Design for Thermoforming
A complete engineering reference for designers and product engineers. Apply these guidelines early in your design process to reduce tooling cost, scrap, and cycle time — and avoid the most common thermoforming failures.
Why design for thermoforming matters
Thermoforming is a fundamentally different process from injection moulding or CNC machining. Material stretches, thins at corners, and must release from a mould under vacuum or pressure. Parts designed without thermoforming constraints in mind routinely fail at three points: sharp corners that crack, insufficient draft that prevents release, and deep features that thin the wall below structural limits.
The good news: all three failure modes are entirely preventable with simple design rules applied at the concept stage. The guides below cover each constraint in detail, with worked examples, data tables, and rules of thumb used by Machinecraft's engineering team across 50+ years of machine manufacturing.
These guides are written for the engineer designing the part — not the machine operator. If you are evaluating which thermoforming machine to buy, see the Machine Selection Guide or use the Machine Finder.
Quick reference: the 5 golden rules
| Rule | Minimum | Recommended | Why it matters |
|---|---|---|---|
| Draft angle (flat surface) | 1° | 3–5° | Part release without distortion |
| Draft angle (textured surface) | 3° | 5–7° | Texture grips mould — needs more draft |
| Inside corner radius | 25% of sheet thickness | 3–5× sheet thickness | Prevents stress cracking & thinning |
| Outside corner radius | 1.5× sheet thickness | 3× sheet thickness | Smooth material flow around corners |
| Max draw ratio (no plug) | — | 0.5:1 (depth:width) | Above 0.7:1 requires plug assist |
All 6 design guides
Each guide is a standalone deep-dive with data tables, worked examples, and practical rules.
Frequently asked design questions
What is design for thermoforming (DFT)?
Design for thermoforming (DFT) is the practice of designing plastic parts with thermoforming-specific constraints in mind — including draft angles for part release, minimum corner radii to prevent stress cracking, wall thickness distribution based on draw ratio, and material selection for the intended application. Applying DFT principles early reduces tooling cost, scrap, and cycle time.
What draft angle is required for thermoforming?
A minimum of 2–3° draft is required for most thermoformed parts. For textured surfaces, 5° or more is recommended. Deep-draw parts (draw ratio > 1:1) may need 7–10° to release cleanly without distortion. Male moulds generally require more draft than female moulds.
What is the minimum corner radius for thermoforming?
The minimum inside corner radius should be at least 25–30% of the sheet thickness. For a 3mm sheet, the minimum inside radius is 0.75–0.9mm, but 3–5mm is recommended for structural integrity. Outside corner radii should be at least 1.5× the sheet thickness. Sharp corners cause material thinning and stress concentration.
How does draw ratio affect wall thickness in thermoforming?
Draw ratio is the ratio of part depth to the smallest plan dimension. A 1:1 draw ratio means the depth equals the width. As draw ratio increases, material stretches more and wall thickness decreases — particularly at corners and the bottom of deep features. For a 3mm sheet with a 1:1 draw ratio, corner wall thickness can drop to 1.0–1.5mm. Plug assist is used for draw ratios above 0.7:1 to pre-stretch material into deep areas.
Which thermoforming material is best for outdoor applications?
ASA (Acrylonitrile Styrene Acrylate) and ABS/PMMA alloys are the best choices for outdoor thermoformed parts due to their UV resistance. HDPE is also UV-stable and chemical-resistant. Standard ABS without UV stabiliser will chalk and fade within 2–3 years of outdoor exposure. For extreme outdoor environments, consider PETG or PC for impact resistance alongside UV stability.