Key principles for draft angles, radii, undercuts, wall thickness, and design for manufacturability in thin and heavy gauge thermoforming.
BRT USA Engineering Team · Thermoforming & Tooling
Published July 9, 2026
Successful thermoformed parts start in CAD. Whether you need a medical device tray or a heavy gauge equipment cover, design rules differ from injection molding — sheet stretching, vacuum draw, and trim tolerances all influence what we can manufacture repeatably for you. This guide walks through the DFM decisions we review on customer drawings every day at Blue Ridge Thermoforming.
Draft is the taper on vertical walls that allows a formed part to release from the mold without binding. Unlike injection molding where near-zero draft is sometimes achievable, thermoformed sheet cools against the tool surface and can vacuum-lock without adequate relief. As a rule of thumb, plan for 3–5° draft on interior walls for shallow parts and 5–7° (or more) for draws deeper than the part width.
Sheet thinning concentrates at sharp corners and the bottom of deep draws. Generous fillet radii (minimum 0.125" on heavy gauge, proportionally smaller on thin gauge) distribute material and reduce webbing risk. Wall thickness thermoforming targets should stay as uniform as practical — step changes create chill marks and differential shrinkage that show up as warpage after trim.
Sharp 90° corners, zero draft walls, deep undercuts without a plan, abrupt wall transitions, holes too close to formed edges.
Rounded transitions, consistent draft, split assemblies for undercuts, gradual wall changes, post-form CNC for tight features.
Undercut design thermoforming requires either a multi-piece tool, a loose insert, or a design change. Most production parts avoid true undercuts in the forming step and instead use 5-axis CNC trimming, routing, or assembly of two formed halves. Locating holes and mounting features are often machined after forming for tighter tolerances than the forming process alone can hold.
Before we build your tooling, confirm draft, radii, draw depth, material choice, and trim allowances with our engineering team.
Send a STEP file or drawing and our engineering team will review draft, material, and process fit — typically within one business day.
Most thermoformed parts need 3–5° draft on vertical walls. Deep draws and textured surfaces often require 5–7° or more. Our engineers confirm the right draft for your material, depth, and tool type during DFM review.
Thermoforming starts from flat sheet that stretches over or into a tool. Draft, radii, and draw depth matter more than mold shutoffs. Features requiring tight tolerances are often trimmed or machined after forming rather than formed in.
True undercuts are limited without multi-piece tooling. Design teams usually split the part, use loose cores for low-volume runs, or add secondary CNC trimming to achieve the final geometry.
Specify starting sheet gauge and critical formed wall thickness after draw. Heavy gauge often runs 0.125"–0.500" sheet; thin gauge packaging may start at 0.010"–0.060". Uniform walls reduce warpage and improve repeatability.