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By introducing performance additives into the mix, suppliers of metal-stamping lubricants create formulations that, besides reducing friction and prev...   Read More

Extreme-Pressure Additives
Extreme-pressure (EP) additives, activated at certain forming pressures and temperatures, deter friction during tough metal-stamping operations. We de...   Read More

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Extreme-Pressure (EP) Additives

Lou Kren,

Extreme-pressure (EP) additives, activated at certain forming pressures and temperatures, deter friction during tough metal-stamping operations. We describe what EP additives are and how they work, and how metal stampers can determine when they may be needed. 

Extreme-pressure (EP) additives contain molecular compounds that react with metal surfaces under extreme friction conditions, producing a protective film that prevents welding and surface damage. The additives activate as pressures and temperatures rise during the metal-stamping process. When properly applied, the compounds in EP additives enhance the ability of metalworking fluids to provide extended tool life and improved surface finishes.

EP additives are often used in combinations to accommodate the temperature and pressure profiles of specific metal-stamping process.

Types of EP Additives

Sulfur EP additives--Sulfur compounds react with metal surfaces under very high temperatures and pressures, and the resulting film persists to extreme conditions. Modern sulfur additives offer light-color and low-odor properties, thus providing good aesthetic qualities. Older generation sulfur additives, with dark-color and strong odor profiles, are still applied in cost sensitive applications.

Phosphorus EP additives--Phosphorus compounds react with metal surfaces under moderate temperatures and pressures, and the resulting films provide extreme-pressure and anti-wear properties to the lubricant. Characteristics of phosphorus-based EP additives range from completely water-soluble compounds to products specifically engineered for oil-based systems.

Chlorine EP additives—Chlorine compounds react with metal surfaces under moderate temperatures and pressues, and the resulting films persist temporarily. Chlorinated hydrocarbons such as paraffin oils are the most common type applied in lubricant applications. Chlorine has a long history of effective use as an EP additive; however, concerns about the health, safety, and environmental profile of chlorinated additives is beginning to limit their acceptability in the market.

Overbased sulfonates--Overbased sulfonates display strong attraction to surfaces during metal forming and removal operations, and react to provide strong films that deliver EP and anti-weld properties to the lubricant. This group of products is particularly effective when used in combination with sulfur EP additives, creating synergistic effects.

How EP Additives Work

article15-250x175Each EP additive is activated by specific temperatures and pressures. “Lubricants containing EP additives also include other additives that work in the seconds or milliseconds before temperatures and pressures reach the point where the chlorine, sulfur or phosphorous EP additives are activated,” explains Frank Kroto, global technology manager for Lubrizol Corp.

When the EP activates with “for instance, an iron molecule in the workpiece material, the phosphorous, sulfur or chlorine will chemically react with the iron to form compounds such as iron sulfide or chloride, thus forming a chemical coating on the part material,” says Tom Oleksiak, research chemist at Lubrizol.

The EP additives, with their chemical reactions in given pressure ranges, weaken workpiece metal surfaces enough to allow the metal surface to slide, bring the lubricant along to wherever the metal flows within the die. This action keeps a lubricant coating present to prevent metal-to-metal contact and the resulting contact welds that harm parts and tooling.

The lubricant must be formulated with the proper EP additives to account for the temperature and pressure ranges of a particular stamping process, even below ranges where EP additives  take effect.

“EP-additive activation may occur in milliseconds, but the lubricant must contain lubricity additives that allow it to function effectively prior to that point,” stresses Kroto. “If not, the stamping operation will result in material tears or rips, or damage to the die even before the point where the EP chemistries are activated.”

Of course, EP additives cannot activate where they are not present, so the stamping lubricant must be formulated and applied to reach all areas where metal-to-metal contact may occur during the stamping process. At the same time, a lubricant with too much viscosity or excessive levels of esters (fatty acids) can cause problems, too.

“Part and tool failures may result if the lubricant prevents the EP additives from activating,” says Oleksiak. “If the base oil’s viscosity is too high, it will absorb heat and tend to thin less. So rather than thin as temperatures and pressures rise, the oil film just collapses and is pushed away, before the EP additives have had an opportunity to activate. This results in part and tool damage.”

EP Additives Important in Stamping Higher-Strength Steel

“When stamping higher-strength steels, a lot of heat is generated in the process, so maintenance of the lubricant film under higher temperatures and pressures is important,” explains Oleksiak. “Especially in a situation such as the stamping of higher-strength steel, extreme-pressure additives become more important.”

The key to performance of EP additives rests in their ability to activate at certain points during the stamping process. So the EP additives, to be effective, must match with the temperatures and pressures experienced in a particular stamping process.

“During a metal-stamping operation a progression of temperatures and pressures are generated, and various EP additives activate at different points in the progression,” says Kroto. “That is why a combination of EP additives may be employed—to provide friction protection across various pressure ranges. So the chemistry of a lubricant is especially important in forming higher-strength steels, with their higher forming pressures. If the chemistry is not activated, it will not perform, and the result is failure to stamp a good metal part, and possible tool failure as well.” 

What Formulation Do I Need for My Metal-Stamping Application?

That’s a simple question, but, unfortunately, there is no simple answer.

“You can’t take just a series of lubricants with a fatty acid-ester setup plus an EP additive and make a chart noting ‘good, better, best’ for every application,” says Kroto. “These things are learned in the field, at the stamping plant. That is why lubricant suppliers to metal stampers offer large product lines with thousands of products. A good number of them have specific applications, but even stampers at the same location may require different lubricant formulations due to the combination of machinery in use and the alloys being formed, as well as many other factors.”

The best approach is to examine lubricant selection when changes are in the offing or when problems occur. Lubrizol’s Kroto and Oleksiak provide some instances below where alternative lubrication options should be considered. 

When to Consider Alternative Lubrication Options

Changes in part-material metallurgy

This includes introduction of harder or softer metal alloys as part materials. The same lubricant cannot provide the same performance across various alloys

Changes in tooling

Dies with new materials and coatings are introduced

Changes in production-line speed

Stampers may note that their plant is not operating as efficiently as sister plants, or a certain stamping-press line lacks performance on par with other lines.

Production bottlenecks

A certain stamping-related operation may be slowing down performance of the entire stamping line. Even a 10 percent productivity gain in a certain operation may provide a measurable boost in overall production.

Problems with downstream impact

Not enough, too much, or the wrong type of lubricant may affect downstream operations such as welding, or may require extra time for part finishing. Perhaps washing operations are operating overtime to clean parts.

 

Discussion Question:

How do you assess performance of EP additives in your stamping operation? Does your operation designate a point person for lubricant-selection initiatives?

What do you think?

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    Hi Andrea. We are checking that and will have an answer for you soon.
    Lou

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    I´d like to know if there is one correlation between a content of sulfur in the formulation and four ball load.
    Andrea

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