Draft Angle Calculator

Draft angles are the small per-side tapers built into the vertical walls of an injection-molded part. They exist for one reason: to let the part eject cleanly from the mold without scuffing, sticking, or breaking the cavity surface. Without enough draft, the part drags against the steel as the ejector pins push it out. The result is drag marks, witness lines, white stress areas, and in worst cases, parts that snap apart on ejection or molds that wear out years before they should.

Our free draft angle calculator above gives you a per-side angle recommendation in seconds. The guide below explains what those numbers actually mean, when you can push them tighter, and when you absolutely cannot.

What is a Draft Angle in Injection Molding?

A draft angle is the degree of taper applied to any wall, rib, boss, or feature that runs parallel to the direction the mold opens. The angle is measured per side from vertical. A 1 degree draft means the wall leans 1 degree away from perpendicular over its full height.

On a 50 mm tall wall, 1 degree of draft adds 0.87 mm of dimensional change from bottom to top. That sounds like nothing, but it is the difference between a part that ejects in 200 milliseconds and a part that hangs in the cavity, breaks an ejector pin, or comes out with a 2-inch white stress band running up the side wall.

Standard Draft Angle Recommendations by Surface Finish

The single biggest factor in your draft requirement is surface finish. A polished mold cavity is glass-smooth and lets parts release with minimal draft. A heavily textured cavity behaves like sandpaper. The general rule for SPI and VDI 3400 surface finishes:

The rule of thumb every toolmaker knows: add 1 degree of draft for every 0.001 inch (0.025 mm) of texture depth. Mold-Tech MT-11030, a common medium texture, is 0.0035 inch deep. That texture alone demands 3.5 degrees of draft before you even account for material or part geometry.

Material Effects on Draft Requirements

Different resins shrink differently and have different coefficients of friction against tool steel. As a baseline:

• Soft, high-shrink materials (PP, PE, TPE): shrink hard onto cores. Add 0.5 to 1 degree above the texture-based minimum.
• Engineering thermoplastics (ABS, PC, PC/ABS): the texture-based minimum is usually fine. The published numbers above were calibrated for ABS.
• Glass-filled materials (PA6 GF30, PBT GF30, PP GF20): behave more like metal. You can usually run 0.5 degree less than unfilled, but the fibers abrade the cavity over time.
• High-temp polymers (PEEK, PEI, PPS): minimal shrinkage but high stiffness. Use the texture minimum.
• Elastomers (LSR, TPU, TPE): need the most draft of any category. 3 to 5 degrees minimum, sometimes more, because the part deforms during ejection rather than sliding off cleanly.

If your part is going to be molded in China, this matters even more. Chinese toolmakers default to lean toward less draft because it produces a sharper-looking part for sample approval. You will see beautiful T1 samples, then fight ejection issues at the 5,000-shot mark when the cavity polish breaks in. Always specify draft requirements explicitly in your RFQ. Our free RFQ builder bakes draft requirements into the spec sheet.

Wall Depth and the 50 mm Rule

The deeper your part, the more draft you need. This is not arbitrary. Friction is proportional to the contact area between part and cavity, and the contact area grows linearly with wall depth. Our calculator adds an automatic increment for parts deeper than 50 mm, because below that depth the surface friction is the dominant factor, but past 50 mm part shrinkage onto the core starts adding significant grip force.

For deep features like ribs, screw bosses, and walls in housings or enclosures: aim for at least 1 degree per side at the base, even for polished surfaces. If you cannot add draft to a deep wall for design reasons, the solution is to break it into multiple shorter walls or add a parting-line break.

Common Draft Angle Mistakes

From hundreds of DFM reviews over the past 15 years, these are the failures we see most often:

• Zero draft on cosmetic-A surfaces. Designers protect the show face by removing draft. The result is drag lines that ruin the cosmetic anyway. Add 0.5 to 1 degree and use SPI A-2 polish to hide it.
• Same draft on textured and polished features. Texture and polish on the same wall demand different drafts. Specify each surface separately on the drawing.
• Draft going the wrong way on ribs. Ribs on the core side need draft from the base toward the tip. Ribs on the cavity side need draft from the tip toward the base. Mixing these creates undercuts.
• Forgetting draft on through-holes and bosses. Cores that form holes also need draft. A perfectly cylindrical core grips the part. 0.5 degree on cored holes is the minimum.
• No allowance for shutoffs and parting lines. Steel-on-steel shutoffs need extra draft to prevent flashing. Aim for 3 degrees minimum on shutoff faces.

Frequently Asked Questions