Graphic Overlay Material Selection

There is a plethora of screen printable films on the market these days, all touting for their share of the graphic overlay and membrane keypad market. It can be a difficult job deciding which material is best suited to your application.

Each material has unique properties, and if not selected correctly they can have a detrimental impact on the life of the product in question.

Furthermore, not all screen printable films are created equal and the apparent cost saving offered by inferior products may have implications in the long run. Can your graphic overlay printer provide detailed data supporting the performance of the products they are producing? At Foster Graphics we can. We are very careful about material selection and deal only with suppliers prepared to back up product performance with facts.

We have experience producing graphic overlays and panels that have been deployed in a range of situations. As each application is different so too is the solution we can provide. Some design criteria we will discuss with any new design:

  • Will the product or equipment be exposed to UV light? Many commonly used graphic overlay materials are not well suited to outdoor UV exposure conditions.
  • Will the overlay or key pad come into contact with chemicals such as cleaning products (chlorine bleach), fuels such as diesel or chemicals such as toluene or ammonia? Not all screen printable films are designed to cope with exposure to chemicals.
  • Does the graphic overlay or key pad need to be embossed? The embossing process and subsequent mechanical movement due to key pressing places a significant strain on the polymer chains making up the graphic film. If not selected correctly the film will quickly work harden, become brittle and crack.
At Foster Graphics we have the experience and technical know-how that will help you decide the best material selection for your application.

Commonly Used Printing Film


Polycarbonate is a thermoplastic that was developed in the 1950’s and has been used in products as diverse as fighter aircraft canopies to space suit helmets. It also finds widespread application in a more down to earth role in the production of graphic overlays and control panels.

Polycarbonate is a polymer and as such is extremely tough and hard wearing. The long polymer chains of the material mean it is shatter resistant and the selection of films with fine velvety texture means it is difficult to scratch.

Lexan is a well known brand name for polycarbonate and it is not uncommon to see polycarbonate referred to as ‘Lexan’.

Polycarbonate films commonly used for graphic overlay production range from around 150 micron to 500 micron thick (0.15mm to 0.5mm). Note that 1 micron is around 0.0394 mil, so 500 micron would be equivalent to 19.68 mil.

Generally polycarbonate is used where no mechanical movement of the overlay is needed and the possibility of chemical contact is low.

Combined with a high quality adhesive, polycarbonate overlays can be produced that are tough and will give many years of service.


Polyester is a biaxially orientated film, which means that the strong polymer chains in the material are aligned in multiple, but perpendicular directions. This leads to a dramatic increase in the strength of the film.

Polyester is becoming the film of choice for high quality membrane keypads and graphic overlays due to its many features.

Polyester is resistant to almost all common chemicals and is susceptible to damage only from strong acids. This chemical resistance also makes the film resistant to solvents in printing inks so special keying agents are used on the film to promote good ink adhesion.

Polyester is an excellent electrical insulator exhibiting a very high dielectric strength and resistivity.

Because of the biaxial nature of the polymer chains in the film, polyester is extremely strong compared to other commonly used substrates and has a superior ability to cope with flexing. This property makes it ideally suited to embossing where the embossed area will be subject to significant mechanical flexing. It is not uncommon to see polyester rated to cope with many millions of activations when embossed.