Fact or Friction?

The tube and pipe bending process is full of variables and options, all of which you must understand and manage successfully to make the end product.

Any of these variables can make a significant contribution to the success or failure of the manufacturing process. Certain variables are beyond your control, such as the material selected, geometry of the tube, and the equipment currently on hand to do the job. One variable you can control, though, is the bending lubricant. A huge advantage of having this control is that the proper lubricant can make significant positive contributions to the entire process—bending and all subsequent manufacturing steps. After you bend the tube, you're probably going to clean it, weld it, or assemble it in some fashion. The bending lubricant can be beneficial or detrimental to these operations. Making the best lubricant choice can make the troubles of the past a distant memory.

Processes and Materials

Tube and pipe bending lubricants, chosen and used correctly, can be a productivity enhancement, not just an additional cost. What if the scrap rate could be reduced by 20, 40, or even 75 percent? What would be the impact on this job's profitability? Material alone can be 50 percent to 75 percent of the end part cost. If the part is stainless steel or another exotic material, the costs could go even higher.

If you have been bending tube or pipe for many years, you probably remember a time when honey oil was the main bending lubricant. You might get a successful bend, but the time and cost associated with cleaning the parts were significant. Additionally, what if the part required welding after bending? Bad welds, blowouts, and numerous scrap parts were common. Today's bending lubricants can make life easier and save you money.

Process Considerations. Several factors come into play when you're selecting tube and pipe bending lubricants. Probably the most important are health and safety, so review and understand the chemical content of the lubricant and any restrictions that may be present.

Second, determine if the lubricant will have adverse effects on the material, tooling, or equipment. Third, evaluate the compatibility between the lubricant and all subsequent operations. Fourth, record tooling life and set parameters for producing quality parts. Fifth, don't forget to determine how waste lubricant needs to be disposed.

Last, complete a cost analysis to determine the overall effectiveness of the lubricant. Do not stop with comparing purchase prices of lubricants; review the entire operation and the impact of each one.

Material Considerations. Tube bending requires a static film for a dynamic motion. To put it simply, bending needs an antiwipe film that does not break, yet allows the material to flow. It's important to compare the characteristics of the metal with those of the lubricant to get a good match.

Zinc-plated materials require additional boundary film to prevent metal-to-metal contact. Certain additives form a boundary film, which is a highly resistant, tenacious layer on the moving surfaces. Depending on the additive, this layer can support the load and prevent major wear or breakdown. Other plated materials, such as aluminized steel, require additional lubrication, along with a boundary film and extreme-pressure additives.

Many plated materials generate particles. These particles must be either flushed out or encapsulated by the lubricant. Pastes and gels do a fine job of capturing these particles, thereby minimizing metal-to-metal contact.

Because stainless steels have higher tensile strengths and yield strengths than carbon steels, bending stainless requires more energy, which generates more heat. All too often near the end of a tough bend, the heat builds up and causes the lubricant to run away from the bend, or slump. Therefore, it is necessary to use lubricants with extreme-pressure additives, which reduce the amount of heat generated.

Selection Criteria

Ease of Use and Process Compatibility	Bending Severity
1. Disappearing compounds	1. Pigmented pastes
2. Vegetable oils, seed oils	2. Compounded oils
3. Chemical solutions	3. Pastes and gels
4. Pastes and gels	4. Soluble oils
5. Soluble oils	5. Chemical solutions
6. Compounded oils	6. Vegetable oils, seed oils
7. Pigmented pastes	7. Disappearing compounds

Figure 1
Selecting the best lubricant for a particular job is a matter of selecting the most important process criteria. Seven broad lubricant families are listed in order of user friendliness and process compatibility in the left column. They are listed in order of their ability to endure severe bending in the right column. For the most part, the two lists are opposites.

Today's tube and pipe lubricants come in a variety of chemistries and forms. The common question is, Which lubricant is the best? The answer depends on your criteria.

Figure 1 shows two different criteria. Lubricants that are the easiest to use and are the most compatible with subsequent manufacturing processes are listed in the left column. Those that can stand up to the toughest bending jobs are listed in the right column. With just one exception, the two lists are opposites.

Defining the criteria and the goals is necessary before selecting the proper lubricant for the job.

Disappearing Compounds. Disappearing compounds, also known as evaporatives or vanishing oils, minimize residue. Parts are clean and dry after bending. These lubricants are suited for light-duty forming applications, copper alloys, and coated stock requiring minimal lubrication. Be aware that these lubricants tend to be high in volatile organic compounds (VOCs), so their flammability is a concern.

Vegetable and Seed Oils. Vegetable oils and seed oils are good for light- to medium-duty work. Many bending applications are successful with a minimal amount of this type of lube. They are environmentally safe to use and have good weld-through characteristics. Many times this family is used in specialty application equipment such as microapplicators and through-the-mandrel applicators.

Chemical Solutions. The water content of chemical solutions cools the tooling and tubing during the bending process. They are oil-free and can be used as supplied or cut with water. They offer excellent flushing characteristics. In many instances, tubes bent with chemical solutions can be welded or brazed without cleaning, saving a process step and reducing costs. If the parts need to be cleaned after they are bent, you can get by with lower temperatures and fewer chemicals than you would need with some other lubricants, equating to cost savings.

The downside concerns bending aluminum. If the lube dries out it will fail, which could result in scrapped parts and broken tooling.

Gels and Pastes. Pastes can be made with or without oil. They can be packed full of additives to offer excellent boundary film protection for extreme-pressure bending. Their consistency offers good encapsulation of particulate and no-slump characteristics. Cleanability is enhanced with additives, and many offer good weld-through capability without cleaning.

For the most severe jobs, pastes can be pigmented for a tough physical barrier. The downside is the expense associated with cleaning the part. In some cases, this cost prohibits using a gel or a paste. That said, some pigmented pastes can be formulated with easy-to-wash characteristics.

Gels have proven to be the next generation of bending lubricants. Gels are solid yet jelly-like materials. By weight, a gel is mostly liquid, but behaves like a solid. As long as you do not store gels for a long time or allow them to dry out, they offer excellent coverage, don't slump, and are easy to remove.

Soluble Oils. Soluble oils are versatile lubricants; they can be used straight, diluted with water to form an emulsion, or mixed with a small amount of water to form an invert emulsion or paste. The basic chemistry of soluble oils enables maximum levels of boundary film additives and extreme-pressure additives. The emulsifying ability makes removal easy. However, when in liquid form, the application of a soluble oil is difficult to control.

This type of lubricant also has the potential to make a mess. Soluble oils are not forgiving; if they run off the tooling or the workpiece, disaster will strike. Weld-through is possible, but it depends on the dilution used.

Compounded Oils. Compounded oils perform varying degrees of work. They are suitable for light- to heavy-duty bending, depending on the compounding, the quantity of extreme-pressure additives, and viscosity. The viscosity and tackiness of the oil determine whether they stay where you put them. The drawback is that if they stay put while you are bending with them, they tend to stay put later when you try to remove them.

Because many formulations exist, selecting the right one can be a difficult task. It's a matter of asking all the necessary questions. Does the oil drip under heat? What sort of fumes or smoke does it create during welding? Does it clean up easily? Does it have zero-discharge characteristics during cleaning? Zero-discharge oils separate quickly.

Their demulsification properties make them ideal for aqueous wash systems because the lubricant floats for easy removal, keeping the wash and rinse tanks clean.

Making the Choice

From the six chemistry families you have quite a few to choose from. Making the best choice requires a review of your entire process—not just bending, but subsequent operations as well, such as welding and cleaning. Don't settle for the same old choice. Evaluate different chemistries and products to find the best lubricant for your process and parts. Purchasing the cheapest bending lubricant rather than the right one for your application could cost you two to three times the savings when you factor in additional labor, machine time, extra material to replace scrapped pieces, weld flaws, and cleaning. Don't focus only on the lubricant's cost; instead, determine the value you get in improved productivity.