How Big a Printed PET Face Can Be Before It Sags
A printed PET face is a thin panel held at its edges and nothing else. Under its own weight it bows, and the bowing gets worse far faster than most people expect: sag rises with the fourth power of the span. Double the width of an unsupported face and the droop is not twice as deep — it is sixteen times.
The formula, and what it assumes
The textbook case is a simply supported strip of uniform thickness under its own weight:
δ = 5 w L⁴ ÷ (384 E I)
with E = 2,800 MPa for PET sheet, density 1.35 g/cm³, I = b·t³/12 for a unit width, and w the sheet’s own weight per square metre. No back support, no tension, room temperature, no ink load. A real face is held a little tighter than the ideal, and often has a light guide plate or a back sheet behind it — both of which reduce sag. So read these numbers as the worst case you are designing away from, not as a prediction of what the panel will do on the wall.
Droop under its own weight
1 m span, simply supported, PET sheet
Sag at the centre of a 1 m unsupported span
Doubling the gauge cuts sag to a quarter, because stiffness rises with the cube of thickness while the weight only rises linearly. It is the cheapest fix per millimetre of face.
Sag by span and gauge
| Gauge | 1.0 m span | 1.5 m span | 2.0 m span |
|---|---|---|---|
| 0.3 mm | 8.2 mm | 41.6 mm | 131.4 mm |
| 0.5 mm | 3.0 mm | 15.0 mm | 47.3 mm |
| 0.8 mm | 1.2 mm | 5.8 mm | 18.5 mm |
| 1.0 mm | 0.7 mm | 3.7 mm | 11.8 mm |
Same assumptions. The jump between columns is the fourth-power law: going from 1 m to 2 m multiplies sag by sixteen, not by two.
What that means on a real face
A 2 m × 3 m printed face on 1.0 mm sheet bows about 12 mm at the centre of the long side if nothing supports it. Twelve millimetres is enough to change the distance between the sheet and the LEDs by more than a third, which is how a face that looked even in a sample ends up with a bright band down the middle. On 0.5 mm it is about 47 mm, and the panel starts to read as a drum rather than a sheet.
GAG behaves within about 6 % of APET here, because its stiffness comes from the APET core; the PETG skins change the surface, not the bending. So the table above applies to both, and the choice between them for a large face is about folding and welding, not about sag.
Three fixes, in order of cost
Thicker sheet. From 0.5 to 1.0 mm cuts sag to a quarter and costs less than a mid-rail, as long as your press and cutter handle the gauge.
A back support. A light guide plate or a back sheet sitting 10–20 mm behind the face turns a 2 m span into two 1 m spans, and the sag drops sixteen-fold. This is why lit boxes tolerate bigger faces than unlit panels.
A mid-rail or a split panel. Cheapest on very large faces: break the face into two panels with a rail between them, and you have halved both spans without buying thicker sheet.
Frequently Asked Questions
Does the print itself add weight?
UV ink adds grams per square metre, not kilograms — it does not move these numbers. A heavy flood white pass on a large face is still under 10 g/m².
Does temperature change it?
PET softens as it warms, so a face near a hot light guide plate sags more than the table says. Keep the sheet from sitting directly on the LEDs, and give the box a little venting.
Is 0.3 mm ever the right answer on a big face?
Only with a back support within about 300 mm, or on a face small enough to stay inside the 0.9 m span above. Otherwise it is a face that will photograph badly under LEDs.
Not sure which gauge your face needs? See our printable PET sheet range — 0.3 to 1.0 mm, APET, PETG and GAG — or send the face size and we will tell you the span and the support spacing.
Products referenced