Sheet metal drawing design (drawing process and mold)
Sheet metal drawing definition:
Deep drawing, also known as drawing, stretching, extension, etc., is a process in which flat blanks or hollow parts of a certain shape are molded into various hollow parts through deep drawing. It is one of the most widely used processes in stamping production.
Characteristics of the drawing process:
The drawing process is divided into non-thinning drawing and thinning drawing. In actual production, deep drawing without thinning is widely used.
（1）No thinning and deep drawing: the thickness of each part of the formed part is basically the same as the thickness of the blank before drawing.
（2）Thinning and deep drawing: The wall thickness of the formed part is significantly thinner than the original blank thickness, but the bottom thickness of the drawn part remains unchanged.
Classification of deep-drawn parts (cylindrical, curved, box, non-rotating surface)
In stamping production, there are many types of deep drawing. Various deep-drawn parts can be divided into four basic types according to their deformation mechanics characteristics, as shown below:
Sheet metal drawing deformation process
Take the drawing process mold of flangeless cylindrical parts as an example, as shown below
Note: The bottom of the barrel is divided into a non-deformation area. This part of the material is subject to bidirectional tensile stress during deep drawing. The material thickness will be thinner, but the thinning amount is very small, generally only 1% to 3%.
The force on the material during the sheet metal drawing process (including: drawing force, compression force, bending force and friction force)
The thickness and hardness of the cylinder wall of the cylindrical part after drawing will change to a certain extent (reference for slight changes).
Changes in hardness and wall thickness of deep-drawn parts along the height direction (reference to slight changes).
Typical structure of drawing die
The drawing dies working on single-action presses can be divided into drawing dies for the first drawing and drawing dies for subsequent drawings. These two types of molds are divided into those with edge pressing devices and those without edge pressing devices.
(1) Simple drawing die without edge holder (note: remove the mold calibration positioning ring during work).
(2) Drawing die with edge holding device
(3) Blanking and drawing composite die
Typical blanking and drawing composite mold, formal composite mold structure. The upper mold part is equipped with convex and concave dies, and the lower mold part is equipped with blanking dies and drawing convex molds.
(4) Continuous deep drawing of strip and its mold
Continuous strip drawing means that the drawing is carried out directly on the strip and the parts are punched out from the strip after they are drawn. The production efficiency of this deep drawing method is very high, but the mold structure is very complex, so it is suitable for mass production and small parts.
Key points of sheet metal drawing design
(1) Shape of drawn parts
The shape of the drawn parts must be kept as simple and symmetrical as possible. Axis-symmetrical deep-drawn parts deform evenly in the surrounding direction, are easy to process, and have the best manufacturing performance. For deep-drawn parts of other shapes, drastic contour changes should be avoided as much as possible.
(2) Depth of drawn parts
Generally speaking, the drawing depth generally doesn’t exceed 0.2 times the diameter. If the depth is too great, it will easily break.
(3) Corners of deep-drawn parts
The corner parts of each adjacent wall of the drawn part should be transitioned with an arc of appropriate size to prevent the corresponding parts of the mold from being prone to wear and stress concentration. Right-angle connections may easily cause the drawn part to be punctured.
The design criteria are: the deep drawing root fillet R1≥T, the deep drawing top fillet R2≥2T, and the deep drawing corner fillet R3≥4T. Once the above criteria are met, the larger the fillet radius, the easier it is to draw.
(4) Dimensional accuracy of deep-drawn parts
The dimensional accuracy of sheet metal parts during deep drawing shouldn’t be too high. At the same time, due to changes in the wall thickness of sheet metal parts, sheet metal parts can only guarantee the internal or external dimensions of the feature during deep drawing, but can’t guarantee both the internal and external dimensions at the same time.
(5) The value of the radius of the die for deep drawing processing
When sheet metal parts are being deep-drawn, the value of the die radius (Rd) should be 60% to 80% of the die radius in the previous deep-drawing process. That is: Rd(n)=(0.6~0.8)Rd(n-1)
(6) Speed during deep drawing processing
When sheet metal parts are being deep-drawn, the speed shouldn’t be too fast. Too fast may cause the product to break. Generally, the downward drawing speed is 0.05m/s~0.2m/s.
(7) Punch exhaust during deep drawing processing
When sheet metal parts are deep-drawn, a vacuum will be formed between the flat bottom surface of the punch and the blank, resulting in vacuum pressure. This pressure makes it difficult for the cylinder to peel off from the punch, so a hole must be drilled into the bottom surface of the punch to eliminate the vacuum pressure.