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Overview of processes.

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Presentation on theme: "Overview of processes."— Presentation transcript:

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


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