Estimated reading time: 9 minute
Friction on Stamping Process
The stamping process is very important, the sheet is always in contact with the die. This contact is not static, but dynamic. Because the metal sheet flows over the die surface, there is relative motion between the sheet and the die. Even though sheet and mold surfaces appear smooth without visual aid, under the microscope, their surfaces show complex shapes.
Sheet and tool surfaces have roughness distributions that consist of a series of peaks and valleys of varying heights, depths, and spacings, as shown in Figures 1 and 2. The roughness distribution of sheet metal will vary depending on the type, grade, and coating of the material, while the roughness distribution of tools will vary depending on the type of material and how they are processed.
Due to these irregularities of the sheet and tool surfaces, there is resistance to relative motion. In simple terms, this resistance to relative motion is called “friction”, which is why lubricants are applied to metal plates to reduce their resistance and therefore friction. The ratio between the frictional force and the contact force of two moving objects is represented by the coefficient of friction “μ”, the value of which depends on the tribological system itself and the forming process, such as the temperature of the sheet, the stamping speed, the contact pressure and the strain of the sheet.
Lubrication on Stamping Proces
We know where the friction is coming from and why we need to lubricate the sheet before stamping. We will now focus on how the amount of lubrication affects the quality of the panels during the forming process. You can get a better understanding of the lubrication effect through the pictures below.
The sheets shown in Figures 3, 4, and 5 were simulated using the same exact simulation setup, and the part quality was different because the amount of lubricant had changed. The sheet shown in Figure 3 has severe wrinkling at the corners due to the high lubricity applied to the sheet prior to stretching.
The higher the amount of lubrication, the lower the resistance to movement, i.e. the material then moves freely over the tool surface in an uncontrolled manner, creating wrinkles. Conversely, when the amount of lubrication applied to the sheet is very low, the resistance to motion is very high. This high resistance forces the sheet metal to stretch beyond the desired amount, producing substantial thinning and, in some cases, extensive cracking, as shown in Figure 4.
Therefore, it becomes critical to use the proper amount of lubrication when pulling panels, as is finding the optimum amount of lubrication required. Figure 5 shows a sheet without wrinkles and cracks when the lubricant is properly applied.
Just like any other manufacturing process, applying lubricant to the sheet creates some inconsistencies like noise. This means, if the user decides to use 1g/m2 of lubricant on the sheet, thus producing a defect-free panel, what is the probability that the robot will spray the exact amount of lubricant on the panel each time? For example, if the accuracy of the equipment is 85%, the deviation of the lubricant will be 0.85 – 1.15g/㎡, if the panel is very sensitive to friction, there may be some problems. Therefore, it is critical to find a safe range of lubrication quantities and ensure that the equipment is spraying lubricant within the given range.
When considering an AHSS stamping tribology system, there are three main points to consider, namely: 1. The effect of friction and tribology on spring back; 2. AHSS stamping produces higher temperatures, which again affects friction behavior; 3. The use of different tool materials in AHSS stamping has new effects on the friction behavior informing and simulation. These three phenomena should be considered in stamping simulations, which can only be achieved by using advanced stamping friction models.
Of course, AHSS has more spring back when stamping delicate parts. Springback can be heavily influenced by the friction behavior set in the sheet metal forming simulation. This is why you should improve the friction behavior in punch simulations. This, in turn, yields better rebound predictions. Friction determines the amount of restraint in the part, and based on this, spring-back behavior is affected. Also, it is important to consider that during AHSS stamping, higher contact pressures between the tool and the sheet are often observed, which is why friction becomes so important, and friction causes an increase in temperature in the material, which For mild steel, this order of magnitude does not occur. Therefore, a proper description of temperature variation and its effect on frictional behavior is critical for simulating the stamping of AHSS.
Additionally, AHSS stamping materials require the use of tool steels that are not typically used on medium strength steels. Now we have to consider the tribological effects of harder tools made of a certain carbon and chromium content, rather than tools made of cast iron. This mold material also has an effect on tribological properties. This is why the user must take this into account as well as lubricant selection during the simulation setup. A good friction model should take into account all these interrelationships when generating the friction model.
If you have an advanced friction model in your forming simulation, then you need to introduce a realistic tribology system in your sheet metal forming simulation. You’ll then get more accurate crack, wrinkle, thinning, and spring back predictions, all tied to the friction model you’re using.
In the process of deep drawing, due to the relative movement between the workpiece and the surface of the mold, adhesion will occur under the action of a certain pressure. When stainless steel is deep-drawn, this phenomenon is more serious, resulting in scratches on the surface of the product and the appearance of the mold surface. “Bonding nodules”, in order to protect the surface quality of products, control friction, wear, and remove scratches, the most effective means is lubrication. The first point of selecting lubricant is that the lubricating film does not break and lubricates throughout the sheet metal deep drawing process.
“Anti-viscosity and friction reduction” is the basic starting point for selecting lubricants. Under the premise that other conditions meet the deep drawing process, the quality of lubrication will directly affect the drawing force, die life and product quality, etc., and even become the key to the success or failure of the deep drawing process. According to the information, among various processes, the deep drawing process consumes the most amount of lubricant. During the deep drawing process, due to the relatively large deformation of the material, the lubricant is required to have excellent performance.
Let’s take a look at the different lubricants:
Here are the characteristics of several common stamping lubricants:
|Mineral oil||1. Widely recognized and used in the industry|
2. Chlorine and sulfur are very effective lubricating additives at extreme pressures
3. Usually keep the workpiece moist, and the workpiece adhesion is not serious during use
|1. Outdated technology, little progress in research and development|
2. Difficult to mix, unstable emulsion
3. Contains harmful and flammable ingredients
4. Difficult to clean and direct welding
5. Increased processing fees
|Volatile oil||1. It can evaporate from the workpiece|
2. Easy to clean
|1. Flammable and toxic|
2. Causes serious skin diseases
3. Less protection for tools
4. Not completely disappearing
5. Exceeds the limit for the amount of VOC in the air
6. Greatly increase the VOC content in the workshop
|Borated soap dry film||1. Mixed with lubricating oil or used alone during stamping|
2. Very effective lubrication products
|1. Build upon the mold|
2. Generate borax particles at the punch
3. Add extra cost when cleaning the mold
4. Foaming when cleaning
5. Will become soft and sticky in a wet environment or when encountering lubricants
6. Mistaken as heavy metal in wastewater
|Soap compound||1. Universal tool protection product|
2. Low price
3. Still effective after dilution
|1. Technology of the 1830s|
2. Does not contain EP additives
3. The pigment is attached to the tool, and the stamping part is prone to bonding
4. The surface viscosity of the workpiece is large
5. Corrosion to soft metals
6. Foaming while cleaning
During the lubrication process, different vaporization will occur as the temperature rises, taking away a lot of heat, thereby reducing the temperature of the mold; as the vaporization continues to generate, the lubricant will continue to accumulate to the high-temperature point, forming a tougher The lubricating film protects the mold more effectively, thereby extending the service life of the mold. It will also improve the quality of the workpiece surface without scratches, as shown in the figure below.