Print-Grade Polyester (Mylar): Applications and Properties
Where polyester film wins and where it fails. Print-grade vs industrial-grade Mylar.
May 8, 2026 · K&R Plastics Substrate Team · 8 min read
Mylar is a trade name. Polyester film is the material. The two terms get used interchangeably across the industry, and that's mostly fine — but the substrate behind both names is more specific than people assume. Polyester film is dimensionally stable in ways no other plastic substrate matches, prints cleanly when properly treated, and survives temperature ranges that destroy PVC and PETG.
This guide is for substrate buyers who need to know when polyester is the right pick and when it isn't. We'll cover the chemistry briefly, the difference between print-grade and industrial-grade polyester, the five applications where polyester wins, and the cases where you should look elsewhere.
What polyester film actually is
Polyester film is biaxially-oriented polyethylene terephthalate (BOPET). Same polymer family as APET, PETG, and RPET sheet — but the production process is fundamentally different.
Sheet PET is cast or extruded with the polymer chains in random orientation. Polyester film is stretched in two perpendicular directions during production, aligning the polymer chains in both the machine direction (MD) and transverse direction (TD). That biaxial orientation is what gives polyester film its signature properties:
- Dimensional stability across temperature
- High tensile strength in thin gauges
- Tear resistance once oriented
- Optical clarity matching the best clear plastics
- Service temperature range from -100°F to 300°F+
- Excellent dielectric properties
Mylar is DuPont's trade name for BOPET film. Other brand names exist (Melinex, Hostaphan, Skyrol, Lumirror) but the chemistry is the same. When a print or fabrication shop says "Mylar," they almost always mean print-grade BOPET — usually 1 to 14 mils thick.
Print-grade vs industrial-grade
Polyester film is sold across a wide range of grades, and the difference between them is mostly about surface treatment and thickness:
Industrial-grade polyester (untreated)
- Used for laminating, electrical insulation, dielectric film, capacitor film
- Surface is hydrophobic — ink and adhesive don't stick well
- Cheaper per square foot
- Typical gauges: 0.5–14 mils for film, heavier for industrial sheet
Print-grade polyester (surface-treated)
- Used for printed graphics, durable labels, membrane switches
- Surface treatment varies: chemical primer, corona, plasma, or proprietary topcoat
- More expensive — surface treatment costs money
- Typical gauges: 1–14 mils
The difference matters. Untreated polyester is what you order for laminating layers or electrical applications. Print-grade polyester is what you order for any direct-print application. Substituting industrial-grade for print-grade saves money up front and costs the difference back in adhesion failures and rejected parts.
When ordering, specify the grade and the treatment. Vague "Mylar 5 mil" requests routinely get filled with whatever the distributor has in stock, and the surface chemistry varies enough between mills that two "treated" rolls don't always behave the same on press.
Five applications where polyester wins
1. Graphic overlays
Membrane switches, instrument panels, control panels, and decorative overlays are polyester's home turf. The substrate is dimensionally stable enough that printed registration holds across temperature cycles, treated grades take ink reliably, and the thin gauges (3–10 mils) fit the assembly stack without adding bulk.
A polycarbonate overlay can substitute for polyester in some membrane switch applications, but polyester wins on cost and dimensional stability. Polycarbonate wins when the overlay needs to flex repeatedly without fatigue.
For new graphic overlay projects, default to print-grade polyester at 5–7 mils unless there's a specific reason to choose otherwise.
2. Durable labels
Polyester labels survive environments that destroy paper or vinyl labels: outdoor weathering, temperature extremes, chemical exposure, repeated wash cycles. Treated polyester takes most label-printing processes (toner, indigo, UV inkjet, thermal transfer) and pairs with permanent acrylic adhesives for industrial label applications.
Where polyester label film wins:
- Asset tags and equipment labels
- Outdoor-rated product nameplates
- Chemical drum labels
- Automotive under-hood labels
- Industrial machinery labels rated for steam clean
Where it loses:
- Indoor consumer product labels (vinyl is cheaper and prints just as well)
- Anywhere clear film doesn't matter and paper would do
- High-volume short-life labels (paper is half the cost)
For long-life industrial labels, polyester is the right substrate. For grocery shelf tags, it's overkill.
3. Membrane switch face film
The face film on a membrane switch carries the printed graphics, the dome layer, and (sometimes) hard-coated scratch resistance. Polyester does this well because:
- The printed graphic stays in registration across the assembly's life cycle
- The film can be scored and folded for the perimeter seal
- Treated grades take screen-print, indigo, and UV inkjet reliably
- Hard-coated polyester face films are commercially available
- Service temperature exceeds the membrane switch's design envelope
Polycarbonate competes here when the application demands more flex fatigue (think: a button pressed millions of times). Polyester is the default for lower-cycle membrane switches.
4. Hot-stamping foil carrier
Hot-stamping foils are a thin metallic or pigmented layer on a release coating, on a polyester carrier film. The polyester film is the structural backbone — it has to hold the foil layer flat, release cleanly under heat and pressure, and not stretch or warp on the press.
Polyester works for this because the dimensional stability of BOPET film is unmatched at the sub-1-mil gauges typically used in foil. No other substrate handles those gauges with the consistency required for high-speed hot-stamp production.
This is more often a converter's substrate decision than a brand's, but for shops running their own foil production, polyester carrier film is the standard.
5. Drafting film and stencils
A niche but persistent application: polyester drafting film, polyester stencils for fabric printing, polyester laser-cut templates. The dimensional stability, ink receptivity (treated grades), and resistance to handling damage make polyester the right substrate where paper would warp or tear.
Industrial pattern-makers, fabric printing shops, and aerospace template fabrication still spec polyester for these uses because nothing else has matched its stability at thin gauges.
Where polyester loses
Polyester film is great at thin gauges and dimensional stability. It loses against other substrates when:
Bulk and rigidity matter more than thinness. Polyester maxes out at roughly 14 mils as a film. For rigid sheet applications above that, PETG, polycarbonate, or PVC are the substrates.
Cost is the primary driver and you don't need polyester's specific properties. Polyester is more expensive per square foot than PVC, HIPS, or APET. If you don't need its dimensional stability, temperature range, or thin-gauge performance, you're overpaying.
You need a substrate that thermoforms. Biaxially-oriented polyester is dimensionally locked in. It doesn't form into 3D shapes the way amorphous substrates do. For thermoforming applications, switch to PETG or APET.
You need printability without surface treatment. Untreated polyester is hydrophobic and ink-resistant. If you don't have access to treated grades, PVC or HIPS will print without the treatment headache.
Recyclability is a top priority. Polyester film is recyclable in PET streams, but the thin gauges and surface treatments often disqualify it from standard collection. PET sheet (APET, RPET) is more cleanly recyclable.
Polyester vs PETG: a common confusion
Both are PET. Both are clear. Both are recyclable. The difference is form factor and processing:
| Property | Polyester (Mylar) film | PETG sheet |
|---|---|---|
| Form | Biaxially-oriented film | Amorphous sheet |
| Typical gauge | 0.5–14 mils | 15 mils – 250 mils |
| Thermoformable | No | Yes |
| Service temperature | -100°F to 300°F+ | -40°F to 160°F |
| Dimensional stability | Best in class | Moderate |
| Glycol modification | No | Yes |
| Maximum sheet thickness | ~14 mils | 0.250"+ |
| Best applications | Overlays, labels, thin-gauge graphics | Displays, packaging, faceshields |
The two substrates rarely compete for the same job. Polyester wins below 14 mils and where dimensional stability matters. PETG wins above 15 mils and where thermoforming or impact matters.
Print compatibility on treated polyester
Treated print-grade polyester runs cleanly on:
| Print method | Compatible | Notes |
|---|---|---|
| Toner / laser | ✓ | Treated grades; pre-condition stock to avoid moisture-driven curl |
| HP Indigo | ✓ | ElectroInk-qualified treated grades; verify by mill |
| UV inkjet | ✓ | Treated grades; UV is the most forgiving process |
| Solvent / eco-solvent inkjet | ✓ | Treated grades; aggressive solvents may etch surface |
| Screen | ✓ | Treated grades; specify ink system compatibility |
| Offset / litho | ✓ | Treated grades; one of the original substrates for printed Mylar |
| Thermal transfer | ✓ | Treated grades; the standard substrate for industrial labels |
Untreated polyester won't reliably print on any of these processes. Always specify treatment when ordering.
Polyester page → · Download datasheet ↓
Common mistakes
Ordering "Mylar" without specifying grade. "Mylar 5 mil" is a request that doesn't pin down treatment, supplier, or surface chemistry. Specify: "treated print-grade polyester, 5 mil, [supplier or grade]." Distributors will appreciate the clarity.
Substituting industrial-grade for print-grade to save money. It looks the same and costs less per square foot. Then the print fails the tape test. Get the treated grade.
Trying to thermoform polyester film. Doesn't work. Biaxially-oriented PET is dimensionally locked in at room temperature and tears or shrinks unevenly when heated to forming temperature. Use PETG or APET sheet.
Defaulting to polyester when PVC or HIPS would do. Polyester is genuinely better than PVC in some applications, but for indoor signage, presentation materials, or commodity printed graphics, polyester is overkill and you're paying for stability you don't need.
Storing treated polyester for too long before printing. Surface treatments fade. For corona-treated polyester especially, print within 30 days of treatment date for reliable adhesion. Older treated stock may need re-treatment or reject.
When polyester is genuinely the right answer
Default to polyester film when at least two of these are true:
- The application requires gauges below 14 mils
- Dimensional stability across temperature matters
- The end-use environment is harsh (chemical, temperature, weather)
- The graphic must hold registration tightly through the part's life
- Service temperature exceeds 160°F continuous
- The printed surface needs scratch resistance from a hard-coat layer
- The substrate is laminated, foil-stamped, or acts as a carrier in a multi-layer assembly
Default to something else when only one is true. Polyester earns its premium in applications that need its specific properties; in applications that don't, it's the wrong substrate.
Need help selecting a polyester grade for your application? Request samples of treated print-grade polyester at your target gauge to qualify on your press or assembly line. Or talk to a substrate specialist — we stock multiple grades of treated and untreated polyester for graphic overlays, labels, membrane switches, and industrial applications.