The Role of Lubrication in Metal Forming
Lubrication mainly consists of the lubricants applied at the mill, often called mill oil. Unless specifically requested and paid for to be a forming lubricant, these are primarily intended to prevent oxidation, including aluminum oxide (white rust), ferrous oxide (orange rust), and zinc oxide (with a white powder appearance). The intended purpose of lubrication is to prevent oxidation until the part is formed.
Lubrication performs a number of functions: creating a separation between the die surface and the part, reducing friction in high-strain areas, corrosion control, and sometimes cleaning the part. The latter is becoming less common.
Optimizing Blanks with Metal Forming Lubricants
The best way to optimize a blank is to design it using the incoming mill oil. Designing a blank this way eliminates the need to worry about plug nozzles, application pressures, or fluid flow rates reducing and eliminating many process variables.
Enhancing Material Performance with Die & Sheet Metal Lubricants
One of the most common applications for lubrication is to enhance the materials ability to slide over the die surface. One of the most important metrics in measuring the ease a material can slide is the coefficient of friction (COF). A rough die and rough sheet metal can create interlocking peaks, resulting in a high resistance to force movement. Conversely, highly polished surfaces can create a hydraulic seal, holding the pieces together due to the minimal air gap between them.
The Importance of Proper Lubrication Levels
If we have an extremely polished die and specified roughness on the sheet metal, the lubrication should fill the valleys, with the fluid level reaching the top of the peaks. As the peaks wear down upon contact, they expose the lubrication, reducing the coefficient of friction. Therefore, the roughness averages provided by the mill and the specified amount of lubrication at the plant are crucial. The specified amount of lubrication is usually measured in milligrams per square foot, typically around 75 to 100 milligrams per square foot.
Avoiding Common Lubrication Pitfalls in Metal Forming
When the lubricant amount is below 75 milligrams per square foot, the metal parts may not be sufficiently lubricated, leading to exposed points above the fluid. On the other hand, if the amount exceeds 100-125 milligrams per square foot, there would be an excess of lubricant. Too much lubricant can lead to the formation of a hydraulic barrier, which may be acceptable from a coefficient of friction standpoint but may lead to excessive lubricant accumulation in low areas of the die can result in hydraulic deformation, causing bubbles and distortions in the part. Moreover, excessive lubricants not only increase costs but also creates housekeeping challenges. It may collect in flanges and contaminate surfaces, requiring an additional cleaning process and maintenance efforts. While this does not significantly impact the forming process, it adds to the overall stamping and assembly cost.
Controlling Temperature with Die & Metal Forming Lubricants
Another critical process variable lubricant helps control is temperature. Excessive heat can cause dimensional changes in the die, so it’s important to ensure the die is at its normal operating temperature before beginning production. This is especially crucial when working with high-strength materials such as DP 980 steel, where intense friction is generated to bend and pull the material.
The cracks seen in chrome parts during manufacturing are not compression fractures, but rather thermal fractures caused by excessive heat building up in the die used to form the part. To address this issue, we need to focus on protecting the die from erosion and ensuring proper lubrication to help dissipate heat. Good lubrication should enable the heat generated to stay within the part and be transferred out of the die each time a part is produced. On the other hand, poor lubrication can lead to excessive heat buildup in the die, causing the lubrication to break down unless it contains a special high-pressure additive.
According to Newton’s third law, the energy extracted from the press is translated into stretching in the part during production. As the part reaches the bottom of the stroke, the kinetic energy transforms into heat generated by the die and the part.
Selecting the Right Lubricant for Your Metal Forming Process
Different types of lubricants are available, from standard mineral oils to those with phosphorus or sulfur bases, fatty esters, or chloride-type bases. However, some lubricants are becoming less acceptable for health and environmental reasons, so it’s crucial to consider alternatives. When selecting a lubricant, factors such as the coefficient of friction and the temperature at which it functions best should be considered. For instance, a lubricant with a sulfur base may be suitable for high-temperature operations, as it maintains a consistent coefficient of friction between 1000°F and 1800°F. On the other hand, a lubricant with a phosphorus base is effective at lower temperatures, around 500°F, while standard oils perform well up to 120°F but start to lose viscosity and increase friction as they heat up.
The choice of lubrication is critical and should be based on the temperature range produced in the die. Extreme pressure and extreme temperature lubricants are available, but they come at an additional cost. High-pressure lubricants are designed to flow to the heated areas and provide better lubrication where it’s most needed. However, if the die design is optimized with proper radii, wall angles, and finish, it may eliminate the need for these expensive high-pressure lubricants.
Optimize Lubricants for Metal Forming Success with Help from Phoenix
At Phoenix, we understand the critical role that proper lubrication plays in your metal forming processes. Our team of experts is equipped with the knowledge and resources to help you select and optimize the right metal forming lubricants and die lubricants tailored to your specific needs.
Ready to optimize your lubrication strategy? Reach out to us today to discuss how we can support your metal forming operations and ensure your processes run smoothly and efficiently. Let’s work together to achieve the best possible results for your business.
If you’re interested in learning how The Phoenix Group can drive improvement at all levels of your organization, contact us today.
