Draft angle and shut-off surfaces

What draft actually does

Draft is the taper applied to any face that slides against the tool during ejection. As the part cools it shrinks onto the core, which increases the normal force between plastic and steel. Without a taper, the whole face has to slide while still in contact, and the friction is high enough to scuff the surface, drag the part out of shape or stop ejection altogether.

With a taper, contact is lost almost immediately as the part moves. The remaining drag is small and predictable. That is why draft is not a tolerance item to be traded away under pressure on a tight drawing. It is the feature that makes the part releasable at all.

The amount needed depends on the depth of the feature, the shrinkage of the material, the surface finish of the steel and whether the surface is on the core or the cavity. Deep cores need more draft than shallow ones because the part shrinks onto a longer contact area and the ejection stroke is longer.

Working draft values by feature

The table below reflects values that hold up across common engineering thermoplastics. They are minimums for a clean part on a well maintained tool, not targets. Where a surface is visible, textured or likely to be polished during maintenance, add margin.

Two habits cause most draft problems. The first is applying draft only to the outside of the part and leaving core-side walls vertical. The second is forgetting internal features: a rib drawn with parallel sides, a boss with a straight bore or a slot with vertical walls all need their own taper.

Texture changes the draft requirement

A textured surface is a series of small peaks and valleys etched into the steel. The plastic grips the peaks as it shrinks, so the effective friction is higher than on a polished face and the draft must rise with the depth of the texture. The common rule of thumb is one additional degree of draft for every 0.025 mm of texture depth, applied per side.

A light matte finish might add half a degree. A coarse leather grain 0.1 mm deep can add four degrees or more, which visibly changes the shape of a wall. That is why texture should be chosen before the tool is cut, not specified on the drawing as a note and settled at first article.

Texture also affects where the draft is measured. A drafted wall with a deep grain has a nominal angle and a local angle at the base of each peak. The toolmaker cuts the nominal angle and the etcher removes material, so the finished surface can end up tighter than intended. Agree the draft allowance with the texture supplier and record it on the tool drawing.

What a shut-off surface is

A shut-off is where two tool surfaces meet to blank off a region of the cavity. It forms an opening in the part, such as a window in a housing, a slot for a terminal or the gap between two moulded features. The two surfaces touch under clamp force, and the melt must not cross the line where they meet.

A shut-off is therefore a sealing feature as well as a shaping feature. It needs a land, which is a narrow flat where the two surfaces bear on each other, and it needs a match angle so the surfaces wedge together and align as the tool closes. A shut-off with no land and no angle is a knife edge: it wears quickly, flashes early and is difficult to maintain.

Shut-offs appear in more places than designers expect. Every slide creates one. Every core that meets the cavity around an opening creates one. Every lifter face creates one. A part with several openings and a slide can have a dozen shut-off lines, each with its own wear and venting behaviour.

Shut-off design rules

Keep the land between about 1 mm and 3 mm on a typical part. Narrower lands wear and flash; wider lands need more clamp force and are harder to match. The land should sit where the steel is supported, not on a thin upstand that deflects under clamp pressure.

Add a match angle of 1 to 3 degrees to the closing faces so they self-align. The angle should be on the side that can be adjusted, usually the insert rather than the main block. Where two shut-offs meet at a corner, radius the intersection rather than leaving a sharp point, which is where the steel erodes first.

Vent the shut-off. Air trapped against a shut-off line has nowhere to go and produces a burn mark or a short fill at the edge. A shallow vent, typically 0.02 mm deep, running to the atmosphere, is often enough. The vent must be cleaned when the tool is serviced, so it needs to be accessible.

Think about the parting line at the same time. The shut-off often ends at the parting line, and any mismatch there shows as a step on the part. If the step is on a visible face, the tool needs a shut-off design that hides the mismatch, for example by placing the line on an internal edge.

• Land width of 1 to 3 mm, supported by solid steel underneath.
• Match angle of 1 to 3 degrees on the closing faces.
• Radius the corners where shut-offs meet.
• Vent the shut-off to atmosphere, 0.02 mm deep is a common starting point.
• Draw the parting line on the part, not just on the tool.

Draft, tolerance and the dimension you actually measure

Draft moves dimensions. A wall drawn at 1 degree per side on a 30 mm tall feature is 0.52 mm wider at the base than at the top. If the drawing calls that wall at 20.00 mm with a plus or minus 0.10 mm tolerance and does not say where the dimension applies, the moulder and the inspector will disagree.

Dimension drafted features at a defined plane, usually the parting line or the base, and say so. Where the functional requirement is a clearance at the top of the wall, dimension the top. Where it is a fit at the base, dimension the base. A general note that all dimensions apply at the parting line is better than a drawing with no rule at all.

Draft also interacts with flatness and straightness. A wall that is drafted on one side and vertical on the other is a wedge, and it will bow as it cools. If the part must sit flat on a surface, put the draft on the non-functional face and keep the seating face square.

What to check in a design review

Open the model and look at every face that touches steel. A section analysis or a draft analysis tool will colour the vertical faces and save time. The aim is not to reach a specific number everywhere, but to find the faces where draft is zero or negative, because those are the faces that will not release.

Then check the shut-offs against the wall thickness and the gate position. A shut-off next to a gate sees high pressure and flashes first. A shut-off at the end of fill sees low pressure and may not seal at all. Both need attention at the design stage rather than at trial.

Finally, ask the toolmaker to mark the draft and shut-off strategy on the tool drawing, including land widths, match angles and vents. That drawing is the record that lets a maintenance team restore the tool to its original condition instead of guessing.

1. 01

   Run a draft analysis

   Find every face at zero or negative draft, including ribs, bosses and core pins.

2. 02

   Check texture

   Add draft for the specified grain depth before the tool is cut.

3. 03

   Map the shut-offs

   List each shut-off, its land width, match angle and vent path.

4. 04

   Fix the dimension plane

   State where drafted dimensions apply, at the base or at the top of the feature.