Why Apply Mos2 Lubricant To Mold Walls For Fine-Grained Al-Matrix Powders? Essential Guide For Defect-Free Pressing

Applying Molybdenum Disulfide (MoS2) is essential for mitigating the intense friction caused by the high specific surface area of fine-grained powders. Without this lubrication, the significant resistance between the powder and the mold walls leads to destructive shear cracks and delamination during the ejection phase. MoS2 drastically reduces the coefficient of friction, ensuring the fragile "green body" (the pressed powder) can be removed intact and without defects.

The high surface area of fine powders creates excessive wall friction that disrupts the pressing process. MoS2 serves as a critical barrier that lowers ejection forces, prevents structural failure, and ensures uniform pressure transmission.

The Challenge of Fine-Grained Powders

High Specific Surface Area

Fine-grained Aluminum Matrix Composite powders possess a high specific surface area. This means there is significantly more total surface area contacting the mold walls compared to coarser powders.

Escalating Wall Friction

This increased contact area generates substantial friction during the pressing cycle. As the punch compresses the powder, the material resists sliding against the steel die walls, creating a "stick-slip" effect that hinders smooth movement.

The Ejection Hazard

The most critical moment occurs during ejection (demolding). If the friction remains high, the force required to push the compact out of the die creates excessive shear stress, causing the part to crack, fray at the edges, or delaminate (separate into layers).

How MoS2 Solves the Problem

Reducing the Coefficient of Friction

MoS2 acts as a solid lubricant, forming a thin, slippery film between the metal powder and the mold wall. This significantly reduces the coefficient of friction, transforming a high-resistance interaction into a smooth sliding motion.

Improving Pressure Transmission

By reducing lateral friction at the walls, MoS2 allows the compaction pressure to be transmitted more uniformly throughout the powder bed. This minimizes density gradients, ensuring the part is equally dense in the center and at the edges.

Extending Mold Life

Beyond protecting the part, the lubrication layer creates a buffer for the precision tooling. This reduces wear on the steel walls over repeated cycles, significantly extending the service life of expensive molds.

The Risks of Inadequate Lubrication

Structural Integrity vs. Friction

It is a common pitfall to assume that increasing compaction pressure can overcome friction. In reality, without adequate lubrication, higher pressure often exacerbates tensile stresses during ejection, guaranteeing edge chipping or total part failure.

The "Green Body" Vulnerability

The pressed part (green compact) is mechanically weak before sintering. It relies solely on mechanical interlocking of particles. If the ejection force—driven by unmitigated friction—exceeds the weak internal bonds of the powder, the part will disintegrate regardless of the pressing force used.

Making the Right Choice for Your Goal

To maximize the quality of your Aluminum Matrix Composites, apply MoS2 based on your specific processing priorities:

If your primary focus is Defect Prevention: Apply MoS2 to lower the ejection force, directly preventing shear cracks and delamination in fragile green bodies.
If your primary focus is Part Uniformity: Use the lubricant to minimize lateral friction, allowing pressure to distribute evenly and reducing density gradients within the compact.
If your primary focus is Equipment Longevity: Ensure consistent lubrication to protect precision mold edges from abrasion and extend the lifespan of your tooling.

Proper lubrication is not just a processing aid; it is the fundamental enabler for pressing high-surface-area powders successfully.

Summary Table:

Factor	Impact on Fine-Grained Powders	Role of MoS2 Lubricant
Surface Area	High specific surface area increases contact friction.	Forms a thin, slippery film to lower friction coefficient.
Ejection Phase	High shear stress causes delamination and cracks.	Reduces ejection force, allowing for smooth demolding.
Pressure Distribution	Wall friction leads to density gradients.	Improves uniform pressure transmission through the compact.
Tooling Wear	Excessive abrasion shortens mold lifespan.	Acts as a protective buffer to extend mold service life.

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