Last updated: 2026-09-04 · 5 min read

Draft Angle Calculation — How Do You Determine the Minimum Draft Required for Each Surface, and What Happens When the Drawing Specifies Zero Draft?

The minimum draft angle depends on three variables: surface finish (polished 0.5–1°, textured 1.5–3°), wall depth (add 0.5° per 25 mm beyond baseline), and resin shrinkage (high-shrink PP/PA add 0.5°, glass-filled GF30 add 0.5–1°). A polished wall at 25 mm depth in ABS needs 1° per side; the same wall with medium texture in PA66-GF30 needs 3° per side.

Key Facts

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  • Three-variable calculation: baseline by surface finish (polished 0.5–1°, light texture 1.5°, medium 2°, heavy 3°) + depth increment (0.5° per 25 mm above baseline) + resin adjustment (high-shrink +0.5°, GF-filled +0.5–1°). Total draft per side = sum of all three.
  • Dimensional impact: 1° draft on a 10 mm wall changes dimension by 0.175 mm (10 × tan 1°); on a 25 mm wall, 0.44 mm. The taper is held within standard tolerance (±0.1–0.2 mm per 25 mm) when the draft direction is specified on the drawing.
  • Zero-draft walls require a stripper plate ($2,000–5,000 added mold cost), mirror-polished cores (SPI-A1), and slower ejection (5–10% cycle increase). For volumes above 10,000 pieces, adding 1° draft in the DFM review is always the lower-cost solution.

    • Process parameter — A controllable variable in the injection molding process: melt temperature, injection speed, holding pressure, cooling time, mold temperature. Each parameter affects part quality — deviations cause defects. Parameters are set during T1 and locked for production. The related guide covers adjacent topics.

    How Do You Calculate the Minimum Draft Angle for Each Surface?

    Six surface types with baseline draft, depth increment and resin adjustment:

    Surface Type Wall Depth Baseline Draft Depth Add Resin Add Total per Side
    Polished (SPI-A1 to B2) Below 10 mm 0.5° 0° 0° 0.5°
    Polished (SPI-A1 to B2) 10–25 mm 1.0° 0° 0° 1.0°
    Polished (SPI-A1 to B2) 25–50 mm 1.0° +0.5° 0° 1.5°
    Light texture (VDI 24–27) Any depth 1.5° +0.5° per 25 mm 0° 1.5–2.5°
    Medium texture (VDI 30–33) Any depth 2.0° +0.5° per 25 mm 0° 2.0–3.0°
    Heavy texture (VDI 36+) Any depth 3.0° +0.5° per 25 mm 0° 3.0–4.0°

    Resin adjustments are added on top of the table values: high-shrinkage resins (PP, PE, PA) add 0.5°; glass-filled resins (GF30, CF30) add 0.5–1.0° due to fiber friction. Worst case — deep-walled, glass-filled part with heavy texture — needs 3.0° + 0.5° + 1.0° = 4.5° per side.

    The dimensional impact is small: 1° draft on a 10 mm wall changes the dimension by 0.175 mm (10 × tan 1°). On a 25 mm wall, the change is 0.44 mm — within standard tolerance (±0.1–0.2 mm per 25 mm) when the draft direction (toward core or cavity) is specified on the drawing.

    What Happens When the Drawing Specifies Zero Draft — and How Do You Fix It?

    1. Identify the function of the zero-draft surface. If the surface mates with another part (press-fit interface, sealing face), the zero draft may be intentional — the designer wants a sharp edge for the seal.
      In this case, add draft only below the mating surface: the draft starts 1–2 mm below the seal face, preserving the sharp edge at the top while still allowing ejection.
    2. Add draft in the DFM review. If the zero-draft surface is not a functional requirement, recommend adding the minimum draft (1° per side for polished surfaces).
      The change is usually accepted — 1° taper adds less than 0.2 mm dimension change at 10 mm wall depth, which is within most tolerance bands. This costs nothing and eliminates the ejection risk.
    3. Use a stripper plate or sleeve ejector. If the designer refuses to add draft (common on medical device parts where the wall must be perfectly vertical), the mold uses a stripper plate that pushes the part off the core without relying on draft.
      The stripper plate adds $2,000–5,000 to the mold cost and requires additional mold height, but it allows zero-draft walls to be ejected without scraping.
    4. Polish the core to SPI-A1 mirror finish. If the wall is deep and has minimal draft (0.25–0.5°), polishing the core to a mirror finish (SPI-A1, 600 grit) reduces the friction coefficient enough to allow ejection.
      This is a workaround — it works for shallow walls below 15 mm but becomes unreliable for deeper walls where the shrinkage lock is stronger.

    For production volumes above 10,000 pieces, zero-draft designs are uneconomical: the mold cost increases $3,000–8,000 (stripper plate, polishing, stronger ejector), cycle time increases 5–10% (slower ejection), and the risk of part damage during ejection is 5–15% per shot. Adding 1° draft in the DFM review is always the lower-cost solution.

    Flash — Thin excess material that escapes between the mold parting line or around ejector pins. Cause: insufficient clamp force, worn parting line, excessive injection pressure, or contaminated mold surface. Flash thickness: 0.02–0.1mm. Requires secondary trimming to remove. The related guide covers adjacent topics.

    The minimum draft for a polished wall in standard resin is 1° per side — this adds less than 0.2 mm of dimension change at 10 mm depth and eliminates 90% of ejection problems. Zero-draft walls are almost always a design oversight, not a functional requirement.

  • Key parameter identified — critical variable confirmed against spec.
  • Tolerance within achievable range — commercial ±0.15mm or tighter as needed.
  • Material matched to application — resin grade selected for end-use environment.
  • Process window validated at T1 — parameters locked after first trial.
  • Inspection method defined — CMM, visual, or functional test specified.

    • Frequently Asked Questions

    Does the draft angle affect the part’s critical dimensions?

    Yes — adding draft changes dimensions at the top and bottom of the wall. For a 10 mm wall with 1° draft per side, the top is 0.175 mm wider than the bottom; for 25 mm, the difference is 0.44 mm. Within standard tolerance (±0.1–0.2 mm per 25 mm), but for tight-tolerance parts, specify the draft direction on the drawing so the critical dimension is held at the correct end.

    If the critical dimension is at the top of the wall, specify “draft toward cavity”; if at the bottom, specify “draft toward core.”

    Which resins need more draft?

    High-shrinkage resins (PP, PE, PA6, PA66) shrink onto the core more tightly than low-shrinkage resins (PS, PC, PMMA), increasing the ejection force. Add 0.5° to the baseline for high-shrinkage resins. For glass-filled resins (GF30, CF30), the fibers create additional friction — add 0.5–1.0° to the baseline. The worst case is a deep-walled, glass-filled part with textured surface: baseline 2° (medium texture) + 0.5° (GF30) + 0.5° (deep wall) = 3.0° per side minimum.

    Values are quoted from the resin supplier’s document for the grade you select.

    Can you mold a part with no draft at all?

    It is technically possible using a stripper plate, mirror-polished cores and a strong ejection system — but it is expensive and unreliable. The mold cost increases $3,000–8,000, cycle time increases 5–10%, and the risk of part damage during ejection is 5–15% per shot. For volumes above 10,000 pieces, the accumulated risk and cost make zero-draft designs uneconomical. The correct solution is to add 1° draft per side in the DFM review — it costs nothing and eliminates the ejection risk.

    For prototype quantities below 100 pieces, zero draft is acceptable — the mold can be hand-polished and the parts hand-ejected without significant risk.

    Key Takeaways

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  • Minimum draft: 0.5° (polished, shallow) to 3°+ (heavy texture, deep walls).
    Baseline: 1° per side for polished surfaces below 25 mm.
  • Add 0.5° per 25 mm wall depth above baseline; add 0.5° for high-shrink resins; add 0.5–1° for glass-filled resins.
  • 1° draft on 10 mm wall = 0.175 mm dimension change; on 25 mm wall = 0.44 mm — within standard tolerance.
  • Zero-draft walls: stripper plate adds $2,000–5,000 mold cost + 5–10% cycle time.
    Adding 1° draft costs nothing.

    • If your part has walls with insufficient draft or zero draft, send the 3D drawing and the resin grade for a DFM review. The review identifies each wall that needs draft, calculates the minimum angle for the resin and texture specified, and recommends the most cost-effective solution.

    Sources: Society of Plastics Engineers (SPE) — Draft Angle Design Guidelines · ASTM International — Injection Molding Tolerance Standards · Osswald, Injection Molding Handbook · HASCO Mold Components — Draft Angle Recommendations · DME Mold Components — Texture Draft Requirements

    — Arthur, Tooling Engineer at aCatMould