When forming in a progressive die, blank holder pressure, pressure conditions, and raw materials all affect the ability to obtain consistent stretch results without wrinkling.
Q: We are drawing cups from grade 304 stainless steel.On the first stop of our progressive die, we draw to about 0.75 inches deep.When I check the thickness of the flange perimeter of the blank, the difference from side to side can be as high as 0.003 inches.Every hit is different and doesn’t appear in the same spot.I’ve been told it has something to do with the raw material processing, probably the outermost edge of the main coil.How can we get a consistently shaped cup without wrinkling?
A: I see your question raises two questions: first, the changes you get in the lottery process, and second, the raw materials and their specifications.
The first question deals with fundamental tool design flaws, so let’s review the basics.Intermittent wrinkling and post-draw thickness changes on cup flanges indicate insufficient tooling blanks in your progressive die drawing station.Without seeing your die design, I would have to assume that your draw punch and die radii and their respective gaps meet all standard design parameters.
In deep drawing, the blank is sandwiched between the drawing die and the blank holder, while the drawing punch draws the material into the drawing die, drawing it around the draw radius to form the shell.There is a lot of friction between the die and the blank holder.During this process, the material is compressed laterally, which is the reason for the wrinkling and radial elongation of the blank holder against the flow of the material.If the holding pressure is too high, the material will break under the pull of the stretch punch.If it is too low, wrinkling will occur.
There is a limit between the shell diameter and the blank diameter that cannot be exceeded for a successful drawing operation.This limit varies by the percent elongation of the material.The general rule is 55% to 60% for the first draw and 20% for each subsequent draw.Figure 1 is a standard formula for calculating the blank holder pressure required for stretching (I always add at least 30% of the additional force as a safety factor, it can be reduced if needed, but it is difficult to increase after the design is complete).
The blank holder pressure p is 2.5 N/mm2 for steel, 2.0 to 2.4 N/mm2 for copper alloys and 1.2 to 1.5 N/mm2 for aluminium alloys.
Variations in flange thickness also indicate that your tool design is not strong enough.Your mold boots must be thick enough to withstand the pull without buckling.The support under the die base must be solid steel, and the tool guide pins must be large enough to prevent any lateral movement of the top and bottom tools during stretching.
Check out your news too.If the press guides are worn and sloppy, it doesn’t matter if your tool is strong – you won’t be successful.Check the press slide to make sure the full stroke length of the press is true and square.Verify that your drawing lubricant is well filtered and maintained, and that the tool application amount and nozzle position are fixed.Thoroughly inspect all print tools to ensure correct surface finish, coating and symmetry.And pay special attention to drawing radii; their geometry and surface finish must be perfect.
Also, while customers tend to view the 304L and standard 304 as interchangeable, the 304L is the better choice for drawing.L stands for low carbon, which gives 304L a yield strength of 0.2% of 35 KSI and 304 of 0.2% of 42 KSI.With a 16% reduction in yield strength, 304L requires less force to yield when forming and setting the formed shape.It’s just easier to use.
Are shop stamping or tool and die issues confusing you?If so, please send your questions to kateb@thefabricator.com and have them answered by Thomas Vacca, Director of Engineering at Micro Co.
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