Last updated: 2026-09-02 · 5 min read

How Much Draft Angle Do You Need — and How Do You Calculate It for Your Mold?

Draft angle is the taper added to vertical walls so the part releases from the mold without drag marks, scuffing or ejection damage — and the minimum depends on wall depth, surface texture and resin shrinkage: 1° per side for smooth surfaces up to 50mm deep, 1.5° for textured surfaces, and 2°+ for deep draws or sticky resins like PVC and TPE. Adding draft is free at the design stage; removing it after the mold is cut requires welding and re-machining, which costs more than the original design change.

Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. See also related guide.

Key Facts

  • The minimum draft for a smooth mold surface is 0.5° per side for shallow features (under 10mm) and 1° per side for deeper features (10–50mm) — anything less risks drag marks and ejection damage.
  • Surface texture adds to the required draft: each SPI finish class adds approximately 0.5–1.5° — a VDI 3400 texture (rough) needs 3° total draft, while a polished SPI A-1 surface needs only 1°.
  • Resin shrinkage affects draft: as the part cools, it shrinks onto core features, increasing ejection force — resins with high shrinkage (PP, PE, POM) need more draft than low-shrinkage resins (PS, PC).

    • Parting line — The line where the two mold halves (A-side and B-side) meet. Parting line location affects: gate placement, ejector pin marks, witness lines, and cosmetic appearance. A well-designed parting line minimizes visible witness marks. The related guide covers adjacent topics.

    Draft Requirements by Surface and Depth

    Surface Finish Min Draft (per side) Typical Application
    Polished (SPI A-1/A-2) 0.5–1° Optical lenses, cosmetic surfaces
    Smooth (SPI B-1/B-2) 1° Consumer electronics, appliance housings
    Light texture (SPI C-1/C-2) 1.5° Automotive interiors, tool housings
    Heavy texture (SPI D-1/D-2) 2–3° Leather grain, deep texture panels
    EDM / abrasive blast 1.5–2° Medical housings, industrial covers

    How to Calculate the Required Draft

  • Start with wall depth: for every 25mm of vertical wall, add 0.5° of draft — a 50mm deep wall needs 1° minimum; a 100mm deep wall needs 2°.
  • Add texture allowance: if the surface is textured, add the texture’s draft requirement (0.5–1.5° depending on roughness) to the base draft.
  • Check resin shrinkage: high-shrinkage resins (PP 1.5–2.5%, POM 1.8–2.2%) shrink onto cores and need 0.5° more draft than low-shrinkage resins (PS 0.4–0.6%, PC 0.5–0.7%).
  • Verify at the DFM review: the mold maker’s DFM report flags any wall without adequate draft — this is the last free chance to add it before steel is cut.
  • Interior vs. exterior walls: interior walls (cores) need more draft than exterior walls (cavities) because the part shrinks onto the core — add 0.5° to core walls as a rule of thumb.

    • What This Means for Your Quotation

    The DFM review confirms the draft angle on every wall before steel is cut — and the quotation should state the minimum draft requirements based on the part’s surface finish, wall depth and resin. When the drawing shows zero-draft walls, the DFM flags them and the designer decides whether to add draft or accept the ejection risk.
    Adding draft at the design stage costs nothing; adding it after the mold is built costs weld, re-machine and re-texture — typically 3–5× the cost of the original design change.

    Frequently Asked Questions

    Can I get away with zero draft on a short wall?

    On walls under 3mm deep with a polished surface, zero draft may work — but the risk increases with every cycle as the mold wears. What ejects cleanly at T1 may drag at shot 10,000. Adding 0.5° draft on short walls is insurance against ejection problems in production, and it costs nothing at the design stage.

    Does draft affect the part’s dimensions?

    Yes — draft changes the wall thickness at the top and bottom of the feature. A 1° draft on a 50mm wall changes the dimension at the top by approximately 0.9mm. The designer must account for this in the drawing tolerance; the mold maker must machine to the specified draft angle, not to a vertical wall with a note saying “add draft.”

    What if the customer drawing shows zero draft?

    The DFM review flags it and recommends the minimum draft for the material, depth and texture. If the customer cannot accept the dimensional change from draft, the alternative is a shut-off with a stripper plate or air poppet — but these add mold complexity and cost. The DFM should present both options so the decision is informed.

    Key Takeaways

    1. Minimum draft is 0.5–1° per side for smooth surfaces, 1.5–3° for textured surfaces — calculated from wall depth, texture roughness and resin shrinkage.
    2. Interior walls (cores) need more draft than exterior walls because the part shrinks onto the core — add 0.5° to core walls as a standard allowance.
    3. The DFM review is the last free chance to add draft; adding it after the mold is built costs weld, re-machine and re-texture — 3–5× the design-stage cost.

    Send aCatMould the part drawing, and the DFM review flags every wall without adequate draft — with the minimum angle calculated from the surface finish, wall depth and resin, so the mold is built with the draft needed for clean ejection, not discovered as a drag mark at T1.

    Sources
    Plastics Technology — draft angle design guidelines
    SPE — mold design and draft angle guidance
    MoldMaking Technology — DFM and draft requirements

    Sources: Plastics Technology · Plastics Technology Online · Society of Plastics Engineers (SPE) · ThomasNet