Why Are Flexible Rubber Molds Used In Conjunction With Isostatic Pressing Technology? Achieve Uniform Alloy Density

Flexible rubber molds serve as the critical interface for transferring hydrostatic pressure during the isostatic pressing of Ti-6Al-4V alloy powder. They are used primarily because they enable the application of uniform, omnidirectional force to the powder, ensuring that the material densifies evenly from all sides rather than just one.

Core Takeaway The use of flexible rubber molds allows liquid pressure to compress metal powder equally from all directions. This eliminates the internal density variations common in rigid molding, directly preventing structural failures like warping or cracking during the subsequent sintering phase.

The Mechanics of Uniform Densification

The Role of Omnidirectional Pressure

In isostatic pressing, the "isostatic" component refers to applying equal pressure from every side.

Flexible rubber molds are essential here because they act as a pliable membrane. They transmit the pressure from the surrounding liquid medium directly to the Ti-6Al-4V powder without resisting the force, causing the particles to rearrange and compress synchronously in all directions.

Eliminating Density Gradients

Traditional rigid molds often create "density gradients"—areas where the powder is tightly packed in some spots but loose in others.

This unevenness usually stems from friction between the powder and the rigid die walls. Rubber molds eliminate this issue effectively. Because the mold moves with the powder as it compresses, there is no wall friction to impede the force, resulting in a homogenous internal structure.

Achieving Synchronous Densification

When pressure is applied via a rubber mold, the Ti-6Al-4V powder undergoes "synchronous densification."

This means the entire volume of the green compact shrinks at the same rate. This uniformity is vital for creating a green body with sufficient mechanical strength to be handled and ejected without deforming.

Impact on Sintering and Final Quality

Preventing Warping and Cracking

The quality of the final alloy is determined largely by the uniformity of the green compact before it enters the furnace.

If a compact has uneven density, it will shrink unevenly during sintering. By using rubber molds to ensure consistent density upfront, you significantly reduce the risk of warping, layering defects, or cracking when the material is heated.

Enhancing Geometric Stability

A uniformly dense green body is geometrically stable.

This stability is crucial for ensuring the final component retains its intended shape after vacuum arc melting or sintering. The rubber mold ensures that the foundation—the green compact—is free of the internal stresses that lead to deformation.

Understanding the Trade-offs

Precision vs. Uniformity

While flexible rubber molds excel at density uniformity, they differ mechanically from high-precision rigid presses.

Rigid dies (often used for gears or complex net shapes) rely on fixed punches to define dimensions strictly. Flexible molds offer superior internal structural integrity but rely on the liquid pressure for shaping, which prioritizes material properties over micron-level geometric containment.

Making the Right Choice for Your Goal

To maximize the quality of your Ti-6Al-4V components, align your tooling choice with your primary manufacturing objective.

If your primary focus is internal structural integrity: Prioritize flexible rubber molds with isostatic pressing to eliminate density gradients and prevent internal defects.
If your primary focus is preventing sintering failures: Use flexible molds to ensure uniform shrinkage, which directly mitigates the risk of cracking or warping during heat treatment.
If your primary focus is complex geometric handling: Ensure the green compact achieves sufficient mechanical strength through adequate pressure (e.g., 1000 MPa in CIP) to withstand ejection without deformation.

Ultimately, flexible rubber molds are the standard for isostatic pressing because they turn external hydraulic force into internal material consistency.

Summary Table:

Feature	Flexible Rubber Molds (Isostatic)	Rigid Steel Dies (Traditional)
Pressure Direction	Omnidirectional (360°)	Uniaxial or Biaxial
Density Gradient	Virtually Eliminated	High (due to wall friction)
Shrinkage Control	Uniform synchronous shrinkage	Uneven; prone to warping
Internal Quality	High structural integrity	Risk of layering and cracks
Best Application	Structural alloys & battery research	High-precision net shapes

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