Rubber molds act as the critical, high-elasticity interface between the hydraulic fluid and the Tungsten Heavy Alloy (WHA) powder during Cold Isostatic Pressing (CIP). Their primary function is to transmit uniform hydraulic pressure to the metal powder while simultaneously sealing it against contamination and facilitating the safe removal of the fragile pressed part.
In CIP, the rubber mold serves as a flexible transmission medium that converts isotropic hydraulic force into precise mechanical compaction, ensuring the WHA powder achieves uniform density without direct contact with the pressurizing fluid.
The Mechanics of Pressure Transmission
Achieving Isostatic Uniformity
The core advantage of CIP is the application of pressure from all directions simultaneously. Rubber molds possess high elasticity, allowing them to deform under hydraulic load.
This flexibility ensures that pressure is transmitted evenly to every surface of the WHA powder. This results in uniform densification of the particles, regardless of the component's geometry.
Adaptive Deformation
As the pressure increases, the metal powder compacts and the overall volume of the part decreases.
Because the rubber mold is flexible, it shrinks along with the powder. This maintains continuous, consistent contact with the material throughout the entire compression cycle, ensuring no voids are left behind.
Sealing and Surface Integrity
The Hydraulic Barrier
Direct contact with hydraulic fluid would contaminate the WHA powder and ruin the sintering process.
The rubber mold acts as a superior hermetic seal. It isolates the dry powder from the high-pressure liquid medium surrounding it, ensuring the chemical purity of the alloy is maintained.
Preserving the Green Body
Once the pressure is released, the WHA powder has formed a "green body"—a compacted part that holds its shape but is structurally fragile.
Rubber molds facilitate non-destructive demolding. Because the mold is elastic, it can be peeled away or will relax back to its original shape, releasing the green body without inducing surface cracks or stress fractures.
Understanding the Trade-offs
Dimensional Precision vs. Density
While rubber molds excel at achieving uniform density, they lack the rigid constraints of a steel die.
Consequently, CIP creates "near-net-shape" parts rather than final-dimension parts. You must account for post-process machining to achieve tight geometric tolerances, as the flexible mold allows for slight dimensional variations.
Surface Finish Limitations
The surface of the final green body will directly reflect the texture and condition of the inner rubber wall.
Over time, rubber molds can degrade or lose elasticity. Using worn molds can lead to surface irregularities on the WHA part, requiring more aggressive finishing steps later in production.
Making the Right Choice for Your Goal
To maximize the effectiveness of rubber molds in your WHA production line, consider your specific processing priorities:
- If your primary focus is Internal Integrity: Ensure the rubber formulation has high elasticity to guarantee uniform pressure transmission and consistent density throughout the part.
- If your primary focus is Yield Rate: Prioritize molds with superior release properties to prevent damage to the delicate green body during the demolding phase.
The rubber mold is not just a container; it is the active transmission tool that defines the structural quality of the pressing.
Summary Table:
| Function | Description | Key Benefit |
|---|---|---|
| Pressure Transmission | Converts hydraulic force into isotropic mechanical compaction | Uniform densification and zero voids |
| Hermetic Sealing | Isolates WHA powder from the hydraulic fluid medium | Maintains chemical purity and prevents contamination |
| Adaptive Deformation | Shrinks with the powder during volume reduction | Ensures consistent contact throughout compression |
| Green Body Protection | Facilitates non-destructive, elastic demolding | Prevents surface cracks and structural fractures |
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References
- A. Abdallah, M. Sallam. Effect of Applying Hot Isostatic Pressing on the Microstructure and Mechanical Properties of Tungsten Heavy Alloys. DOI: 10.21608/asat.2017.22790
This article is also based on technical information from Kintek Press Knowledge Base .
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