August 2011 / Volume 63 / Issue 8|
By Alan Richter, Editor
Controlling foam when applying high-pressure coolant requires trade-offs to achieve effective metalcutting.
Five deadly sins are related to metalworking fluids, according to Dave Enright, industry manager for metalworking fluids, Chemetall. They are rust, residue, dermatitis, foul odors and foam. This article focuses on combating foam, particularly when applying high-pressure coolant, which is typically from 1,000 to 1,200 psi with higher pressures possible.
Those applications can have foaming problems because a high-pressure pump with rapid fluid circulation can severely agitate coolant, which requires inherently low-foaming fluids, stated Cincinnati-based Cimcool in a technical report titled “High-Pressure Delivery of Metalworking Fluids.”
Courtesy of Münzing
In addition, a sufficiently large fluid reservoir is needed to prevent pump cavitation and supply enough fluid based on machine tool horsepower, the report noted. The coolant manufacturer recommends an “ideal” fluid retention time of 10 minutes to minimize turbulence and maximize filtration. For example, a 50-gpm filter system would require a 500-gal. tank for a 10-minute retention time.Frothy Suds
The trend, however, is toward smaller coolant sumps in machine tools with high-pressure delivery systems to reduce the footprint. “You see a lot more 30-gal. sumps as opposed to 100-gal. sumps,” said Steve Badger, DeWitt, Iowa-based chemist for ITW Rocol North America, Glenview, Ill. “The smaller the sump, the more times that sump is being turned over, so the less time the foam has to dissipate in the sump.”
There are fewer machines with a large enough sump to allow the fluid sufficient time to rest before it’s pumped again, concurred Randy Templin, vice president, customer service for the Americas for coolant manufacturer Blaser Swisslube Inc., Goshen, N.Y. However, he added that users with a sufficiently sized sump do see the benefit. “We’ve had customers use some of our fluids that have a fairly high foam potential with high-pressure delivery without producing foam due to the machine design,” he said.
Although a small sump on an individual machine tends to exacerbate foaming, large central coolant systems are not immune. In those systems, the large volume of coolant returns from the machines and the resulting turbulence creates a significant amount of aeration, which must be minimized to reduce foaming, explained John S. Wiley, business development manager for metalworking fluids manufacturer QualiChem Inc., Salem, Va. “You can get some pretty big bubbles and poor heat transfer if you don’t have the right coolant,” he said.
Large bubbles break faster than small ones, according to David Gotoff, product manager, metalworking for Chemetall, New Providence, N.J. He noted that, in laboratory simulations, as long as the bubbles break within 30 to 45 seconds of forming, foam problems in production environments shouldn’t occur, but that’s not the case if foam persists for more than 2 minutes. “As long as you’re breaking the foam faster than you’re creating it, the system will operate well and be in good equilibrium,” he said. “It’s when that foam hangs on that you have problems, and you don’t have the right chemistry in place for that system.”
Another cause of foam is tramp oil that enters the sump and reacts with the metalworking fluid. When that contamination occurs, foam may not be present immediately but will eventually appear. Proactive maintenance helps control it. That includes skimming the tank surface with a rope or belt, using a shop vacuum to clean the surface or removing the tramp oil with a centrifuge or coalescing system, Gotoff noted.
Ed Jones, COO and technical director for Hangsterfer’s Laboratories Inc., Mantua, N.J., sees tramp oil contamination as more of an issue for older machines modified with high-pressure pumps than modern equipment, which tends not to leak as much. Therefore, skimming is not as critical for the newer machines, he noted.
Jones pointed out that builders typically deliver new machines with a coating of Cosmoline or some other waxy rust preventative, which can contaminate the coolant and counter its antifoaming properties if the rust protection is not properly removed. “Sometimes a customer might have to drain the coolant after a few months just to get rid of all the junk in that system,” he said.
Jones added that Hangsterfer’s offers a line of coolants formulated for high-pressure applications.Foam Problems
While coolant foam won’t invade workers’ nostrils with the scent of rotten eggs or inflame their skin, it can cause a host of other unwelcome consequences. Those include reducing a fluid’s lubricity characteristics, obstructing proper skimming of tramp oil and interfering with a shop’s ability to manage chips by preventing them from settling on the bottom of a sump and carrying the chips to where the filtered coolant is located. “Then you’re pumping chips through your system and plugging the coolant nozzles and causing downtime to clean the system,” Templin said.
He added that foam creates an insulating layer or air on top of the coolant, causing it to heat up. In addition, high-pressure pumps generate a significant amount of heat in a fluid system. A high-temperature fluid doesn’t effectively cool the tool/workpiece interface, and coolant that’s around 100° F can cause dimensional changes to the workpiece material and machine, noted Ed H. Rolfert, project engineer for Cimcool Fluid Technology’s R&D laboratory, Cincinnati. “Then you have part-quality issues.”
Courtesy of Blaser Swisslube
In general, elevated coolant temperatures also speed up any corrosion process that already may be occurring, making it more pronounced. It also hastens coolant evaporation, according to Jerry P. Byers, R&D manager for Cimcool. However, the heat doesn’t break down the coolant. “We haven’t seen fluids get hot enough that it causes them to separate,” he said.
The most evident foaming problem is when coolant bubbles out of the equipment and onto the shop floor. “Then you have a big problem with the slip hazard; you have a safety issue,” said Chemetall’s Gotoff. “That’s a worst-case scenario, where the foam is really unmanageable.”
And that scenario doesn’t depend on the sump size. “Foam doesn’t have any preference,” Templin said. “You could have a small machine with a 20-gal. sump that’s foaming terribly down the aisle of the shop or a 50,000-gal. system foaming out, causing huge issues.”
Unfortunately, most users only become aware of the aeration problem when the foam is on the shop floor. “Coolants, on the whole, don’t get much attention in the shop unless they’re causing problems,” said QualiChem’s Wiley. Most shops don’t realize the entrained air is negatively impacting tool performance and chip formation, even if there’s no foam coming out of the tank, he added.Coolant Formulations
Regardless of whether the metalworking fluid is classified as a soluble, semisynthetic or synthetic oil, a coolant manufacturer must formulate it to effectively control foaming and release the entrained air, according to Wiley.
Even so, ITW Rocol’s Badger recommends semisynthetic fluids because they have less oil than soluble oils. “Basically, more oil in a formula equals a higher potential to foam,” he said. That makes a synthetic even better at controlling foam, but a synthetic’s lubricity and corrosion protection are not as good, according to Badger.
Courtesy of Cimcool
In addition to being added tank-side to the sump as a stand-alone product to control foaming, antifoaming compounds, or “defoamers,” can be a metalworking fluid formulation ingredient. “Our goal is to provide flexibility and control to a metal- working fluid formulator by providing defoamers ready to formulate into a coolant concentrate,” said Russell Wescott, sales director for chemical additive manufacturer Münzing Inc., Bloomfield, N.J. “So before the concentrate gets diluted with water and gets put into the sump, it already has a defoamer. With this strategy the fluid manufacturers’ can reduce the foam generation up to 70 to 80 percent upfront and, in some cases, up to 100 percent.”
According to Ravi Joshi, metalworking and industrial fluids R&D manager for Münzing, the role of a defoamer is to break the foam bubble at the liquid/air interface or at the liquid/liquid interface (entrained air). “A defoamer is a complex and multiple-component chemistry comprised of actives, emulsifiers and carriers,” he said. “It is challenging to find the right balance of compatibility and incompatibility to provide the expected defoaming and stability in a coolant concentrate. The key is not only the type of defoamer being used, but its use level and dispersibility in coolant. Both factors play important performance and stability roles.”
Joshi added that the company’s Foam Ban HV-Series and Foam Ban HP-Series defoamers are comprised of 3-D siloxane and other additives, which aid in antifoam dispersion and allow the antifoam to be rinsed from metal parts to avoid staining, “fish eyes” and other surface defects.
However, coolant filtration systems can remove antifoaming particles, reducing their effectiveness over time, Wescott cautioned. “What we’ve succeeded to some degree in doing is developing defoamers that can withstand the coolant filters and can continue to provide effective defoaming over time with relatively minimal clogging of the filters,” he said.
As a general rule, a filtration system that removes particles larger than 40µm starts to have a negative impact on tool life, and one that filters particles smaller than 15µm starts to remove antifoaming compounds, according to Jones at Hangsterfer’s. But even though the latter system is designed for that level of filtration, he added that particles can start forming a filter cake and “blind” those filtration pores if not maintained. “It can become 5µm in a day or sometimes within hours, but definitely within a few days,” Jones said.
Therefore, a coolant formulated with defoamers must have the proper chemical mix to control foam even after many or most antifoam particles are removed and not rely solely on defoamers, Jones explained. “That’s why we have to build a coolant from the ground up, so even if particles get filtered out, it still doesn’t foam,” he said.
Jones noted that antifoam additives work best by floating to the coolant surface, where they can contact the foam. However, a significant amount of tramp oil on the surface prevents those additives from contacting the foam and doing their job.
Defoaming additives can also be removed with the tramp oil when cleaning that contaminant from the coolant using a skimmer or other device, according to Dr. Yixing “Philip” Zhao, senior scientist for Chemetall.
He added that defoamers tend to be unstable because they are insoluble particles in the emulsion.
“The best you can do is disperse defoamers and keep them dispersed,” said Chemetall’s Enright.
Therefore, Chemetall minimizes the amount of antifoaming compounds used in its coolant formulations, Zhao noted. “We try to balance the formula itself before adding defoamers, if needed,” he said.
That includes the correct ratio of nonionic surfactants, which help stabilize a semisynthetic fluid’s emulsion, provide effective wetting properties, act as a detergent to clean the overall operation, reduce residues and allow the machined parts to be easily cleaned. However, nonionic surfactants also tend to cause foaming. “Thus, we need to balance the formulation well to obtain the ideal performance features,” said Chemetall’s Gotoff.
Although high molecular-weight fatty acids and esters can destabilize an emulsion, Zhao noted that the company incorporates them because they provide effective coolant lubricity. That requires carefully balancing the coolant formulas to provide that lubricity while maintaining a stable emulsion with low-foam characteristics. “Thus, we often use design of experiments to analyze data, seek correlations and optimize our formulas to achieve the best performances, including low-foam properties,” he said.
Enright noted that balancing the raw materials is a challenge because adjusting one characteristic impacts another. “So you could end up with zero foam and no lubricity,” he said, using an extreme example.
To minimize foam, Cimcool’s Byers explained that the company studies various coolant ingredients and avoids high-foaming ones when formulating fluids. For example, fatty acid soaps can be used as emulsifiers for oil, and nonionic detergents can help with machine cleanliness, but they can cause a lot of foam. Therefore, Cimcool says it uses special, low-foaming ingredients in its metalworking fluids for high-pressure applications. “We look at the emulsifiers that hold the oil in the product, dispersing it into water, and try to find materials that are low foaming and still have good emulsifying properties,” he said.Waterways
Because up to 95 percent of a metalworking fluid is water, the type and quality of water the fluid is mixed with plays a significant role when managing foam. Generally, hard water tends to foam less and allows the foam to break easier than soft water, mainly because hard water includes more mineral content—primarily calcium and magnesium. Therefore, some part manufacturers add calcium acetate or other calcium salts to the metalworking fluid. “Although magnesium is part of the hardness, it doesn’t offer any antifoam properties,” said Blaser’s Templin.
“Ideally, the product has enough calcium or other antifoam compound built in so you don’t have to add anything tank-side, but quite often a calcium addition at the very beginning is enough to take care of the foam,” Templin said. He cautioned, however, that adding too much calcium can split the emulsion, separating the oil from the water.
Chemetall’s Gotoff noted that the formation of magnesium and calcium salts can alter fatty acids and esters, creating residues that can stain parts and causing the acids and esters to come out of solution and lose their emulsion-stabilizing and lubricity-enhancing characteristics. He described that scenario as a sign of hard water creating a premature sump failure, requiring a more frequent recharge. “Some companies formulate a coolant for an application and have one version for soft water and a second version for hard water because of that issue,” Gotoff said.
Courtesy of Cimcool
Conversely, hard water can be softened. “Some people use soft water, which is just a bad choice all the way around,” Byers said. That’s because the softening agents remove the calcium and magnesium, which react with detergent in the fluid and form scum, but leave the chlorides, sulfates and other ions, which cause fluid instability and corrosion issues, he explained.
More shops are finding it necessary—but beneficial—to treat their water. Technically, the best water to use for coolants contains no minerals, according to QualiChem’s Wiley. He explained that water contains minerals and other dissolved solids that negatively impact coolant chemistry, and removing these contaminants can provide cleaner machines, reduce coolant consumption and minimize odor problems.
Shops can control water quality with a deionizing system or reverse-osmosis filter. Unfortunately, using treated water increases the likelihood of foaming, unless a shop is using the correct coolant, according to Wiley. “The need to treat water varies by geography, but on the whole is becoming more common. For instance, the water is terrible in Arizona; it’s about as bad as you’re going to find anywhere in the United States,” he said. “Also, water quality sometimes changes from one month to the next, depending on rainfall.”
Wiley added that investing in a water treatment system usually has a quick payback because significantly less coolant is consumed using deionized or reverse-osmosis water.
In addition, coolant remains stable for a longer time in reverse-osmosis water than hard water because hard water ions interfere with the emulsion, according to Chemetall’s Enright.
Courtesy of Cimcool
For customers using deionized or reverse-osmosis water, ITW Rocol’s Badger recommends initially charging a machine with tap water to provide a reasonable level of natural water hardness and then adding reverse-osmosis or the less-expensive deionized water to the makeup coolant. “That way your hardness level doesn’t increase. Otherwise, as the water evaporated, if you were to add more tap water the hardness would constantly be going up,” he said.
Coolant concentration also plays a role in foam control. According to Wiley, most properly formulated and applied metal-working fluids should not foam at concentrations from 5 to 15 percent, but are more likely to foam when the concentration is greater than about 20 percent. However, he pointed out that such an atypically high concentration could indicate a different coolant should be used.
Foaming can also be an issue for neat cutting oils, but they typically don’t bubble out of the sump. “The bigger issue with neat oil is entrained air and then you’re pumping air to the tool and cavitating your pump,” Templin said. “We’re seeing more requests for low-foaming oils and oils that are able to release entrained air quickly.”
A foamy metalworking fluid can certainly wreak havoc on a machining operation, but only if it’s allowed to. “Coolant is absolutely like everything else in a machine shop,” Wiley said. “You have to control it to have a repeatable process.” CTEAbout the Author: Alan Richter is editor of CTE, having joined the publication in 2000. Contact him at (847) 714-0175 or firstname.lastname@example.org.
From terrible to terrific: a foam story
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