stamping die preventive maintenance
Introduction to deep drawing process
Deep drawing: pressing the sheet into a hollow part (the wall thickness is basically unchanged)
Deep drawing process: the material on the plane (flange) is transferred to the side wall of the cylinder (box), so the outer dimensions of the plane change greatly
Drawing coefficient: the ratio of drawing diameter to blank diameter “m” (deformation degree from blank to workpiece)
Two: the main factors affecting the drawing coefficient
- Mechanical properties of materials (yielding strength—-elastic deformation; tensile strength—-plastic deformation; elongation coefficient; section shrinkage rate)
- The relative thickness of the material (T/D<1 D is the diameter of the blank)
- Number of deep drawing: Due to the phenomenon of cold work hardening, except for the annealing process in the middle, the m value of each time is generally increased (m1<m2<m3, it can be seen that m means to take a small value as much as possible)
- Deep drawing method: the influence of the presence or absence of the blanking plate on the m value is that the m of the blanking plate is preferably smaller; if the punch R is too small, it is more likely to cause dangerous section rupture
- Fillet radius of convex and concave die: a large die R can reduce the frictional force of forming, but when it is too large, the reduction of the pressing area will cause wrinkling
- The smoothness and lubricating conditions of the deep drawing working surface, the drawing speed such as the clearance: when the drawing speed is too fast, the flange material cannot be converted into the side wall in time, and it is easy to crack
Flangeless cylindrical parts with or without the m value of the blank holder
Three: The first drawing coefficient of flanged cylindrical parts is affected by the following factors
- Where d convex / d1: the relative diameter of the flange should include the trimming allowance
- h1/d1=relative height (narrow flange: d convex/d=1.1~1.4)
- r/d1=relative fillet radius (wide flange: d convex/d>1.4)
- t/D=relative thickness
For wide flange cylindrical parts, it is generally required to draw the required flange diameter in the first deep drawing. At this time, the smaller m1 should be used as much as possible, that is, the sufficient deformation capacity should be used, and the flange diameter should be kept in the subsequent deep drawing. Invariant (flange invariant principle)
Four: deep drawing process arrangement
- Parts with thinner material and larger depth of drawing than diameter: the method of increasing the height by reducing the diameter of the cylinder, the radius of the fillet can be gradually reduced
- Parts with thicker material and similar deep drawing depth and diameter: the fillet radius can be reduced during the process of gradually reducing the cylindrical diameter while maintaining the same height
- When the flange is large and the circle radius is small: it should be achieved by multiple shaping
- When the flange is too large: if necessary, the bulging forming method is adopted to reflect the principle of “flange unchanged”, so that the flange formed by the first deep drawing does not participate in the subsequent deep drawing deformation, and the wide flange is drawn. The material that first enters the die (that is, the material that forms the wall and the bottom) should be 3~10% more than the material actually required for the final deep drawing.
Note: The upper limit is taken when the drawing times are calculated by area, and the lower limit is taken otherwise. These excess materials will gradually return to the flange in subsequent deep drawing, causing the flange to become thicker but to avoid head cracking, and the localized thinning area can be corrected by shaping. Therefore, it is very important to strictly control the drawing height of each drawing.
Five: box-shaped parts deep drawing
The corner part is equivalent to the deep drawing of the cylindrical part, and the straight wall part is equivalent to the bending deformation
Six: deep drawing lubrication theory
1: One-sided lubrication
In the process of deep drawing, there is friction between the material and the die. At this time, there are five friction forces:
- F1—-The upper surface of the die is pressed, the lower surface of the ring and the blank
- F2—-The corner of the die
- F3—-At the side wall of the die
- F4—-Punches and Spaces
- F5—-Punch fillet
- A Friction force F1, 2, 3 is opposite to the direction of deep drawing deformation, which not only increases the drawing coefficient, but also increases the drawing force and wears out, scratching the surface of the die and the workpiece, so it is harmful
- B F4, 5 follow the direction of deep drawing and have the effect of hindering the thinning of the material at the dangerous section surface. Based on this analysis, the side of the die should be lubricated during the deep drawing operation, and the punch is not allowed to be lubricated. On the one hand, in actual production, the surface of the die and the blank holder are sometimes polished as much as possible, and the surface of the punch is intentionally roughened for deep drawing.
- C Single-sided lubrication is only suitable for the deep drawing of some cylindrical parts, but not for the whole drawing and various deep drawing forming (such as: shallow cylindrical parts, box-shaped parts and curved surfaces with bulging deformation). Deep drawing of parts)
2: Double-sided lubrication theory
Double-sided lubrication for deep drawing of box parts
From the perspective of deformation, the deformation of the deep drawing of the cylindrical part is uniform, and the deformation area is required to be smoothly deformed and the plastic deformation of the force transmission area should be minimized to improve the forming limit. At this time, only single-sided lubrication can meet the requirements.
In the deep drawing of box-shaped parts, due to the characteristics of uneven deformation in the deformation area, the deformation potential of the force transmission area is used to compensate for the non-uniformity of the two parts of the deformation area by using double-sided lubrication conditions, which can improve the The bearing capacity of the force transmission zone can also promote the smooth plastic deformation of the entire deformation zone, so the deep drawing forming limit of the box-shaped part is improved to a certain extent.
Seven: Pulling and pulling crack maintenance method
1) Split form:
- A: Neck tear
- B: The head is pulled apart
- C: The head fillet is cracked
- D: The frame mouth is cracked
2) Maintenance method
1: Analysis of the causes of “A, B” neck and head pulling cracks:
- 1) The height of the previous draw is not enough (the amount of material drawn in is too small)
- 2) The fillet of the die is too small or the transition between the R angle and the straight face is not smooth. Countermeasures:
- 1) The height of the previous draw is appropriately increased
- 2) Polish the die fillet and it is not appropriate to increase the R angle
2: Analysis of the causes of the “C” head fillet cracking:
- 1) The height of the previous punch is not enough
- 2) The R angle of the punch is too small (relative to the previous draw), and the material at the rounded corner cannot withstand excessive instantaneous transformation
- 3) The size of the punch A of the previous pump is smaller than that of the current pump. Countermeasures:
- 1) The height of the punch of the previous pump should be appropriately increased
- 2) Properly increase the R angle of the punch and correct the A size (make sure it is greater than or equal to the previous draw)
- 3: Analysis of the reasons for the cracking of the “D” frame mouth:
- 1) The process hole is too close to the extraction wall
- 2) The R angle of the punch is too large or too smooth (the friction force decreases when the bottom material flows, resulting in excessive flow)
- 3) Countermeasures for too small pressing force of bottom material:
- 1) Displacement process hole
- 2) The R angle of the punch is reduced and the amount of hair is deliberately shaved
- 3) The spring force of the floating block of the mold core is strengthened
4: The second pull crack
Reason: The transition from the first pumping to the second pumping is too large, and the material cannot withstand the pressure of the instantaneous conversion and ruptures.
Countermeasure: sharpen the head of the first pumping punch (as shown in the picture)
5: Analysis of the causes of wrinkle repair methods caused by pumping:
1) The flange pressing force is too small
2) The flange pressing area is too small
3) Countermeasures for material accumulation caused by pulling cracks during the extraction process:
1) Spring strength of stripper plate (red is recommended)
2) Stripping plate preload (1 T+0.02~0.04mm)
3) Repair the crack first
1) When repairing the crack, first check the deformation of the first pumping flange – to maximize the conversion of the flange material into the material of the side wall.
2) To determine the drawing height of each drawer, the height of the punch should be used as the criterion (and make a record) when confirming the draw height of each drawer. *Based on the height of the punch in the last draw (if the height of the existing product is OK); the last second draw should be 0.10~0.20mm higher than the last draw and the first few draws should be gradually reduced.
3) Check the finish of the working surface of the die.
4) When repairing the crack, it is necessary to comprehensively consider and carefully analyze the cause, and must not “medicate the head with a headache, and treat the foot with a pain in the foot”.