CO2 Composite Spray Machining: An Introduction (Part 1)
- David Jackson, Cool Clean Technologies

Background

Carbon dioxide (CO2 gas, solid, liquid) as a machining fluid and its beneficial impacts on machining operations with regards to both lubricating and cooling qualities has been studied for the past 60 years. One of the earliest documented metal cutting processes utilizing a carbon dioxide spray is described by Thompson Products (later TRW) in the early 1950’s (1). The literature contains several good examples of the benefits that can be realized with a CO2 based machining fluid. With regards to tool wear, a reduction in CBN tool wear has been noted when CO2 gas is admitted to the atmosphere of the cutting zone, removing oxygen and reducing oxidation (2). With regards to tungsten carbide tool wear, it has been observed that liquid CO2 sprayed at base of carbide tool tip retards crater wear (3). In another investigation, it was observed that CO2 gas increases tool life by allowing a larger, protective BUE to form on HSS (4). Gases such as CO2 not only lubricate, but also cool. This point has been illustrated with cooled gases in many applications (5). For example, tool life increases when CO2 gas is cooled to -40° C to -60° C, even when cutting forces rise (6-8).

Limitations and shortcomings associated with conventional carbon dioxide machining fluid sprays of the past include, among others, a lack of fluid compositional control, limited lubrication ability, lack of temperature and penetration control, and limited machine-tool adaptability. These issues are being addressed with a new CO2 machining spray technology – CO2 Composite Spray Machining comprising gas-solid, gas-solid-liquid and gas-liquid compositions.

Conventional Coolants and Lubrication

Literally thousands of different cooling lubrication formulations are available on the market for the many different types of machining processes, equipment, cutting tools and materials. Besides machinability issues related to cooling lubricants, selection factors include machine/tool compatibility, sump stability, foaming characteristics, filterability, toxicity, biodegradability, odor, misting, surface wetting, staining, surface cleanliness and disposal issues. Cooling lubrication formulations are tested and selected based on its ability to provide the best mix of all of these characteristics, the tradeoffs being between machining and non-machining performance characteristics.

Alternatives to current practices are getting more serious consideration in response to environmental and operational cost pressures. Alternatives to flood machining include dry machining, near-dry machining (NDM) or minimum quantity lubrication (MQL) machining. The MQL approach utilizes a small amount of an oil of one type or another which is entrained as microscopic droplets in an airstream and delivered as a coherent dry (air only), near-dry, and wet machining spray. Bio-based lubricating oils derived from soybeans or other vegetable products are also being utilized successfully with MQL. Natural oils have numerous MQL advantages, including a polar chemistry which reacts more favorably with metal surfaces, unsurpassed lubricity, and an abundant U.S. agricultural growing capacity. MQL performance studies in machining processes such as milling, grinding and drilling show great promise. However issues related to tool adaptation and cooling capacity continue to be barriers to widespread adoption of MQL and other near-dry or dry machining technologies.

CO2 Composite Spray Machining

A new carbon dioxide (CO2) based cooling lubrication technology – comprising CO2 composite sprays - has been developed and is in its early market introduction period. CO2 composite spray technology employs robust spray compositional and energy control capability which have been developed to resolve many of the limitations found in more advanced cooling alternatives such as LN2 and CO2 machining sprays, while delivering beneficial physicochemical machining actions and benefits provided by conventional MQL. CO2 composite spray machining employs unique and beneficial combinations of lubrication and cooling technologies:

- Minimum amounts of carbonated coolants and lubricants
- Coanda effect for additive injection and spray trajectory control (see figure below)
- Precise machining spray temperature control
- Precise cooling lubrication composition control
- Electrostatic charging (Passive/Active) of cooling lubrication compositions for improved droplet formation and cutting zone deposition
- Pressure and flow control for enhanced penetration, flushing and lubricant deposition

CO2 composite spray technology includes three general kinds of compositions:

1st Kind (Dry): Gas (Air)-Solid(CO2)
2nd Kind (Near-Dry): Gas (Air)-Solid(CO2)-Liquid(MQL)
3rd Kind (Flood): Gas (CO2)-Liquid(Coolant/Lubricant)

This paper focuses primarily on CO2 composite spray systems of the second kind – a gas-solid-liquid composition – and fluid phenomenon associated with same. A gas-solid-liquid composition combines a source of propellant gas (i.e., compressed air), lubrication additives (i.e., soy oil), and solid and/or gaseous CO2 (i.e., coolant) in various proportions to form an infinitely adjustable cooling lubricant spray. For lubricant-based CO2 composite sprays, a Coanda-Coaxial lubricant injector and spray applicator are applied as an external spray. Spray applicators employ a passive electrostatic charging mechanism to enhance droplet uniformity, spray force and machined surface deposition. Alternatively, an electrostatic charging system may be employed to provide combination spray charging capability. An important performance aspect associated with composite CO2 machining sprays, and unlike conventional LN2 and CO2 (flood) machining sprays, is that dilute mixtures containing solid coolant particles and subcooled lubricants more easily access and interact (high frequency impacts) with cutting surfaces and interfaces. Concentrated particle or fluid streams tend to “pack” the surface during impact which retards physical actions such as outflow velocity and heat exchange.

Chemistry and Control

CO2 composite spray chemistries combine several chemical and physical cooling and lubrication ingredients and are formed and delivered on-the-fly. The sprays are infinitely adjustable and may include liquids, extreme pressure solid additives, and reactive gases which are combined with a propellant gas and injected into a metered flow of charged CO2 gas-solid aerosol. Each ingredient contributes a specific physical and/or chemical dimension, including cooling capacity, penetration power, boundary layer reactivity, lubricity, viscosity, spray particle size and density. CO2 composite sprays have variable geometry including adjustable physical spray characteristics from dry to wet composition, room temperature to near-cryogenic temperature, and spray pressures ranging from 10 psi to 150 psi, or much higher if desired.


Unlike simple atmospheric gases like nitrogen and oxygen, CO2 demonstrates very strong hydrocarbon fluid and water solubility. For example, CO2 gas exhibits greater than 600% higher solubility in oils (i.e., mineral oil) as compared to compressed air. CO2 modifies lubricant and coolant MQL or flood properties to produce mixtures having lower surface tension, lower viscosity, and increased heat capacity. Moreover, CO2 itself behaves as a reactive boundary layer lubricant, forming carboxylic acid functional groups during tribochemical reactions.
CO2 composite spray technology provides infinitely adjustable cooling-lubricant compositions of CO2 coolant, propellant gases, and minimum quantities of any type of lubrication additive(s). Adjustable spray pressure, temperature, coolant particle size and lubricant additive concentration allow a machinist to customize a cooling lubricant composition for a particular machining application. One or more individually controllable machining spray applicators may also be employed.

The next blog article will cover performance aspects of CO2 composite spray technology including heat management, spray characteristics and laboratory test results.

I may be reached via e-mail at david.jackson@coolclean.com. For more information on CO2 composite spray technology, visit http://www.2cooltool.com/.

References

1. U.S. Patent No. 2,635,399, West, April 21, 1953

2. V.N. Ponduraev, Russian Eng. J., 59 (3), 1979, pp. 42-44
Study of Cubic Boron Nitride (CBN) Tools

3. W.S. Hollis, Int. J. Mach. Tool Des. Res., 1, 1961, pp. 59-78
Study of Carbide Tool

4. N.N. Zorev and N.I. Tashlitsky, Machinability, ISI Spec., Rep. 94, Iron Steel Institute, London, 1967, pp. 31-34

5. Tribology in Metalworking, Friction, Lubrication and Wear, John Schey, American Society for Metals, 1983, pp. 624-625

6. I. Ham, K.Hitomi and G.L. Thuering, Trans. ASME, 83, 1961, pp. 142-154

7. L.Walter, Can. Mach. Metalwork., 76(8), 1965, pp. 94-97

8. F.A. Monahan et al, Am. Mach., 104 (May 16), 1960, pp. 109-124