SB.7 Polymers Flashcards
What are man-made polymers reffered to as
Plastics. Plastics are derived from coal, gas, cellulose & crude oil.
Fractional distillation
Separates crude oil into useful hydrocarbons. The diff components separated are known as fractions. The fractions of crude oil include petrol, kerosene, bitumen. (Large chains, high bp so leave quickly)
Before fractions can be used, they are broken down by cracking. Cracking breaks large hydrocarbons into individual hydrocarbons called monomers
Polymerisation
Individual monomers are joined to form a longer polymer chain
2 categories of plastics
Thermoplastics
Thermosetting plastics
Thermoplastics
Easy to mould & shape when hot. Can be formed into complex shapes & reformeed multiple times. Lightweight, strong & waterproof. Good strength:weight ratio. Easy to clean
Uses: plastic seating made from polypopylene
Thermoplastics
Easy to mould & shape when hot. Can be formed into complex shapes, reformed multiple times. Lightweight, strong, waterproof. Good strength:weight ratio. Easy to clean
Uses: plastic seating made from polypropylene
Thermosetting plastics
A thermoset is irreversibly hardened & cannot be reformed. Hard but often brittle. Good heat & electrical res. Good chemical res
Uses: electrical components, plugs
Modifying plastics: stabilisers
Can be added to make plastic more resistant to heat & light. Overtime, UV light degrades plastic, turning brittle & faded. Affects strength & look of plastic.
Modifying plastics: plasticisers
Give material improved flexibility & are commonly used in PVC to improve applications
How can colour be added to plastics
Adding pigments
Stock forms of plastics
Film: used for packahing, bags
Granules/pellets: used for extrusion & inj moulding
Foam: flexible or rigid, thermal insulation
Rods/tubes: can be bent to shape
Powder: mixes well w additives & dyes
Sheet: durable, lightweight, can be cut/bent/vacuum formed
Standard components
A manufacturer would buy standard components rather than making items themselves for improved efficiency & cost.
Adv of using standard components
Easier to obtain & likely to be stocked by many retailers
Parts are more likely to be manufactured in large numbers, making them more cost effective due to economies of scale
Vacuum forming
Plastic sheet is placed above the mould & clamped securely
Eletric heater turned in to heat plastic sheet, becomes flexible
The air is vacuumed out below the plastic & mould
Once cools and hardens, can be removed from mould
3D printing
3D printers use CAD files which are converted into a series of co-ordinates called G-code that the printer will follow to build up object in layers
Also known as additive manufacturing
Drape forming
Plastic sheet is heated until soft, where it can be formed over a mould. It takes on shape if the mould w/o stretching the plastic & retaining dimensional thickness. Slow process w/o need of a vacuum, low cost
Extrusion
This creates a continuous flow of plastic, that is pushed through a die to produce specific shape. Used for cables, pipe
Injection moulding
Molten material is injected into a mould. Once plastic has cooled & hardened, mould is opened to release shape. Can handle complex parts & shapes to produce consistent products in large quanitities.
Used for bottle caps, toys
Bending
Line bender heats sheet of thermoplastic over a strip heater until its soft. It can then be bent to a chosen angle. When plastic cools, it retains the shape. Acrylic sheets suitable for this
Similarity between injection moulding & extrusion
Plastic pellets are pushed through a heated barrel
Welding (plastics)
Heat softens the polymers & they can be joined w a plastic filler material in form of long thin cable. Hot gas welding gun produces thin jet of hot air to soften the polymers
Resin casting
Liquid synthetic resin is mixed w a curing agent, poured into a mould & allowed to harden. Unlike extrusion or inj moulding (which forces plastic into a shape), casting relies on gravity to pull the resin in to all parts of mould. Used for industrial prototypes, precise models & moulds for dentistry.
Adv: quick production time
Disadv: use is limited to smaller scale items
Blow moulding
A tube of heated plastic, known as parison is fed into a hollow mould. The parison is pinched at the bottom so it can be filled w warm air & inflates to fill the mould, cools & takes on the shape. Used for plastic bottles, containers (anything w space inside)
What are plastics referred to as
‘Self finishing’ as they are manufactured w their final surface
Surface treatments of plastic
If scratched or dull, light markings can be restored by using mild abrasive & a soft polishing cloth
Designs can be added to surfaces using offset lithography & flexography
Can be painted (before, should be clean & free of grease, primer applied)
Hydro-graphic printing
Applies a printed design onto 3dimensional surface. Film containing the design is placed on surface of water in a dipping tank. The pattern will curve around the surface of the product when submerged
