Why Is A Laboratory Hydraulic Press With A Steel Mold Required For Nbt-Sct Green Bodies? Achieve Structural Integrity

The laboratory hydraulic press with a steel mold serves as the critical mechanism for transforming loose NBT-SCT powder into a cohesive, manageable solid. By applying directional pressure, this equipment compresses the mixture into a cylindrical block—typically 10 mm in diameter and 4 mm thick—establishing the necessary shape and mechanical stability for all subsequent manufacturing steps.

The primary role of this process is to convert raw, aerated powder into a "green body" with sufficient structural integrity to be handled without crumbling, laying the foundation for high-pressure densification.

The Role of Preliminary Forming

Establishing Geometric Definition

Loose NBT-SCT powder has no inherent shape. The steel mold provides a rigid boundary that defines the precise geometry of the sample.

When the hydraulic press applies pressure, it forces the powder to conform exactly to the mold's internal dimensions. This ensures that every sample begins with a consistent 10 mm diameter and 4 mm thickness, which is crucial for reproducibility in scientific testing.

Creating "Green Strength"

Before a ceramic material is fired (sintered), it is fragile. The hydraulic press creates basic mechanical strength, often referred to as "green strength."

By compressing the particles together, the press increases the contact points between them. This mechanical interlocking ensures the sample creates a solid block that acts as a coherent unit rather than a pile of dust, allowing it to be transferred to other equipment without falling apart.

Initial De-airing

Loose powder contains a significant amount of trapped air. If this air remains during high-temperature sintering, it can lead to voids and cracks.

The directional pressure applied by the press forces a significant portion of this air out of the powder matrix. This preliminary de-airing is a vital preparatory step that reduces the risk of defects in the final ceramic component.

Preparing for Advanced Processing

The Foundation for High Pressure

This hydraulic pressing stage is rarely the final step; it is usually a precursor to secondary compaction.

Processes like Cold Isostatic Pressing (CIP) require a pre-formed shape to work effectively. The hydraulic press provides this pre-formed "green body," ensuring the sample has the structural baseline needed to withstand even higher pressures later in the workflow.

Understanding the Trade-offs

Uniaxial Pressure Limitations

While effective for initial forming, a hydraulic press typically applies uniaxial pressure (pressure from one direction).

This can sometimes create density gradients within the sample, where the powder closer to the moving piston is denser than the powder further away. This is why it is often used as a preliminary step rather than the final densification method.

Friction Effects

The interaction between the powder and the walls of the steel mold can generate friction.

If not managed, this friction can lead to uneven edges or slight deformities in the green body. However, for the standard 10 mm by 4 mm cylindrical blocks used in NBT-SCT fabrication, this effect is generally manageable and accepted as part of the process.

Making the Right Choice for Your Goal

When utilizing a laboratory hydraulic press for NBT-SCT fabrication, tailor your approach to your specific processing needs:

If your primary focus is sample handling: Ensure the pressure is high enough to maximize particle interlocking, preventing the green body from crumbling during transfer.
If your primary focus is final density: Treat this strictly as a preliminary shaping step to remove air, relying on subsequent isostatic pressing for uniform density.
If your primary focus is geometric consistency: regularly inspect the steel mold for wear to guarantee the 10 mm diameter remains precise across all batches.

The hydraulic press does not just shape the powder; it establishes the structural reliability required to produce a high-quality, defect-free final ceramic.

Summary Table:

Feature	Function in NBT-SCT Forming	Key Benefit
Steel Mold	Defines 10mm x 4mm geometry	Ensures consistent sample dimensions and reproducibility
Uniaxial Pressure	Increases particle contact points	Develops 'green strength' for safe handling and transfer
Compression	Forces out trapped air	Reduces risk of voids and cracks during final sintering
Preliminary Forming	Creates solid cylindrical block	Prepares sample for secondary Cold Isostatic Pressing (CIP)

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References

Phan Gia Le, Won‐Jin Moon. Growth of single crystals in the (Na1/2Bi1/2)TiO3–(Sr1–xCax)TiO3 system by solid state crystal growth. DOI: 10.1007/s40145-021-0481-2

This article is also based on technical information from Kintek Press Knowledge Base .