A multistart screw mechanism functions as a kinetic converter, directly transforming the standard vertical axial movement of a press into a precise rotational motion for the mold punch. This mechanism creates a synchronized dual-force environment, applying rotational shear forces to the iron powder surface simultaneously with axial pressure to achieve combined loading compaction.
The core value of this mechanism lies in its ability to generate deep shear flow. By coupling downward pressure with rotation, it enables the high-density molding of complex geometries—specifically extremely thin or high-aspect-ratio components—that standard vertical compaction cannot achieve.
The Mechanics of Combined Loading
Converting Linear Force to Rotation
The fundamental role of the multistart screw is to alter the physics of the press stroke. Utilizing specific geometries, such as an 18-degree thread lead angle, the mechanism mechanically couples the punch to the press slide.
As the press moves vertically, the screw forces the punch to rotate. This ensures that the rotational speed is perfectly synchronized with the speed of the axial compression.
Generating Shear Forces
Standard compaction relies solely on top-down pressure, which often results in uneven density. The rotation introduced by the screw generates shear forces that act tangentially on the iron powder.
This shearing action disrupts friction between particles. It forces the powder to move laterally, not just vertically, resulting in a phenomenon known as deep shear flow.
Critical Manufacturing Implications
Achieving Powder Leveling
One of the primary challenges in powder metallurgy is ensuring the powder is spread evenly before high pressure is applied. The synchronized rotational action provides active powder leveling.
This reduces density gradients within the part. It ensures that the material structure is uniform from the center to the edges of the component.
Enabling High-Aspect-Ratio Designs
Without shear forces, thin or tall parts (high-aspect-ratio) are prone to cracking or low density regions. The multistart screw mechanism overcomes this limitation.
By facilitating fluid-like movement of the powder during compaction, it allows manufacturers to produce extremely thin components that maintain high structural integrity and density.
Optimizing Process Variables
Regulating Force Vectors
While the screw provides the motion, the system relies on spring elements positioned between the screw and the press slide to fine-tune the application of force.
These springs generate additional axial preload. More importantly, they allow engineers to regulate the specific ratio between the linear downward movement and the rotational twist.
Maximizing Press Efficiency
Proper calibration of these spring elements is essential for process efficiency. By adjusting spring stiffness, the mechanical force vectors applied to the iron powder preform can be precisely managed.
When optimized, this increases the effective utilization of the pressing force to a range of 90% to 95% or higher, ensuring minimal energy is wasted during the compaction cycle.
Making the Right Choice for Your Goal
To leverage a multistart screw mechanism effectively, you must align the hardware settings with your specific component requirements.
- If your primary focus is Geometric Complexity: Prioritize the synchronization of the screw's lead angle to ensure sufficient shear flow for thin or high-aspect-ratio features.
- If your primary focus is Process Efficiency: Focus on tuning the stiffness of the spring elements to maximize force utilization rates toward the 95% threshold.
Ultimately, the multistart screw transforms the mold from a simple crushing tool into a precision instrument capable of complex, high-density particle rearrangement.
Summary Table:
| Feature | Function in Combined Loading |
|---|---|
| Kinetic Conversion | Transforms vertical press stroke into precise rotational motion. |
| Shear Force Generation | Enables deep shear flow for uniform powder distribution. |
| Lead Angle (e.g., 18°) | Synchronizes rotational speed with axial compression speed. |
| Spring Elements | Regulates force vectors and provides axial preload for efficiency. |
| Process Outcome | Produces thin, high-aspect-ratio parts with 90-95% force utilization. |
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References
- Sergey N. Grigoriev, Sergey V. Fedorov. A Cold-Pressing Method Combining Axial and Shear Flow of Powder Compaction to Produce High-Density Iron Parts. DOI: 10.3390/technologies7040070
This article is also based on technical information from Kintek Press Knowledge Base .
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