cold isostatic pressing

Squeeze a water balloon and the pressure inside is the same everywhere at once — that is Pascal's law, and it is the whole idea of cold isostatic pressing. Instead of ramming powder between two stiff punches (where friction spoils the pressure), you seal the powder inside a flexible rubber bag, drop the bag into a chamber full of oil or water, and pump the fluid to high pressure. The fluid presses on every square millimetre of the bag equally, so the powder is squeezed inward uniformly from all sides. There is no die wall dragging on the powder and no privileged direction, so the green body comes out far more uniform in density than anything a rigid die can make.

The pressures are high — typically 100 to 400 MPa, well above uniaxial pressing — and the payoff is uniform green density around 55 to 65 percent of theoretical with almost no gradient. Two industrial styles exist. In wet-bag CIP the rubber mould is filled, sealed, and lowered into the pressure vessel by hand or crane: slow, but it makes long or complex shapes. In dry-bag CIP a fixed membrane is built into the vessel so parts can be loaded and pressed rapidly, which is how millions of spark-plug insulators are made every day. Because the mould is a soft bag, the finished shape follows the bag, so dimensions are less exact than a machined die gives.

CIP earns its place wherever uniformity or awkward geometry matters more than tight tolerances: long thin tubes and rods, sputtering targets, refractory crucibles, grinding balls, radomes, and the green blanks for spark plugs. The usual workflow is CIP to a uniform oversized blank, then green-machine it to near-final shape while it is still soft and easy to cut, then fire. Its siblings run hotter — hot isostatic pressing applies the same all-round fluid (gas) pressure at high temperature to close the last pores during densification — but CIP itself is a cold shaping step only.