250 logger enclosures: process choice, break-even and an impossible tolerance
A small firm needs 250 enclosures for a soil-moisture logger. Each enclosure is 160 × 110 × 45 mm, open at the back, with a 34 mm circular window and four M3 screw bosses. The drawing specifies the window as 34.0 ± 0.3 mm. Three quotations are on the table. Vacuum forming in 2 mm HIPS: machined MDF tool £350, £4.20 per part, three-week lead time, achievable tolerance ±0.5 mm. Injection moulding in 2 mm ABS: tool £6,800, £1.10 per part, ten-week lead time. FDM printing in PETG: no tooling, £9.50 per part, 3 h 40 min per part, and the firm owns two printers. The firm also has a vacuum former, a pillar drill and hand tools. Sales confirm 250 units and hope for 3 000 over three years. The customer wants first deliveries in five weeks.
1. Calculate the break-even between vacuum forming and injection moulding, and recommend a process for 250 units and for the 3 000-unit scenario. Include the risks, not just the arithmetic.
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Break-even: 350 + 4.20n = 6 800 + 1.10n → 3.10n = 6 450 → n = 2 081 parts. Above about 2 080 units, injection moulding is the cheaper route.
At 250 units: vacuum forming = 350 + (250 × 4.20) = £1 400. Injection = 6 800 + 275 = £7 075. Vacuum forming is £5 675 cheaper, so on cost alone it wins decisively.
At 3 000 units: vacuum forming = 350 + 12 600 = £12 950. Injection = 6 800 + 3 300 = £10 100, plus a far lower unit time and much better repeatability. Injection wins.
Risks that the arithmetic hides:
- Cash and commitment. £6 800 of tooling on a 250-unit order is a bet on the optimistic forecast. If the design changes - a new connector, a different boss position - the tool is scrap or needs an expensive modification.
- Lead time. Ten weeks does not meet the five-week delivery, so injection cannot serve the first order regardless of cost.
- MDF tools wear. A machined MDF vacuum tool is good for hundreds, not thousands, of pulls, so the 3 000-unit case would need a second tool or an aluminium tool.
Recommendation: vacuum form the 250 units now and design the part so that the same CAD can go to an injection tool later. Revisit injection when confirmed orders exceed about 2 000 or when a design freeze is in place; if a redesign is likely, consider an aluminium vacuum tool as the intermediate step.
2. The 34.0 ± 0.3 mm window cannot be held by vacuum forming, which achieves ±0.5 mm. Set out the options and say what you would do about the specification.
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First check whether the tolerance is real. A window is usually there to let an LED or a sensor be seen; if its function only needs ±0.5 mm, the ±0.3 mm is habit, not requirement, and the correct engineering action is to challenge it with the designer and record the change. Holding an unnecessary tolerance is the most expensive mistake in the whole job.
If the tolerance is genuinely needed, three routes:
- Form a blank, then cut the window as a secondary operation. A drilling jig with a 34 mm holesaw on the pillar drill can hold about ±0.2 mm; a CNC or laser trim would be ±0.1 mm. Cost roughly £0.60–£0.90 per part, which at 250 units is £150–£225 - still far below the £5 675 injection penalty.
- Mould a recess and bond in a machined ring or lens. The ring is made to tolerance and the formed part only has to be roughly right. Adds a part, a bond and an inspection step.
- Change the feature. Make the window a larger opening hidden behind a printed bezel, so the visible edge is the bezel and the formed hole no longer defines the appearance.
What to do with the drawing: issue a change note. State the functional requirement, the process capability of the chosen route, and the agreed tolerance; get the customer to sign it. Do not simply ship parts outside the drawing, and do not silently widen a tolerance on the shop floor - both destroy the traceability that a design-and-make project is assessed on.
3. Evaluate FDM printing for this order using the firm's capacity, and identify when it would still be the right choice.
Read the worked answer
Capacity: 3 h 40 min = 3.67 h per part. 250 × 3.67 = 917 printer-hours. With two printers that is 458 h, or about 19 days of continuous 24-hour printing - and real printing is not continuous, so at 12 h per day it is nearer five and a half weeks. That misses the five-week delivery on its own, before any failures, reprints or maintenance.
Cost: 250 × £9.50 = £2 375, against £1 400 for vacuum forming. So it is slower and more expensive than the recommended route.
Where FDM still wins:
- Prototypes and pre-production units. Five to ten printed parts in week one let the customer hold the product, check the window position and test the bosses before any tool is cut. That is the cheapest risk reduction available and it is exactly what the process should be used for here.
- Design churn. If the connector layout is still moving, printing absorbs changes that would scrap a tool.
- Low volume or spares. Units 251–260, or warranty replacements in year three, are ideal FDM jobs.
Judgement: use FDM for the first article and any pilot batch, then vacuum form the 250. Do not choose it for the whole order on the grounds that it has no tooling cost - at this quantity the tooling is cheap and the machine time is the expensive resource. Note too that printed PETG is anisotropic: the layer lines make bosses weaker in Z, so the screw bosses would need orientation or metal inserts.