Machine Design
CO₂: Clean for Cooling and Cool for Cleaning

CO₂: Clean for Cooling and Cool for Cleaning

Clean for Cooling

Liquid coolants have many benefits in machining, but they fall short when a work piece must remain dry. Machinists have tried to get around this problem by using cold air guns, but this dry method doesn’t lower temperatures quickly enough. Enter CO₂ as a viable alternative coolant.

CO₂ cooling delivers solid crystals of dry ice into the heat zone, where the tool and work piece meet, at a temperature of -110°F. The dry ice penetrates the vapor-heat barrier to improve heat transfer. This cooling effect keeps the cutting edge sharper and extends the tool’s life compared to conventional dry or minimum quantity lubrication processes. CO₂ coolant can be delivered in two ways: For drilling, it is delivered through ports in the tool. For milling and other machining processes, an external delivery is utilized.

Fig. 1
Solid particles of CO₂, or dry ice, exit through the ports in the spindle used in a drilling operation to keep the tool cool and extend its overall life. CO₂ does not require special tooling and is compatible with any standard drill bit.

In the aerospace and automotive industries, manufacturers who work with composites opt away from using conventional liquid fluid coolant. Composite have a porous surface which can trap coolant and requires an extensive cleaning procedure. Drilling composites dry eliminates the necessary cleaning, which is why using dry CO₂ cooling is acceptable—e.g., it cools without leaving behind any trace residue.

This process works well with stack ups. Using the example of a composite titanium stack up, a drill penetrates easily through the composite without creating much heat. Once it hits a titanium layer, however, the drill meets resistance and generates a significant amount of heat. The concern for manufacturers is that this heat buildup greatly diminishes tool life. In addition, metal chips exiting can damage the composite layer, resulting in poor hole tolerances and delamination.

Fig. 2
A side view of the stack up shows a composite on top of titanium. As the drill goes deeper in the stack up, an increase in heat is shown in red. Without proper cooling, the heated tool and chips can cause the composite’s properties to become jeopardized.

CO₂ technology addresses both of these issues. First, the CO₂ moves through the drill, maintaining an ambient temperature. As the drill continues to move through the layers, it cools the hole—making it harder, and significantly reducing the damage done by the metal chips. Plus, the process remains completely dry while allowing tighter hole tolerances and increases in tool life.

Cool for Cleaning

CO₂ is also finding use as a cleaning agent. In the automotive industry, several major car makers have found that CO₂ cleaning saves them money and time while lowering their negative impact on the environment.

CO₂ is used for pretreatment of plastics; it can clean parts such as automotive trim, door handles, dashboards, and headlights, prior to a painting or coating. CO₂ spray effectively removes all contaminants that may affect the quality of the paint, such as light oils, water marks, dusts, and fingerprints.

Fig. 3
CO₂ spray cleaning systems are integrated directly into the manufacturing line. The parts heading for the entrance to the painting booth are cleaned with the help of two robots that work simultaneously to remove all the foreign objects on the surface.

CO₂ spray is delivered at pressures high enough to clean parts effectively, but low enough not to damage parts’ surfaces. Unlike traditional cleaning processes, CO₂ evaporates completely, leaving no residue to necessitate a second cleaning. It also cleans without condensation, so there is no need for cleaning zones in most plants. After cleaning with dry CO₂, parts can move directly into painting/coating, saving energy and time associated with expensive drying processes.

Aqueous cleaning has been the norm for industries that relied on traditional methods for cleaning and sterilization. Although cleaning with water has been very successful in many applications, the process is showing drawbacks as newer materials, like porous implants, are adopted. Water-based cleaning uses water and ultrasonics to clean. Water can become trapped in the substrate after cleaning, leaving water spots. When that happens, an additional drying procedure is required. 

Fig. 4
Similar to pretreatment plastics cleaning, CO₂ cleans optic lenses of water spots and light particulate without damaging the surface. Other industries include, but not limited to electronics, semiconductor, medical, and aerospace.

Liquid CO₂ (LCO₂) provides a solution where a part is completely immersed and concealed in a highly pressurized vessel at around 600-900 psi. The low viscosity and surface tension of LCO₂ lets it flow easily through porous parts, rinsing away contamination without leaving behind residue.

After pressure is released from the vessel, the CO₂ evaporates and a clean, dry, porous part remains—all in about 20 to 45 minutes. As an added environmental benefit, the CO₂ is recaptured after use, reducing the amount of energy used to recover it.

LCO₂ can also be used to clean materials by extracting unwanted compounds (such as unreacted silicone oils) with minimal damage or denaturing. The biggest advantage of LCO₂ is its low levels of toxicity. LCO₂ cleaning works well on medical implants, catheters, silicone tubing, and silicone disks to outgas. Compared to expensive vacuum bake out systems, LCO₂ takes significantly less time (1 to 6 hours, compared to 24 to 72 hours).

In summation, CO₂ is changing the manufacturing landscape in more ways than one. It’s time to start taking notice.

Jon Wikstrom is CEO of Cool Clean Technologies, Eagan, Minn.


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