Common quality issues in deep drawing and measures to prevent these quality problems mainly include the following aspects:
·Wrinkling
Wrinkling is one of the common issues in the deep drawing process, and its causes are:
1. Mainly due to excessive deformation of the blank, tangential compressive stress causes the sheet material to lose stability. When the tangential stress exceeds the material's critical stress, the material loses stability and results in bending and wrinkling.
2. A small relative thickness of the material also easily leads to wrinkling.
3. The deep drawing coefficient; the smaller the deep drawing coefficient, the easier it is for the material to wrinkle.
4. The shape and parameters of the deep drawing die; excessively large radii and gaps of the punch and die can cause wrinkling. Conical dies are less prone to wrinkling. The unreasonable design of pressure ribs on stamped parts, with ribs that are too small or incorrectly positioned, cannot effectively prevent the rapid flow of the sheet material, leading to wrinkling. An unreasonable design of die positioning can result in the inability to hold the material during the stretching process or the edges being pressed too small, causing wrinkling during stretching.
Measures to prevent wrinkling include:
1. Using a blank holder: By setting a blank holder in the die, the free fluctuations of the blank in the thickness direction are restricted, preventing wrinkling. When using a pressure device, it tightly presses the deformed part of the blank and applies a pressure force to prevent the flange from bulging and causing wrinkling. The magnitude of the pressure force must be appropriate. Pressure devices are divided into elastic pressure and rigid pressure types. Elastic devices are suitable for shallow drawing, while rigid devices are suitable for deep drawing.
2. Using a conical die: A conical die helps with the tangential deformation of the blank, increasing its resistance to instability.
3. Increasing the thickness of the sheet: By increasing the thickness of the sheet, its ability to resist instability under pressure is enhanced.
4. Using stretch ribs or deep drawing ledges: Setting stretch ribs or deep drawing ledges in the die increases radial tensile stress, reducing tangential compressive stress and lowering the tensile stress on the cylinder wall. Setting extension ribs on the pressure surface is an effective and practical method to adjust and control deformation resistance. Extension ribs can effectively regulate the flow of material, ensuring that the flow resistance of the material is uniform during the stretching process, allowing the amount of material flowing into the cavity to meet the needs of the finished part, preventing wrinkling from excess material or cracking from insufficient material. For complex shaped stretched parts, especially those with small flanges, extension ribs should be set to enhance radial tensile stress to control wrinkling; the positioning of the extension ribs should be on areas with low radial tensile stress, i.e., areas where the sheet material flows easily. For parts with small flanges, to set extension ribs, some additional material (process supplementary material) can be added, which can be removed during trimming. For stretched parts with significantly varying depths, extension ribs should be placed in areas with less material feed, allowing excess material to be drawn into the die cavity, thus preventing wrinkling.
5. Using reverse drawing: By increasing bending and friction, radial tensile stress is increased, and tangential compressive stress is reduced.
·Drawing breakage
Drawing breakage is caused by tensile stress exceeding the material's tensile strength, resulting in sheet material fracture. Measures to prevent tearing include:
1. Improving the mechanical properties of the material: By increasing the tensile strength of the material to resist tensile stress, good material properties can reduce the occurrence of wrinkling.
2. Reasonable die design: Properly formulating the deep drawing process and designing the die structure rationally.
3. Increasing the radius of the punch and die corners: Appropriately increasing the corner radii of the punch and die reduces stress concentration.
4. Increasing the surface roughness of the punch: Enhancing the surface roughness of the punch shifts the dangerous fracture surface towards the mouth of the cylinder, reducing the likelihood of tearing.
Other quality issues and causes
In addition to wrinkling and tearing, the deep drawing process may also experience the phenomenon of ear formation. Ear formation is caused by the anisotropy of the sheet material; in the direction with a lower directional coefficient in the thickness direction, the sheet material thickens, resulting in a lower cylinder wall height; while in the direction with a higher directional coefficient, the thickness of the sheet material changes little, and the cylinder wall height is higher. Therefore, the phenomenon of ear formation becomes more severe.
Through the above measures, common quality issues in the deep drawing process can be effectively prevented, improving product quality and production efficiency.
