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Switching drawing lubricants takes time, effort for research, analysis

A lubricant sales rep sits down in your office. He wants to discuss a new line of lubricants developed specifically for drawing tube in the size range you happen to produce. He says that you can expect a bright finish and extended tool life. He concludes his pitch with, “It’s 20 percent less expensive than your current product.” After he leaves, you surround yourself with your draw mill team to discuss the new lubricant. An engineer states that you could save $15,000 a year by making the switch. What is your decision?

A Thorough Trial, Draw by Draw

Not so fast. Changing a lubricant in a tube drawing operation is a difficult task. Most companies use one lubricant for many reductions, so even the simplest trial would encompass the heaviest and lightest reductions with a scattering of midrange reductions. This would provide an idea of how the new product performs, but a thorough trial would also encompass the heaviest and lightest of each type of draw.

While a particular lubricant might work well on your 1.25-in.-OD by 0.110-in.-wall-thickness tube with a 32 percent total area reduction mandrel draw, it may create 360 degrees of scratching on a 45-degree included sink. A safe alternative to millwide lubricant changes would be to evaluate the potential lubricant bench by bench. This means starting with the bench that does the heaviest draws and moving to less intense draws.

Before embarking on a series of trials, it’s important to ask how much of your team’s time the research will take. In addition to pulling the team away from other duties, profitability is a concern. Every hour spent drawing with a test lubricant is one less hour dedicated to production. Also, every foot of tube used to test a lubricant is a potential foot of scrap.

A thorough set of trials can be a costly endeavor.

Beyond the Draw

A good lubricant does more than make good tube. Efficiency and productivity are part of the evaluation. The processes for applying, removing, and disposing the lubricant being considered—that is, the time and effort expended for each of these steps—must be part of the evaluation.

Application. Viscous lubricants are difficult to work with. They put a strain on pumps and pipe fittings and may require investment in a new pump. Among commonly used drawing lubricants, viscosities are so high that hand coating using brushes and spatulas is common.

Fortunately, lubricant application on ODs is relatively easy. Applying enough lubricant to the point transition forms a doughnut that distributes a consistent film along the length of the tube. ID lubrication is more difficult. On larger, straight mandrel draws, hollow back-rods supply lubricant directly to the ID. As the tube diameter decreases, manufacturing a hollow back-rod becomes increasingly difficult, and for the smallest IDs, it’s impossible. In such cases, the lubricant must be pumped into the tube before the draw. Lubricating the mandrel and back-rod before the draw is another way to lubricate the tube’s ID. Coil drawing requires a pump to fill the ID with lubricant for a long reduction.

Removal. The true cost of a lubricant change incorporates the cost of cleaning. If the lubricant is compatible with the degreasing fluid, cleaning might become a moot point. However, if the lubricant isn’t compatible with the degreasing operation, more research is necessary to see if it will affect the cleaning process, temperature, or cycle times. For example, whereas the current product may be 100 percent soluble in trichloroethylene, the one under review may have ionic insolubles. Floating particles in the degreaser may require a secondary cleaning step such as hand wiping, which increases the cycle time.

Following EPA regulations for trichloroethylene, many tubing producers have sought greener cleaning chemicals. In other words, the degreaser choice directly affects other choices made on the draw bench, especially lubrication. It is advisable to determine the most effective drawing lubricant and build your degreaser strategy around removing it.

Figure 1
This tube’s shiny surface (far half) is visually appealing, but it is the result of heavy metal-to-metal contact, indicating that the lubricant doesn’t match the application. A matte finish (near half) is a good indicator of a lubricant doing its job.

A good test is to heat a sample of a degreasing fluid to its operating temperature and add an amount of lubricant reflecting the normal dirt load of the mill. Solubility is determined by the amount of haziness, dropout, and floating particulate matter. After the lubricant is dissolved, place it on a bench until it reaches room temperature and keep an eye out for haziness. Lubricants are typically more soluble at higher temperatures. A partially insoluble lubricant can perform effectively, but it may have some disadvantages.

Surface Quality. Surface quality of a tube is the best way to determine performance of a lubricant. All tube drawing relies on boundary lubrication, a regime in which the hydrodynamic film is minimal or nonexistent. Instead, the load is carried by asperities on the surface. The ability of a lubricant to separate the tube surface from the tool surface determines the lubricant’s effectiveness.

A poor lubricant allows significant metal-to-metal interaction, which results in burnishing, scratching, or breaking. Beware, however, that in some cases a lubricant that doesn’t match the application will create a mirror finish rather than scratches.

While a mirrorlike finish looks great, it’s an indicator of poor lubrication. A shiny surface is caused by heavy tool-to-product interaction and ultimately shortens the tool’s service life, resulting in the added cost of reworking the tooling more frequently and eventually replacing the tooling sooner than you would otherwise. The mirror surface finish has a secondary effect of complicating lubrication. A smooth surface, with fewer asperities, pulls less lubricant into the reduction interface. Tubing with a smooth surface is more prone to cause tool galling and failure during subsequent drawing operations.

Bear in mind that a mirror finish doesn’t indicate a low-quality lubricant; it merely means that the lubricant isn’t compatible with that specific operation. A lubricant that performs poorly on breakdown passes might be suitable for final draws.

When evaluating lubricants, it’s important to remember that a matte surface finish is a good visual indicator that the lubricant has done its job effectively (see Figure 1).

Product Contamination. Contamination on or just below the surface is possible when using new lubricants.

Depending on the application, this may be critical. Understanding the contamination potential of a lubricant is important for all precision component manufacturing.

Some forms of spectroscopy provide useful information about surface contamination. To look for surface contamination, it’s necessary to draw, clean, and anneal a tube with the existing lubricant and the one under evaluation. Then, inspect both and compare. Many of the best extreme-pressure additives use foreign, sometimes inorganic chemistries. These lubricants regularly exceed contamination limits found in nuclear and medical specifications. If the test lubricant contains these chemistries, be sure they are removed before an annealing process and before shipping the finished product.

Disposal. If a lubricant is designated as nonhazardous, disposal generally falls under a nonhazardous waste oil designation, and it is common practice to incinerate it. Check the lubricant’s safety data sheet (SDS) for proper disposal instructions. The same goes for the cleaning solution. If it’s nonhazardous, and if the lubricant is nonhazardous, then any rags or other absorbents are considered nonhazardous and can be disposed accordingly.

However, if either the used lubricant or the cleaning solution is considered hazardous, it’s necessary to follow the relevant federal guidelines for disposing hazardous material. Keep in mind that it’s necessary to follow state and local guidelines as well.

To Change or Not to Change

The research needed to consider changing lubricants can be time-consuming and costly. In some cases, manufacturers have no choice. If a compound used in a lubricant is found to be hazardous and is phased out, lubricant manufacturers must find alternatives, and tube and pipe manufacturers must test them.

It happens all the time. Consider tetraethyl lead, once used commonly in gasoline to improve vehicle performance, which was phased out in the 1970s. Gasoline refiners had to scramble to find octane-boosting alternatives. A more recent case is that of trichloroethylene, an excellent industrial solvent. It has several uses, gentle to severe—dry cleaning for clothing to removing oils and greases in metalworking applications—but a study conducted by the Agency for Toxic Substances and Disease Registry and one conducted by the EPA found that it affected the central nervous system and associated it with several types of cancers. Some of these findings have been contested, and while the EPA is currently reassessing the cancer classification of this compound, the publicity and subsequent controversy have led many to reduce or eliminate its use.

Similarly, many stainless steel tubing manufacturers in the U.S. are considering new lubricants following the EPA’s decision to maintain its May 2016 elimination date for medium- and long- chained chlorinated paraffin. As the industry waits for the final decision from the federal agency, formulators will be hard at work developing new products for the trials to come.

About the Author

James Brooks

Product Development Chemist

4202-24 Main St.

Philadelphia, PA 19127

215-487-1100