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ENME 417 > Polymers > Flashcards

Polymers Flashcards

1
Q

What are plastics/polymers?

A
  • low density, low strength, low stiffness and electrical and thermal insulating properties. also chemical insulating
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2
Q

Plastics Greek word

A

plastikos meaning it ca be molded and shaped

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3
Q

Early Polymers

A

natural organic materials - cellulose.

synthetic polymer - phenol formaldehyde 1906

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4
Q

Modern Polymers

A

1920s, polymers were produced using petroleum products (ethylene)

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5
Q

Structure of Polymers

A

A monomer is the basic building block.
Organic compounds in which carbon atoms are attached covalently.
Repeating monomers created through polymerization

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6
Q

Polymerization

A

Thermoplastic and Thermosets

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7
Q

Thermoplastics

A

Linear - have a linear bonding structure

Branched - have branched structure hence have higher strength and lower densitiy

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8
Q

Thermosets

A

Crosslinked - adjacent chains linked covalently. Increases strength and dimensional stability
Networked polymers - have three or more active covalent bonds for improved strength

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9
Q

Crystallinity

A

Polymers can be crystalline or amorphous
As crystallinity increases, polymers become stiffer, harder and less ductile
Crystallinity can be imparted by processing but never 100%
Amorphous structures can be transparent and as crystallinity increases opaqueness increases

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10
Q

Glass Transition Temperature

A

As temperature increases amorphous polymers undergo a change

Low Temp - hard, glass, brittle
High Temp - Rubbery, leathery

Transition occurs at Glass transition Temp.
Crosslinking makes this temperature less defined

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11
Q

Thermoplastics

A

strength determined by secondary bonds
increasing temp, causes bonds to break and can be shaped

Upon

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12
Q

Thermoplastics

A

Upon cooling strength determined by secondary bonds
increasing temp, causes bonds to break and can be shaped

Upon cooling the original strength and hardness returns. Can cause thermal aging.

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13
Q

Thermoplastics

A

strength determined by secondary bonds
increasing temp, causes bonds to break and can be shaped

Upon cooling the original strength and hardness returns. Can cause thermal aging.

Constant load applied causes the polymer to stretch at a high strain rate and elongates over time.

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14
Q

Thermoplastic Examples

A

Acetals
Acrylics
Polycarbonates
PVC

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15
Q

Acetals

A

Good strength, stiffness, chemical resistances.

App - Bearings, cams, rollers, Delrin

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16
Q

Acrylics

A

possess moderate strength, good optical properties.

App - Lighted signs, skylights,plexiglass

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17
Q

Polycarbonate

A

Versatile, with great impact resistance.

App : Windshields, Safety Helmets (Lexan)

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18
Q

PVC (Polyvinyl Carbonate)

A

inexpsnive, water resistant, not suitable for high temps

Apps: Pipes, conduits, flexible hose (tygon)

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19
Q

Thermosets

A

Long chained molecules, crosslinked in a strong covalent bond.
Cross linking process is irreversible
Strength is not affected by temperature or rate of deformation. High temp will cause burning
Have bettern mechanical properties and dimensionally stable

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20
Q

Thermoset example

A

Aminos
Phenolics
Polysters

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21
Q

Aminos

A

Rigid, hard and resistant to abrasion and creep.

App: Countertopps, appliance housing (Melamine)

22
Q

Phenolics

A

High Resistance to heat, waer and chemicals
App: handles, knocs, conectors
(Bakelite)

23
Q

Polysters

A

Good mechanical properties and electrical properties. Used with fibres or available as castin resisn.
App: Clothing, textiles, swimming pools, boats (cool max)

24
Q

Elastomers (ie rubber)

A

A polymer capable of recovering substantially in size and shape after a load as been removed
properties: low glass transition temp, large deformations without rupture, soft, low elastic modulus

highly linked structure
good in vibration insulation, non skid surfaces and abrasion resistant.

maybe crosslinked for added strength (vulcanisation : sulfur added at high temps)

25
Q

Elastomer examples

A

Natural rubber
Synthetic rubber
Silicones

26
Q

Natural rubber

A

latex based good for high friction abrasive applications but has low resistance to oil, heat and sunlight. App: shoes

27
Q

Synthetic Rubber

A

improved environmental resistance over nat.rubber and higher temperature range. Apps: Tires, seals, belts

28
Q

Silicones

A

Highes temperature range. but inferiour in other properties. App: Seals, gaskets, thermal insulation

29
Q

Silicones

A

Highes temperature range. but inferiour in other properties. App: Seals, gaskets, thermal insulation

30
Q

Processing of Polymers

A

Very similar to metals.
Can be molded, cast, machined, joined with ease
polymers require less energy to process but their properties are highly influenced by processes
polymers start out in pellet form or as liquid for reinforced plastic parts

31
Q

Polymer Extrusion

A

raw thermoplastic pellets are fed into the hopper and then a barrell.
the barrel houses a screw that conveys the pellets down the barrel
the shearing action as well as external heaters cause the pellets to melt
in the last section of the screw the pressure builds up and a the polymer is extruded through a die

32
Q

Extrusion Dies

A

Once the melted plastic reached the end of the barrel, it is forced through a die and cooled

uniform cooling helps minimize distortion.
various die shapes produce different proviles or even thin sheets and films.

the phenomena of die swell causes plastic to be shaped differently than the extrusion die

33
Q

Extrusion Dies

A

Once the melted plastic reached the end of the barrel, it is forced through a die and cooled

uniform cooling helps minimize distortion.
various die shapes produce different proviles or even thin sheets and films.

the phenomena of die swell causes plastic to be shaped differently than the extrusion die

34
Q

Co extrusion

A

h

35
Q

Co extrusion

A

simultaneous extrusion of multiple polymers through a single die

each material may have their own properties and funtions
ie: barier to oil and water, electrical inslation and conduction, sealing and labelling

plastic coated eletrcial wires are produced this way

36
Q

Co extrusion

A

simultaneous extrusion of multiple polymers through a single die

each material may have their own properties and funtions
ie: barier to oil and water, electrical inslation and conduction, sealing and labelling

plastic coated eletrcial wires are produced this way

37
Q

Injection Molding (very common)

A

Similar to hot chamber die casting
Polymer pellets are plasticized to create “the melt”
the melt is forced through a nozzle into the mold
Use a reciprocating screw type injector
For thermoplastics - the mold is cooled to speed up processing. For thermosets -the mold is held got to promote polymerization and crossing linking

shrinkage is a concern (1-7%) and molds are made oversized

38
Q

Injection Mold Geometry

A 
Gate
Mold Cavity
Cavities
Main runners
Sprue
Guide Pin
39
Q

Typesof Injection Molds

A

Cold Runner 2 plate
Cold Runner 3 Plate
Hot Runner Mold

40
Q

Cold Runner 2 Plate

A

simplest mold desprue and runners must be removed by hand

41
Q

Cold Runner 3 Plate

A

Runner system is removed from part via intermediate stripper plate
Runners and parts are separate and solid

42
Q

Hot Runner Mold

A

Runner system is kept in melted state after parts have solidified

43
Q

Blow Molding

A

Combines Extrusion and injection molding

44
Q

Extrusion Blow Molding

A 
A thin tube (parison) is extruded and clamped into a mold
high presure( 350-700 MPa) then forces the parison outward t fill the cavity. Upon coling the part is injected
45
Q

Injection Blow Molding

A

Instrad of extruded parison in injected molded then transferred to a blow mold and expanded using high pressure gas.
Ex: milk containers

46
Q

Rotational Molding

A

similar to extrusion molding but at a lower cost
Thin walled metal mold is made and powdered plastic is placed inside the mold. The heat and rotatin is applied about 2 perpendicular axis.
Long thin walled parts may be made from this process. Cycle times are longer but mold and equipment costs are lower

47
Q

Rotational Molding

A

similar to extrusion molding but at a lower cost
Thin walled metal mold is made and powdered plastic is placed inside the mold. The heat and rotatin is applied about 2 perpendicular axis.
Long thin walled parts may be made from this process. Cycle times are longer but mold and equipment costs are lower

48
Q

Thermoforming

A

A family of processes where thermoplast sheet is formed over a mold via the application of heat and pressure or vacuum
This plastic sheet is head until soft then placed over a mold and pressure is applied.
Holld parts ma be made by forcing to plastic sheets together against the mold and applying force.

Limitations: No parts with holes

49
Q

Compression Molding

A

A preshaped charge i placed into a heated mold cavity and held under pressure until cured.

Flash is formed which mustbe removed in subsequent operations
Thermosets are usually chosen for compression molding because of londer cycle times and heated die

molds are less complex and cost less tha injection molding
undercuts are possible because molds can be mad to open sideways

aps: dishes, handles, pipe fittings

50
Q

Compression Molding

A

A preshaped charge i placed into a heated mold cavity and held under pressure until cured.

Flash is formed which mustbe removed in subsequent operations
Thermosets are usually chosen for compression molding because of londer cycle times and heated die

molds are less complex and cost less tha injection molding
undercuts are possible because molds can be mad to open sideways

aps: dishes, handles, pipe fittings

51
Q

Polymer Casting

A

Some thermoplastics and thermosets maybe cast into rigid or flexible molds.

For thermoplasics a mixture of monomer and catalyst is poured into themold and allowe dto polymerize at ambient pressure

Use flexible molds allow complex, intricate shapes to be formed

inserts and other components may be cast into the patr allowing versatility

form molding of cups, food containers and packing materials are made using this process

ENME 417 (4 decks)

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