The Warping Phone Stand
A Year 10 pupil injection-moulds a phone stand from recycled polypropylene (PP) granules supplied by the school. The first 40 parts cool in a 25 °C workshop and measure flat within 0.4 mm across the 90 mm base. A visiting technician moves the batch to a storeroom at 8 °C overnight. The next morning 12 of the 40 stands show a 2.5 mm bow across the base and two have fine white stress-whitening lines near the hinge. The pupil wants to switch to the same shape in polylactic acid (PLA) because 'it is more eco'. The mould is steel, the cycle is 35 s, and the school has no drying oven.
1. Explain the bowing and whitening, and judge whether the fault is the material, the moulding conditions, or the design.
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The fault is mainly process and design, not the PP itself. PP is semi-crystalline: on cooling it shrinks noticeably and unevenly, because thick sections stay hot while thin ones freeze first. A 90 mm flat base with a hinge (a local thick mass) creates differential shrinkage, so the base bows. The overnight drop from 25 °C to 8 °C is not the cause of the bow; it simply revealed built-in frozen-in stress as the part continued to relax and shrink. The white lines are stress whitening - local yielding where the hinge geometry concentrates stress during ejection or bending. Judgement: PP was a defensible choice (tough, cheap, recyclable), so changing material is not the first fix. First fix the design (uniform wall thickness, ribs instead of a thick hinge boss, generous radii), then the conditions (longer hold/cool time, slower ejection, warmer mould).
2. The pupil proposes PLA. Give two technical reasons this is a poor swap in this workshop, and one condition under which it would be acceptable.
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Reason 1 - moisture. PLA is hygroscopic; wet granules hydrolyse at melt temperature, dropping molecular weight and giving brittle parts. It normally needs drying (often several hours in a desiccant dryer). The school has no drying oven, so the pupil cannot control the variable that matters most. Reason 2 - thermal limits. PLA has a low heat-deflection temperature (roughly 55–60 °C). A phone stand left on a car dashboard or beside a charging phone can soften and creep, whereas PP copes far better. PLA is also brittle, so a thin hinge is a poor feature. Acceptable condition: PLA is reasonable if the part is redesigned with thicker, well-radiused sections, granules are supplied pre-dried and stored sealed, and the product is used indoors away from heat - and even then, the steel mould's shrinkage allowance (set for PP) will make PLA parts dimensionally off, so the mould would need re-cutting. That cost usually rules it out for a school project.