Choosing the overlay for your membrane or decal design
The two main materials used for membrane/decal overlays are polycarbonate and polyester. These materials each have distinctive properties and when choosing your overlay material, each application must be evaluated before deciding which technology you will require.
Polycarbonate for graphics is available in a variety of thicknesses from .005” to .030” with good clarity and a selection of matte finishes and textures. Polycarbonate can be hardcoated to allow additional protection from abrasion and solvents, considering that raw polycarbonate are easily scratched and solvent resistance is rather limited.
An ideal application of polycarbonate would be designed to incorporate a non-textured, optically clear window. A hard coating should always be utilized in this application to minimize scratches. Hard coatings can be selectively textured providing resistance to marring and fingerprints in active areas and yet retaining an optically clear viewing area that would still be hard coated. Note that although a hard coat will provide increased solvent resistance compared to raw polycarbonate, specific solvent resistance requirements should be evaluated early in the design.
Although polycarbonate is relatively stable up to ~ 150 degrees C, exposure above 85 degrees C over the long term can lead to an increase in yellowness and hazing.
Low temperatures have little effect on polycarbonate film.
One of the big advantages of polycarbonate is that it is easily formed and is the basis for many in-mold decorating (IMD) applications. From a membrane switch design view, embossing puts undue stress on polycarbonate film and the continual flexing of the material that occurs over the lifetime of an embossed overlay can cause stress fractures and eventually a breakdown of the film in the embossed area. An embossed polycarbonate has a much shorter actuation life than polyester overlay.
Below is a chart that compares the mechanical properties of polyester vs polycarbonate. The ‘folding endurance’ is a good comparison of the differences in mechanical durability
Typical Material Selection For Components In Membrane Switches
- Graphic Overlays: Polyester (Preferred), And Polycarbonate
- Spacers: Polyester (Typical), Polycarbonate
- Lenses: Polycarbonate
- Printed Circuits: Polyester
1 = Resistant
2 = Fair Resistant
3 = Poor Resistant
ADVANTAGES / DISADVANTAGES
In summary, polycarbonate is a good choice for IMD or overlay applications where mechanical flex is at a minimum. Its advantages lie in its wide thickness range and availability of textures and finishes.
Polyester Polyester is only suitable for optical windows up to 0.010”. Above this, and the film begins to turn milky white. Although harder than polycarbonate, polyester can still scratch easily and like polycarbonate, is also available with hard coat finishes to protect the surface from scratches.
In comparison to polycarbonate, polyester films are inherently more chemical and mechanical resistant and can survive long term temperature exposure up to 120 degrees C. Its is virtually unaffected by all commercially used solvents and due to its inherent strength, an embossed polyester overlay does not suffer fatigue and can have up to 20 times the flex life of polycarbonate. Tests have produced actuations of five million and following embossing guidelines, actuations in excess of one million are normal. The high strength of polyester does limit forming to certain heights, depths or draws.
To summarize, polyester films are significantly tougher than polycarbonate films and are more suitable for demanding applications such as a membrane switch application or where the overlay will be subjected to harsh chemicals. Its limited thickness range can limit its application.
Embossing is forming raised areas on an overlay to provide functional and aesthetic features. Embossing is generally incorporated into a graphic overlay for membrane switch assemblies to provide a tactile feel and button location for the user. LED windows are also embossed to allow the LED to be viewed from the side. Embossing can also be performed on logos or text to create three-dimensional effects.
This guideline will focus on embossing for membrane switch assemblies
Polycarbonate and polyester are the two most common materials chosen for an overlay. Embossed polyester has more than 20 times the flex life of polycarbonate and much better tactile response.
The following design guidelines for embossed parts will help to minimize stress and maximize key life in membrane switch applications utilizing either polyester or polycarbonate as the overlay material.
Material Thickness Polyester thicknesses up to 0.010” can be readily embossed and polycarbonate films up to 0.020” can be embossed in certain situations. For thicknesses greater than 0.020” thermoforming or hydro forming should be used.
Embossed Width The width of a rail (ridge, rim) emboss should be at least five times the material thickness. The minimum rail width for a 0.010” thick material would be 0.050”. If adhesive and liner are included behind the rail, their thickness must be included when determining the rail thickness.
Note: Line widths less than the recommended thickness can result in warping or buckling of the overlay.
Embossed Height The height of an embossed area should be no greater than 1.5 times the material thickness for polycarbonate, or 1 times the material thickness for polyester. Polyester’s extraordinary strength makes it difficult to maintain a height greater than 1 times the material thickness.
Note: Excessive emboss heights increase the risk of material failure. Greater embossed heights can be reached with polycarbonate, but are not recommended for membrane switch applications.
Embossed Spacing A minimum space of 0.187” should be held between embossed areas and internal cut outs or the die cut edge.
Note: Typically the number of embossed areas is not a concern if the proper spacing and embossing criteria are observed.
Embossed Configurations The inside and outside corners, and any intersecting lines, shall maintain a minimum radius of .060”. The inside and outside corners should use the same center for the radius. The radius at the end of any line will be a function of the line width.
Note: Sharp corners increase the risk of material failure.
Embossing Tolerances The standard registration of emboss to printing is +/- .015”. Thought should be given to designing the embossed area intentionally smaller than the printed key by .015” on all sides. This prevents the emboss from straying off the key area. Standard registration of emboss to internal cut outs or the die cut edge is +/- .020”. The emboss height will be held to within +/- .005” with a minimum emboss height of .003”.
In summary, embossing can add a great deal to the functionality and aesthetic quality of an overlay or label design. However, when working with polycarbonate films, it is important to realize that embossing raised areas for membrane switch keys will significantly reduce the life of the overlay. Since switch size and travel, film thickness, embossed height, and working environment can all be factors, it is recommended that life testing be performed on any project utilizing polycarbonate where more than a few thousand actuations will be necessary.