1. Add enough draft
Draft is the slight taper on vertical walls that lets a part release from the tool. Thermoforming needs more draft than injection molding because the part shrinks onto a male tool or into a female cavity. Aim for several degrees on deep walls; too little draft causes scuffing, webbing, and release problems on a large part.
2. Use generous radii
Sharp internal corners thin the material and concentrate stress. Generous radii let the sheet flow into the corner without over-thinning, which protects both wall thickness and part strength. As a rule, larger is better, design the biggest radius the part function allows.
3. Design for even wall distribution
A thermoformed wall thins as it draws. Deep draws and tall features pull material away from the corners and bottom. Plan the part so the draw is as even as possible, and tell your former where minimum wall matters so the process, plug assist, pre-stretch, can be tuned to protect it.
4. Minimize undercuts
Undercuts trap the part on the tool and may require moving tool sections or secondary operations to release. On a large part, every undercut adds tooling complexity and cost. Design them out where you can; where a function truly needs one, flag it early so tooling accounts for it.
5. Stiffen with formed-in geometry
You cannot add a separate rib the way injection molding does, but you can form stiffening geometry directly into the part, beads, flutes, steps, and crowns. On large panels these features do the structural work, controlling oil-canning and adding rigidity without added weight or parts.
6. Set realistic tolerances
Thermoforming tolerances are looser than injection molding and scale with part size and draw. Over-tight tolerances drive cost and scrap with no functional benefit. Put the tight tolerance only where the part mates or mounts, and open it everywhere else.
7. Choose material early
Material drives draft, shrink, forming temperature, and finish. Picking it late forces rework. Decide early whether the part needs the impact resistance of ABS, the chemical resistance of HDPE, the toughness of Kydex, or the clarity of an acrylic, and design to that material's behavior from the start.