Plastics Review Questions Flashcards
What are some reasons why plastic-shaping is so important
- variety of shaping processes
- Many parts formed by molding which are net shape
- Less energy required than for metal
- Simplified handling, due to lower temp required
- finishing not required (except in unusual circumstances)
Main process of plastic shaping processes
Plastic Injection Molding
Upon what parameters does viscocity depend
viscocity is a fluid property that relates the shear stress experienced during flow of the fluid to the rate of shear
How does viscocity of a polymer melt differ with most newtonian fluids
For a newtonian fluid, viscocity is constant at a given temp
For polymer melt, viscocity decreases with shear rate
What does viscoelasticity mean when applied to a polymer melt
hot plastic expands when exiting the die opening
define die swell in extrusion
extruded material tries to remembers its former shape and attempts to return to it after leaving die orifice
Describe plastic extrusion process
- plastic pellets enter the hopper
- the pellets fall from hopper to the feeder system
- screw in the feeder system pushes the plastic pellets towards breaker plate. At the same time, friction between pellets and opposing surfaces create heat to melt plastic along with additional heaters in the barrel
- compression section - polymer transformed into liquid consistency, air entrapped amongst pellets extracted from melt, material compressed
- material is pumped into the die opening
Identify the sections of the extruder
- feed section - where stock is moved from hopper and preheated
- compression section - where polymer melt is transform to liquid consistency, air escapes from plastic melt, material compressed
- metering section - melt homogenized, sufficent pressure develops to pump it through die opening
what are the functions of the screen pack and breaker plate at the die end of extruder
screen pack
series of wire meshes supported by the breaker plate containing small axial holes
- filter contaniments and hard lumps
- build pressure in metering section
- straighten the flow of polymer
- remove memory
what are the forms of extruded shapes and corresponding dies
solid profiles
hollow profiles
wire/cable coating
what is the distinction between plastic sheet and plastic film
plastic sheet
0.5mm - 12.5mm
plastic film
<0.5mm
what is blown film process for producing film stock
Process for making thin polyethylene film for packaging
cobines extrusion and blowing to produce a tube of thin film
describe calendering
process for producing sheet and film stock out of rubber or rubbery thermoplastics
initial feedstock passed through a series of rolls to work the material and reduce its thickness to desired gage
polymer fibers and filaments are used in several applications. What are the most important?
textiles
what is the technical difference between fiber and filament
fiber is a long, thin strand of material whose ength is finite
filament is a strand of continous length
briefly describe injection molding process
- mold is closed and clamped
- shot of melt is injected under high pressure to mold cavity. Plastic cools and solidifies. Ram maintains high pressure
- screw rotates and retracts. Part solidifies completely
- mold is opened and part ejected
name principle components of injection molding machine
- clamping unit
- concerned with the operation of the mold
- plastic injection unit
- like an extruder
what is the function of gates in injection molds
constrict flow of plastic into cavity
what are advantages of three-plate mold over two plate
- flow of plastic is through a gate located at the base of the cup shaped part, allows for more even distribution of melt into sides of the cup, also avoids weakness created by weld lines
- allows for more automatic operations
Describe defects in injection molding
short shots
flashing
sink marks and voids
weld lines
What are difference in injection molding of thermosets to thermoplastics
In thermosets: Barrel length is shorter to avoid premature curing and solidification of the TS polymer
What is reaction injection molding
mixing of two highly reactive liquid ingredients and immediately injecting mixture into a mold cavity, where chemical reaction leading to solidification occurs
what kinds of products are produced in blow molding
plastic bottles
plastic jugs for drinks/milk/juices
what is the form of starting material in thermoforming
plastic sheet
what is the difference between a positive mold and a negative mold
positive mold: convex cavities
negative mold: concave cavities
why are molds generally more costly in mechanic thermoforming than in pressure/vacuum thermoforming
Mechanical using matching positive and negative molds
what are some general design considerations that product designers keep in mind when designing components of plastic
- styrength and stiffness
- impact resistance
- service temperature
- thermal expansion
- degradatiom from sunlight and other forms of radiation
Injection molding
- tolerances
- holes
- draft
- corner radii and fillets
- economic production quantities
- part complexity
- wall thickness
extrusion
- wall thickness
- hollow sections
- corners
