The introduction of a specifically designed ejector is the defining factor in preserving the structural integrity of NiTi green bodies. During the cold pressing process, this specialized component is critical because it eliminates the friction between the die wall and the powder particles that typically occurs during sample ejection.
The ejector facilitates smooth demolding after high-pressure compaction, preventing uneven friction distribution that causes circumferential micro-cracks, thereby guaranteeing the production of defect-free, high-density samples.
The Challenge of High-Pressure Compaction
The Friction Variable
When NiTi powders undergo high-pressure compaction, the particles are pressed tightly against the die walls.
Upon ejection, this pressure creates significant resistance. Without intervention, the friction between the die wall and the particles becomes a destructive force.
The Formation of Micro-Cracks
If friction is not managed effectively, it distributes unevenly across the sample surface.
This uneven distribution generates stress gradients within the compacted powder. These stresses result in circumferential micro-cracks, compromising the physical continuity of the green body before sintering can even occur.
The Role of the Ejector
Ensuring Smooth Demolding
The specifically designed ejector changes the mechanics of how the sample leaves the die.
By effectively eliminating wall friction, it assists in the smooth demolding of the sample. This controlled release is the primary defense against the shearing forces that tear delicate powder structures apart.
Guaranteeing High Density
Producing a green body is not just about shape; it is about maintaining density.
The primary reference identifies the ejector as an "essential process guarantee." It ensures that the high-density structure achieved during compaction is preserved, rather than being degraded by defects during the removal phase.
Common Pitfalls to Avoid
Underestimating Ejection Stress
A common error in powder metallurgy is focusing solely on the compression force while neglecting the ejection force.
Failure to use a specifically designed ejector leads to a high rejection rate. Even if the pressing parameters are perfect, uneven friction during removal renders the sample unusable due to structural defects.
Making the Right Choice for Your Goal
To maximize the quality of your NiTi green bodies, the ejection phase requires as much engineering attention as the compression phase.
- If your primary focus is preventing defects: Implement a specifically designed ejector to eliminate the friction that causes circumferential micro-cracks.
- If your primary focus is material density: Ensure your demolding process is frictionless to preserve the high-density compaction achieved during the pressing stage.
Success in cold pressing NiTi lies not just in how hard you press, but in how smoothly you release.
Summary Table:
| Feature | Role of Specialized Ejector | Risk Without Ejector |
|---|---|---|
| Demolding Process | Smooth and controlled release | High resistance and mechanical shearing |
| Wall Friction | Effectively eliminated | High friction against die walls |
| Structural Integrity | Prevents circumferential micro-cracks | Formation of stress gradients and defects |
| Sample Density | Preserves high-density compaction | Degrades density due to structural damage |
| Final Quality | Defect-free NiTi green bodies | High rejection rates and unusable samples |
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References
- Rajeev Singh, Ajay Kumar Sharma. Physical and Mechanical Behavior of NiTi Composite Fabricated by Newly Developed Uni-Axial Compaction Die. DOI: 10.1590/1980-5373-mr-2020-0549
This article is also based on technical information from Kintek Press Knowledge Base .
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