Cracked acrylic phone stands
A student is producing 20 acrylic phone stands for a local market. Each stand is laser-cut from 5 mm cast acrylic sheet, then a 5 mm hole is drilled for a rubber foot. The student drills at 1800 rpm with a standard twist drill, no lubricant, and the acrylic sheet is clamped only by hand. Cracks radiate from the holes on 6 of the first 10 stands. The market is in three days, and the student has 10 sheets of 5 mm acrylic and no polycarbonate. The drill bit is new but has a standard 118° point. The holes are positioned 10 mm from the edge of the stand. The student has already cut all 20 stand blanks on the laser cutter, so the acrylic is expensive to replace.
1. Explain why the acrylic cracked, referring to material properties and the drilling process.
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Acrylic is an amorphous thermoplastic: it is brittle, has low thermal conductivity, and softens at relatively low temperatures. A standard twist drill has a positive rake angle that tends to grab the plastic and generate heat rather than cut cleanly. At 1800 rpm the drill rubs, heat builds around the hole, and the acrylic contracts on cooling, creating tensile stress. Hand clamping allowed the sheet to vibrate and flex, so the drill grabbed and cracked the sheet. The cracks radiating from the holes are typical of stress cracking from heat and point loading. The fact that 6 of the first 10 cracked suggests the process is not controlled, not that the material is inherently unsuitable.
2. Recommend changes to the drilling process that could save the remaining stands, and justify each.
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Use a sharp drill bit designed for acrylic, such as a 60–90° point with zero rake, or a step drill; this reduces grabbing. Slow the drill to about 500–800 rpm and use a firm backing board of MDF or ply to support the exit side. Clamp the acrylic to the bench, not by hand, so it cannot vibrate. Apply water as a coolant, or peck drill in short bursts to clear swarf and let the bit cool. Drill a small pilot hole first. Trade-off: slower drilling reduces output, but 20 stands are still feasible in three days and the reject rate should fall. These changes control heat and support, which are the direct causes of the cracks.
3. Would switching to 3 mm acrylic solve the problem? Explain.
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No. Thinner acrylic is less stiff and more likely to flex and crack under the same drilling forces; it also reduces the load-bearing capacity of the stand. The main cause is heat and poor support during drilling, not the sheet thickness. If the 5 mm stand is over-designed, 3 mm might be acceptable for a light phone, but it would need a slower, supported drilling process anyway. A better change would be to keep 5 mm cast acrylic and fix the drilling, or move to a tougher material such as polycarbonate if impact resistance matters. Polycarbonate costs more and scratches more easily, so the trade-off is cost and appearance against toughness.