After so many years in the industry, I've summarized the core of continuous drawing dies into four words: "flow control" and "synchronization." Let's review these techniques and key points honed through practical experience:
I. Process Calculation is the Soul: The "Navigation Map" of the Drawing Process
Before starting the design, these calculations must be thoroughly memorized:
Total Drawing Coefficient and Pass Allocation
Ultimate Formula: m<sub>total</sub> = d/D (Workpiece Diameter/Blank Diameter). This is the total compression ratio.
Golden Rule for Pass Allocation: Use a small initial drawing coefficient m<sub>1</sub> (e.g., 0.5-0.55), increasing it incrementally with each subsequent pass (m<sub>2</sub>≈0.75-0.8, m<sub>3</sub>≈0.8-0.85). The secret lies in stretching to the limit in the first pass to create space for subsequent processes and reduce the total number of workstations.
Ironclad Rules for Process Layout
"Stretching must be followed by shaping, and shaping must be followed by punching": The drawing process changes the material thickness and introduces springback; therefore, high-precision punching should never be performed before or during the drawing process. "Stretch first, then form": Other local forming processes such as flanging and bulging must be performed after the stretching process; otherwise, they will severely interfere with material flow.
Blank Development and Pre-cutting Design
Continuously stretched blanks are strips, not individual circular pieces. The "bridges" and "pre-cutting" of the strip are crucial.
Pre-cutting Forms: Round workpieces commonly use "double-ring cuts" or "figure-eight cuts." Their core function is to "isolate" the material from the strip during stretching, reducing mutual pulling between adjacent stretched parts and preventing wrinkling and cracking.
Overlap Value: 30%-50% larger than ordinary progressive dies, ensuring the strip still has sufficient strength for feeding after multiple stretches.
II. Layout Diagram as the Skeleton: The Blueprint Determining the Success or Failure of the Die
This is where the designer's skill is most evident.
The Choice Between "One-to-Two" and "One-to-One" Feeding
One-to-Two (Dual Feed): Extremely high material utilization, but requires extremely high precision in guiding the material strip and balancing stress on the die. Suitable for products with large batch sizes and cost control.
One-to-One (Single Feed): The king of stability, with sufficient strip rigidity, smooth feeding, and long die life. Highly recommended, especially for complex drawing parts.
The Art of an Empty Station
An empty station must be set up between two drawing stations! This is crucial for releasing material stress and facilitating the setting of the drawing gap adjustment mechanism. It cannot be omitted.
"Tool Reception" and "Giveaway"
A "giveaway cavity" must be precisely milled into the die plate around the drawing punch to precisely shape the semi-finished product from the previous drawing, preventing interference.
The scrap cutter on the strip must be skillfully "received" to ensure that the scrap material can be smoothly cut and fall, without creating a "neck" that affects feeding.
III. Structural Design: The Devil is in the Details
Floating Material and Guiding System
Strong Floating Material: Due to the height of the drawn parts, high-strength spring-loaded floating material pins must be used, with precise positioning on the floating material plate to ensure the strip can be steadily lifted to a sufficient height for smooth feeding.
Guiding First, Pressing Later: In the drawing station, a guide pin must first be used to roughly position the semi-finished product from the previous process, then a stripper plate is used to press the material, and finally the punch enters. Incorrect sequence will result in die collision.
Details of Drawing Punch/Die
Round Corners: The initial round corner radius (rp) for the punch is (4-6)t, increasing thereafter; the initial round corner radius (rd) for the die is (6-8)t. Round corners must be polished to a mirror finish; this is crucial for reducing frictional resistance and preventing tearing.
Clearance: The drawing clearance Z (single side) is usually (1.1-1.3)t. A larger value is used for the first drawing to accommodate material thickness.
Ventilation Holes: Ventilation holes must be drilled on the punch! Diameter φ1.0-φ2.0, to prevent product deformation or carry-out due to vacuum adsorption.
Fine-tuning and Compensation Mechanism
Stretch Height Fine-tuning: During assembly, a thin shim (plug gauge) is placed between the die base and the stretching die to precisely adjust the stretching depth of each pass. This is an essential method for die debugging.
Anti-lateral Force Design: During multi-station stretching, lateral forces are enormous. Stop keys/wear-resistant blocks must be installed in the die base to prevent die displacement.
IV. Core Techniques and Debugging Principles
Balance between "Anti-wrinkle" and "Anti-breakage"
Wrinkling: Increase blank holder force or add stretching ribs (grooving on the blank holder ring) to increase material flow resistance.
Breakage: Reduce blank holder force, increase fillet radius, improve lubrication, or reduce the single-pass stretching coefficient.
Debugging is about finding the perfect balance between these two.
Lubrication is the "miracle cure"
In the stretching area, oil pits or oil channels must be designed to ensure sufficient lubrication during stretching. The choice of lubricant (drawing oil, grease mixture, etc.) directly affects success or failure.
Strip End Treatment
When the strip reaches the last few steps, it may sag due to insufficient strength. A strip support bracket needs to be designed at the end of the die or on the machine.
V. Design Checklist (Practical Essentials)
[ ] Are the total drawing coefficient and pass distribution reasonable? Has the initial drawing been fully utilized?
[ ] In the layout, are there empty workstations between drawing processes?
[ ] Are there vent holes on the drawing punch?
[ ] Are the rounded corners of all drawing dies marked with mirror polishing requirements?
[ ] Is the spring force of the float pin sufficient? Are there limit pins to prevent over-floating?
[ ] Has the template made precise clearance for each semi-finished product shape?
[ ] Is a drawing height adjustment shim structure designed?
[ ] Are positions for tension ribs/grooves reserved on the pressure ring or template (for addition during debugging)?
[ ] Has support been considered at the end of the strip?
Finally, a saying summarizes the essence of continuous stretching dies:
"Layout determines the overall situation, rounded corners determine life or death, floating material ensures stability, and debugging determines the outcome."
These skills and key points are accumulated through countless sleepless nights of debugging and mold repair. Making continuous stretching dies requires boldness and meticulousness, combining theory with practice; every detail deserves repeated consideration. I hope these shared experiences can inspire or resonate with you. Welcome to continue the discussion!
