Characteristics and User Applications
October 9, 2007
A third-generation dry-film lubricants consist primarily of blends of various polar and low- to nonpolar plymers with different molecular weights. They are designed to form extremey thin, highly cohesive bonds to the metal surface. This article explains how they differ from previous generations of dry-film lubricants and presents several application examples.
In the simplest terms, stamping lubricants are either "wet" or "dry," and that's how many in the industry still classify them. While that simple approach really isn't very helpful in understanding the choices available today, it does provide some common ground on which to begin the discussion.
Wet lubricants, most of them petroleum-based, have been around in various forms for hundreds of years. They still are quite common because they often work where more modern lubricants don't.
Dry-film lubricants started out as compounds based on stearate soaps or animal fats that were applied wet and then dried to leave a relatively thick coating on the metal. In truth, these films are not dry at all, and the first-generation dry lubricants using this technology occupy a middle ground somewhere between the traditional wet oils and the later, truly dry lubricants.
The second generation of dry-film lubricants replaced some or all of the soap or animal fat with long-chain polymers. Whether solvent- or waterborne, these were the first stamping lubricants to be truly dry when the metal entered the press.
Recently a third generation of dry-film lubricants was introduced that consists primarily of blends of various polar and low- to nonpolar polymers with different molecular weights (MW) (see Figure 1). They are designed to form extremely thin films with high cohesive energy density (CED) with strong bonds to the metal surface. These lubricants operate on a different principle than the second-generation products, having some previously unavailable characteristics.
Two concepts—CED and thermoplastic interpenetrating polymer networks (TIPN)—are critical to really understanding the differences between the various dry lubricants. Fortunately, neither is as complicated as it sounds.
CED simply measures the strength of the molecular bonds holding a material together. It's useful, if not precisely accurate, to think of CED as the amount of energy it takes to tear a molecule of the material away from all of the other molecules around it.
Metals have very high CED values, while materials like rubber and plastics have lower CED values. CED values also are useful for comparing lubricants, and in the laboratory there is a clear progression in CED from first- to second- to third-generation materials.
Actually, a more critical measurement is a combination of the MWs of the polymers used and their CEDs. Second-generation lubricants tend to be based on medium-MW polymers, which also have low to medium CED values.
Third-generation lubricants are based on more sophisticated formulations using multiple polymers with complementary structures, polarities, melting temperatures, and MWs. The complementary properties of these polymers cause them to form TIPN as they dry on the surface of the metal.
Because the various polymers are essentially interlocked in the TIPN, the resulting film has a higher CED than would be expected based on the values of the individual polymers alone. The film also exhibits many of the individual properties of the constituent polymers, such as their lubricity and adhesion.
It is possible for such a material to be simultaneously hydrophilic and hydrophobic in a completely controllable manner. The synergistic effect of the interlinked polymers is one of the keys to the enhanced performance of these materials.
The polymer chemistry of third-generation products was developed to address the following challenges:
The following examples illustrate the capabilities of these relatively new lubricants, while also pointing out areas in which improvements are still necessary.
DP 600 Automotive Cross Member. A second-generation lubricant lowered exiting part temperature by 10 degrees F compared with a previously used drawing compound. However, buildup had to be chiseled out of the tooling after 50 hits, and blanks were rusting in the humid conditions of the plant.
Substituting a third-generation lubricant reduced part temperature an additional 60 degrees F, increased the production rate 20 percent, eliminated rusting, and produced no buildup on the tooling. The employees also commented on how much cleaner the working environment was and how much less frequently they had to change wet gloves.
HSLA Frame Parts. Running these parts with drawing compound on a six-station press produced so much smoke that employees were sickened, despite the use of exhaust fans. Parts temperatures were higher than 200 degrees F as they exited the press.
Second-generation lubricants eliminated the smoke but produced an excessive buildup on the feed rollers, caused blanks to stick together, and permitted parts to rust in storage.
Substitution of a third-generation lubricant eliminated the feed roll buildup and blank sticking while preventing rust on stored parts for up to 30 days. The overall scrap rate was reduced to less than 1 percent.
Aluminum Tubes. Done with wet lubricants, this bending operation generated more than 80 percent scrap, and all of the good parts had to be washed before a subsequent welding operation.
First-generation dry lubricants exhibited excessive buildup on the mandrel, which caused many split tubes. Second-generation dry lubricants reduced the mandrel buildup and cut scrap to 30 percent, but parts still had to be washed before the welding operation.
Third-generation lubricants produced no buildup on the mandrel and did not have to be washed off before welding. The scrap rate was cut to less than 1 percent.
DP 600 Automotive Rails. The stamper coated these rail blanks with Prephos before flooding them with drawing compound but still experienced a 20 percent scrap rate from splitting and galling, along with excessive tool wear.
The situation became critical when a shipment of blanks came in that were at the extreme specification margin and could not be formed. At that point the stamper invited several lubricant suppliers to coat a sample of 25 blanks with their products.
All blanks coated with first- and second-generation dry lubricants split and hung up in the dies, producing a 100 percent failure rate. Of 25 blanks coated with a third-generation dry lubricant, 21 were formed successfully.
As a result of the test, the company coated all of the 15,000 blanks in the 450,000-pound shipment with the third-generation product, and 99.1 percent of them were formed successfully.
Galvanized Quiet Steel® Tubs. The stamper experienced wrinkling and splitting of tub corners on 70 percent of the parts produced using a wet drawing compound. The problem was so severe that the customer authorized a patch for the corners of tubs that would otherwise be scrapped.
A second-generation dry lubricant showed no improvement in wrinkling and splitting and also caused the zinc galvanizing to adhere to the tooling, introducing a new source of scrap. Substituting a third-generation lubricant reduced the scrap rate to less than 2 percent and eliminated both the need for corner patches and the zinc lift-off in the tooling.
Deep-drawn Stainless Steel. Using a heavily chlorinated drawing compound to draw 0.30-in. stainless material to a 12-in. depth resulted in a 25 percent scrap rate.
A first-generation dry lubricant reduced the scrap rate to 20 percent, but the buildup on the tooling required cleaning after 400 hits. A second-generation dry lubricant cut the scrap rate to 15 percent, but exhibited galling caused by lubricant film lift-off and caused additional problems in a subsequent washing operation.
A third-generation dry lubricant nearly eliminated scrap, exhibited no buildup, and was successfully removed with a mild alkaline wash at 140 degrees F.
Adjustments were made to tooling, binder pressure, and punch taper during all of the dry lubricant tests, but the third-generation product consistently produced parts that most closely mirrored the tooling.
Third-generation dry lubricants offer a controllable combination of chemical structure, hydrophilic/hydrophobic balance, polarity, physical bonding, and the TIPN behavior that provides opportunities for improving the performance and profitability of stamping operations while helping to reduce environmental impacts.
These lubricants are not the answer to every requirement. However, until the fourth generation is introduced, they represent the newest developments in polymer chemistry and offer a range of practical, environmentally friendly uses in the stamping industry.