Unit 2 - Metal Flashcards
What is a metal?
A group of materials which are solid at room temperature (except for mercury) and have a crystalline structure, and are capable of metallic bonding
What are the useful properties of metal?
- Good mechanical properties (strong, tough, hard, and stiff)
- Ductile and malleable which makes them easy to form
- Opaque and impermeable which can make them an absolute gas and light barrier
- Thermal and electrical conductors
- Well-established recycling stream with no loss in properties
- Vary in density and corrosion resistance
What is the most abundant metallic element, and in what form is it usually found in nature?
Aluminium - found in nature as bauxite
What are the two stages of aluminium production called?
Stage 1 - Bayer process
Stage 2 - Hall-Heroult process
Describe the Bayer process
- Bauxite is crushed into a fine powder and mixed with caustic soda (sodium hydroxide) to form a slurry
- It is transferred to a pressure vessel and boiled for several hours to form sodium aluminate - this is know as the digestion stage (important to note that impurities such as iron oxide will not dissolve and are removed for treatment and storage)
- Sodium aluminate is seeded with pure aluminium hydroxide crystals onto which further aluminium hydroxide crystallises
- Crystals are dried at 1000°C to drive off water, leaving pure aluminium oxide (alumina)
Describe the Hall-Heroult process
- This stage is to liberate the aluminium from the alumina (which is a very stable compound)- it requires a large amount of energy to break the chemical bonds and so is done in an electrolytic cell
- Alumina is dissolved in molten cryolite - this is a chemical agent that decreases the melting point of alumina from 2000°C to 950°C
- The electrolytic cell is lined with refractory bricks to protect from the high temperatures and then graphite anodes are lowered in
- When electricity is passed through the solution, 99% of aluminium is deposited at the negative cathode - 1T of aluminium is produced for every 2T of alumina
- Cells typically contain 24 anodes in two rows which are operated continuously - to prevent molten material from solidifying, the liquid aluminium is syphoned off regularly
- Electrodes are sacrificial and must be replaced regularly
What are the 4 methods of processing aluminium?
- Casting
- Hot rolling
- Continuous casting
- Cold rolling
Describe the process of casting aluminium
- Molten material is poured into a mould to solidify - composition can be controlled by the addition of elements to form an alloy
- Slabs are up to 13m long and weigh up to 25 tonnes
Describe the process of hot rolling aluminium
- Takes place at 60% of a metal’s melting point, so around 400°C for aluminium - this is because it is easier to deform
- Done to reduce the thickness to ~6mm thick
- Slab is then reheated to 500°C to go through a scalping mill to remove oxide from the surface
- Then goes back and forth through a reversing mill to do the initial thickness reduction
- Then goes through a series of tandem rolls to get to the final desired thickness
- Temperature has now dropped to 300°C and the sheet is rolled into a coil
- Rolls are lubricated with oil emulsions to reduce wear
Name the two types of continuous casting aluminium
- Roll continuous casting
- Belt continuous casting
Describe the process of roll continuous casting aluminium
- Casting liquid aluminium directly onto a single water-cooled rotating drum or through a pair of cooled rolls
- Up to 10mm thick
Describe the process of belt casting continuous aluminium
- Casting aluminium onto a cooled belt made from steel or glass cloth
- Much slower than roll casting
Describe the cold rolling process for aluminium
- Done for foils for requiring very low micron
- Performed cold so the sheet has sufficient strength as it passes through the rollers
- Is done as a coil to coil process done at 100°C
- Rolls are highly polished - the thickness is controlled by the distance between the rollers
- As the aluminium passes between the rollers, the structure is changed, therefore making it stronger
- Can be done down to 6µm
- The foil has a bright finish on the surface in the contact with the rollers and matte on the inside
Why does aluminium need to be tempered?
- During rolling, aluminium is said to be ‘work-hardened’ - this means that the structure has changed through the application of force making it more difficult to deform
- These internal stresses must be relieved by annealing
Describe the process of annealing aluminium
- The aluminium is heated to above its recrystallisation temperature and held for a period of time to allow a process called diffusion to happen and the structure to reach equilibrium
- The properties of an annealed coil are the same in all directions
What type of temper is used for packaging foils?
Soft temper (slow cooling)
What type of temper is used for blister foil and aseptic packaging?
Hard temper (fast cooling)
What is the main ore of iron?
Iron oxide - haematite
Describe iron extraction for steelmaking
- Iron oxide is ground into small chunks and transported to steelworks by ship
- Extraction takes place in a reaction vessel called a blast furnace
- These are made up to 60m tall and 15m in diameter
- Made from refractory-lined and water-cooled steel
- Iron ore, coke (a form of carbon), and limestone are continually fed into the top of the blast furnace.
- Halfway up, nozzles called tuyeres blow 1000°C hot air to the centre to aid chemical reactions - The reversible Boudouard reaction takes place:
2CO <–> CO2 + C - Heavy liquid iron sinks to the bottom. Slag (formed by limestone reactions) floats to the top and is removed
- The iron produced is known as pig iron and contains 5% carbon and sulphur - it is tapped into a ladle and has calcium oxide added to remove the sulphur
What is steel?
Iron which contains up to 2% carbon (anymore than this is known as cast iron)
What are the two methods of converting pig iron to steel?
- BOS - basic oxygen steelmaking
- EAF - electric arc furnace
Describe the process of BOS steelmaking
- Accounts for 70% of crude steel production
- Step 1 - pig iron is transferred to a converter - this is a refractory-lined steel vessel; up to 30% scrap can be added at this stage
- Step 2 - a good is bought over the top of the vessel and a water-cooled lance is lowered to charge the surface
- Capacity is around 400 tonnes
- Step 3 - high pressure oxygen is blown which reacts with the excess carbon in the pig iron - this reduces the carbon content to form steel
- Step 4 - vessel is tilted on lugs to tap the steel
- Can produce up to 400 tonne of crude steel in 25 minutes
- The primary fuel for the process is the exothermic chemical reactions which occur in the vessel
Describe the EAF process of steelmaking
- Utilises up to 100% scrap and is also water-cooled
- Electrodes are lowered in; an electrical arc is struck to cause induction heating to melt the charge
- The voltage is increased and fluxing agents are added
Name the four methods of processing steel
- Casting
- Hot rolling
- Cold rolling
- Heat treatment
Describe the casting process for steel
- Molten steel is poured into a tundish
- Nozzles direct liquid steel into water-cooled copper moulds which rapidly chill and solidify the surface
- Oxidation contamination is a big issue so the steel is protected by a layer of artificial slag
- Rollers rotate the semi-cooled steel and it is cut to the desired length by gas cutters
Describe the hot rolling process for steel
- Slab is re-heated to 1250°C and scalped (oxide layer on top removed)
- Slab goes through a reversing mill for thickness reduction
Describe the cold rolling process for steel
- Slab is re-heated to 900°C (or sometimes ambient) in preparation for thickness reduction
- Passes through 20 roll stacks and is coiled upon exit
- Directionality is introduced and annealing may be required part-way through the process
Describe the heat treatment process for steel
- Necessary to control the properties of steel prior to any other processing
- Annealed at 910°C to return the steel to equilibrium
Name the 4 most common alloys of steel
- Blackplate
- Tinplate
- Tin-free steel
- Polymer-coated steel plate
Describe blackplate
- Low carbon steel (0.25% carbon) with no other coatings
- Poor resistance to corrosion, even when a lacquer is applied
- Used for products such as wax, oil, and grease
Describe tinplate
- Also called electrolytic tin plate - it is blackplate and tin coating
- Tin gives the corrosion resistance by providing a barrier to the atmosphere
- Can be used with or without a lacquer but thickness should be adjusted accordingly
- Tin levels must be below 200ppm (statutory limit)
Describe tin-free steel
- Also called electrolytical chromium-coated steel (ECCS) - it is blackplate and chromium
- Cheaper than tinplate but always requires a lacquer, but chromium surface is rough so provides a good surface for coating
- Chromium layer must be removed for welding
- Most commonly used for can ends
Describe polymer-coated steel plate
- One or both surfaces have a polymer coating which is done via direct extrusion or lamination
- Usually PET or polypropylene between 12-40µm
What two things is electroplating used to make?
- Tinplate
- ECCS
Describe the process of electroplating
- Electrolyte (liquid) is broken down into its constituent atoms
- Positively-charged metal ions are deposited on the cathode
- In continuous process, the negative cathode charge is transferred to the blackplate strip - this is guided by charged rollers
- Each time it enters the electrolyte, a thin layer or tin is deposited on the surface
- Tin either comes from sacrificial anodes or from the electrolyte
Name the 12 steps for making a 3-piece welded can and explain each step
-
Shear
- Steel enters in coils of the correct thickness which are then inspected
- Then goes through straightening rollers, then through a reciprocating tool which cuts into sheets
-
Coat and cure
- Sheets are coated with a lacquer via one of two methods:
- Method 1 - thermal cure: mandatory for food contact on heat-processed containers; cured by heating to 210°C for 15 minutes to drive-off the solvents
- Method 2 - UV-curing: not for food contact, but are used for lip-contact on drinks cans; photo-initiators give an instant chemical cure
- Sheets are coated with a lacquer via one of two methods:
-
Slitting
- Cuts each sheet to the size of 1 can body
-
Form and weld
- Flat blanks are fed through 3 roller to form a loose cylinder with a 0.4mm overlap along the seam
- Area being welded is held between 2 copper rollers with flowing alternating electric currents
- Electrical resistance heats and softens the can so the weld is formed to 1.4x the original thickness - this is called electric resistance welding
- Multiple spot welds are produced to give one long and continuous weld
- Copper is used to prevent build-up of tin metal on the contact surfaces of the rollers which would otherwise alter the electrical resistance and affect the weld quality
- This is called soudronic welding
-
Coat and cure
- Coating must be removed to form a weld on the edges of the can; therefore it is essential that the coating is reapplied in this area to protect the join from corrosion or other damage
- Usually done as a side stripe down the internal length of the can
-
Parting
- Only used for short height cans
- Cans that are shorter than the diameter may be unstable during processing and fall over, so to avoid this, multiple cans are made from one body
-
Necking
- Done to reduce the end diameter, usually on drink cans which allows a smaller end to be used and thus save materials
- Designed in a way that allows the cans to stack on shelves
- ‘Neck out’ is used for the tops of cans with easy-open lids (ring-pull)
-
Flanging
- Produces a ‘lip’ around the top and bottom to allow the can end to sit on the body before seaming
-
Seaming
- Can end is fixed onto the base of the 3 piece can
-
Beading
- Circumferential ridges are rolled into the body of the can
- These provide more strength and prevent denting
- Detrimentally, they do reduce compression strength (resistance to vertical crush)
-
Inspect
- Inspected and tested to ensure a good seal
-
End of line
- Other processes may include printing, coating, and palletisation
Describe the process for making a 3-piece built-up can
- Have a folded seam (also called a lock seam) and are used for dry products such as biscuits
- Coiled sheet is cut into master blanks which are coated and cured before slitting into individual blanks
- ‘Hooks’ are rolled into the edges where the side seam will be; and to the ends to allow can end attachment
- Blank is positioned over a mandrel and a hinged mould closes to deform the blank around the mandrel
- Mandrel closes in a way that means the hooks overlap slightly
- Mould is released slightly to lock the hooks together and a hammer applies pressure to give a good seal
- Hot melt adhesive may also be used
Describe the process for making a 2-piece drawn and re-drawn can
- Typically used for short cans such as those for tuna
- A press cuts circular blanks and draws them into cups in a singular operation - the surface area and thickness stay the same
- Cups are then re-drawn into a taller can with a smaller diameter and the flange is trimmed
Name and describe the 5 finishing processes for 2-piece cans
-
External coating
- Can pass inverted through a lacquer waterfall and are then cured in an oven
-
Hanging
- Top of DWI can must be flanged outwards to accept a can end
- Carried out in a process called spinning
-
Wall-beading
- Passed through a beader to give strength to resist heat processing conditions
- Patterns are a compromise between increasing lateral force and decreasing compression force
-
Internal lacquering
- Applied by an airless spray via 2 nozzles (one for the base and one for the walls)
- Cured in a horizontal belt oven
-
Printing
- Printed up to 8 colours plus a protective varnish
- Not possible to register print on a cylinder, so the image must first be transferred to a blanket
- Inks must be heat-cured via a pin-chain drying system for around 2 minutes at 160-210°C
- Oven is tilted to ensure cans do not fall of during the drying process
What finishing processes must beverage cans go through?
Base coating, internal lacquering, necking, and flanging
What two things can be made with 2-piece impact extrusion?
Aerosols and tubes
Describe 2-piece impact extrusion
- Impact extrusion forms a can or tube from a solid slug of metal
- Slug is placed in a mould and impacted with great force, after which the top is reformed to produce an opening suitable for a cap or valve
- Then trimmed, washed, internally-coated, and printed
- Transferred between stages via a pin-chain
- Metal is hardened during these processes and so must be annealed before use
Name the 6 components of an aerosol can
- Can body (and a base if made from 3 pieces)
- Cylinder top (dome)
- Valve cup
- Valve consisting of an actuator, body, spring, and housing
- A dip tube cover
- Propellant and cover
Name and describe the two main types of aerosol
- 2/3 phase - vapour phase of the propellant expels the product from the container; the propellant can either be separate from the product (immiscible) or dissolved in the product (miscible)
- separate: propellant is contained in a separate chamber from the product
Describe the process of making a tube
- Circular blank with a central hole (where the lid will attach) is placed in a die
- Sometimes it is just a thin area of metal, not a hole, which forms a temper-evident seal that is broken by the consumer
- Punch is depressed onto the blank and metal is forced up around the mandrel
- Tube is released by a stripper plate and the end is folded and crimped to provide a seal
Name and describe the two types of general line (lock seam) cans
- Performance
- 3 piece welded and steel-only cans which are suitable for edible oils and industrial products
- Formed into a square or oblong shape
- Both ends are fixed by the can-maker - 1 has a pour spout or other opening device
- Non-performance:
- Wide variety of shapes and lid systems
Name the 4 types of metal closure
- ROPP - roll on pilfer proof
- Composite closure
- Twist-off
- Crown cork
Describe a ROPP closure
- Common for bottles, especially to replace cork, as they are convenient to open
- Provided to the bottling plant as a smooth-walled deep draw aluminium tube
- Aluminium cap is moulded around the screw threads of the bottle to form a screw closure
- Perforations are added to the base to provide tamper evidence
Describe a composite closure
Composed of an outer aluminium body with a plastic insert with screw-threads moulded in
Describe a twist-off closure
- Formed in a shallow draw process, followed by hemming and curling to provide lugs
- Lugs screw onto the thread at the top of the bottle
- Many have a sealing compound around the rim to provide a seal
Describe a crown cork closure
- A circular blank is cut from a pre-lacquered sheet
- Sealant is applied as a liquid or hot solid
- Closure is placed in a press tool to form the flutes; these eventually form the seal closure when they are crimped around the top of the bottle
Describe a non-easy open can end and how it is made and attached
- Ends which are fixed to the walls - they are fairly cheaper than other types of ends
- Manufacturing speed is up to 500 can ends per minute for a single lane
- Ends are stamped out of pre-printed or pre-lacquered material
- The edges are then curled over and a compound is piped around the outer circumference to form a fixed gasket which ensures increased safety
Name and describe the purpose of the 3 parts of a non-easy open can end
- The seaming panel is the outer curl which will fit over the flange of the can and be formed into the double seam
- Chuck panel is important to allow the end to be opened with a can opener - usually 3mm deep
- Multiple expansion panels are important so that the can end can expand without inverting which is called ‘peaking’
- After, the can must return to normal and be flat which is called the pull back pressure
Name and describe the three properties of a non-easy open can end
- Peaking resistance - the ability to not permanently deform
- Pull back pressure - return flat after heating
- Flip out pressure - prevent movement if the air pressure increases
Describe an easy-open can end and how the parts are formed
- End shell and tab are produced separately and then bought together
- The shell requires 10 sets of tooling and the tab requires 25 sets of tooling
- The shell is stamped out from a metal sheet
- Rivet is formed in a drawing process and is done in several stages to prevent splitting
- Step 1 - Strip is pierced and cut
- Step 2 - Tab is formed and riveted to the can end
- Step 3 - Tab approaches the end shell at right angles and the rivet clamps it to the tab
- Score line is produced towards the end of the process and must be deep enough to allow the centre panel to be easily removed (but cannot be too deep otherwise it will split)
- An instance of controlled failure
Name the 7 important features of an easy-open can end and describe them
- Finger hole size - needs to be big enough
- Position of tab attachments - should not dig into the fingers
- Centre panel design - allow flexibility but not around the score line
- Tab break - this punctures the score and must be in the correct position; the shape can affect the opening efficiency
- Anti-tab rotation features - end cannot be opened if the tab rotates
- Pop force and tear force
Describe a peelable disphragm end
- About the same price as easy-open ends but are easier to use
- Consists of a standard plain end with a centre panel cut-out
- Laminated aluminium foil is used as the aluminium needs a heat-seal coating
- A tear tab is attached to allow easy peeling
- Must be covered by a plastic lid which increases the cost
Describe can end double seaming and how it is done
- A method of attaching can ends to the body in a 2-stage operation
- Seal is both microbiologically safe and pressure resistant
- First operation:
- End is placed on the body and then rotated in the seamer to bend the metal into the first seam
- Second operation:
- Rotated again to form the double seam
- Complete seam consists of 2x body thickness and 3x end thickness
- Compound is used to fill residual gaps left
Describe the secondary packaging used for metal can ends
Stacked in layers on pallets using layer pads and strapping
Describe the secondary packaging for metal closures such as metal caps
Jumble packed in polyethylene bags inside corrugate cases
Give 4 functions of a can coating
- Provide an inert barrier to the environment to prevent corrosion, prevent taint, and prevent discolouration by tin or iron sulphide
- Provide sulphur absorption
- Protect decoration (no water absorption, delamination, damage, or discolouration)
- Provide a lubricated surface to aid forming
What type of coatings are used for can coatings, explain why, and give three examples
High-solvent to reduce emissions of voltatile organic compounds to the air
Resin, phenol, and epoxy
Give 4 in-line quality assurance tests for rigid metal packaging
- Crack and pinhole tester
- Pressure testing for 3-piece to ensure good seam quality
- Light testing used for 2-piece cans
- Air testing
Give 4 off-line quality assurance tests for cans
- Dimensions
- Strength (axial compression and panelling resistance)
- Coating integrity (weight and adhesion)
Give 5 off-line quality assurance tests for can ends
- Dimensions
- Strength as buckle resistance
- End profile following internal pressure testing
- Coating integrity
- Easy-open force required to pop
Give 4 reasons a wet food packed in a can might fail its shelf-life testing
- Taint pick-up from lacquer or metal
- Metal pick-up into product
- Suplhur staining of light-coloured product, particularly meat and fish
- Corrosion of can wall or end
Describe the can shelf-life testing process
- Pack testing process
- The typical shelf-life is ~3 years so need to use accelerated product
- Cans are stored at 35°C to increase the rate of chemical change to 4x (compared to ambient) for 6 months
- However, can sometimes give false results as some live organisms react differently in these conditions
- Pack testing protocol
- Sample packs filled with product and processed
- 75% stored at elevated temperature; rest at ambient temperature
- Accelerated test opening plan
- 33% opened after 1 month; 33% after 3 months, and 33% after 6 months
- Ambient testing opening plan
- 50% opened after 12 months
- 50% opened after 24 months
- Sample packs filled with product and processed
How thick does aluminium foil need to be to be an absolute barrier to moisture, gas, and light?
20µm
Name the two types of unsupported aluminium trays
- Wrinkle wall
- Smooth wall
Describe wrinkle wall aluminium containers
- A commodity item formed in a simple pressing operation
- The strength of the tray depends on the gauge of the material, the temper (hardness), the side wall folds, and the rim formation
- Side wall folds are not ‘designed’ but rather a result of random distortion from pressing
Name and describe the 3 types of closure used for wrinkle wall aluminium containers
- Mechanical folding - the flange is folded over a lid (as in takeaways)
- Snap-on closures - the flange is folded over and raised to allow the cover to ‘snap-over’ the rim
- Heat-seal closures require a flat rim for sealing and a lacquer or plastic film - liquid but not gas tight
Describe smooth wall aluminium containers
- Produced in a drawing process where actual stretching of the metal occurs
- Tooling is more expensive
- The smooth sealing surface makes it suitable for gas flushing and sterilisation (such as for pet food)
Give 4 advantages of wrinkle wall aluminium containers
- Lightweight foil reduces cost
- Maximise strength
- Variety of closures
- Cheaper
Give 4 advantages of smooth wall aluminium containers
- More strength in oven
- Shatterproof when frozen
- Leakproof hermetic seal
- Extended shelf-life using MAP
Give 6 advantages of using aluminum foil
- Absolute barrier to gas, moisture, and light
- Good dead-fold
- Good thermal transfer
- Good heat resistance
- Food contact safe
- Non-absorbant so does not retain flavour or aroma
Give 2 disadvantages of using aluminium foil
- Tears easily
- Lacks compression resistance so many require support
What thicknes is unsupported foil and give three uses of it
9-200µm
Household foil, blister packs, and lidding (with a lacquer)
How thick is foil when used in a supported laminate, and give three reasons why it is used
<50µm
Barrier, appearance, and decoration
Give a disadvantage of using foil in a laminate
Poor / no recyclability
Name the 3 types of supported foil laminates
- Retort pouches
- TetraPak
- Pharmaceutical packs
What are retort pouches designed for?
Microbiologically-sensitive products that are sterilized
Why do retort pouches have nylon in?
Puncture resistance
Give the structure of a TetraPak and the advantages of them
74% paper, 22% PE, and 4% aluminium; absolute barrier to light, flavour, and oxygen
Describe support foil used in a pharmaceutical pack
Uses a hard-temper foil of 20-30µm thick which is heat-sealed to a polymeric-thermoformed base
Describe pinholes in aluminium foil
- The thinner the foil, the more pinholes
- Do not significantly alter the barrier
- Important to conduct testing after transit as flex can introduce pinholes
Name the two ways in which metal packs can be decorated before forming
- Printed using offset lithography
- Embossed
How are metal packs decorated after forming?
Dry offset letterpress
