What is sheet metal drawing process?
Simply put, sheet metal drawing is a sheet metal forming process that uses molds to “draw” metal sheets into specific three-dimensional shapes.
It does not cut the material, but rather causes the sheet metal to undergo controllable plastic deformation under external force, thereby obtaining the desired part shape.
From a process classification perspective, sheet metal drawing is a type of sheet metal forming process, typically completed under the constraint of a mold. During processing, the metal sheet flows while being stretched, and its shape gradually transforms from a two-dimensional plane into a three-dimensional structure.
This process has several very typical characteristics:
- Large range of shape changes : Pulling can process originally flat sheet metal into structural components with obvious height or depth.
- Thickness variation is relatively controllable : Under reasonable design, the overall thickness of the part remains relatively uniform, and the main purpose is not to “thin” it.
- Most parts are axisymmetric or nearly axisymmetric in structure : common shapes include cup-shaped, cylindrical or closed shell-shaped parts.
It should be noted that sheet metal drawing does not pursue complex outer contours, but is more suitable for parts with integral forming and high requirements for structural continuity.
The differences between drawing and deep drawing, as well as their specific applicable scenarios, will be explained further in subsequent chapters.
Basic process of sheet metal drawing
Sheet metal drawing is not a complex operation completed in one go, but a sequential forming process. Throughout the process, the material is always stretched and flowed, rather than being cut or removed.
1. Preparation of sheet materials (raw material)
Drawing processes typically begin with a blank of a suitably sized metal sheet.
The core objective of this step is singular: to ensure that there is enough material to be stretched and distributed during subsequent forming of the sheet.
The shape and size of the blank directly affect the success of subsequent forming, but at this stage, the sheet material itself is still flat.
2. Place it into the mold (concave mold/convex mold)
The prepared sheet material is placed between molds and fixed and guided by the molds. At this point, the role of the molds is not to “cut”, but to limit the deformation direction and final contour of the sheet material.
Under the constraints of the mold, the sheet metal can only deform along the expected path, which is the key to obtaining a stable shape in the drawing process.
3. Stretch forming (material flow)
This is the core stage of the drawing process. Under the action of external force, the sheet is gradually drawn into the mold, the material is stretched and flows, and the original planar structure begins to transform into a three-dimensional shape.
It is important to note that:
- The material was not “pressed in”.
- Instead, it is stretched and redistributed under controlled conditions.
If not properly controlled, the following may occur at this stage:
- Wrinkling (due to excessive material buildup)
- Rupture (localized overstretching)
- Uneven thickness variation
These issues will directly affect the quality of the finished product.
4. The state after molding is completed
After the drawing process is completed, the sheet material has formed a stable three-dimensional structure, and the overall outline is basically determined.
The parts at this time usually have:
- Continuous overall structure
- Obvious depth or height
- Relatively uniform wall thickness distribution (under reasonable process conditions)
Whether further processing is needed depends on the specific product requirements, but the drawing process itself has already completed the main shape shaping task.
The difference between ordinary drawing and deep drawing
In sheet metal drawing processes, not all drawing is considered “deep drawing.” Whether something is called deep drawing depends not on the name, but on the degree of forming and the difficulty of processing.
Generally speaking:
- Conventional drawing: Used for forming requirements with relatively limited shape changes.
- Deep drawing: Used for forming complex structural parts with large forming depth and high degree of deformation.
The two can be distinguished from the following four aspects.
1. Forming depth
Ordinary pulling
The forming depth is relatively shallow, and the sheet material is transformed from a two-dimensional shape into a three-dimensional shape, but the overall height or depth is limited, and the shape change is controllable.
Deep drawing
The forming depth has increased significantly, and the parts have a larger height or cavity depth. The sheet metal needs to be pulled into a deeper part of the mold to complete the forming.
2. Degree of deformation
Ordinary pulling
The material deformation is relatively low, the overall stretching process is relatively mild, and the shape change is mainly concentrated in local areas.
Deep drawing
The material needs to withstand greater tension and flow, resulting in a larger overall deformation range and higher requirements for process control.
3. Requirements for material properties
Ordinary pulling
The requirements for material plasticity are relatively relaxed; sheet metal materials with conventional properties can meet the processing needs.
Deep drawing
Higher requirements are placed on the ductility and stability of the material; otherwise, cracking or instability is more likely to occur during the forming process.
4. Processing difficulty and cost
Ordinary pulling
The process is relatively simple, the processing difficulty is low, it is suitable for routine production needs, and the overall cost is controllable.
Deep drawing
The processing difficulty has increased significantly, requiring stricter process control, and the trial production and adjustment costs are relatively high.
If a part is simply transformed from a two-dimensional shape to a three-dimensional shape with limited forming depth, ordinary drawing is usually used; however, when a part has a deeper structure and a large shape change, deep drawing is often required.
Common Product Forms of Sheet Metal Drawing
The most direct way to determine whether a sheet metal part was produced using a drawing process is not by looking at the industry, but by looking at its shape. If a part possesses the following typical characteristics, it can generally be determined that it was formed by drawing.
Cup-shaped or shell structure
This is the most typical form of a pull-out product.
Parts typically have:
- A single, integrally formed bottom
- Continuous sidewalls extending upwards
- The structure is intact and there are no signs of splicing.
This type of shape is commonly found in various shell and container-shaped parts, and is very common in industrial products and home appliances.
Cylindrical structure
Cylindrical or near-cylindrical parts are also a common result of the drawing process.
The characteristics of this type of part are:
- Regular cross-section
- Relatively uniform wall thickness
- Clear axisymmetric structure
It is typically used for components that require high structural strength and overall consistency.
A closed structure with a certain depth
When a part is not only tall but also has a deep internal space, it often needs to be formed in one or more steps through a drawing process.
These types of products typically exhibit the following characteristics:
- The opening size is relatively fixed.
- The internal cavity is relatively deep
- The overall structure is continuous and smooth.
They are commonly found in automotive and aerospace-related parts.
Pulling parts with simple edge structure
After the main body of some drawn parts is formed, the edges may have the following characteristics:
- Simple edge flip
- Transition fillet
- Strengthen edge structure
These features are usually designed to enhance strength or facilitate subsequent assembly, but the overall shape is still mainly formed by drawing.
Advantages and disadvantages of sheet metal drawing
Sheet metal drawing is not a “universal forming method,” but it does have irreplaceable advantages in suitable scenarios.
Whether or not to use a drawing process often depends on the part’s structure, material properties, and production method.
The main advantages of sheet metal drawing
Suitable for complex three-dimensional forming
The drawing process can form flat sheet metal into parts with obvious depth and continuous structure in one step, which is especially suitable for cup-shaped and shell-shaped products.
High material utilization rate
The entire process is mainly based on material stretching and flow, with almost no chips produced. Compared with cutting-type machining methods, it is more conducive to controlling material waste.
The finished products have good consistency and are suitable for mass production.
Provided that the process and mold are stable, the size and shape of the drawn parts have high repeatability, which is suitable for the needs of mass production.
The main disadvantages of sheet metal drawing
High requirements for material properties
The material needs to have good ductility; otherwise, it is prone to cracking or instability during the drawing process.
Mold costs are relatively high
The drawing process is highly dependent on the mold. Once the structure is complex, the initial investment and adjustment costs of the mold are quite significant.
Improper design can easily lead to molding failure.
If the structure or size proportions of the parts are unreasonable, problems such as wrinkling, cracking or uneven thickness may occur during the forming process, increasing the risk of rework.
In summary, sheet metal drawing is well-suited for parts with continuous structures and large forming depths, but it is not suitable for all sheet metal products. In practical applications, it is often necessary to combine deep drawing processes, other sheet metal forming methods, or customized sheet metal processing solutions, and make a comprehensive evaluation before making a choice.