Plastics

Question 1

Polymer Structures

Answer 1

1 linear (thermoplastics)
2 branched (thermoplastics) –> branches do not connect
3 thermosets
4 elastomers

Question 2

Thermoplastic polymers

Answer 2

• solid at room temp
• melting at jsut a couple of hundred degrees
• remelting possible

Question 3

thermosetting polymers (thermosets)

Answer 3

• initial heat allows to luiqidify

- high temperature produce a chemical reaction –> cross linking, once cross linked not able to go back, one way process

Question 4

Elastomers (rubbers)

Answer 4

extreme elastic behaviour
heating similar to thermosets plastics (cross linking etc.)
- capable of large elastic deformation

Question 5

Importance of polymers

Answer 5

• net shaping process
• good material properties
• cost competitive
• req. less energy for processing

Question 6

Limitations of polymers

Answer 6

• low strength compared to metals

- limited service temp. —> UV impact (sunlight )

Question 7

Additives

Answer 7

• Fillers
• UV light absorbers
• Antioxidants

Question 8

Fillers (additives)

Answer 8

-solid materials, powder
possible reasons:
- alter mechanical properties
-reduce cost
-Improve stability

Question 9

Thermoplastic polymers

Answer 9

raw material: pellets or powder

• disadv: thermal aging
• adv: can be molten multiple times

Question 10

Thermoset polymers

Answer 10

Comparisn to thermoplastics:

• more rigid
• more brittle
• higher service temp

Question 11

Forming and shaping processes polymers

Answer 11

• Extrusion
• Injection molding
• blow molding
• rotational molding
• thermoforming process
• compression modling
• transfer molding

Question 12

Extrusion

Answer 12

• forcing molten plastic throug a die
• gets heaten up by srew
  Applicability:
• mostly thermoplastics and elastomers
• produces tubes, pipes, hose, sheets
  CONTINIOUS CROSS SECTION

Question 13

Injection Molding

Answer 13

-molten plastic is injected into a mold using a ram or a screw as a plunger
- creates parting line and ejection pin on product
–> highest production rate
–> high variety of different products
Applicability:
-produces many components:
cars, toys etc
- high variety on part size
- Only economical on large production quantitiy (Molds are expensive)
Injection molding defects:
-Short shots (too less material)
-Flashing (not enough injection power)
-sink marks
-weld lines

Question 14

Hot Runner vs Cold Runner molds

Answer 14

Cold runner:
- Sprue & runners are waste materials after process
- can often be reground and reused
Hot runner:
Heater keep sprue and runner system melted during process (very expensive and hard to control)

Question 15

Blow molding

Answer 15

Injection and Extrusion blow molding:
products: thin wall containers
Injection:
Injection --> Blowing 
Extrusion:
Extrusion --> blowing

–> high production rates low costs
–> injection blow molding more economical
ONLY THERMOPLASICS because of cross linking, no double process possible

Question 16

Thermoforming Process

Answer 16

thermoplatic sheet is heated to its softening point and then pressed against a cavity (mold) with vaccum
–> medium to high prod. volume

Question 17

Compression molding

Answer 17

Only for thermosets and elastomers because heating and cooling process for thermoplastics would make process unefficient
–> produces simple part geomitries

Question 18

Transfer molding

Answer 18

similar to compression molding but with a sprue and runner system; mold produces one or multiple parts per circle