Deep drawing is a combination of sheet metal manufacturing and metal forming processes.
It uses a punch and a dies to pull (draw) the workpiece into the desired shape. The metal sheet is placed between two dies and the punch is pressed against it to create the desired contour. Deep drawing is used to make cups, bowls and other hollow shapes from sheet metal. It is also used to produce precision parts with complex shapes. First of all, deep drawing is the process of converting a sheet metal blank into a cylindrical container with a flat or hemispherical base. It is a combination of metal forming process and sheet metal process. If the aspect ratio of the product is greater than 0.5, this drawing operation is called deep drawing. On the contrary, it is called shallow drawing. In addition, it basically consists of a dies, a punch, and a blank holder. After the pulling force is applied, the blank of the material is formed and formed into the punch or the shape made on the punch. In order to avoid friction resistance between the blank and the dies, a lubricant is used. The application of deep drawing process is kitchen utensils, cylindrical containers, fire extinguishers, etc.
The components of the deep drawing process are as follows:
Blanking:
It is the workpiece on which the drawing operation is done. It is a piece of blank placed on the dies hole.
The diameter of the blank is denoted by “D_{b}” and the thickness is denoted by “t”. After the tensile force is applied, the blank is formed and the punch is formed.
Punching:
The punch is used to apply tensile force on the blank. The shape of the product depends on the shape of the punch.
The punch pushes the blank into the dies cavity. The diameter of the punch is denoted by Dp.
Punch:
It is generally made of cast iron or tool steel. The blank is bent on the round shape of the dies.
The gap between the dies and the punch should be enough to allow stretching and avoid shear failure of the material. The corners of the dies should be rounded to avoid shearing.
Blankholder:
The blankholder is used to fix the blank surface on the dies surface. The blanking force should be optimal.
If it is greater than the optimal limit, it will cause shearing at the dies corners. If it is lower than the optimal limit, it will cause wrinkling defects on the surface of the product flange.
The optimal holding force is one-third of the drawing force.
F_{h}= frac{F_{d}}{3}
1. Place the sheet metal blank on the dies surface.
2. The blank is fixed to the dies by the blank holder.
3. The punch moves down into the dies operation to complete the drawing operation, so the blank will be formed and take the shape of the punch or the shape on the punch.
4. In some cases, a single stroke will not give the product the required height. Therefore, in this case, the drawing operation is completed in multiple stages.
Drawing Calculation
Blank Diameter (Db):
For cylindrical:
dp = inner diameter of the cylinder.
h = cylinder height
r = corner radius
Considering that the surface area of the blank and the inner surface area of the cup are equal.
The surface area of one side of the blank = the area inside the cup
frac{pi }{4}D_{b}^{2}= frac{pi }{4}D_{p}^{2}+pi d_{p}h
D_{b}= sqrt{d_{p}^{2}+4d_{p}h}
In order to consider the corner radius (r), the blank size is given by the following formula,
1. Case 1: When frac{D_{p}}{r}geq 20, the effect of the fillet radius is ignored.
D_{b}= sqrt{d_{p}^{2}+4d_{p}h}
2. Case 2: When `15< frac{D_{p}}{r}< 20
D_{b}= sqrt{d_{p}^{2}+4d_{p}h}-0.5r
3. Case 3: When `10< frac{d_{p}}{r}< 15
D_{b}= sqrt{d_{p}^{2}+4d_{p}h}-2r
For hemisphere:
In this case, it is equal to the surface area of the blank and the hemisphere.
frac{pi }{4}d^{2}= 2pi r^{2}
d_{b}= sqrt{8}r
Maximum Pullout Force (Fd) :
The maximum pullout force applied to the punch for the pullout operation is given by,
F_{d}= pi d_{p}t+sigma _{ut} [ frac{D_{b}}{d_{p}}-0.7 ]
Where,
dp = punch diameter
t = sheet thickness
sigma_{ut} = ultimate tensile stress of blank material
Db = blank diameter
0.7 = friction correction factor.
Blank Holder Force:
The blank holder force should be sufficient to avoid wrinkling defects and avoid shear failure. It is generally equal to one-third of the pullout force.
F_{h}= frac{F_{d}}{3}
Gap between dies and punch:
The gap between dies and punch should be optimal to avoid shear failure of blank.
The gap is 10% larger than the sheet metal thickness.
Gap, (C) = (1.1) xt
Defects in drawing
Defects that occur in blank material of deep drawn metal stamping are as follows
1. Shearing: It is shear failure of blank at corners of dies and punch. This happens when stress in blank exceeds limit stress.
2. Wrinkling/Corrugation: When blank holding force is less than optimum clamping force, it causes wrinkling of product collar.
Frequently Asked Questions about Deep Drawing
1. What is the difference between deep drawing and shallow drawing?
Deep drawing is a sheet metal forming operation in which blank of material is formed into cylindrical or hemispherical container, while bar drawing is the process of reducing diameter of cylindrical bar.
2. What is deep drawing used for
Deep drawing is used to make kitchen utensils, cylindrical and hemispherical containers, fire extinguishers.
3. What is sheet metal deep drawing?
Sheet metal deep drawing is the process of converting a sheet metal blank into a cylindrical container with a flat or hemispherical base.
4. How do you distinguish between shallow drawing and deep drawing
If the ratio of the height to the diameter of the product is greater than 0.5, this drawing operation is called deep drawing, and if the ratio of the height to the diameter of the product is less than 0.5, this operation is called shallow drawing.
5. What is the deep drawing gap
The gap is the distance between the dies and the punch, and it should be optimal to avoid shear failure in the blank.
The gap is 10% larger than the sheet metal thickness.
6. How to prevent wrinkles in deep drawing
To prevent wrinkle defects during deep drawing, the holding pressure on the blank should be optimal.
7. What is the HD ratio in deep drawing
It is the ratio of the height (H) to the diameter (D) of the product.
If the ratio of the height to the diameter of the product is greater than 0.5, this drawing operation is called deep drawing.
Of course, the deep drawing process is a versatile manufacturing technology used to create a variety of parts from sheet metal. Of course, the deep drawing process produces parts with tight tolerances and requires almost no additional finishing operations. This means that the parts can be delivered directly to customers without going through further manufacturing processes. The inherent simplicity of the process enables manufacturers to automate many tasks without losing production quality. Therefore, deep drawing is an efficient and cost-effective process for creating sheet metal parts. Here, if you need large quantities of deep drawn sheet metal parts or products, you can choose Mulan Group to help. Mulan Group is a professional Chinese stamped metal parts manufacturer with 12 years of export trade experience. The sheet metal parts products produced are exported to many countries in the world and establish long-term cooperative relationships. Choosing Mulan Group is your best decision.
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