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Flashcards in Plastics Review Questions Deck (27)
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1

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)

2

Main process of plastic shaping processes

Plastic Injection Molding

3

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

4

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

5

What does viscoelasticity mean when applied to a polymer melt

hot plastic expands when exiting the die opening

6

define die swell in extrusion

 

 

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

7

Describe plastic extrusion process

  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

8

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

9

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

10

what are the forms of extruded shapes and corresponding dies

solid profiles

hollow profiles

wire/cable coating

 

11

what is the distinction between plastic sheet and plastic film

plastic sheet

0.5mm - 12.5mm

 

plastic film

<0.5mm

12

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

13

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

14

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

textiles

15

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

16

briefly describe injection molding process

  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

17

name principle components of injection molding machine

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

18

what is the function of gates in injection molds

 

constrict flow of plastic into cavity

19

what are advantages of three-plate mold over two plate

  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

20

Describe defects in injection molding

short shots

 

flashing

 

sink marks and voids

 

weld lines

21

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

22

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

23

what kinds of products are produced in blow molding

plastic bottles

 

plastic jugs for drinks/milk/juices

24

what is the form of starting material in thermoforming

plastic sheet

25

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

positive mold: convex cavities

 

negative mold: concave cavities

26

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

Mechanical using matching positive and negative molds

27

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