Dry Pressing and Isostatic Pressing Questions and Answers

Answer: A

The term dry is relative: a small amount of moisture and an organic binder are needed for the granules to deform and bond. Twenty per cent would be a plastic body and half would be a casting slip, so the three routes are quite distinct.

Answer: B

The granules first slide into a denser packing, then deform and break down, and finally the primary particles themselves rearrange. Each stage shows as a distinct region on a plot of density against the logarithm of pressure.

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 cause. At low pressure the granules simply rearrange and slide into a denser packing; at intermediate pressure they deform and fracture; at high pressure the primary particles themselves are forced closer and further gain becomes very hard. Each mechanism has its own slope on the semi-logarithmic plot, so the curve breaks into regions rather than running straight.

Answer: D

Pressure is absorbed by wall friction as it travels down the compact, so the region nearest the punch is denser. The effect grows with the depth-to-diameter ratio and is the main limit on what single-action pressing can do.

Answer: A

Wall friction means the pressure falls with distance from the moving punch, so the far end is consistently less dense. Double-action pressing halves the distance the pressure must travel and improves the uniformity considerably.

Answer: B

Each punch has to drive pressure only half the depth, so the gradient is halved and the weak plane moves to the middle. It remains a compromise rather than a cure, which is why isostatic pressing exists for demanding work.

Answer: C

Correct answer: 20 mm

With double-action pressing both punches move, so each has to compact only half the depth: \[ \frac{40}{2} = 20\ \mathrm{mm}. \] That halving is the whole point of the arrangement. Because friction at the die wall absorbs pressure with distance travelled, halving the distance greatly reduces the density gradient, which is why deep parts are pressed from both ends.

Answer: D

Correct answer: equally from all directions through a fluid

The powder is sealed in a flexible mould and immersed in a fluid that is pressurised, so the pressure is hydrostatic and acts equally on every surface. There is no punch, no die wall and no preferred direction, which is what gives the uniformity the next item describes - and it is also why the process can form long or complex shapes a die could not.

Answer: A

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

Both are true and the reason is the mechanism. In a die, friction against the wall consumes a growing fraction of the applied pressure with depth, so the far end of the compact sees much less than the near end and comes out less dense. With no rigid wall there is nothing for friction to act against, so the pressure is the same everywhere and the green density is uniform - which in turn means uniform shrinkage on firing.

Answer: B

The flexible bag cannot hold a precise shape, so the compact usually needs machining before firing. Isostatic pressing buys uniformity of density at the cost of dimensional control. The two methods answer different requirements rather than competing.