Dry Pressing and Isostatic Pressing Questions and Answers

Answer: C

Cutting a soft green body is quick and uses cheap tooling, whereas machining the fired ceramic requires diamond and considerable time. Allowance must be made for the shrinkage that firing will cause. Green machining removes material at a fraction of the later cost.

Answer: D

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

Both are true and the reason gives the economics. Green ceramic cuts like chalk with ordinary tooling at high speed; a sintered ceramic is harder than most tool steels and must be ground with diamond, slowly and at considerable cost. Anything that can be machined before firing therefore is - allowing for the shrinkage - and only the final close tolerances are ground afterwards.

Answer: A

The bag is loaded, sealed and lowered into the vessel, which suits large or varied work but gives a slow cycle. Dry bag pressing fixes the membrane in the vessel and is far quicker, which suits high-volume items such as spark plug insulators.

Answer: B

Correct answer: dry bag isostatic pressing followed by green machining

A spark plug insulator is a long, slender shape with a central bore, which is exactly what isostatic pressing does well and a die does badly. The dry bag arrangement keeps the flexible mould in the machine so that the cycle can be automated to high volume, and the external profile is then turned in the green state before firing.

Answer: C

An inert gas at high pressure surrounds the part while it is held at sintering temperature, so pores are closed by pressure as well as by diffusion. It is used to reach the last fraction of a per cent of theoretical density.

Answer: D

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. Ordinary sintering is driven only by surface energy, which becomes very weak once the pores are small and isolated. Hot isostatic pressing adds a large external gas pressure acting to close them, so residual porosity of a per cent or two can be removed entirely - which is why it is used for critical components where a single pore would be a failure origin.

Answer: A

Gas would simply enter open pores and apply no net pressure, so the surface must be sealed first. Components that are still porous are canned in metal instead, which is then removed after the cycle. Canning adds cost, so pre-sintering is preferred where it is possible.

Answer: B

The pressurising gas enters the pores and exerts no compacting force at all. Either the part is sintered to closed porosity first or it is sealed inside a metal can for the cycle. Open porosity would simply admit the gas and defeat the process.

Answer: C

The area is about 1,963 square millimetres, and 200,000 newtons divided by that is roughly 102 newtons per square millimetre. Pressures of this order are typical for dry pressing technical ceramics. Higher pressures reach diminishing returns and risk end capping.

Answer: D

Stored elastic energy releases as the compact leaves the die and shears a cap off one end. Reducing the pressing rate, improving lubrication and tapering the die exit all help to control it. The fault appears at ejection rather than under load.