Plastic Forming and Extrusion Questions and Answers

Answer: A

Correct answer: deforms under stress and keeps the new shape

Plasticity is permanent deformation without cracking: the body yields when worked and holds the form it is given. Elastic behaviour returns the shape, brittle behaviour breaks, and a liquid holds no shape at all - so plasticity is the narrow middle condition all these forming methods depend on.

Answer: B

Correct answer: plate-like clay particles sliding over a water film

Clay minerals are thin plates, and a film of water between them lets them slide past one another while surface tension holds them together. Both halves matter: the shape of the particles allows the sliding and the water film both lubricates and binds. Take the water away and the body is rigid; add too much and it flows.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. Equiaxed alumina particles are roughly spherical and slide past one another only by rolling, which gives none of the cohesion a clay body has. Such powders are therefore made workable by adding an organic plasticiser that supplies artificially what the particle shape does not.

Answer: D

The plastic limit marks the point at which it begins to deform without crumbling and the liquid limit the point at which it begins to flow. The range between them is the plasticity index and describes how workable a clay is.

Answer: A

Subtracting the plastic limit from the liquid limit gives 25. A larger index means a wider range of water content over which the body can be worked, which is generally an advantage in forming. Too wide a range brings its own drying problems, however.

Answer: B

A very plastic clay is easy to shape but shrinks a great deal on drying and is prone to cracking. Bodies are blended to reach a workable middle rather than to maximise plasticity alone. Grog and sand are the usual leaning additions.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. The water film that gives plasticity is lost on drying, and the particles close up by that amount - so the more plastic the body, the more it shrinks and the more likely it is to crack. Blending in a less plastic material trades some workability for a body that survives drying.

Answer: D

Correct answer: mix it uniformly and remove entrapped air

A pug mill works the body under vacuum, so it homogenises the water content and the composition and draws out the air. Both are needed: an uneven body extrudes unevenly, and trapped air becomes a void that expands and splits the piece later.

Answer: A

Correct answer: False

Trapped air is compressed as the body passes through the auger and then expands as it leaves the die, blistering the column or splitting it. Even air that survives extrusion becomes a void which expands on drying or firing. De-airing under vacuum is standard for exactly this reason.

Answer: B

It compacts and advances the plastic mass so that it emerges through the die under considerable pressure. Cutting is done afterwards by a separate wire cutter running across the moving column. Column speed and cutter speed have to be matched exactly.