Common defects and solutions of stretching dies

Cracks at the corresponding part of the punch shoulder

This defect occurs when the deep drawing load reaches the material's breaking load due to insufficient material strength. The defect occurs at the corresponding part of the punch shoulder R (rp), that is, the part closer to rp than the impact mark line. The impact mark line of the fractured part is different from other parts, and can be observed and inspected in the following ways: it may be extended; there may be a shiny part on the upper and lower surfaces of the flange; or wrinkles may be formed. In addition, there may sometimes be a shiny part on the side wall. Initial transverse cracking, in the shape of a tongue.

Causes and elimination methods:

(1) Product shape.

① The deep drawing depth is too large. At present, the limit of deep drawing of cylindrical and square tubes is determined in the design stage. Therefore, when deep drawing near the limit, a smooth and flat material should be used, and the die fit and grinding, lubricating oil, buffer pressure, press accuracy and other on-site conditions should be considered to conduct test deep drawing.

② The punch radius (rp) is too small. a. Correct rp to an appropriate value. b. If the rp on the drawing is too small, first draw to an appropriate value, then add a process to form the required size.

③ The die size (rd) is too small. a. Adjust rd to an appropriate value. b. If the rd on the drawing is too small, first draw with an appropriate rd value, then add a process to form the required size.

④ The corner radius (rc) of the square tube is too small. a. Reduce the drawing depth; b. Add an extra drawing process; c. Use a higher grade material; d. Increase the sheet thickness.

(2) Stamping conditions.

① The blank holder force is too large. When the blank holder force is too large, wrinkling will not occur on the flange surface. The surface roughness of the anti-wrinkle pressure plate, die fit, clearance, rp, rd, type and application conditions of processing oil, and the distribution of blank holder force caused by the buffer pin all affect the anti-wrinkle pressure. If all the above conditions related to drawing are suitable, the blank holder force will decrease, and cracking will not occur before wrinkling. ① Excessive blank holder force: The flange surface will become shiny, making it easy to identify.

② Poor lubrication: Deep drawing is closely related to lubrication, especially when it involves thinning. The temperature rise of the product must be controlled. In well-conditioned deep drawing processes, the choice of lubricant is not a problem; however, in poorly conditioned processes, improper lubricant selection can cause cracking.

③ Poor blank shape: Determining the blank shape is crucial during the trial deep drawing stage. The blank shape must be limited to the minimum dimensions. When using a square blank for cylindrical deep drawing, the limit drawing ratio is approximately 0.58. Furthermore, if the drawing ratio is too stringent, scratches at the rp (right corner) area can cause cracking. Chamfering can prevent this. When deep drawing square cylinders, a square blank can be used first to produce a nice-looking product, but if the drawing limit is reached, cracking will occur near the rp. If cracking has already occurred, a portion of the four corners of the blank can be cut off. However, cutting too much will cause flange wrinkling, leading to wall cracks. ④ Poor blank positioning. Even if the blank shape is good, if the position is not properly adjusted or the orientation is incorrect, misalignment between the punch and the blank can occur, leading to cracking or wrinkling. Additionally, when forming large deep-drawn parts (material SUS304) using a 500-ton hydraulic press, low-viscosity oil is sufficient for deep drawing. When using high-viscosity oil, the RP (rp) section will crack when the drawing reaches 1/4 of the height. Compared to mild steel plates, stainless steel is more susceptible to speed variations, but with adequate cooling and lubrication, other factors are more important than speed in actual operation. During high-speed blanking, even with normal clearance, the entire shear surface of the cut is ideal.

⑤ Poor die installation. This defect is caused by improper die installation and misalignment of the upper and lower dies. Nowadays, almost all dies are equipped with guiding devices, and malfunctions due to die misalignment are rare.

⑥ Uneven length of buffer pins. During use, buffer pins often become uneven in length due to bending, impact damage, etc. During deep drawing, the longer sections of the buffer pins are prone to breakage due to concentrated loads. To check for uneven buffer pin lengths, during mold adjustment, the pins are manually shaken back and forth. It's easy to understand why the longer pins, bearing the concentrated weight of the pressure ring, become very heavy.

⑦ Uneven buffer pad. When the pin positions of the press's buffer pad are recessed, or scrap material falls onto the buffer pad from the pin holes, the buffer pressure cannot be controlled. If the press has a movable worktable, simple cleaning or maintenance can prevent such accidents. However, if it has a fixed worktable and is not maintained for a long time, accidents often occur upon use.

⑧ Improperly fitted buffer pins. Buffer pins should ideally be fitted around the punch; however, there must be an appropriate gap. If the pressure ring is thin and the buffer pins are improperly fitted, the flange of the product will be subjected to strong tensile force at a buffer pin location, causing it to break. At this point, the end shape of the flange will extend locally in a tongue-like manner, which is a very simple and obvious principle. Additionally, if the buffer pin configuration is inconsistent with the shape of the punch perimeter, the flange surface will wrinkle, often leading to breakage. Ultimately, when the blank holder is very thin, the position of the pin has a significant impact; therefore, ensuring sufficient strength in the blank holder is the most fundamental issue.

⑨ Wrinkling causes breakage.

a. The blank size is larger than the blank holder. When the blank size is larger than the blank holder, wrinkling occurs on the exposed part of the blank after drawing begins. This wrinkling functions similarly to the "draw rib," and continued drawing will cause it to break. During the trial drawing stage, to determine the position of the "draw rib," the blank is sometimes intentionally left exposed outside the blank holder. Generally, even with localized bulging of large blanks, the principle remains that the blank is held down by the blank holder before applying pressure.

b. Insufficient blank holder force. When the blank holder force is insufficient, wrinkling will occur on the blank surface. This wrinkling often leads to breakage when it passes through the die corner radius (rd). Therefore, in this situation, wrinkling and cracking are intertwined. When deep drawing square tubes using stainless steel sheets with a high degree of work hardening, a bright section develops wrinkles near the rd (dimension radius). These wrinkles are the cause of cracking. If the rd section cracks, the first priority is to increase the blank holder force to eliminate the wrinkles; this is of paramount importance. Never increase rd or decrease the blank holder force. The bright section is caused by the increased thickness of the blank, resulting in concentrated loads. Therefore, while increasing the blank holder force, the contact points between the dies should be leveled to eliminate the thickened material. If the load is distributed, wrinkling on the flange surface can be eliminated, making material flow easier.

c. The die radius (rd) is too large. When rd is too large, work-hardened wrinkles will appear in the rd section, which, in turn, act as draw beads, causing the drawn part to crack. Therefore, during deep drawing, rd should be as small as possible to facilitate deep drawing. d. The clearance between the blank holder sidewalls is too large. For example, in deep drawing with flange clamping on cylindrical parts or partial flange clamping on square tubes, the clearance between the punch and the sidewall of the blank holder must be smaller than the fillet radius (rd) of the die. If the clearance is too large, the material will not adhere tightly to rd during deep drawing, but will bulge upwards, resulting in wrinkles. These wrinkles will then be pressed into a certain shape after entering the clearance, becoming a cause of breakage. Therefore, the sidewall of the blank holder must have a reasonable clearance during processing. The clearance for the flange clamping portion of cylindrical parts and the flange clamping portion of square tube corners must be designed to be smaller than rd.

⑩ Poor press precision.