In membrane switch, embossed graphic panel and backlit panel projects, the first serious engineering decision is not the circuit layout or the dome selection — it is the front surface film. The choice between polycarbonate (PC) and polyester (PET) directly governs key life, resistance to cleaning chemicals, display readability and dimensional stability on large panels. This guide compares polycarbonate and polyester overlay material selection using measurable parameters rather than marketing language.
The short answer is simple: PET for surfaces that move repeatedly, PC for fixed and optically critical surfaces. That rule alone is not sufficient, though. The details below show exactly where the decision reverses.
Where the behavioural difference between polycarbonate and polyester comes from
PET film is biaxially oriented and semi-crystalline. Its molecular chains are aligned in the plane of the film, which raises fatigue resistance and solvent resistance and sharply lowers thermal expansion. Polycarbonate is an amorphous polymer: the chains are randomly arranged, which makes the material nearly optically isotropic and impact resistant, but leaves it vulnerable to repeated flexing and to solvent-induced stress cracking. In front panel and graphic overlay work, the polycarbonate film offered in the market under a range of trade names is the same base material.
Flex life is set by the film, not the dome
Metal domes typically survive 1–5 million actuations and poly domes 100,000–500,000 actuations. The overlay, however, can fail long before the dome does. A 0.15–0.20 mm PET top layer with correct emboss geometry can operate in the million-cycle range, while a PC top layer in the key area tends to develop micro-cracks and hairline delamination of the printed layer after a few hundred thousand actuations. Base your life calculation on the weakest layer in the stack, not on the dome datasheet.
Chemical resistance does not come from the base film
A common misconception is that chemical durability comes from the polymer itself. In practice, how the surface reacts to isopropyl alcohol, detergents, hydraulic oil or disinfectants is largely determined by the hard coat. Uncoated PC hazes, cracks or loses its print when exposed to ketones (acetone, MEK), aromatic solvents and alkaline cleaners. Hard-coated PET is markedly more stable in the same environment. If the machine has a defined cleaning procedure, selecting the material around that procedure is far cheaper than replacing overlays in the field.
Technical comparison: PC and PET overlay films
| Property | Polyester (PET) | Polycarbonate (PC) |
| Common thickness range | 0.125 – 0.20 mm | 0.175 – 1.00 mm |
| Structure | Biaxially oriented, semi-crystalline | Amorphous |
| Repeated flex resistance | Very high; million-cycle range | Limited; micro-cracking risk |
| Embossing suitability | Suitable for pillow and rim embossing | Limited; stress whitening at corners |
| Coefficient of thermal expansion | approx. 17 ppm/K | approx. 65–70 ppm/K |
| Typical continuous service temperature | -40 … +105 °C | -40 … +115 °C |
| Solvent / chemical resistance | High | Low to moderate; stress-crack prone |
| Optical behaviour | Birefringent; patterning under polarised light | Near isotropic; suitable for display windows |
| Rigidity | Low, flexible film | High; self-supporting above 0.5 mm |
| Surface hardness | 2H–3H with hard coat | Soft uncoated; 2H–3H hard coated |
| Outdoor / UV | Good in hard-coated grades | Yellowing risk unless UV stabilised |
| Cost per m² | Generally higher | Generally lower |
Choosing by application
Keypad top surface
In every membrane switch keypad, whether metal dome or poly dome, the top layer flexes on each actuation. PET is the default material here. Thickness also drives feel directly: a 0.125–0.175 mm film transmits the metal dome snap cleanly to the finger, while an overlay above 0.20 mm softens the collapse point, raises actuation force and dulls the feedback. Expecting a crisp tactile click through a thick PC overlay is not realistic.
Embossed graphic panel
Pillow and rim embossing is typically formed to 0.3–0.8 mm height, and PET carries that permanent deformation without cracking. PC can be embossed as well, but its amorphous structure brings a risk of stress whitening at sharp transitions and fracture in the printed layer. Wherever embossing is required, PET is the default.
LCD / TFT display window
This is the item most designers discover late. The biaxially oriented structure of PET produces optical birefringence. Nothing is visible to the naked eye, but when the film sits over an LCD/TFT display emitting polarised light — or when the operator wears polarised sunglasses — rainbow-like interference patterns appear on the screen. On construction machinery, agricultural equipment and forklifts working outdoors, this single criterion rules PET out for the window area. PC, being amorphous, does not create the pattern.
If you need PET for the key area and PC for the window, there are two practical routes: cut the film away completely in the window area (open window) and bond a separate clear PC window, or split the product into a PC graphic panel plus a separate PET membrane keypad.
Backlit panels and dead-front design
For icons that are entirely invisible when off and appear only when the LED is lit, tinted or smoked PC film with roughly 10–40% light transmission is typical. PC's availability in thicker gauges and a wide tint range is an advantage here. LED service life is typically in the 50,000–100,000 hour range; to avoid disturbing light distribution, prefer a plain or fine diffusing texture over a deep matte finish in the window area.
Production details that get overlooked in design
Differential thermal expansion is a real force. A 300 mm panel heated from 20 °C to 60 °C grows about 0.8 mm in PC and about 0.20 mm in PET. If the carrier is aluminium (roughly 23 ppm/K), the difference against PC becomes shear stress in the adhesive layer. On large panels a 0.13 mm class (5 mil) acrylic transfer adhesive absorbs that movement, whereas a 0.05 mm class (2 mil) adhesive can lead to edge lifting.
Calculate total stack thickness up front. A typical six-layer membrane switch falls in the 0.9–1.4 mm range. Specifying 0.50 mm PC instead of 0.175 mm film changes both the mounting clearance and the actuation force.
Match cutting tolerance to the material. Die cutting is typically ±0.2 mm and laser cutting ±0.3 mm. PC thicker than 0.5 mm can show burrs and white edges when die cut, so the cutting method and edge quality should be agreed at the drawing stage.
Sealing depends on how the film conforms. Where IP65/IP67 is targeted, the overlay is extended 1–2 mm beyond the sub-panel and an uninterrupted adhesive frame is maintained. Rigid PC does not conform to surface irregularities as well as thin PET, so the flatness of the mounting plate becomes more critical.
Material selection checklist
- How many actuations must the key area survive? Start with PET above 100,000.
- Is embossing required? If yes, PET.
- Is there an LCD/TFT window? Do operators wear polarised eyewear?
- Which cleaning chemicals will be used: alcohol, ketones, alkaline detergent, hydraulic oil?
- Does the longest panel edge exceed 250 mm? If so, assess thermal expansion and adhesive thickness together.
- Is a dead-front effect needed? What target light transmission?
- Will the overlay be self-supporting or bonded to a metal or plastic housing?
- What surface finish is required: matte, semi-matte, selective gloss window?
- Will the product operate outdoors? UV exposure requires a stabilised PC grade.
- What is the total stack thickness and the available mounting recess in mm?
- Is the target IP65 or IP67, and is the edge overhang shown on the drawing?
Working through this checklist alongside your drawing turns the material decision from a guess into an engineering input. Share your actuation cycle target, cleaning procedure and assembly dimensions with Tuşhan Elektronik and we can define the appropriate film type and layer construction with you and prepare a quotation.