Product Design (Finishes)

Overview of Finishes


This means applying a finish to the surfaces of materials to -

- To protect the material from corrosion

- Make the product water repellent

- Improve resistance to wear & fatigue

- Improve ability to absorb or reflect heat

- Improve ability to insulate against heat or electricity

It also means applying a finish to -

- To enhance the appearance of the surfaces (eg) applying teak oil to a patio chair to bring out the grain

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Self Finishing Materials

Self-finishing/coloured materials (No finishing needed) –

- Some materials, mainly polymers do not need the application of a finish

- A polypropylene chair seat is textured from the pattern on the inside of the injection moulding mould

- Polythene washing up bowl is the same colour all through the material because that is the colour of the polythene

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Cutting Processes

- Saws & other machines produce rough edges (burrs) which have to be removed.

- Modern methods of cutting materials – laser & plasma cutting do not produce rough edges (burrs)

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Finishes on metals – Natural Barriers

- Oxides exist on most metals to form a protective layer

- Iron oxide is porous so steel carries on rusting when it becomes wet

- Stainless steel does not rust

- Steel can be ‘blued’ by heating up to 300°C and then quenching in oil

- This gives a fine oxide layer which protects the steel

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Finishes on metals – Applied Barriers

Steels in particular need protecting from corrosion

Finishes can be applied by:-

- Brushing

- Rolling

- Spraying

- Dipping

- Electroplating

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- Ions from the coating material form on the base material

- E.g. Chromium is used to enhance & protect car parts, bathroom fittings, and kitchen equipment

- Gold – Silver – Tin – Zinc & Copper can all be used as coatings

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Anodising on aluminium

- Anodising makes aluminium more durable and resistant to scratches

- An electric current passing through the electrolyte builds up a tough oxide layer

- Can be coloured with dyes then lacquered

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Dipping and Spraying & Zinc Plating - Galvanising

Dipping and Spraying -

- Metals can be coated with other metals

- Base metal must be clean - washed in detergent, then pickled, then rinsed  

Zinc Plating - Galvanising -

- Steel is dipped in molten zinc 450-460°C

- Usually the first protective layer for car bodies

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Enamelling & Tin plating

Enamelling - 

- Finely ground glass is sprayed onto a metal product – gas cooker, steel bath.

- This is fired in a kiln

- This gives a scratch & heat proof surface but it can chip

- Glass enamels can be fused onto jewellery to give an attractive & colourful finish

Tin Plating - 

- Steel sheets are passed through baths of molten tin at 315-320°C

- Used in manufacture of food cans

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- Surfaces must be clean & free of rust & oil

- Car bodies are first galvanised

- Primer coat keys into the metal surface

- Then an undercoat to give a good colour & smooth base for the top coat

- Then a top coat is applied

- Car bodies also have a hard coat of lacquer

- Spraying gives a better quality finish than brushing

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Powder coating

- Positively charged powder is sprayed onto a negatively charged product

- This gives a very strong bond

- Product is then baked in an oven giving a harder & tougher finish than conventional paints

- Used on ‘white goods’, gates & fences

- No solvents or propellants so environmentally sound

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Plastic dip coating

- The product is heated to 230°C

- It is then dipped into a fine dust of fluidized polythene

- Heat from the product melts the polythene into an even plastic coat over the product

Injection moulding

- A thicker layer of plastic coating can be applied by injection moulding

- This is used on the handles of tools

- Gives a better grip & electrical insulation

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Finishes on woods

- Unprotected wood expands & contracts when it absorbs water then dries

- This causes cracks & splits

- Also resins that hold the cells together break down weakening the wood

- Bacteria & fungi also cause the wood to rot & decay

- Hardwoods are generally more resistant to the environment than softwoods

- Teak contains oil helping to repel rain

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Finishes on woods – Applied barriers


- As for metals

- Primer – Undercoat – Top coat

- Rub down with glass paper between coats

- Oil based paints are hard wearing, non - porous and available in a range of colours

- Polyurethane paints are used on toys, because they are harder wearing, tougher & scratch resistant

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Finishes on woods – Applied barriers Continued...

Where natural grain of the wood are to be visible varnishes & polishes are used:-

- Polyurethane varnish brush applied, tough, hardwearing, gloss, matt, satin

- Sanding sealer – cloth wad applied light, clear polish

- Teak oil – cloth wad applied can be used on all wood, indoors & out

- Wax polish – applied on top of other polishes for a final polish

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Wood Preservatives

Wood Preservatives

3 main groups

- Tar- oil derivatives – creosote

- Water soluble preservatives

- Organic solvent preservatives

All can be applied by:-

- Brushing

- Dipping

- Spraying 

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Water soluble preservatives & Tanalising

Water soluble preservatives –

- These solvents come in a wide variety of colours

- Need to maintained more regularly

Tanalising –

- Wood is pressure treated in large tanks

- Dry wood is impregnated with copper sulphate solution under pressure for deep penetration – not just on the surface

- Wood is then steam dried 

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