10.4 Finishing

○ paints

○ polymer coatings

○ varnishes, including UV and spot varnishing

Acrylic spray paints

Although polymers are self-finishing it may still be necessary to apply a different colour or surface finish for aesthetic or practical reasons. Acrylic paint is a fast drying, water soluble paint that becomes water resistant when dry. It is readily available in a wide range of colours including pearlescent, metallic and fluorescent.

Acrylic paints are widely used in the automotive industry for bumpers, wing mirrors and other mainly plastic parts. They can add a layer of UV protection, help to avoid weathering and increase the aesthetic properties of the object as well as allowing for mass production and colour matching of parts.

Cellulose and acrylic paints

Paints provide colour and sometimes a textured finish to metals.

They can also provide a barrier against corrosion for metals such as Steel and Iron.

The surface of the metal must be cleaned, de-greased and primed for the paint to adhere to the surface properly.

To get a full, rich colour an undercoat is often applied before the colour and, in the case of cars and other shiny products, a clear lacquer may be applied over the paint layer.

Thermoplastic elastomer (TPE) Overmouldings

Overmoulding is when a second polymer is moulded over a specific part of a product.

There are two main ways of doing this:

Use two injection moulds
Twin shot injection moulding

Two injection moulds

One mould is made for the product , such as a toothbrush body, and one mould for the grip areas. The toothbrush is injection moulded in one material such as Polypropylene, then the product is moved to a second mould where the grip is injection moulded on to the body using a TPE.

Twin shot injection moulding

This process uses an injection moulding machine with a mould designed to produce the product in one cycle. The mould has two separate component cavities which can be rotated through 180 degrees so the line up with the twin injection points.

It is a continuous cycle as one part is being overmoulded while another part is being made in the first mould cavity.

Polymer dip coating

This is used on a variety of products such as wire coat hangars and outdoor equipment.

The metal is heated then the hot product is dipped into a tank of fine polymer powder which has air blown into it to ensure even distribution of the powder.

The hot metal melts the powder giving a tight fitting and even polymer finish.

Metal dip coating

Metal can also be dipped into other metals for coating.

In this case the product is dipped into a tank of molten plating metal. This process is often used for inexpensive metals such as low carbon Steels primarily to provide a barrier against corrosion.

Food cans are often Tin plated because it is less reactive to food goods

*Galvanizing

Using a Zinc plating is known as Galvanizing. It is used to produce car bodies, buckets, screws, hinges and much more. It give a corrosion resistant finish

Powder coating

This process gives a much better finish than dip coating.

A negatively charged product is sprayed with positively charged polymer powder so it sticks to it like a fine dust.

It then passes through an oven which bakes the fine layer of powder into an even coat of polymer. Used to coat chair and table legs as well as many domestic metal products such as fridges and washing machines. *We saw this at triumph motorbikes

Varnishing

Metal varnishing is a method used to provide a clear surface finish to metals that will also provide some resistance to corrosion.

They would be applied exactly the same as a paint or spray. Varnish is effectively paint without any pigment leaving it completely clear. A clear lacquer could also be used for a finer finish.

Sealants

Metal sealants are tough polymer based sealant coatings that protect polished surfaces from decay and tarnishing.

Silicon based sealants are often used in the automotive industries as an additional bodywork barrier against weathering and rust. It is generally applied with a cloth or machine pad to produce a film that is then left to cure for 15 mins then buffed to a shine.

Preservatives

Metal preservatives can provide temporary in-process or final post-processing protection for metal surfaces. They are ideal for moulds and dies to prevent fingerprints showing on surfaces. They can be applied with a cloth, spray or via immersion.

anodising

This is a finishing process commonly used for Aluminium products to enhance the natural oxide layer and provides increased hardness and toughness.

It is similar to electroplating but penetrates the surface of the material which can then be finished with a lacquer or varnish to further protect the finish.

finishes for woods

Wood is a natural product and is therefore prone to attack from moisture, insect and weathering.

Although wood is often used for its attractive grain, it can be further enhanced by the use of wood stains or colour washes to change its colour or shade. These are often combined with varnish to provide additional protect for the wood.

More importantly, finishes for wood should preserve the material. Wood can degrade in dry conditions as well as wet condition and is susceptible to wet and dry rot.

The following chart lists the main finishes for woods. How they are used depends on the function and aesthetic requirements of the product. For example, a chopping board would not be oiled or waxed as it would contaminate the food. A cricket bat is oiled as a varnish would chip away in use.

Finishes for paper and board

There are many products that contain graphics to identify and promote a product or company. It is also used to give information about a product on the packaging such as weight, contents, ingredients etc.

Such products will be produced using a commercial printing process.

Varnishing, UV varnishing and spot varnishing

Varnish is a clear, non-pigmented ink only used on pre-coated papers and boards to enhance aesthetics as well as some protection against dirt and fingerprints. It is available in gloss, satin and matt finishes.

UV varnishing provides a smooth finish and is cured using UV light once the ink has dried.

Spot varnishing is applied to specific areas to make text or logos stand out. It can be done with normal or UV varnish.

summary

There are three terms associated with finishes.
Materials may be self coloured.
Materials may be self finishing.
Often these types of material will not require a surface finish applied but they may be applied for aesthetic reasons.
Materials may have a surface finish applied or they can be left unfinished.
Finishes can be applied, plating, coatings, preservatives or a combination of these.
There are to main reasons for finishing a material: to protect (preserve) or to enhance (aesthetic).

Revision Notes

For each material group, you need to be familiar with the range of finishes available and that are suitable.
A range of printing processes are available and are used depending on the finish required and/or the scale of production.
Materials can be self-finishing/coloured or they may have an applied finish.
Many polymers are self finishing/coloured
Ferrous metals will require a finish to protect from decay
Non-ferrous metals wont necessarily need a finish
Wood finishes can enhance the look of the wood and provide protection from weathering, rot and insect attack.