1.5.1 Paper finishes, printing and polymers

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.

Letterpress

is one of the earliest methods of printing and involves raised letters onto which ink is coated. Paper is then pressed against the plates holding the letters. It is still used today for specialist printing such as personalised wedding invitations, but it can usually only print one colour at a time.

Laminating

There are two main lamination methods in relation to papers and boards.

Lamination via encapsulation
Lamination via a surface coating

Embossing

This creates a raised design on the surface and is an aesthetic effect although it can have practical applications such as applying braille to packaging.

Debossing

This is essentially the opposite of embossing and produces an indented depression rather than a raised surface. It may also be classed as an embossing process by some manufacturers as it essentially uses the same kind of tool and process.

Foil blocking

This process involves the application of heat and pressure to a metallic paper (foil) to create areas of depth and texture. This adds aesthetic impact to the product and give it a sense of ‘quality’. It is used on business cards, high quality menus and leaflets as well as book covers, gift cards, wedding stationary etc.

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.

Printing Processes

Screen Printing

Screen printing has a number of different applications where inks or other materials need to be printed onto a flat surface such as:

transfer of designs onto paper and fabrics.
transfer of designs onto ceramics.
printed circuits for electronics products.
the process was originally called silkscreen printing as ink was forced through a template via a silk screen and squeegee. Nowadays it is much more complex and can print onto curved surfaces.

Pad printing

Pad printing is often compared with screen printing. Pad printing is how lego figures are printed.

See Lego Figure

Flexography

is a relief printing process similar to letterpress with the raised letters but in this case raised text and images are photo etched onto a rubber material which is glued onto steel rollers. The inks used are thick and viscous which allows printing onto poorer quality materials such as waxed films and cellophane.

This process is primarily used for packaging, carrier bags but also books and magazines.

Offset Lithography

is probably the most versatile and economic of the commercial printing processes and uses the same four colours (CMYK) through a series of up to five presses.

Offset lithography is often used to print high-quality promotional materials such as branded coasters, mouse mats and product advertisers. It can also be used to print hotel over door hangers, plastic menus and tent cards among other things

Digital printing

processes include electro-photographic and inkjet printing.

Electro-photographic printing

involves the depositing of toner into the paper or card. Dry or liquid toners can be used but they are fixed by heat and absorption which explains why paper comes out of printers warm

Inkjet printing

involves spraying electrostatically charged ink droplets directly onto the paper relying on absorption to fix the inks.

Finishes for Polymers

Adding colour during the moulding process can be done because polymers are synthetically produced, colour can be introduced during the creation of the polymer.

Polymers are self- finishing as they take the surface of the mould they are made in.

With sheet materials this is often a perfectly smooth mirror-like finish, however, they can also have textures to give a frosted look or any other texture that is also in the mould.

Polymers are self coloured.

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.

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.

Gel coats can be added to moulds when laminating GRP

GRP - Glass fibre reinforced plastic

During unit 1.2.11 we learned about composites and in particular about how Glass-Fibre shapes are made by ‘laying up’ as in the diagram on the left

Pigments can be added during this moulding process by adding gel coats during the laminating process as shown below.

Smart pigment

Pigment is what gives a material its colour, it’s in nature and even in our skin; it is what makes something a particular colour or hue.

By replacing traditional pigments with smart pigments we can affect the colour of a materials by subjecting them to heat or light.

Smart pigments have a variety of practical uses and can be incorporated into fabrics and polymers in the same way that you might mix in a paint or dye to colour a material while it is being formed.

Thermochromic pigment

‘Thermo’ relates to heat and ‘chroma’ to colour. Therefore Thermochromic pigments change colour when exposed to heat. Common examples are baby spoons, kettles and thermometers.

Phosphorescent pigment

Phosphorescent materials have the unique ability to absorb light and then use that light to provide low level illumination in dark or low light conditions. The pigment can be used in paint or polymers, and products using this pigment are commonly referred to as a ‘glow in the dark’.