Two features decide whether an injection-molded part can be made cheaply: the draft angle — the taper on every vertical wall that lets the part release from the steel — and the parting line — where the two halves of the mold meet. Get them right and a part ejects cleanly from a simple two-plate tool. Get them wrong and you are paying for side actions, hand-polishing, cosmetic defects, or a tool that has to be re-cut. Neither one is difficult; both are routinely forgotten until a molder quotes the part.

Why Draft Is Not Optional

A molded part shrinks onto the core as it cools, gripping the steel. Ejector pins then push it off. If a wall is perfectly vertical, the plastic drags along the steel the entire way out. The results are predictable: scuffed drag marks down the wall, stress-whitened or torn features, stuck parts that stop the press, ejector-pin push marks, and accelerated tool wear. A few degrees of taper break that contact almost immediately and the part slides off cleanly.

Draft is also what makes texture possible. Textured surfaces have far more mechanical grip on the steel than polished ones, so they need extra draft — a common rule of thumb is roughly one additional degree per 0.001 inch of texture depth. This is why texture selection and draft have to be decided together rather than sequentially; see mold texture selection.

How Much Draft Do You Actually Need?

There is no universal number, but these are reliable starting points:

  • Minimum for smooth, shallow walls: about half a degree to one degree. Half a degree is a floor, not a target.
  • General-purpose default: one to two degrees on all vertical faces. Use this unless something forbids it.
  • Textured surfaces: three to five degrees is common, more for coarse textures.
  • Deep draws and tall ribs: more draft the deeper the feature, because total drag scales with depth. Tall thin ribs are frequent offenders.
  • Shutoffs and mating faces: these need draft too, and they are the ones designers most often leave vertical.

Material matters. Stiff, high-shrink, or glass-filled resins generally need more draft; soft flexible materials often tolerate less but bring their own ejection issues. Ask the molder rather than assuming.

The practical workflow point: apply draft while you model, not afterward. Retrofitting draft into a finished solid model changes wall thicknesses, breaks mating fits, and shifts every dimension that downstream parts referenced. Model the walls tapered from the start and let the fits follow.

The Parting Line and Where It Shows

The parting line is the seam where the two mold halves close. Two things are true about it: it will always be visible to some degree, and draft runs in opposite directions on either side of it. That means the parting line placement determines the entire drafting strategy for the part.

Good practice:

  • Put it on a natural edge. A hard corner, a step, or a design break hides a seam far better than the middle of a smooth face.
  • Keep it off critical surfaces. Not across a sealing face, not through a bearing surface, not down the middle of a clear window, not where a user's thumb rests.
  • Keep it simple. A flat parting line is cheap. A stepped or contoured one costs more to cut, is harder to keep sealed, and flashes more readily as the tool wears.
  • Design the seam intentionally. A small deliberate step, groove, or radius break at the parting line makes minor flash and mismatch invisible. Trying to blend a seam invisibly across a flat gloss surface never works in production.
  • Remember flash grows with tool life. A seam that is acceptable on the first thousand parts should still be acceptable at a hundred thousand.

Undercuts, Side Actions, and What They Cost

An undercut is any feature that would prevent the part from lifting straight out of the mold: a side hole, a snap hook facing sideways, a recessed groove, a threaded boss. Mold makers solve these with sliders (side actions), lifters, cams, or collapsible cores — every one of which adds tooling cost, adds a moving component that wears, adds cycle time, and leaves witness lines on the part.

Cheaper alternatives worth trying first:

  • Reorient the feature so it forms in the direction of mold opening.
  • Use a shutoff or pass-through core — a projection from one half touching the other to form a side opening without a slider. Often free.
  • Move the parting line so the undercut is no longer an undercut.
  • Split the part into two simple mouldings that assemble. Two simple tools plus an assembly step sometimes beats one complicated tool.
  • Design a bump-off. A shallow undercut in a flexible material can sometimes strip off the core without any mechanism.

Snap-fit hooks are the most common source of avoidable side actions in consumer products, so orient them deliberately — see our snap-fit design guide. Every added mechanism also shows up in the quote and the lead time, both of which are covered in injection molding cost.

Fitting This Into the Design Process

The practical sequence for a moldable part looks like this:

  1. Decide how the part will be pulled from the mold — the direction of draw — before detailing geometry.
  2. Place the parting line where the shape gives you a natural break.
  3. Draft every wall from that plane as you model, defaulting to one to two degrees and more for texture.
  4. Hunt for undercuts and eliminate the ones you can with reorientation or shutoffs.
  5. Keep walls uniform in thickness, since draft and thickness interact — a drafted wall thins as it rises, and thin sections do not fill. See wall thickness for injection molded parts.
  6. Review with the molder before the tool is quoted, not after.

The payoff for doing this properly is that the defects you never have to debug — drag marks, sink, short shots, ejection damage, and warp — are largely designed out in advance. The catalog of what happens otherwise is in injection molding defects, and the broader discipline is plain design for manufacturing. Your tooling material choice affects how forgiving the tool will be about all of this too — soft aluminum tools tolerate less abuse than hardened steel, as covered in aluminum vs steel injection molds.

A Cheap Sanity Check

Before you release the model, run a draft analysis in your CAD package with the pull direction set. It colors every face by its angle relative to the draw and finds the vertical wall you missed in about ten seconds. Then send the model for a formal moldflow or DFM review from the molder. Both checks cost a fraction of a tool modification, and a tool modification costs a fraction of a new tool.

Projects House designs plastic parts for the mold from the first sketch — draw direction, parting line, draft, and undercut strategy set before detailing begins — and reviews existing models before tooling is committed. If you have a part heading to a molder and want it checked first, send it through our contact form.