A pressure-controlled blank holder is mandatory in hot pressing molding because it regulates the flow of material. By applying a specific pre-tightening force to the edges of the composite sheet, it controls how fast the material enters the mold as the punch descends. This mechanism is the primary defense against structural defects during deep drawing or complex forming operations.
The blank holder acts as a critical hardware safeguard during manufacturing. By exerting precise control over the material's flow rate, it balances the tension needed to prevent wrinkling against the freedom required to avoid fiber breakage.
The Mechanics of Material Control
Regulating Flow Rate
When a punch enters a mold, it draws material inward. Without regulation, this material would flow unpredictably. The blank holder acts as a brake, ensuring the material flows at a controlled, consistent rate.
Applying Pre-tightening Force
Before the forming process begins, the blank holder secures the edges of the composite sheet. This applies a specific "pre-tightening" force. This initial tension is essential for preparing the material to accept the shape of the mold.
Defect Prevention and Quality Assurance
Preventing Wrinkling
If the material is allowed to flow too freely into the mold cavity, it tends to buckle. This results in wrinkling on the final part. The blank holder restricts this movement just enough to keep the sheet taut and smooth.
Avoiding Fiber Breakage
Conversely, if the material is held too tightly, it cannot move with the punch. This causes the fibers to stretch beyond their limit. Precise force control ensures the material yields enough to form the shape without snapping or breaking fibers.
Eliminating Surface Roughness
High-quality composite components require a pristine finish. Uncontrolled material movement often leads to surface roughness. By stabilizing the sheet, the blank holder ensures the final surface remains uniform and defect-free.
Understanding the Trade-offs
The Risk of Excessive Force
While securing the material is vital, applying too much pressure effectively locks the sheet in place. If the blank holder clamps down too hard, the punch will tear the material rather than form it. This leads to immediate part failure through fiber breakage.
The Risk of Insufficient Force
If the pressure is too low, the blank holder fails to generate the necessary friction. The material slides into the mold too easily. This lack of tension causes the material to bunch up, resulting in severe wrinkling and structural instability.
Making the Right Choice for Your Goal
To achieve high-quality results in composite structural components, you must tune the blank holder force to the specific needs of your geometry.
- If your primary focus is complex forming: You must prioritize a balanced flow rate to allow the material to conform to intricate shapes without tearing.
- If your primary focus is surface quality: You must ensure enough pre-tightening force to maintain tension and prevent roughness or wrinkling.
The blank holder is not merely a clamp; it is the definitive variable for ensuring the structural integrity of deep-drawn composite parts.
Summary Table:
| Feature | Effect of Correct Force | Risk of Excessive Force | Risk of Insufficient Force |
|---|---|---|---|
| Material Flow | Controlled & Uniform | Material Locked / Immobility | Unpredictable / Rapid Flow |
| Surface Quality | Smooth & Taut | Potential Tearing | Wrinkling & Bunching |
| Structural Integrity | High / Defect-free | Fiber Breakage / Cracking | Structural Instability |
| Process Role | Regulates Punch Entry | Material Failure | Surface Roughness |
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References
- Shun‐Fa Hwang, Hsien‐Kuang Liu. Thermoforming Simulation of Woven Carbon Fiber Fabric/Polyurethane Composite Materials. DOI: 10.3390/app14010445
This article is also based on technical information from Kintek Press Knowledge Base .
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