PLASTICSDesign and Manufacture

PLASTICS THE BASICS

Natural plastics: fossilised tree resin and latex

Form of rubber

Synthetic plastics: Chemically manufactured from carbon based materials such as crude oil, coal and gas

Basic chemicals used in creating plastics are from coal and crude oil.

Obtained by using heat and chemical catalysts

PLASTIC PROPERTIES

Ideal for mass production of quality products Light in weight Resistance to corrosion Low thermal conductivity Translucent, transparent and opaque Electrically resistant Easily formed Recyclable

FORMS PLASTIC COMES IN

Profiled sheets, rods, tubes and bars Moulding compounds Thin layers of films and sheets Foam Casting compounds such as ingots Paint, varnish and lacquer used for finishing Filaments and fibres Composites that contain reinforcing material

THERMOSETTING PLASTICS

Formed by chemical reactions that leave them in a relatively fixed state.

They cannot be softened or reformed when reheated.

Can withstand high temperature, excess heat will cause material to decompose.

Used when products needs resistance to extremes in temperature, electrical current, chemicals and wear.

Heated

Softens

Shaped

Permanently hard

THERMOSETTING PLASTICS Some examples of thermosetting plastics

are: Epoxy Resin ER(Araldite) – good adhesive, low shrinkage,

strong when reinforced. Used in bonding, laminating, surface coating, printed circuit boards

Polyester Resin PR(Orel Beetle) – stiff, hard, brittle, resilient as laminated GPR, formed without heat or pressure. Used in panels for car bodies and boat hulls, casting and embedding

Melamine formaldehyde MF(Formica Melaware) – waterproof, tasteless, odourless, mark and scratch resistant. Used in worktops, tableware, buttons, electrical insulation.

Urea Formaldehyde UF(Aerolyte Cascamite) – good adhesive, stiff, hard, brittle, good electrical insulator. Used in electrical fittings, paper and textile coating, wood adhesive.

Phenol Formaldehyde PF (phenol methanal) – Hard, brittle plastic with dark colour, glossy finish. Resists heat. Used in parts for domestic appliances, bottles tops, kettle/iron/saucepan handles.

THERMOPLASTICS Softens when heated, hardens

when cooled down Can be repeated indefinitely When soft they are easily

formed under pressure. PLASTIC MEMORY – this is the

property of the thermoplastic returning to its original shape when softened after being shaped.

Heated

Softens

Shaped

Hardens

THERMOPLASTIC Polyethene HDPE – High density, tough, resists chemicals, feels

waxy. Used for rigid objects: buckets, bowls, sterilised containers.

Polyethene LDPE – Low density, soft and pliable, electrical insulator. Used for flexible objects: bags, bottles, cable, sheathing, toys.

Polypropene PP – rigid, light, good chemical resistance, resistance to fatigue, bending. Used for crates, seats, string, rope, medical equipment, hinges, kitchenware, film.

Polystyrene PS – light, buoyant, stiff, water/weather resistant. Used for packaging, containers, insulation and toys.

Polyvinyl chloride UPVC – rigid, abrasive resistance, water/weather resistant. Used for piped, gutters, bottles, roofing, window frames.

Poly-methacrylate PMMA (Acrylic/Perspex) – Stiff, hard, clear, durable, scratches easily, easily machined and polished. Used for signs, lighting, reflectors/lenses cases, jewellery.

Nylon – Hard material, good resistance to wear and tear. Solid nylon has low friction qualities and high melting point. Used in curtain rail fittings, combs, clothes and gear wheels.

ELASTOMERS

Particular branch of thermoplastics Has elastic properties Similar to rubber in how it reacts to pressure

and their texture Withstands large amounts of deformation Can be stretched several times to their

length. Uses:

Waterproof seals Diaphragms for mechanical purposes Flexible handles Sportswear Foam padding

COMPOSITE MATERIALS

Consists of two or more substances Combines to produce properties that no

single substance can achieve. One component forms the matrix (base)

while the other provides reinforcement Properties are controlled by size and

distribution of reinforcing substance.

EXAMPLE OF COMPOSITE MATERIAL

Using carbon fibres embedded in Resin Carbon-fibre reinforced plastic CFRP

Properties are: Low in density Light in wieght High tensile strength Better corrosion resistance and fatigue performance

than most metal alloys

Used in aerospace industry and car frameworks.

PLASTIC PROCESSINGPlastics part 2

INJECTION MOULDING Allows large quantities of plastic components to

be made quickly Most important industrial process in mass

production of plastic goods. Costs of machining original moulds are very high

therefore necessary to sell large numbers of products to recover costs.

There are a few features which help identify this process:

Sprue marks Draw angles Mould split lines Ejection pin marks Webs used for strength Variation of finishing techniques from high quality shine to

texture

INJECTION MOULDING

1. Granules of plastic powder are poured or fed into a hopper which stores it until it is needed.

2. A heater heats up the tube and when it reaches a high temperature a screw thread starts turning.

3. A motor turns a thread which pushes the granules along the heater section which melts then into a liquid.

4. The liquid is forced into a mould where it cools into the shape (in this case a sphere).

5. The mould then opens and the sphere is removed.

INJECTION MOULDING (USES)

High volume, automated industrial manufacturing process.

Golf tees Spoons Wash basins Buckets Product casings

EXTRUSION

Used for products with long uniform cross sections.

Variety of metals and thermo-plastics are suited to the extrusion process.

Plastic is easier as it requires less force Polythene, PVC, polypropylene Lead, copper, brass, bronze, aluminium, steel

Features that help identify this process; smooth-walled long sections Line texturisation

EXTRUSION

A machine is used to extrude materials which is very similar to the injection moulding.

A motor turns a thread which feeds granules of plastic through a heater.

The granules melt into a liquid which is forced through a die, forming a long 'tube like' shape.

The extrusion is then cooled and forms a solid shape. The shape of the die determines the shape of the tube.

EXTRUSION (USES)

Curtain rails Drainpipes Electric cable sheathing Fluorescent light covers Hose pipes

BLOW MOULDING Several variations to the process, producing

various sizes Finish is not as high a standard as injection

moulding Good mould allows:

Uniform thickness Complex shape Good quality of production

Fast process Little waste Features that allow for identification:

Mould separation lines Flash lines Any shape of bottle or container Transparent, opaque or coloured

BLOW MOULDING

The process is similar to injection moulding and extrusion. 1. The plastic is fed in granular form into a 'hopper' that stores it. 2. A large thread is turned by a motor which feeds the granules through a heated section. 3. In this heated section the granules melt and become a liquid and the liquid is fed into a mould. 4. Air is forced into the mould which forces the plastic to the sides, giving the shape of the bottle.5.  The mould is then cooled and is removed.

BLOW MOULDING (USES)

Variety of mass produced bottles and liquid containers, using variety of polymers and laminates.

COMPRESSION MOULDING

Used for shaping thermosetting plastics These are strong and brittle but have poorer

impact resistance than thermoplastics Features that identify compression moulding:

Walls of uniform thickness 3-6mm Draft 1˚ minimum Flashes on edges Quality finish on female mould

Plastic used can resist temperature increases.

MOULD THROUGH COMPRESSIONThe stages are as follows: 1. The mould is heated.2. A 'slug' or piece of the plastic is placed into the mould and warms up.3. The hydraulic press begins to move down when the plastic has reached the correct temperature..4. As the upper and lower mould meet, the plastic is compressed into the shape of the mould.5. The upper mould moves upwards and the plastic piece (in this case a dish) is removed.

COMPRESSION MOULDING (USES)

Automotive distributor caps (cars) Camera cases Electrical wall sockets Handles Door knobs Light switches

VACUUM FORMING

Widely used in industrial processes to mould plastic.

No need for expensive moulds or dies Moulds generally used from wood/ steel/

aluminum Process is over in a few minutes Thermoplastics are suited for this process Features that identify this process:

Thin sheets normally used Patterns and textures are transformed from the

mould to the product surface Venting holes cause ‘pips’ on surface Tapers are pronounced Thinning on side surfaces

Vacuum forming

VACUUM FORMING (USES)

Packaging items that have complex shapes Trays Dishes Margarine tubs

Toys Light panels

ROTATIONAL MOULDING Produces “closed and seam free”

components made from plastic polymers Products produced are hollow, stress free and

come in a wide range of shapes and sizes. Moulds made from steel

copper coated for better surface detail Aluminium for better heat transfer Electroformed nickel for best surface detail.

Moulds rotates several axes with speed dependent on production requirements.

Large moulds have air vents to avoid distorting

Suited to small production runs used for prototypes

ROTATIONAL MOULDING (PROCESS) Measured quantity of

plastic power is added to mould

Mould is closed and rotated slowly around 2/3 axes. Allows distribution over hot surface

Plastic melts on contact and builds up even coating on inside surface of mould

Mould surface is cooled, plastic retains its shape

Mould opens and product is removed.

ROTATIONAL MOULDING (USES)

Plastic toys Play equipment Road markers Buoys Large tanks (10,000litres)

OTHER PROCESSES Calendering: Thermoplastics are

squeezed between hot rollers forming thin sheets or film

Casting: useful for thermosetting plastics. Molten plastic poured into mould

Bending: thermoplastics heated along a line using strip heater, folded to desired angle. Bending jig sometimes used to hold in place.

Fabrication: joining together using a variation of fixings and adhesives

Coating: powered thermoplastic melts on surface of heated product.(dip coating)

Forming: Layers o glass fibre matting and polyester resin formed over a mould. Resin cures it hardens giving rigidity to fibre/resin matrix. Thermoplastic formed by heating whole sheet till soft then pressed between two formers.