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The die radius should be between four and 10 times sheet thickness to prevent wrinkles and splits.
Straightening of sheet metal occurs next as the die continues to close. Material that was bent over the die radius is straightened to form the draw wall. Material on the blank holder now is fed into the cavity and bent over the die radius to allow for straightening without fracture. The die radius should be between four and 10 times sheet thickness to prevent wrinkles and splits. The compressive feeding or pulling of the blank circumferentially toward the punch and die cavity is called drawing. The draw action involves friction, compression and tension. Enough force must be present in drawing to overcome the static friction between the blank and blank holder surfaces. Additional force is necessary during the drawing stage to overcome sliding or dynamic friction and to bend and unbend the sheet from the blank holder surface to the draw wall. As the blank is drawn into the punch, the sheet metal bends around the die radius and straightens at the draw wall.
To allow for the flow of material, the blank is compressed. Compression
increases away from the die radius in the direction of material flow because there is more surface area of sheet metal to be squeezed. Consequently, the material on the blank holder surface becomes thicker.The tension causes the draw wall to become thinner. In some cases, the tension causes the draw wall to curl or bow outward. The thinnest area of the sheet is at the punch radius, and gradually tapers thicker from the shock line to the die radius. This is a probable failure site because the material on the punch has been work-hardened the least, making it weaker than the strain hardened material. The drawing stage continues until the press is at bottom dead center. With the operation now complete, the die opens and the blank holder travels upward to strip the drawn stamping off of the punch. Air vents provided in flat or female cavities of the punch allow air to travel under the material as it is lifted by the blank holder. The stamping will have a tendency to turn inside out due to vacuum in the absenceof air vents.
板料冲压成形成功机率着冲压件形状的复杂程度减少而增加,冲压成形的目的是生产具有一定尺寸,形状并有稳定一致应力状态,甚至无起皱无裂纹的冲压件.
冲压有一种至多种成形方式用来成型所需形状,它们是弯曲,局部成形,拉深,局部成形用来成形,凹陷形状或凸包,拉深用来成形,啤酒罐之类的冲压件,随着冲压件的形状越来越复杂,多种成形方法将会被用到同一零件的成型中,事实上,有很多冲压件上同时有弯曲,局部成型,拉深模具成型的特征,通常有三种形式的模具,它们是自由成型,局部成形以及拉深形式.