Plastics Review Questions

Question 1

What are some reasons why plastic-shaping is so important

Answer 1

• 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)

Question 2

Main process of plastic shaping processes

Answer 2

Plastic Injection Molding

Question 3

Upon what parameters does viscocity depend

Answer 3

viscocity is a fluid property that relates the shear stress experienced during flow of the fluid to the rate of shear

Question 4

How does viscocity of a polymer melt differ with most newtonian fluids

Answer 4

For a newtonian fluid, viscocity is constant at a given temp

For polymer melt, viscocity decreases with shear rate

Question 5

What does viscoelasticity mean when applied to a polymer melt

Answer 5

hot plastic expands when exiting the die opening

Question 6

define die swell in extrusion

Answer 6

extruded material tries to remembers its former shape and attempts to return to it after leaving die orifice

Question 7

Describe plastic extrusion process

Answer 7

1. plastic pellets enter the hopper
2. the pellets fall from hopper to the feeder system
3. 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
4. compression section - polymer transformed into liquid consistency, air entrapped amongst pellets extracted from melt, material compressed
5. material is pumped into the die opening

Question 8

Identify the sections of the extruder

Answer 8

• 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

Question 9

what are the functions of the screen pack and breaker plate at the die end of extruder

Answer 9

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

Question 10

what are the forms of extruded shapes and corresponding dies

Answer 10

solid profiles

hollow profiles

wire/cable coating

Question 11

what is the distinction between plastic sheet and plastic film

Answer 11

plastic sheet

0.5mm - 12.5mm

plastic film

<0.5mm

Question 12

what is blown film process for producing film stock

Answer 12

Process for making thin polyethylene film for packaging

cobines extrusion and blowing to produce a tube of thin film

Question 13

describe calendering

Answer 13

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

Question 14

polymer fibers and filaments are used in several applications. What are the most important?

Answer 14

textiles

Question 15

what is the technical difference between fiber and filament

Answer 15

fiber is a long, thin strand of material whose ength is finite

filament is a strand of continous length

Question 16

briefly describe injection molding process

Answer 16

1. mold is closed and clamped
2. shot of melt is injected under high pressure to mold cavity. Plastic cools and solidifies. Ram maintains high pressure
3. screw rotates and retracts. Part solidifies completely
4. mold is opened and part ejected

Question 17

name principle components of injection molding machine

Answer 17

• clamping unit
  
  • concerned with the operation of the mold
• plastic injection unit
  
  • like an extruder

Question 18

what is the function of gates in injection molds

Answer 18

constrict flow of plastic into cavity

Question 19

what are advantages of three-plate mold over two plate

Answer 19

1. 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
2. allows for more automatic operations

Question 20

Describe defects in injection molding

Answer 20

short shots

flashing

sink marks and voids

weld lines

Question 21

What are difference in injection molding of thermosets to thermoplastics

Answer 21

In thermosets: Barrel length is shorter to avoid premature curing and solidification of the TS polymer

Question 22

What is reaction injection molding

Answer 22

mixing of two highly reactive liquid ingredients and immediately injecting mixture into a mold cavity, where chemical reaction leading to solidification occurs

Question 23

what kinds of products are produced in blow molding

Answer 23

plastic bottles

plastic jugs for drinks/milk/juices

Question 24

what is the form of starting material in thermoforming

Answer 24

plastic sheet

Question 25

what is the difference between a positive mold and a negative mold

Answer 25

positive mold: convex cavities

negative mold: concave cavities

Question 26

why are molds generally more costly in mechanic thermoforming than in pressure/vacuum thermoforming

Answer 26

Mechanical using matching positive and negative molds

Question 27

what are some general design considerations that product designers keep in mind when designing components of plastic

Answer 27

• 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