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Through-tool CO2 cooling.

Study shows multiple benefits of CO2 cooling in composites drilling

The use of composite materials in aerospace and automotive applications has grown at a fast rate (and will continue to do so) due to the high strength and low weight of the materials.

The benefit of using composite materials for applications demanding strength, flexibility, elimination of corrosion, at minimal weight is well documented. Using composites rather than aluminum in aircraft structures saves weight on the order of 20%. This weight reduction is combined with the ability to achieve aerodynamically more advantageous shapes through precision molding, resulting in better payload fractions and reduced fuel requirements.

Use of the material continues to grow despite its significantly higher raw material costs and relatively involved processing—frequently requiring expensive autoclaves or other pressure- and heat-producing devices to compact and cure the materials.

Another challenge of composites is their response to being drilled. This is especially true when the material is used in combination with a metal in a stack-up—e.g., composite and titanium. Drilling of the titanium layer(s) creates much heat, which can thermally damage the resin matrix of the carbon fiber reinforced plastic (CFRP) layer(s).

Heat is a key issue in nearly every machining operation, causing wear, reduced tool life, reduced processing speed, and limited throughput in addition to the issue of resin-matrix damage. Traditional liquid coolants, particularly petroleum-based ones, often do not provide enough cooling effect. Also, it is not feasible to clean liquid coolants or oils from large CRFP parts such as fuselage panels.

By using CO2 through-tool cooling, which is a dry process, it is possible to protect both layers, researchers at Cool Clean Technologies have found through work supported by the U.S. National Science Foundation and U.S. Department of Energy. The work shows that CO2 through-tool can significantly increase productivity while maintaining required hole tolerances in both the composite and Ti layers. In addition, improvements in tool life have been demonstrated with CO2 through-tool cooling compared to either emulsion or dry drilling.

In CRFP-Ti stack-ups, the carbon fibers are very abrasive to drill, and significant heat is created in the drilling process. This damages the resin binders of the composite material. Drilling of the Ti layer produces an even more damaging chip, which can erode the corners and inner diameter of the drilled hole and therefore create a sloppy fit between the panel and fastener.

In aircraft, CFRP-Ti stack-ups are most commonly used for the fuselage. Thousands of holes, which are typically 0.64 cm diameter, need to be drilled through the stack-up in order to fasten the panels to the aircraft frame.

The cooling system used in the Cool Clean Technologies’ research includes a cooling fluid consisting of CO2 ice crystals, CO2 gas, and in some cases air. It is sprayed directly to the cut zone where cooling is required. The solid CO2 crystals, which are formed from the expansion of liquid CO2, sublime, and flash to CO² gas. This conversion is capable of absorbing significant amounts of heat: approximately 127-207 kJ/kg.

The system provides variable spray pressures, fluid lubricities, and compositions using a unique liquid-solid-gas spray. The solid crystals of CO2 dry ice are delivered at a temperature of -79°C into the heat zone of the tool/work piece interface. This dry ice in solid form penetrates the vapor heat barrier to optimize heat transfer and thus the cooling effect. No other coolant technology is delivered in solid form, and hence no competing technology has the equivalent mass and momentum to impinge the heat barrier. The end results are:

• Solid dry ice (CO2) crystals blasted into the heat zone to deliver significant cooling.

• Solid crystals have mass that can be forced through vapor barriers and heat zones.

• Cutting tool stays cool from the solid CO2, keeping the cutting edge sharper to extend tool life significantly greater than conventional processes.

As part of their research, Cool Clean Technologies ran tests on CFRP-Ti stack-up drilling. The company found that CO2 through-spindle coolant provides significant benefits over conventional flood coolant:

• Drill temperature reduction by as much as 27°C cooler.

• Hole size variability reduction: 0.025 mm vs 0.152 mm.

• Smoother surface finish: 22% smoother inside hole.

• Tool Life improvement of 10% or more while drilling the composite layer, and 2X or 3X while drilling the titanium layer. This relatively small increase in tool life can be significant, considering that PCD drills typically used to drill CFRP cost around $750 each.

• 100% elimination of post-machining parts washing processes.

This article is based on SAE International technical paper 2014-01-2234 by Nelson W. Sorbo and Jason J. Dionne, both of Cool Clean Technologies.

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