1.3 Enhancement of Materials Flashcards
METALS
work hardening
- when the metal is cold worked eg by bending, rolling or hammering
- improves tensile strength
> crystals are distorted and changed & so cannot move freely within the structure of the metal
> reduced ductility
> cracking/damage in worked area - effects can be removed using annealing
METALS
annealing
- metal is heated then cooled slowly
- allows metal crystals to grow & move into place
- metal loses its elasticity
METALS
case hardening
(summary)
- used for hardening surface of steels with less than 0.4% carbon content
- produces an outer casing of great hardness, improved wear resistance & resistance to surface indentation
- inner core retains softness
METALS
case hardening
(process)
CARBURISING:
- steel is placed in ceramic box & then packed with carbon
- box is heated to 930-950°C- carbon atoms diffuse into material’s structure to build up surface carbon content
- depth of carbon layer determined by length of time exposed to carbon
- longer time = thicker carbon layer
- metal then heated to 760°C & quenched
METALS
quenching
- hot metal is cooled very quickly (in water)
- this seals hard surface case but doesn’t affect properties of inner core
METALS
hardening
- medium & high carbon steels heated to alter crystalline structure
- held at temperature for a given time then quenched in water/oil/salt-water
- increases hardness but also increases brittleness
METALS
tempering
- reduces some excess hardness & brittleness of a hardened metal
- increases toughness & ductility
- metal is heated below the critical point for a given time the slowly air cooled
- exact temperature determines amount of hardness removed
- tempering colour on metal indicates temperature at which brittleness is removed
METALS
case hardening
(workshop technique)
- metal is heated to red
- dipped in carbon powder
- heated again
- quenched
METALS yield point (definition)
- the point on the stress-strain graph that indicates the end of elastic behaviour and the beginning of plastic behaviour
TIMBERS
disadvantages of timber that mean wood enhancement is necessary? (4)
- strength properties decrease when wood is wet
- highly combustible
- susceptible to fungal & insect attack & rot
- anisotropic (wood has different properties in different directions- eg easier to split along grain than against)
TIMBERS
preservatives
- protect wood from fungal & insect attack
- wood can be pre-treated with copper-based preservative which penetrates the wood in order to protect the whole plank
- copper has excellent fungicidal properties
TIMBERS
how is wood treated to make it harder?
- wood is impregnated with polysaccharide
- these cure within the cell structure of the wood
- produce wood with increased hardness, toughness & stability
TIMBERS
how are softwoods treated to make them appeal to customers and how is this more environmentally friendly?
- pigments can be added to make them look more expensive & luxurious like hardwoods, but are more affordable
- softwoods are faster growing than hardwoods so can easily be replanted & regrown
TIMBERS
lamin board
(description)
- thin strips of softwood sandwiched between veneer panels
- strips usually between 5-7mm in thickness
TIMBERS
lamin board
(advantages) (6)
- light, cheap, strong
- bend resistant (stiff)
- easy to work with
- higher quality than blockboard
TIMBERS
lamin board
(disadvantages) (1)
(uses) (1)
- more expensive than chipboard
- USES: shelves
TIMBERS
blockboard
(description)
- strips of softwood glued together & sandwiched between veneer panels (softwood, hardwood, MDF, particle board)
- strips are up to 25mm in thickness
TIMBERS
blockboard
(advantages) (4)
- lightweight
- corrosion resistant
- strong
- environmentally friendly
TIMBERS
blockboard
(disadvantages) (3)
(uses) (5)
- gap between strips of wood makes it difficult to work with
- cheap & low quality blackboard may be vulnerable to insect attack
- shrinking & swelling may occur
- USES: doors, table, shelves, partition walls, paneling
TIMBERS
plywood
(description)
- thin sheets of wood are glued together with the grains at 90° to the previous layer
TIMBERS
plywood
(advantages) (5)
- lightweight
- cheap
- veneer can be added to enhance aesthetics
- strong in all directions
- can be made to any size
TIMBERS
plywood
(disadvantages) (3)
(uses) (3)
- glue used (urea formaldehyde) is carcinogenic
- susceptible to water damage
- edges need to be finished as layers visible
- USES: furniture, structural work, cladding
TIMBERS
structural composite lumber (SCL)
(description)
- strands of wood layered with resins (eg urea formaldehyde)
- pressed & heat cured to produce a stable billet
TIMBERS
structural composite lumber (SCL)
(advantages) (4)
- remains stable
- less prone to warping/shrinking/splitting
- can carry larger loads than traditional lumber
- environmentally friendly as uses recycled chips
TIMBERS
structural composite lumber (SCL)
(disadvantages) (1)
(uses) (3)
- urea formaldehyde is carcinogenic
- USES: beams, joists, rafters
TIMBERS
laminated veneer lumber (LVL)
(description)
- veneers are layered with resins (eg urea formaldehyde)
- pressed & heat cured to produce a stable billet
TIMBERS
laminated veneer lumber (LVL)
(advantages) (6)
- cheap, strong
- can be made to any length
- few defects
- less prone to warping/splitting/shrinking
- can carry larger loads than conventional lumber
TIMBERS
laminated veneer lumber (LVL)
(disadvantages) (1)
(uses) (3)
- urea formaldehyde is carcinogenic
- USES: beams, joists, rafters
TIMBERS
resins with fire retardants
- resin is impregnated with fire retardant cladding (eg for indoor flooring)
POLYMERS
additives- process aids
LUBRICANTS
- wax/calcium stearate reduces viscosity of molten polymer
- make polymer less ‘sticky’
- allows more intricate shapes to be formed
- allows moulding temperature to be lowered- saves energy
POLYMERS
additives- process aids
THERMAL ANTIOXIDANTS
- help prevent oxidising/discolouring due to excessive heat in processing
POLYMERS
additives- enhance aesthetics
PIGMENTS
- time particles mixed in at molten state to give colour to final product
POLYMERS
additives- improve product function
ANTISTATICS
- polymers are insulators so build up static charge which attracts dirt & dust
- antistatic improve surface conductivity by attracting moisture from surroundings
- this reduces static charge
- eg plugs & sockets, electric wire covers
POLYMERS
additives- improve product function
FLAME RETARDANTS
- bromine, chlorine, phosphorous or metal when added to polymers reduce change of combustion/spread of fire
- important in product exposed to heat (eg car engine parts) or potential electrical fires (eg plug sockets)
POLYMERS
additives- improve product function
PLASTICISERS
- allows plastics (eg PVC) to become less hard & brittle at normal temperature use
- also help in processes to allows polymers to be easily formed at higher temps
- eg added to LDPE for food wrap allowing it to be stretched over product
POLYMERS
additives- improve product function
FILLERS
- provide but to product (less product required)
- improve polymer properties
- common fillers:
> provide bulk: sawdust, wood, flour
> increase thermal conductivity: chalk, clay, calcium carbonate
> polymers heat up & cool down more quickly = shorter mould cycle times
POLYMERS
additives- prolong life (prevent degradation)
ANTIOXIDANTS
- held reduce environmental deterioration of polymer from exposure to oxygen
- polymer degradation can lead to increased brittleness, surface cracks & pigment discolouration
POLYMERS
additives- prolong life (prevent degradation)
UV LIGHT STABILISERS
- prevent polymer chains from being broken down by sunlight
- UV attack causes polymers to lose colour & become more brittle
- white pigment becomes yellow, dark pigment (eg green, blue) take on a ‘milky’ effect
- eg PVC hosepipes- become brittle & cracks appear
- other uses include sports stadium seats & synthetic grass
POLYMERS
additives- encourage degradation
BIODEGRADABLE PLASTICISERS
- similar to regular plasticisers
- make polymer more flexible, softer, easier to break down = faster degradation
POLYMERS
additives- encourage degradation
BIO-BATCH ADDITIVES
- oxy-degradable = degrade in presence of oxygen
> often used for high volume single use items (eg carrier bags, food packaging) - photodegradable = degrade when exposed to UV light
- hydro-degradable = degrade in presence of water
