Draft angle is the slight taper applied to vertical walls of a thermoformed part to allow it to release from the mold without scuffing, distortion, or sticking. Insufficient draft is the #2 cause of thermoforming failures (after wall thinning). This guide explains the minimums, the optimums, and the special cases — for every common material and texture combination.
What Is Draft Angle and Why It Matters
A perfectly vertical sidewall (0° draft) creates a cylindrical surface where the sheet is pressed against the mold along its entire length. As the part cools, plastic shrinks. With 0° draft, this shrinkage causes the part to grip the mold like a tightened collar — and it cannot be released without damage.
Draft angle (typically 1° to 7°) widens the opening of the cavity slightly compared to its depth. As the part shrinks, it pulls inward and away from the mold wall, allowing clean release.
The draft direction depends on mold type:
- Female (cavity) mold — draft narrows the opening from outside to inside. Part forms on the inside of the mold.
- Male (positive) mold — draft narrows the part from base to top. Part forms over the outside of the mold.
Minimum Draft Angles by Material and Texture
| Material | Polished Surface (Min) | Light Texture VDI 18-24 (Min) | Medium Texture VDI 27-33 (Min) | Heavy Texture VDI 36-45 (Min) |
|---|---|---|---|---|
| ABS | 1° | 2° | 3° | 5° |
| HIPS | 1° | 2° | 3° | 5° |
| PETG | 1° | 2° | 3° | 5° |
| PMMA (Acrylic) | 1.5° | 2.5° | 4° | 6° |
| PC (Polycarbonate) | 2° | 3° | 4° | 6° |
| PP | 1° | 2° | 3° | 5° |
| HDPE | 1° | 2° | 3° | 5° |
| PC/ABS blend | 2° | 3° | 4° | 6° |
VDI = European texture standard. MT-11000 series (Mold Tech US) maps approximately to these VDI ranges.
Special Cases Requiring More Draft
- Deep draws (H:D > 1:1) — add 1° beyond minimum
- Heavy-gauge thick parts — add 0.5° beyond minimum (more shrinkage = more grip)
- Female cavity with sharp inside corners — add 1° beyond minimum (cooling-induced stress concentration)
- Parts with internal ribs or bosses — apply minimum draft to all rib walls independently
- Glass-filled or mineral-filled grades — add 0.5-1° (filler reduces flow, increases grip)
Why Texture Demands More Draft
A polished mold has a surface roughness around Ra 0.05-0.4μm. The plastic contacts the mold along smooth lines and slides off easily. A textured mold has surface peaks (Ra 5-50μm) that mechanically grip the plastic — small mountains and valleys interlock.
Insufficient draft on textured molds causes a characteristic failure mode: texture drag-marks (vertical scuff lines on the part where the texture peaks scraped material during demolding). These are visible to the eye, ruin Class-A surfaces, and are unrepairable.
Rule of thumb: add 1° of draft per VDI step above 24 (e.g., VDI 30 = base + 1°, VDI 36 = base + 2°).
Draft on Inside vs Outside Surfaces
For most parts, both inside and outside walls need draft — but in opposite directions:
- Female mold: Outside walls of the part must taper inward going down. Inside walls of any features (e.g., recessed pockets) must taper outward going down.
- Male mold: Outside walls of the part must taper inward going up (away from base). Inside walls of any features must taper outward going up.
Draft and Undercuts: When You Can’t Have Draft
Sometimes part function demands a feature that violates draft — a snap-fit lip, a return flange, an internal boss with a flat top. These are undercuts, and they have specialized solutions described in our undercut design guide:
- Slip-rings / loose collars
- Removable plug inserts
- Side actions (pneumatic side-pulls)
- Split-line redesign
Each adds tooling cost ($500-5,000) and cycle time (5-30 sec), so eliminating undercuts via design changes is always preferred when possible.
Practical Draft Application: How Much Material Do I Lose?
A 200mm tall sidewall with 5° draft is wider at the top than the bottom by:
2 × tan(5°) × 200 = 2 × 0.0875 × 200 = 35mm
This 35mm extra at the top is sometimes mistaken for “wasted material.” In reality, it’s the only thing keeping your part demoldable. The cost is real (a part footprint that’s slightly larger than the inner usable space) but unavoidable for thermoforming.
Draft on Class-A Surfaces (Cosmetic Faces)
For visible exterior surfaces (consumer products, retail displays, automotive trim), the draft requirement increases:
- Minimum 5° draft on textured Class-A faces
- Use of female cavity molds only (texture is on the cavity wall, transferred to the part exterior)
- Surface preparation: mold polish + texture application before each production run, with re-polishing every 50,000-200,000 cycles depending on material abrasion
How Draft Angle Affects Cost
| Draft Angle | Demolding Risk | Cycle Time Impact | Tooling Complexity |
|---|---|---|---|
| 0–1° (insufficient) | High (will fail) | Long (slow demolding) | Standard |
| 2–3° (minimum) | Low | Standard | Standard |
| 5° (optimal) | Very low | Fast demolding | Standard |
| 7°+ (excessive) | None | Fast demolding | May require larger sheet (cost) |
Practical recommendation: design at 5° wherever the function allows, drop to 3° where space-constrained, never below 2° on textured surfaces.
Send Us Your CAD for Draft Review
Our application engineers will review every face for adequate draft and flag risks before tooling. Free service for parts entering DFM review.
Draft Angle FAQ
What’s the absolute minimum draft I can specify?
For a polished female mold and easy-flowing material like HIPS, 0.5° has been used in production successfully. But it requires perfect mold release coatings and high-vacuum rapid demolding. We recommend never specifying below 1° on a polished surface or below 3° on any textured surface.
Can different walls of the same part have different drafts?
Yes — this is the norm. You’ll often see 5° on Class-A visible walls, 2° on hidden internal walls, and 3° on functional ribs. Each wall is independent.
Does draft angle affect the part’s structural rigidity?
Marginally. A 5° drafted wall has slightly less effective vertical stiffness than a 0° wall of the same height, because the wall behaves more like a frustum than a column. For most enclosures the effect is negligible (<3%). For load-bearing structural parts, account for it in FEA.
Do you put draft into the CAD or do you add it during DFM?
We always recommend designing draft INTO your CAD model from the start. If the customer’s CAD has 0° walls, we apply draft during DFM and provide marked-up drawings for approval. This adds 1-2 days to tooling kickoff.
Is draft different for vacuum forming vs pressure forming?
Slightly. Pressure forming (with up to 90 PSI mold-side air pressure) reproduces fine details and tighter draft (1-2° instead of 2-3°). The added tooling cost typically only justifies pressure forming for cosmetic Class-A consumer products in volume.