Designing for Deep Draws: Draw Ratio, Wall Thinning & Plug Assists

Deep parts — liners, deep trays, tall covers — are where thermoforming design is won or lost. The physics is simple: a sheet has a fixed amount of material, and the deeper you draw it, the thinner the walls become. Design well and the walls stay usable; design badly and the corners tear.

Draw ratio, in plain terms

Draw ratio is a measure of how much a part stretches the sheet. A shallow tray has a low draw ratio; a deep, narrow liner has a high one. Two rules of thumb:

• Higher draw ratio → thinner walls, especially at the base and the bottom corners, which form last and stretch most.
• Every material has a practical limit. Push past it and you get unacceptable thinning, blushing or splits.

You can’t beat the physics, but you can manage where the material goes.

Managing wall thickness

The main tools for keeping walls uniform in a deep draw are:

• Plug assist. A shaped plug pushes material down into the cavity before vacuum takes over, carrying thickness into the walls and base instead of leaving it at the rim. This is the single most important technique for deep parts.
• Pre-blow (billow). Inflating the heated sheet into a bubble first pre-stretches it evenly, so the draw starts from a uniform starting thickness.
• Starting gauge. Because we extrude our own sheet, we can start a deep part from exactly the right gauge rather than over-specifying.

Radii and draft

• Radii: sharp internal corners concentrate stretching and thin fastest — generous radii spread the material and remove stress risers. As a rule, the deeper the draw, the larger the radii you want.
• Draft: deep walls need adequate draft so the part releases cleanly and doesn’t drag or distort on ejection. Too little draft is a common cause of sticking and marking on deep parts.

Design the base last, mentally

The base and bottom corners of a deep part form last and thinnest, so design with them in mind: don’t put your thinnest-tolerance features or highest loads there without accounting for the thinning. If a feature must be robust at the base, either reduce the draw locally, add a rib, or plan for a bonded reinforcement.

Get it modelled before you tool

We model wall-thickness distribution during DFM, so you see where a deep part will thin before any tooling is cut — and we design the plug and pre-blow around it. That is far cheaper than discovering a thin corner in first articles.

Have a deep-draw part — a liner, a deep tray, a tall cover? Send the drawing and we’ll show you the wall-thickness prediction and the forming approach, or read the full thermoforming design guide.