1. Overview of processes

2. Classification of solidification processes

3. Principle of the process
Structure/Configuration
Process modeling
Defects/quality control
Design For Manufacturing (DFM)
Process variation

4. 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)

5. Shaping Processes For Plastics
Thermoplastics (TP): repeated heating and cooling is possible
Thermosets (TS): no repeated heating and cooling is
possible
As the curing process involves a chemical reaction that changes the molecular structure irreversibly

6. 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 -> net shape processing
Low density relative to metals
Good strength-to-weight ratios

7. Shaping Processes For Plastics
2. Plastics versus Metal (Cont.)
High corrosion resistance;
Low electrical and thermal conductivity;
Melting point of plastics is several hundreds.
Thus requires less energy
Easy for equipment (handling, etc) implementation

8. Shaping Processes For Plastics
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

9. Shaping Processes For Plastics
4. Properties and mechanics
Plastics materials heated up to melt -> flow
Molten state -> polymer melt
High Viscosity
Behavior of 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

10. Shaping Processes For Plastics
Viscosity relation at CONSTANT TEMPERATURE
Viscosity
Pseudoplastic fluid
Newtonian fluid
Shear rate

11. Shaping Processes For Plastics
The relationship between shear stress and shear rate:
Shear stress
Viscosity
Shear rate
Newtonian Fluid
k: a constant corresponding to n
n: flow behavior index, n=1 for Newtonian fluid, n<1 for polymer melt
Pseudo-plastic fluid

12. Shaping Processes For Plastics
Viscosity of “polymer melt” is a function of temperature
Temperature increases  Viscosity decreases
Viscoelasticity
Polymer melts: an ability of recovering
Non-constant viscosity and viscoelasticity: two factors for the difficulty to control the quality of polymer products

13. 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, and the unrelieved stress causes cross-section to expand
Increasing time in channel can reduce die swell

14. Shaping Processes For Plastics
Die swell Dx Dd
The measure of die swell:

15. Shaping Processes For Plastics
Ease of flow - measure
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 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.
Used for grading of polymer melts for a specific type

16. Summary Two types of plastics materials in terms of cross-linking
Better strength-to-weight ratio
The constitutive equation: viscosity is not constant
The challenge in material behavior: die swelling