Cost of a Late Wall-Thickness Change
A consumer device enclosure was quoted at 1.85 USD per part in a 30% glass-filled polycarbonate, based on a 1.6 mm nominal wall. Six weeks into tool build, industrial design asks to thin the visible top wall to 1.2 mm to reduce weight by 40 g. The tool's core and cavity inserts for the top wall are already rough-machined. Simulation of the 1.2 mm wall predicts fill pressure up 35% and a warp increase from 0.4 mm to 0.9 mm against a 0.5 mm limit. Annual volume is 400,000 parts. The customer's launch date is fixed and 10 weeks away.
1. Should the team accept the 1.2 mm wall? Justify with the specific trade-offs.
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Not as proposed, on the evidence given. The change fails the flatness limit by a wide margin and pushes fill pressure up sharply, and it arrives after inserts are rough-cut, so the cost of being wrong is high.
Trade-offs:
- Weight: 40 g saved per part is meaningful to the customer but is a marketing benefit, not a functional requirement. It should be traded against launch risk.
- Warp: 0.9 mm against a 0.5 mm limit is an 80% exceedance. Thinning the wall raises fill pressure, which raises the pressure gradient across the part, which drives differential shrinkage and warp. The prediction is directionally reliable even if the magnitude is uncertain.
- Tooling: rough-machined inserts can still be adjusted, but a thin wall means the core and cavity steel must be re-cut closer to final, eroding the 10-week margin.
- Cost: a 35% pressure increase may require a larger press or a different gate, both of which change the quoted 1.85 USD.
Recommendation: counter-propose a partial thinning, for example 1.4 mm on the top wall only, and ask for a simulation of that variant before committing. If the customer insists on 1.2 mm, present the warp and cost impact in writing and require a formal acceptance of the flatness deviation.
2. What should the team do in the next 10 days to protect the launch date while the wall-thickness decision is open?
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The launch date is fixed, so the team must parallelize and protect optionality rather than wait for a decision.
Next 10 days:
- Run simulations of three variants in parallel: 1.6 mm baseline, 1.4 mm partial, and 1.2 mm full. Each with warp decomposition and fill pressure. This gives the customer a real choice rather than a yes/no.
- Hold the rough-machined inserts at their current state. Do not cut toward 1.2 mm until a decision is made. Leaving stock is the cheapest form of insurance.
- Re-quote the 1.85 USD for each variant, including press size and cycle time, so the customer sees the cost of the weight saving.
- Ask the customer to confirm whether 40 g is a hard requirement or a target, and whether the 0.5 mm flatness limit can flex. These two answers determine the outcome more than any simulation.
- Set a decision deadline at day 7, leaving 3 days to release tooling changes.
Escalation: if no decision by day 7, proceed with the 1.6 mm baseline, which is the quoted and simulated design, and document the assumption. That protects the launch and keeps the change as a future revision.