Cold Isostatic Pressing Process for Molybdenum Crucibles

Loading molybdenum powder into a rubber bag, evacuating and sealing it, then pressing it in a high-pressure vessel from all directions at once: that is the basic route for cold isostatic pressing of molybdenum crucibles. Compared with single-direction die compaction, this loading pattern brings radial and axial shrinkage closer together and leaves a more uniform density base for subsequent sintering.

For downstream users, the isostatic pressing step is neither visible nor tangible, yet it determines the initial wall-thickness distribution of the crucible. Understanding the control points at this stage helps when accepting a blank to judge whether a problem originates in forming or in sintering.

Powder condition comes before pressure settings

The particle-size makeup, oxygen content and apparent density of the molybdenum powder directly affect how the powder fills the bag. If the powder is too fine, flowability is poor and arches readily form inside the bag during filling; if it is too coarse, there are few particle contact points, the pressed body is weak, and edges crumble during stripping.

In practice, molybdenum powders of different particle sizes are blended in set proportions and mixed thoroughly. Too short a mixing time leaves local density fluctuations in the body; too long a time can make fine particles agglomerate as frictional heating raises their temperature. The mixed powder should be sieved to remove agglomerates and then sampled for apparent density, which is the basis for calculating the fill quantity. For a fuller account of powder handling and how it connects to the preceding steps, see the molybdenum powder and forming process section.

Matching the fill to the bag

The shape of the bag and its rigid support determine the initial wall-thickness distribution of the crucible. To keep wall thickness uniform, a limiting sleeve can be fitted outside the bag, or vibration-assisted layer filling can be used: add a layer of powder, vibrate gently to settle it, then add the next layer. Excessive vibration makes fine particles float upward and creates top-to-bottom particle-size segregation.

1. Determine the fill quantity by the volumetric or the weighing method, and keep a record of the difference between the two;
2. After filling, tap the bag wall lightly and check whether the powder surface is level and free of local depressions;
3. Evacuate so that the bag conforms to the powder, and check for trapped powder at the seal before closing;
4. Measure the bag outer diameter and height before pressing, as a reference for comparison after stripping.

Crucibles of different outer diameter and height differ considerably in bag size.

Pressure schedule and stripping

Pressing pressure is usually applied in stages: a low pressure first to expel air, then a ramp to working pressure with a hold. If pressure rises too fast, gas inside the powder cannot escape in time and the body tends to laminate; if the hold is too short, pressure is not fully transmitted through the body and the core region ends up at low density. The upper pressure limit is constrained by bag strength and equipment capability and should not be copied from experience with other sizes.

Bag deformation should also be watched during pressing. If the bag is hard to separate from the body after depressurization, powder may have embedded itself in the bag inner wall, or residual gas may remain from insufficient evacuation. Such problems should be dealt with before the next fill, or they will leave pits on the body surface.

Stripping must avoid mechanical shock. After releasing to atmospheric pressure, separate the bag slowly along the axis; for taller crucibles, support the base with a pallet before stripping to limit deformation caused by their own weight.

Verifying wall-thickness uniformity

After stripping, the body needs geometric inspection: measure wall thickness at several points around the circumference, and outer diameter at upper, middle and lower sections along the axis. Locations with large wall-thickness variation usually correspond to powder segregation during filling or local bag deformation. Problems found should be corrected before sintering, not left to the machining stage.

Density distribution can be checked by taking samples for bulk density measurement, or compared qualitatively by ultrasonic methods; see factory inspection items for the relevant tests. In volume production, it is advisable to record fill quantity, pressing pressure, hold time and body inspection results on the same process card for traceability. Consistency of body quality comes down, in the end, to the powder, the fill and the pressure working together consistently.