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Processes S2 > Composites > Flashcards

Composites Flashcards

1
Q

Definition

and exampls

A

Material composed of two or more distinct phases whose combinatio produces aggregate properties that differ from original components
Examples: plastic moulding components with fillers, rubber mixed with black carbon, wood

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2
Q

Components

A
  1. Primary phase - forms matrix
  2. Secondary phase - imbedded sometimes referred to as a resinforcing agent as usually strengthens material
    - can be in form of fibres, particles and other various geometries
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3
Q

Polymer Matrix

A
  • Provides the bulk form of the part
    o Holds the embedded phase in place (usually enclosing and often concealing it)
    o When a load is applied, the matrix shared the load with the secondary phase, in some cases deforming so that the stress is essentially born by the reinforcing agent
  • Both thermoplastics and thermosets are used as matrices in PMCs
    o Thermosetting polymers are the most common matrix material
     Phenolics, unsaturated polyesters and epoxies
    o Thermoplastics include nylons (polyamides), polycarbonate, polystyrene and polyvinylchloride
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4
Q

Reinforcement Phase

A
  • Function is to reinforce the primary phase
  • Reinforcing phase (imbedded in the matrix) is most commonly fibre, particle or flake shaped
  • Fibres can be short or long (continuous or discontinuous), orientated or random
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5
Q

Fibre Reinforced Polymers (FRP) Examples

A
  • Glass - most widely used
  • Carbon - high elastic modulus
  • Polymers - Kevlar 49
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6
Q

FRP Properties

A
  • High strength-to-weight ratio (W=1/5 of steel yet stregnth and modulus comparabble in fibre direction)
  • Good fatigue strength
  • Good corrrosion resistance, although polymers are soluble in various chemicals
  • Low thermal expansion for many FRPs
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7
Q

FRP Applications

A
  • Aerospace
  • Automotive
  • Sports and recreation: boat hulls, fishing rods, tennis racquets, golf club shafts etc.
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8
Q

Processing of Polymer Matrix Composites

A

Open mould processes
- Manual lay up and spray up
- Automated lay up
Closed mould (similar to plastics moulding)
- Compression moulding
- Resin transfer moulding
Filament winding
Pultrusion processes

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9
Q

Combining Matrix and Reinforcement

A
  1. Materials arrive as seperate entities and are combined durig shaping
    - Filament winding and pultrusion, in which reinforcing phase are continuous fibres
  2. The two component materials are combined into some starting form that is convinient for shaaping
    - Moulding compounds
    - Prepregs
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10
Q

Moulding Compounds

A
  • FRP composite compounds consist of the resin matrix with short randomly dispersed fibres, similar to those used in plastic moulding
  • Most moulding compounds for composite processing are thermosetting polymers
  • Since they are designed for moulding, they must be capable of flowing - cured during and/or after final shaping, not during shape processing
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11
Q

Open-mould Processes

A

Family of FRP shaping processes that use single positive and negative mould surface to produce laminated FRP structure
- Hand lay-up
- Spray-up
- Automated lay-up
Starting materials (resins, fibres, mats, woven rovings) are applied to the mould in layers
Followed by curing and part removal
Common resins are unsaturated polyesters and epoxies, using fibreglass as the reinforcement

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11
Q

Prepregs

A
  • Fibres impregnated with partially cured TS resins to facilitate shape processing
  • Curing is completed during and/or after shaping
  • Fabricated with continuous filaments - increases strength and modulus
  • Available in the form of tape or cross-plied sheets or fabrics
  • Associated with advanced composites - Kevlar and fibreglass
  • Typically stored in a freezre
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12
Q

Hand (Wet) Layup

A
  • Shaping method in which successive layers of liquid resin and reinforcement are manually applied to an open mould
  • Labour intensive
  • Orientation of the fibres is controlled hence resulting in stronger products made by spray up
  • Applications for large parts but low in production quantity - not economical for high production
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12
Q

Hand Lay-up Process

A
  1. Mould is treated with mould release agent
  2. Thin gel coat (resin) is applied to outside surface of mould
  3. Layers of resin and fibre mat or cloth are applied - when gel coat is partially set, each layer is rolled to impregnate the fibre with resin and remove air
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13
Q

Spray-up Process

A
  • Liquid resin and chopped fibres are sprayed onto an open mould to build successive FRP laminations
  • Represents an attempt oto mechanise the application of resin-fibre layers and reduce lay-up time
  • Applications: boat hulls, bathtubs, shower stalls etc.
  • Randomly orientated short fibres mean not as strong as those made by hand lay-up as continuous and directed
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14
Q

Automated Lay-up

A
  • Automated tape-laying machines dispense a prepreg tap onto an open mould following a programmed path
  • Typical products: structural components of aircraft, panelling
  • Can only handle gently curved surfaces (more extreme requires impregnated tows or slit tape)
15
Q

Advantages of Automated Lay-up

(2)

A
  • Removes boring, repetitive work
  • Good control of ply directions - optimise specific strength of part
16
Q

Disadvantage of Automated Lay-up

A

Capital cost - more skilled workforce required, safety precautions are expensive

17
Q

Closed-mould Processes

A
  • Performed in moulds consisting of two sections that open and close each cylce
  • Tooling cost is >x2 of comparable open mould due to complex equipment
18
Q

Advantages of Closed-mould

(4)

A
  • Good finish on all part surfaces
  • Higher production rates
  • Closer control over tolerances
  • More complex three-dimensional shapes are possible
19
Q

Categories of Closed-mould

(3)

A
  • Compression moulding
  • Transfer moulding
  • Reactional injection moulding
20
Q

Compression Moulding

A
  • Identical in every aspect except materials are thermosetting plastics
  • Materials: glass fibres + incured viscous resin paste, resins polyester or vinyl compounds and additives to reduce material shrinkage, increase dimensional stability + reduce material cost
  • Heated mould allows TS resins to flow and cure to form the composite material into shape
  • When moulding is sufficiently cured, mould is opened and part is removed
21
Q

Resin Transfer Moulding (RTM)

A
  • Similar to transfer moulding but with a reinforcement in the mould
  • Resin is mixed with fibres then forced by a piston into the mould
  • (Can be vacuum assisted (VARTM) to reduce air bubbles and voids leading to an improvement in composite quality, in terms of appearance and mechanical properties)
  • Uses positive and negative moulds that provide smooth finish on both sides
  • Alternative to hand lay-up, hand spray-up and compression moulding for low/intermediate production
22
Q

Reinforced Reaction Injection Moulding

A
  • Conventional (RIM) - two reactive ingredients are mixed and injected into a mould cavity where curing and solidification occur
  • Reinforced (RRIM) - similar to RIM but with reinforcing fibres, typically glass ones, in the mixture
  • Transportation products: automotive and truck - body panels, fascia, bumper beams, spare tyre covers, floor pans and other similar products
23
Q

Filament-winding Process

A
  • Continuous fibres are passed through resin bath and wound around a mandrel in a variety of orientations, controlled by fibre feeding mechanism and rate of rotation of the mandrel
  • The mandrel is the special tooling that determines the geometry of the filament-wound part - for removal mandrel must be capable of collapsing after winding and curing
  • Products made have high strength-to-weight ratio and also serve as lightweight pressure vessels
  • Used for hollow, generally circular or oval sectioned components such as pipes, tanks and pressure vessels
  • Process is limited to convex shaped components
24
Q

Pultrusion

A
  • Pultrusion = pull + extrusion
  • Composties are made by pulling the continuous fibre materials through a die that contains a resin bath
  • Pulling action is used to avoid fibre damage
  • SHape of the cross of the die (I or H-shaped, square, rectangular or circular), determines the shape of the composites
  • Composite is cured in the die and final shape is usually bar or rod shaped
  • Products include: golf clubs, ski poles, fishing rods, drive shafts
  • Pulforming is a related process for parts that are curved and may have variations in cross-sections throughout their legnths
25
Q

FRP 5 Methods for Shaping

A
  1. Open mould
  2. Closed mould
  3. Filament winding
  4. Pultrusion
  5. Miscellaneous

Processes S2 (8 decks)

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