Phone stand material selection
A Year 11 enterprise group must produce 30 flat-pack desk phone stands for a school fair, with a materials budget of £60. The stand is a single 6 mm sheet component with two laser-cut slots that interlock by a press fit. In the first batch of 12, eight stands cracked at the slot corners during assembly. The group had used 6 mm cast acrylic costing £48 for the required sheet area. Alternatives quoted are 6 mm birch plywood at £36, which laser-cuts with charred, splintery edges and needs sanding, and 6 mm high-impact polystyrene (HIPS) at £30, which is tougher but flexes noticeably in a 90 mm cantilever. The workshop has a laser cutter, a pillar drill, hand tools and a belt sander, but no vacuum former. The fair is in nine school days. The group wants a decision that keeps the product looking professional and survives being assembled and disassembled by visitors.
1. Recommend which material the group should specify for the 30 stands. Justify your choice by comparing at least three properties and the available processing route.
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Recommendation: 6 mm birch plywood. Compare properties:
- Stiffness: Plywood has a Young's modulus roughly 8–12 GPa along the face grain, so a 90 mm cantilever under a 200 g phone deflects very little. HIPS is around 1.5–2.5 GPa and would creep or sag, especially in a warm classroom.
- Toughness: Acrylic is brittle and notch-sensitive; the sharp slot corners concentrate stress, which is why 8/12 cracked. Ply is less brittle across the face but can delaminate, so the slot needs redesign.
- Processability: The laser cutter can cut ply, but it chars and splinters. At 30 units, sanding edges is about 1 hour of work over nine days, which is feasible. HIPS laser-cuts cleanly but its flexibility undermines the press fit.
- Cost: Ply at £36 leaves £24 for abrasive paper and finishing, still under budget; acrylic at £48 leaves little contingency.
Acrylic is rejected because the failure mode is catastrophic cracking, not because it is expensive. HIPS is rejected because a phone stand's main structural requirement is stiffness, and 6 mm HIPS in a 90 mm cantilever is likely to flex and loosen. Ply is the best compromise if the slot is redesigned. The final decision should be confirmed with a three-piece test batch.
2. The group is worried that the press fit caused the acrylic failures. Explain how the slot geometry and tolerance should be changed for the chosen material, and how you would test the change before cutting all 30.
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Slot geometry: The original press fit was nominal 6.0 mm for 6.0 mm acrylic. For ply, sheet thickness varies (often ±0.3–0.5 mm), and the laser kerf is about 0.1–0.2 mm per side. A nominal 6.2 mm slot gives clearance without being sloppy. Relief holes of 1.5–2 mm radius at the slot ends spread stress and stop a split running into the face veneer. Lead-in chamfers on the slot mouth help assembly and reduce edge crushing.
Testing: Cut three test pieces from the same ply sheet, measure slot width and sheet thickness with digital calipers, and assemble and disassemble each ten times. Check for delamination, cracking and play. If the joint is loose, do not simply shrink the slot; use a slight taper or a small wedge. If tight, increase the slot by 0.1 mm and retest. Only then cut all 30. This is iterative testing, not a guess.
Why it matters: The acrylic failure was a design-for-assembly problem: a brittle material with a sharp, tight slot. Moving to a tougher material does not remove the stress concentration, so the geometry must change as well.
3. Explain why the cheapest material is not automatically the best value for this project. Use the quoted costs and the batch evidence.
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Cheapest is not lowest cost. HIPS at £30 is £6 cheaper than ply and £18 cheaper than acrylic on paper, but the real cost includes:
- Failure and rework: 8/12 acrylic stands failed, a 67% scrap rate. If HIPS flexes and joints loosen, replacements and reputational damage cost more than £6.
- Finishing time: Ply needs sanding, but HIPS may need edge cleaning or a different adhesive; labour has a cost even in a school project.
- Durability: A stand that survives repeated assembly by visitors meets the brief; a cheaper stand that fails does not.
- Appearance: Ply has a visible edge; if the design brief says 'professional', edge finishing or a painted edge may be needed.
So the decision should compare cost per successful stand, not cost per sheet. £36 for 30 successful ply stands is better value than £30 for stands that may not meet the stiffness requirement. The group should also keep a small contingency for test pieces.