Overview of processes

1

Classification of solidification processes

3

Principle of the process

Structure/Configuration

Process modeling

Defects/quality control

Design For Manufacturing (DFM)

Process variation

4

1. Introduction

1. Thermoplastic polymers, or thermoplastics (TP)

2. thermosetting polymer, or thermosets (TS)

Plastics is one of the

Polymers: plastics and rubbers

Plastics materials have general two types

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Thermoplastics (TP): repeated heating and cooling is possible

As curing process involved a chemical reaction that changes the molecular structure

Thermosets (TS): no repeated heating and cooling is possible

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2. Plastics versus Metal

Low density relative to metals

Good strength-to-weight ratios

On a volumetric basis, usage of plastics exceeds metals

Plastics can be easily formed by molding into intricate part geometries

No further processing required -> net shape processing

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2. Plastics versus Metal (Cont.)

- Melting point of plastics is several hundreds.

- High corrosion resistance;

- Low electrical and thermal conductivity;

Thus requires less energy

Easy for equipment (handling, etc) implementation

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3. Plastic shaping processes

continuous extruded products with a 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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4. Properties and mechanics

Plastics materials heated up to melt -> flow

High ViscosityBehavior of polymer melt

Molten state -> polymer melt

Viscosity Relates SHEAR STRESS experienced during fluid flow to RATE OF SHEAR

Most shaping involves fluid flow through small opening

High flow rate high rate of shear high shear stress

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Pseudoplastic fluid

Newtonian fluid

Shear rate

Viscosity

Viscosity relation at CONSTANT TEMPERATURE

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The relationship between shear stress and shear rate:

.

.

Shear stress Viscosity

Shear rate

Newtonian Fluid

nk )(.

K: a constant corresponding to ƞ

N: flow behavior index, n=1 for Newtonian fluid, n<1 for polymer melt

Pseudoplastic fluid

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Viscosity of “polymer melt” is a function of temperature

Temperature increases Viscosity decreases

Viscoelasticity

Polymer melts: an ability of

Non-constant viscosity and viscoelasticity: two factors for

the difficulty to control the quality of polymer products

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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, and the unrelieved stress causes cross-section to expand

Increasing time in channel can reduce die swell

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Dx

Die swell

d

xx D

Dr The measure of die swell

Dd

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Melt Flow Index (MFI):

The weight of material flowing through the die orifice within a specified time and temperature

Used for grading of polymer melts for a specific type

MFI is not just related to a type of polymer melt but more related to a specific state of the polymer melt at particular time and temperature.

Therefore, MFI should not be used for comparison of different types of polymer.

Shaping Processes For Plastics