Overview of processes

Classification of solidification processes

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

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)

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

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

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

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

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

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

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

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

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

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

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

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