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Manufacturing With Polymers Flashcards

1
Q

What are the key properties of a Thermoplastic?

A

Chains of covalently bonded monomers

Weak attractive forces between chains

Amorphous structure

Heat breaks down bonds between chains, softening bulk material

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2
Q

What additives are available for Thermoplastics?

A

Plasticisers - Improve toughness and moulding performance

Fillers - adds bulk, reduces final material cost

Pigments/Dies - makes final material a certain colour

Stabilisers - combats various types of degredation

Flame Retardants

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3
Q

What are the key properties of Thermosetting plastics?

A

Polymer chains undergo ‘cross linking’ with the application of heat

Cross links are not broken down by further addition of heat

Material ‘cures’ at the cross linking temperature

Cannot be reformed once cured

Full condensation polymerisation occurs in-mould

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4
Q

In what raw forms are Thermoplastics available as?

A

Powder

Resin

Paper/cloth impregnated with resin

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5
Q

What additives are unique to Thermoplastics?

A

Hardening Agents - to ensure cross linking occurs

Accelerators - speed up curing process

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6
Q

In general, what benefits do polymers offer compared to other engineering materials?

A

Generally cheaper

Good specific properties compared to many metals

Better for applications where weight is a concern

Customisable for the application

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7
Q

Which is generally more rigid; Thermoplastics or thermosetting plastics?

A

Thermosetting

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8
Q

Are stress - strain curves useful when dealing with plastics?

A

They are useful for sorting materials

Mechanical properties of plastics vary hugely with temperature so data is not useful for predicting long term behaviour

Yield point is not well defined for plastics. Generally take 1% strain as limit (less for brittle materials)

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9
Q

What is the secant modulus?

A

Used instead of the Young’s Modulus for plastics

Select 2% strain value

Slope between that point and origin defines elastic modulus

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10
Q

What is a composite material?

A

A single materials constructed from multiple constituent parts

Generally a matrix combined with reinforcement

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11
Q

What are the main benefits offered by composite materials?

A

Generally have low weight and high stiffness

Easily customised depending on the use

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12
Q

What are the two general types of fibre reinforcement are available for composite materials?

A

Continuous (very long fibres)

Discontinuous (short fibres)

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13
Q

What are the most popular fibre reinforcement materials?

A

Glass

Kevlar

Carbon

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14
Q

What are the most popular resins used for composite materials?

A

Epoxy - the generally better choice

Polyester - cheaper, high shrinkage on cure and high water absorption

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15
Q

Describe the main steps to manufacture using injection moulding.

A

1) Polymeric material fed from hopper into heated barrel
2) Rotating screw moves material along barrel
3) When the mould is closed, plasticised material builds up in front of the screw
4) When build up occurs, the screw stops rotating and physically moves forward, injecting material into the mould
5) When the mould is full, the screw rotates again, bringing new material for the next shot. The mould cools
6) When cool, component is ejected
7) New material is injected into the mould. The process repeats.

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16
Q

Why is a high powered motor essential for injection moulding?

A

Polymer is heated by friction in the barrel (as well as by heaters). Important as polymers are poor heat conductors, so a heated barrel alone would cause uneven heat distribution, resulting in non uniform mechanical properties.

Heating by friction creates a more even temperature distribution

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17
Q

What are the three sections of an injection moulding screw?

A

Feed Zone - moves granules from hopper and compacts then

Compression Zone - squeezes polymer into smaller space, removing air and plasticising

Metering Zone - regulates flow of plastic into the mould

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18
Q

Why does a screw for moulding nylon have a short compression section?

A

Nylon transitions abruptly from a solid to a liquid and doesn’t need the long length.

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19
Q

What key properties does the injection mould itself have?

A

Constructed from a high quality, metallic material

Good wear resistance

Hardened

Highly polished

May have multiple cavities for multiple components to be produced simultaneously

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20
Q

What are the components of an injection mould?

A

Sprue - carries material from barrel to mould

Runners - carry material to other parts of the mould

Gates - used to restrict flow

Ejector pins - eject component after solidification

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21
Q

What process is used to make components with uniform cross section (pipes, tubes, sheets etc)?

A

Extrusion

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22
Q

Describe the extrusion process for a polymer

A

1) Continuously rotating screw feeds granules from hopper along the heated barrel
2) Liquid polymer passed through a die
3) Material is immediately cooled when it exits the die to improve stiffness and prevent it losing shape (use water jets or cooled rollers)

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23
Q

How can wires be coated with insulation?

A

Wire passes into right angled die

Polymer forms around wire

Die determines insulation thickness

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24
Q

Describe injection blow moulding

A

Starting shape produced by conventional injection moulding

Mould opened prior to solidification and the material is then blow moulded

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25
Describe extrusion blow moulding
Thick walled tubing is extruded by conventional extrusion Material is fed between two halves of mould Mould closes on material, cutting it to length Compressed air is blown into closed mould and the plastic takes the shape of the mould cavity
26
Which is better; extrusion or injection blow moulding and why?
Extrusion blow moulding Material is cut to correct length straight away Wall thickness easily controlled Wide variety of compatible thermoplastic materials
27
Describe the main steps of vacuum forming a component
1) Extruded, thermoplastic sheet is clamped over an open mould and heated 2) Once material is at the required temperature, air is removed and atmospheric pressure forces the sheet into shape 3) Final component cooled rapidly with air or water
28
What examples of components are made with vacuum forming?
Food trays Baths Shower trays Packaging
29
Name some plastics used with thermoforming
Polypropylene Polystyrene ABS Polycarbonate
30
Describe the main steps used to produce a component with rotational moulding
1) Certain quantity of moulding plastic placed in a split, hollow mould 2) Mould is rotated biaxially and heated. Rotation is slow enough to avoid a centrifugal effect. Heat causes adhesion to the mould walls 3) Rotation continues until an even layer of plastic coats the mould 4) Heating stops and the mould is cooled with water or air. 5) Component removed
31
What type of component does calendaring produce?
Continuous sheet or film
32
Describe the main steps used in calendaring.
1) Polymer powder and fillers are blended and heated 2) Hot, partially gelled mixture passes between heated rollers and continuous sheet emerges. Sheet contains coarse particles so is further extruded through a fine mesh, producing a dough. 3) Dough is fed into the calendar, which contains 3/4 cast iron or steel rollers which then produce the final thickness Rollers are highly polished. Can produce film down to 0.1mm thick. Equipment is costly.
33
Why are the moulds used in rotational moulding cheap?
They do not need to withstand pressure so can just be mild steel sheet or cast, machined aluminium
34
What is the most common moulding technique for thermosetting plastics?
Compression Moulding
35
Briefly describe the process of compression moulding for thermosetting plastics
1) Two halves of mould are fixed to vertical platens of hydraulic press 2) Granules of moulding compound are placed in mould 3) Mould heated and pressurised, liquifying the plastic, filling the mould 4) Curing occurs while liquid, meaning the component can be removed when still hot without deformation
36
Why do some compression moulded parts require some post-processing?
Some excess material can form at the seam which needs to be machines away
37
Briefly describe the process of resin transfer moulding (RTM) for thermosetting plastics
1) Moulding powder loaded into preliminary chamber connected to the mould 2) Mould closed and heated under pressure 3) Compound is plasticised and injected into the mould where cross-linking occurs
38
Why does RTM generally produce higher quality components than compression moulding?
Liquid plastic better fills the mould compared to granules or powder form
39
Is a sprue and runner system used for RTM?
No, due to the very high pressures involved
40
Briefly describe thermoset extrusion
Similar to metal extrusion Raw material passed through cool barrel (to prevent premature cure) into heated, long die Material cures as it takes shape in the die
41
Why can screw extrusion not be used for thermosetting plastics?
The pressures are too high. Too much heat in the barrel caused by friction could cause premature cure
42
What are expanded plastics used for?
Insulation Cushioning Packaging
43
List and briefly describe methods to create expanded plastic products
1) Thermal decomposition of “blowing agent” additive which gives off nitrogen 2) In Situ chemical reactions producing gases 3) Low pressure release of dissolved gases (added in autoclave) 4) Vapour from a volatile solvent added to material 5) Aerating the material while liquid
44
Describe one method to mass produce expanded plastics on an industrial scale
Injection moulding with the addition of a foaming agent Produces cellular core and dense skin Skin thickness varied with amount of foaming agent
45
What difficulties do plastics add to component design?
Plastics are more sensitive to temperature changes Stress-strain behaviour highly dependant on temperature and humidity Creep behaviour Tough, but notch sensitive
46
What problems can the moulding process cause to the plastic material?
Flow direction causing anisotropy Warp/residual stresses from cooling
47
What geometrical features need to be considered for plastic components?
Tapering (easy release from mould) Undercuts (can make impossible to remove from mould) Wall thickness (another card) Corners (cause stress concentrations, use radii) Mould Filling (gate placement, multiple gates for larger parts, welds where molten plastic meets are weaknesses) Shrinkage (different thicknesses have different cooling rates, causes distortion, flat sections can warp)
48
Why is wall thickness an important consideration for the design of plastic components?
Strength of the component is dependant on the wall thickness Thicker walls lower the production rate due to longer cooling time Voids are more likely to form in thicker walls Thin walls may need to be stiffened with ribs
49
What benefits are offered by plastic reinforcement?
Improved strength and stiffness
50
In a reinforced plastic component, what constituents govern the strength and toughness respectively?
Fibres offer strength Matrix offers toughness
51
What is the main reason composite materials are being developed as a replacement for metals?
Composites have better specific properties compared to metals (strength, stiffness)
52
What materials are generally used for reinforcement fibres?
Glass, carbon, aramid
53
In what forms are fibres generally supplied in?
Filaments (long continuous fibres) Yarn (twisted fibre bundles) Roving (untwisted fibre bundles) Tows (bundles of thousands of filaments) Woven fabrics Mats Whiskers (short, single-crystal fibres)
54
Why are thermoplastic resins difficult to reinforce?
Highly viscous, cannot penetrate fibres well
55
What material is commonly used to reinforce thermoplastics?
Glass fibres (due to its low cost)
56
What form of thermoplastic composite is popular for use in high temperature applications?
PEEK reinforced with carbon fibres
57
Comparing epoxide and polyester resins, which is superior and why?
Epoxides; stronger, stiffer, adhere better to the reinforcement
58
How can thermoplastics be reinforced?
Can use traditional manufacturing methods, but add reinforcement as chopped fibres to raw material
59
What thermosetting composites are used for high stress applications?
Epoxides reinforced with carbon and aramid fibres
60
What different ways can reinforcement be added to thermoset?
As chopped fibres during manufacture Before manufacture Can be supplied pre made as pre-impregnated sheets (pre preg) Dough or sheet moulding compound
61
What is pre preg?
Pre made resin system reinforced with synthetic fibres (glass, carbon, aramid)
62
What applications are pre preg sheets commonly used for?
Aircraft primary structures, tubes, pressure vessels
63
What are the limitations to using pre preg?
Can cure at room temperature, so stored at very low temperatures Difficult to handle Not suitable for automated processes
64
Briefly describe hand lay up using an open mould for thermosets
1) Open mould coated with resin 2) Reinforcement (as cloth, mats or fibre strands) placed onto resin while tacky 3) More resin applied 4) Roller used to impregnate the fibres 5) Layers are added until desired thickness is reached 6) Component cured by heating. Better consolidation achieved using a vacuum bag
65
Briefly describe how pre pregs are used to manufacture
1) Open mould coated in release agent 2) Pre preg material cut to shape and placed on mould 3) If necessary, more layers are added, normally at different angles 4) Peel ply out over lay up 5) Release film added 6) Breather fabric added (allows application of vacuum) 7) Vacuum bag placed around whole assembly and sealed 8) Air removed 9) Assembly consolidated in autoclave at temperature, under pressure
66
Briefly describe Vacuum Assisted Resin Infusion for manufacturing reinforced polymers
1) Dry fibre/mat reinforcement is placed on a single sided mould 2) A rigid or flexible film membrane is placed on top and sealed. 3) A vacuum is applied, drawing the resin through the fibres (an infusion mesh ensures the resin can freely flow when under vacuum)
67
What is filament winding used to produce
Cylindrical or spherical, hollow components
68
Briefly describe the filament winding process
Fibers fed through a resin bath and wound onto a shaped former in a helical pattern
69
What are the benefits of using filament winding?
Very fast Economical Resin content can be metered onto each fibre
70
Briefly describe pultrusion
Raw fibres pulled through thermosetting resin bath through a shaped die. Heated die guides the fibres into shape and cures the resin.
71
What are adhesives?
A material used to attach materials surfaces together
72
What makes a good adhesive?
Good wetting of the surfaces Must cure so as to transmit stress
73
What controls how well an adhesive wets a surface?
Free surface tension and energy of the adhesive compared to the material For good wetting, the surface energy of the material must be greater than that of the adhesive Creates a 0 contact angle, thin film as opposed to droplets
74
What are the four main types of adhesives?
Epoxides Acrylics Phenolics Polyurethane
75
In what forms are epoxy adhesives available as?
Paste Film Powder Liquid
76
What are the two main components of an epoxy adhesive?
Epoxy resin Reactive hardener
77
What situations are most suitable for epoxy adhesives?
High strength Creep resistance Moisture resistance Heat resistance
78
What are the three types of acrylic adhesive?
Anaerobic (cure in presence of metal, exclusion of air) Cyanoacrylates (cure with moisture) Toughened Acrylics (resin on one surface, initiator on other, cure when brought together)
79
What are phenolic adhesives mainly used for?
Wood
80
What pros and cons do polyurethane adhesives have?
Quick to cure Weak Susceptible to environmental attack
81
Why are surface pretreatments sometimes necessary before applying an adhesive?
May be an oxide layer that peels off Low surface roughness may cause poor penetration of the adhesive
82
What surface pretreatments are usually used before applying an adhesive?
Emery Paper Shot Blasting
83
What is the main issue with using adhesives with thermoplastics?
Surface energy of the adhesive is greater than the surface energy of the thermoplastic, causes poor wetting and a large contact angle (droplets)
84
What common metals are reasonable useful with adhesives?
Cast Iron Steel Stainless Steel Aluminium
85
How suitable are PVC and PTFE to use with adhesives?
Poor
86
What are the general requirements for a good, bonded joint?
Large bond area Uniform, continuous bond line Shear or compressive bond loading No tensile or peel loads
87
What are the general advantages to using adhesives?
Can join dissimilar metals Join over large area No stress concentrations Permanent bond Occurs frequently at room temperature Smooth finish
88
What are the general disadvantages to using adhesives?
Need time to reach optimum bond strength Bond affected by moisture Most adhesives are weak at high temperatures

Manufacturing And Materials (3 decks)

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