Chapter 9: Shaping Processes for Plastics Flashcards
What are thermoplastics
Chemical structure remains unchanged - 70% of total plastics
What are thermosets
Undergo a curing process resulting in a permanent change in molecular structure called crosslinking. Once cured, they cannot be remelted
What is a polymer melt and its important properties
Plastic heated to the consistency of a liquid - viscosity and viscoelasticity
What is the viscosity of polymer melts
Fluid property that relates shear stress to shear rate
Most polymer shaping processes inolve what
Flow through small channels or die openings
Viscosity and temperature
Viscosity decreases with temperature increase thus the fluid becomes thinner at higher temps
What is viscoelasticity and an example
Combination of viscosity and elasticity
Ex: die swell in extrusion - hot plastic expands after exiting a die opening (memory)
What is Extrusion
Material is forced to flow through a die orifice to provide a long continuous product whose cross-section is determined by the shape of the orifice
What are the two main extruder components
- Barrel
- Screw
What determines the cross sectional shape of an extrusion
The die orifice
Components of extrusion to create hollow profiles
requires mandrel to form shape
- often includes an air channel blowing air to maintain hollow form for extrudate
- Mandrel help by spider legs
How is wire and cable coating made
Polymer melt is applies to bare wire as it is pulled at high speed through die
- a slight vacuum is drawn for adhesion
What is the thickness for polymer sheet and film
Film - below 0.5 mm
Sheet - from 0.5 mm to 12.5 mm
What are the processes for sheet and film production
- Slit-Die Extrusion of sheet and film
- Blown-Film Extrusion Process
- Calendering
What is slit Die Extrusion of sheet and film
Production of sheet and film by conventional extrusion using a narrow slit as the die opening
Problem in slit die extrusion of sheet and film
Thickness uniformity throughout width of stock due to drastic shape change
What is Calendering
Feedstock is passed through a series of rolls to reduce thickness to the desired gage
What is Calendering noted for and typical material
- good surface finish and high gage accuracy
- typically rubber or rubbery thermoplastics
What is injection molding
polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity where it solidifies and the molding is then removed from the cavity
Key Characteristics of injection molding
- Produces discrete components to net shape
- mold may contain multiple cavities so multiple moldings can be produced each cycle
What are the shape limitations of injection molded parts
-Capacity to fabricate a mold whose cavity is the same geometry as part
- shape must allow for part removal from mold
Injection molding is economical when
only for large production quantities due to high cost of mold
Injection molding is most widely used molding process for
thermoplastics
Two principle components of injection molding
- Injection Unit - melts and delivers polymer melt and operates like an extruder
- Clamping Unit - opens and closes mold each injection cycle
