December 3, 2009
Synthetic lubricants now can take on a greater variety of stamping applications.
When it comes to the function of metal forming lubricants, not much has changed over the years. They reduce friction between the metal and the forming dies; reduce tooling wear; help produce smooth finishes by preventing metal pickup, welding, or scoring; cool the metal to minimize any effect on the metal's gauge or metallurgical properties; and help maintain the final part's desired shape.
What has changed, however, are the capabilities of the various lubricant chemistries now on the market, particularly with synthetics. Always water-based and by definition free of petroleum, such lubricants are clean-running and easy to wash away after forming, reducing overall cycle times. For years these lubricants have found limited success in severe forming applications. But this isn't necessarily the case anymore.
Traditionally, the heaviest-duty forming applications—deep draws with tight radii, stainless steels sensitive to work hardening if cooled quickly, and so on—have required straight oils almost exclusively. Boundary and extreme-pressure additives allow the oil to work well in harsh, high-temperature environments. Over the years, however, proprietary synthetic lubricant chemistries have been tweaked and adapted to take on more challenging forming applications.
Today industry's demand for such synthetic forming fluid has spurred development of new chemistries that now are being used in more extreme conditions, environments thought impossible several years ago. In many circumstances, synthetic lubricants have been used in drawing of stainless steel, as well as soft metal such as aluminum. Hydroforming setups have successfully used synthetic lubricant to form massive (though somewhat shallow) draws of 1-in.-thick material.
Not every circumstance lends itself to synthetic lubricants, of course. Extremely deep-draw or thick-gauge applications often still work best with straight oils, for instance. Nevertheless, more manufacturers have shifted away from oil and toward synthetics over the past five years for several reasons. Synthetic base stocks and esters, additives that creates that desired boundary or barrier between the base metal and die, are biodegradable. Also, the wild fluctuations in oil prices have made manufacturers try to cut oil use where they can, and this includes their stamping lubricants. Most significant, synthetic lubricants are more cost-effective than they once were for more applications.
Various factors are driving these trends.
Additives have been key to the success of synthetic lubricants. Years ago synthetics had trouble with soft metals like aluminum and galvanized steel. The lubricants didn't have any form of corrosion control, so the water-soluble lubricants would ultimately cause white rust to form on workpieces. Antirust agents, as well as new acidity formulations, have eliminated these problems.
Suppliers have developed viable and sound additives for use with synthetic forming lubricants, including both boundary and extreme-pressure additives for use on both ferrous and nonferrous metals. And as the number of nonferrous applications increases industrywide, the demand for synthetics has increased as well.
Synthetic esters are used in solid solutions and emulsions for hydrodynamic and boundary lubrications. Their high molecular weight allows these esters to be used in high-temperature as well as high-stress applications. Chemistries also have been genetically modified to alter their fatty acid distribution for better high-temperature performance.
Extreme-pressure additives also are changing. For years chlorine has been an economical source for extreme-pressure lubrication in all types of forming lubricants. But the chemical has come under increasing scrutiny for environmental reasons, and many companies have restricted or banned its use. The additive also can contribute to corrosion problems.
Phosphorus, on the other hand, is enjoying a resurgence as an additive element. Such lubricants exhibit good solubility characteristics and hydrolytic stability. They have improved wetting characteristics and inhibit corrosion. Polymeric esters with phosphorus offer a balanced blend of hydrodynamic, boundary, and extreme-pressure lubricant properties. In other words, they can work in very tough conditions.
Excess smoke rising from a welding workcell should throw up a red flag. It's not the usual welding fumes. It's the oil-based lubricant left over from the previous stamping operation heating up, and soon it could burst into flames.
There are several solutions, including better cleaning practices and weld prep. But as in any safety situation, it's always best to engineer-out the hazard—that is, the flammable, petroleum-based lubricant. In this situation, a synthetic, nonflammable lubricant could eliminate the hazard.
The dilution factor has changed the cost equation when it comes to synthetics. In 2008 the world saw tremendous spikes in oil prices, and those spikes may happen again. But for the most part, petroleum-based lubricants have a lower price tag than their synthetic counterparts. Calculating the overall cost is a different matter, however, and most significant is how diluted a lubricant can run on the press. Straight oil usually can't be diluted, but synthetics can. The more diluted the synthetic oil, the less expensive it generally is.
Consider, hypothetically, that the price of straight oil lubricant is $10 a gallon; synthetic lubricant is $16 a gallon. A 13-in.-deep cup, a deep draw by any account, may be able to use a synthetic lubricant diluted with 30 percent water, so the press is processing more parts with less lubricant.
Determining the level of dilution possible depends on various factors, and much of it boils down to the severity of the forming operation. Part configuration is a critical factor. Lubricating a draw with a 90-degree radius angle requires a less diluted synthetic lubricant than a draw with a 45-degree radius angle. Deeper parts also require less dilution. Application methods and locations factor in, whether the lubricant is rolled on or sprayed, and whether the lubricant is applied to both sides of the metal. Spraying both sides generally doesn't require high dilution levels, but it does use more lubricant.
Trial and error often produces the best results, and this can include running the part to failure. This may not always be possible, depending on the tooling, application, and organization. But if it is possible, running the part until it fails can reveal the optimal dilution level and often results in using less lubricant. This can lead to significant savings over the life of the press.
The state of synthetic lubrication technology is miles ahead of where it was 10 years ago. Synthetics aren't the solution for everything, of course. Draws of extremely high-strength material may work best with some of the emerging specialized lubricants, including those with extreme-temperature properties. Some draws may still work best with dry film, and still others may work with light-duty vanishing oils, which usually don't require cleaning. But for a growing number of applications, synthetic lubricants make sense from an environmental, safety, and business perspective.
When it comes to lubricants, thousands of chemistries are available for stampers, but all of them boil down to several general types, and many are available with different additives used to reduce friction, dissipate heat, and prevent galling.
Straight Oils. Sometimes called neat oils, these products have a petroleum oil base and are compounded with boundary and extreme-pressure additives (see below). Typically undiluted, they usually are easy to apply and offer good corrosion protection, but they can be difficult to clean.
Water-soluble Oils. Sometimes called emulsified oils, these products are similarly formulated as straight oils but have emulsifiers that form emulsions. This helps provide greater benefits when it comes to additives, along with more efficient cooling. Drawbacks include corrosion and product stability issues. Water-soluble oils generally are easier to clean than straight oils in alkaline wash systems.
Synthetic Lubricants. Water-based and free of petroleum, these products dissipate heat effectively, are clean-running, and can run very well under severe conditions.
Semisynthetic Lubricants. These are blends of water, petroleum oils, and emulsifiers. When mixed with water, they form opaque emulsions. These can be used on both ferrous and nonferrous metal.
Dry-film Lubricants. These are normally composed of soaps or polymers in an aqueous matrix and then dried before being used in metal forming. Their tight films often offer excellent benefits in performance, handling, equipment cleanliness, and cleanability. They do, however, require special application equipment to ensure uniform films and complete drying, and they may not be applicable to all metal shapes.
Chemically Bonded Agents. These include metal-plated coatings that offer outstanding lubrication as the metal deforms and stretches, but they are expensive and require a specialized coating process.
Hydrodynamic. This lubricating additive allows the fluid to act as a film separator under moderate pressures.
Boundary. Polar additives, such as fats or solids like graphite, provide a cushion or separation under heavier pressures.
Extreme Pressure. This additive allows lubricants to form a chemical film that bonds to the metallic substrate and tooling. As the name implies, it works under very heavy pressure and high temperature.
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