How to Choose Sheet Gauge for a Thermoformed Part

“What thickness sheet do I need?” is one of the first questions on any thermoforming job, and the honest answer is: it depends on the finished wall you need after forming, not the number you’d like to start with. Here is how to reason about it.

Starting gauge vs finished wall

A thermoformed part is always thinner than its starting sheet, because the same material stretches over a larger area as it forms. So the starting gauge has to account for the thinning:

• Shallow parts (trays, shallow covers) thin only a little — starting gauge is close to finished wall.
• Deep parts (liners, tall covers) can thin to a fraction of the starting gauge at the base and corners.

You work backwards: decide the minimum acceptable wall in the thinnest area, estimate the thinning from the draw, and pick a starting gauge that leaves you above that minimum everywhere.

What drives the choice

• Draw depth / ratio — deeper draws need thicker starting sheet (or better plug assist) to keep walls usable.
• Stiffness and load — structural parts need more finished wall; you can add stiffness with geometry (ribs, crowns) instead of pure thickness.
• Impact and service — impact duty and temperature push toward more wall or a tougher grade.
• Cost and weight — thicker sheet costs more and weighs more, so you don’t want to over-specify.
• Gauge band of the process — thin-gauge (roll-fed) runs below ~1.5 mm for packaging; thick-gauge (cut-sheet) runs roughly 1.5–12 mm for durable parts.

Geometry beats brute thickness

A common mistake is to fix a thin-wall problem by simply starting thicker everywhere. Often a rib, a crown or a formed lip adds the stiffness you need at far less weight and cost than extra gauge across the whole part. We look at this at the DFM stage.

Why in-house extrusion helps

Because we extrude our own sheet, we are not limited to a supplier’s stock thicknesses. We can dial the starting gauge to exactly what the part needs — and control the grade, colour and any recycled or ABA-core construction at the same time. That means you’re not over-paying for gauge you don’t need, or compromising on a nearest-available thickness.

The practical route

Rather than guess, send us the part geometry and the finished-wall requirement. We model the wall-thickness distribution, recommend the starting gauge and forming approach (plug assist, pre-blow), and confirm it on first articles. Send a drawing or read the design guide for the full picture.