- Compressive stresses do not relax immediately -
When material exits orifice, unrelaxed stresses
cause cross-section to expand
- Increasing time in channel can reduce die swell
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Shaping Processes For Plastics
Die swell
Dd
The measure of die swell
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Shaping Processes For Plastics
Melt Flow Index (MFI) The weight of material
flowing through the die orifice within a
specified time and temperature

• - MFI is not just related to a type of polymer
  melt, but more related to specific status of a
  polymer melt at particular time and temperature.
• Therefore, MFI should not be used for comparison
  of different types of polymer.

- Used for grading within a type of polymer melts