Draft Angle Conflict on a Deep Rib
A housing in ABS has a 38 mm-deep internal rib that stiffens a thin wall. The part designer specified 0.5° draft on the rib flanks to maximize stiffness. The mold designer says the rib will drag and scuff on ejection and wants 1.5°. The rib is 1.2 mm wide at the tip and sits 6 mm from a side wall, so there is limited room for a lifter. Production volume is 150,000 parts per year over three years. Current simulation shows fill pressure at the limit of the 180-ton press, and the toolmaker has already cut the core insert. The customer's cosmetic spec allows light scuffing on internal surfaces but not on the visible outer wall.
1. How should the team resolve the draft-angle conflict without recutting the core insert?
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The conflict is between stiffness (thin, near-vertical rib) and ejection (needs draft), and the core insert is already cut, so the answer must come from process and geometry compromise rather than a clean redesign.
Resolution path:
- Quantify the actual risk. At 38 mm depth and 0.5° draft, the rib flank moves about 0.33 mm over its length. Scuffing depends on ejection force, which depends on shrinkage onto the core and surface finish. A polished flank and a slightly lower mold temperature on the core side can cut ejection force substantially without changing geometry.
- Check whether the 0.5° is functional. If the rib's job is stiffness, reducing its depth by a few millimeters or adding a second, shallower rib may recover stiffness while shortening the drag length. That is a part change, but it can be done on the core insert by adding a pocket rather than recutting the whole insert.
- Use the simulation to test whether a higher melt temperature and slower pack reduce shrinkage onto the rib, which lowers ejection force.
- If scuffing still occurs, it is on an internal surface the cosmetic spec tolerates, so the risk is wear and maintenance, not rejected parts. Budget for periodic rib polishing and track ejection force over the first 10,000 shots.
Recommendation: keep 0.5° for now, polish the rib flanks, tune core temperature, and set a monitoring trigger on ejection force. Revisit geometry only if force climbs.
2. The fill pressure is already at the press limit. How does the draft decision interact with that constraint?
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The two constraints pull in opposite directions and must be evaluated together, not sequentially.
Interaction:
- Lower draft means the rib is closer to vertical, so it adds little to flow area but increases the local wall thickness at the rib base. Thicker sections fill more easily but pack differently and shrink more, which increases grip on the core and raises ejection force.
- Raising melt temperature to reduce fill pressure also raises shrinkage in thick sections, which worsens the grip. So the easy fix for pressure makes ejection harder.
- Adding draft to 1.5° narrows the rib tip, which raises local shear rate and fill pressure at the tip, pushing further against the press limit.
What to do: run a combined fill-pack-warp and ejection-force study rather than separate analyses. Look for a gate location or runner balance change that lowers fill pressure without raising pack pressure in the rib region. If pressure cannot be brought under the press limit, the honest options are a larger press for this tool or a part change that shortens the flow path.
Document the trade in the design record: draft angle, predicted fill pressure, predicted ejection force, and the chosen operating window, so the next engineer understands why 0.5° was kept.