Acrylic keyring batch: nesting, process choice and fume control
A school enterprise has an order for 300 acrylic keyrings for a charity. Each keyring is 60 mm × 40 mm × 3 mm cast acrylic with a 5 mm lanyard hole. The laser cutter has a bed of 600 mm × 400 mm and is loaded with one full sheet at a time; sheets cost €18 each. The cutter takes 5 minutes per sheet plus 20 minutes setting up the file and focusing. The kerf is 0.2 mm and the supplier recommends a 3 mm gap between cut edges. A hand-tool route would take about 12 minutes per keyring, with inconsistent edges and a higher risk of cracking during drilling. The class has four hours of workshop time and one teacher. The charity wants the keyrings in three weeks and will reject any with chipped edges.
1. Calculate the maximum number of keyrings per sheet and the number of sheets needed. State any assumptions.
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Effective pitch: part 60 mm plus 3 mm gap = 63 mm in one direction; 40 + 3 = 43 mm in the other. With no edge margin, floor(600/63) = 9 and floor(400/43) = 9, so 81 parts. Rotating the part gives floor(600/43) = 13 and floor(400/63) = 6, so 78, which is worse. If a 5 mm edge margin is required, floor(590/63)=9 and floor(390/43)=9, still 81, so the result is not sensitive to a small margin. The 0.2 mm kerf is already accounted for in the 3 mm gap if the gap is measured between cut edges; if the 3 mm is between part outlines before kerf, the pitch is 60 + 3 + 0.2 = 63.2 mm, which still gives 9 × 9 = 81. For 300 keyrings, 300/81 = 3.7, so 4 sheets are needed. Assumptions: rectangular nesting, no rotation, one hole per part, and the sheet is used fully.
2. Compare the laser and hand-tool routes for time, cost and quality, and recommend one.
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Time: 4 sheets × 5 min = 20 min cutting plus 20 min setup = 40 min, well within four hours. Cost: 4 × €18 = €72, plus labour. Hand tools: 300 × 12 min = 3,600 min = 60 hours, which is 15 times the available four-hour session and would take 15 weekly sessions. Quality: laser gives consistent edges and hole position; hand tools risk chipping and inconsistent hole placement, and the charity will reject chipped edges. Safety: laser needs extraction and fire watch; hand tools need drilling and polishing hazards. Recommendation: laser cut, but first cut one test sheet to check the 5 mm hole and the 3 mm gap, because cast acrylic can vary in thickness and the hole may come out undersize. The laser route is not just faster; it makes the quality requirement achievable.
3. Identify one safety or environmental control needed for the chosen route and explain why it matters.
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Control: use local exhaust ventilation (LEV) at the laser cutter and leave the machine to run its extraction cycle after cutting. Laser cutting acrylic produces fumes and fine particulates; without extraction they are a health hazard and can deposit on the lens, reducing cut quality. Also keep a CO2 extinguisher and do not leave the machine unattended. Environmental: use cast acrylic offcuts for a second small product or return them to a recycler; do not put PMMA in the general waste. A fire watch for 10 minutes after cutting is required because acrylic can smoulder. For hand tools, the equivalent control would be a dust mask and vacuum extraction when drilling, but the laser route with LEV is better because it captures fumes at source.