Plaster moulds fail after 25 casts in a slip-casting workshop
A workshop produces slip-cast teapots in two-part plaster moulds. Moulds are made in-house: 1 kg pottery plaster is added to 1 litre of tap water, left to soak for 1 minute, then mixed for 5 minutes with a drill mixer at high speed. The slurry is poured into rubber master moulds and left 45 minutes; the moulds are demoulded and dried in a fan oven at 60°C overnight. After about 25 casts, the working surface becomes chalky, cast rims are weak, and teapots stick in the mould. The workshop has an order for 300 teapots and wants to make 10 new moulds quickly with the same method. The plaster store shows bags that have been open for six months. Casting slip is deflocculated with sodium silicate and has a specific gravity of 1.75.
1. Identify the principal mould-making faults and explain how each contributes to the chalky surface, weak rims and sticking. Why did the first 25 casts appear acceptable?
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The main fault is the plaster-to-water ratio. Pottery plaster is normally mixed at about 100 parts plaster to 70–80 parts water by weight. Here 1 kg plaster to 1 litre water is roughly 100:100, far too wet. Excess water gives a low-density, weak, highly porous mould. The surface erodes easily, hence the chalky feel; the mould also saturates quickly, so the cast wall builds slowly and rims stay weak. Sticking follows because the plaster surface is soft and the mould is wet.
The second fault is mixing: 5 minutes at high speed with a drill is too long and whips air into the slurry. Air bubbles create pits and weak spots, and over-mixing can affect the set. Soaking for only 1 minute may leave dry lumps, so the set is uneven.
Third, the plaster has been open for six months. Plaster absorbs moisture from the air and may contain lumps or set erratically. Old plaster often produces soft moulds even if the ratio is corrected.
Fourth, drying at 60°C overnight immediately after demoulding is too aggressive. Plaster needs to cure and dry slowly; forced drying can case-harden the surface and leave the core weak. The first casts worked because the mould was new and dry; once it had absorbed water and the soft surface had been worked, the weakness showed. For 300 teapots, the workshop needs several sound mould sets, not 10 quick moulds made by the same faulty method.
2. Give a corrected mould-making procedure and a mould-care schedule that will carry the 300-teapot order without repeating the failure.
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Corrected mould-making procedure:
- Use fresh plaster from a sealed, dry bag. Discard lumpy or partly set plaster.
- Weigh accurately: for example, 1.4 kg plaster to about 1.0–1.1 kg water (100:70–80 by weight). Do not judge by volume alone.
- Add plaster to water, allow it to soak for 2–3 minutes, then mix for about 2 minutes by hand or with a slow drill. Avoid high-speed whipping.
- Tap or vibrate the mould to release air; pour steadily into the master.
- Leave to set for 45–60 minutes, until the plaster is hard and cool. Demould carefully.
- Air-dry the new mould for 5–7 days at room temperature, away from direct sun. If forced drying is unavoidable, keep it below about 40°C and dry slowly. Do not oven-dry at 60°C overnight.
Mould-care schedule for 300 teapots:
- Make at least 4–6 sound mould sets, not 10 rushed ones. Number them and record casts per mould.
- Rotate moulds so each has time to dry; a mould should not be used twice in a day.
- After casting, rinse the working face with clean water, drain, and dry face-up in still air.
- Keep the working face clean; do not scrape with metal tools.
- Retire any mould showing chalkiness, soft edges, or more than about 40–50 casts for this duty.
- Store moulds dry and dust-free; do not stack them face-to-face without a spacer.
Before the full order, cast 10 teapots from one new mould and check rim strength, release and surface. If those pass, the method is under control.