*4.3. Economic Implications of CO2 Coolants*

Whilst the use of cryogenic MWFs is incentivised by a multiplicity of environmental, operational and social factors, their widespread adoption in industry will of course be subject to their financial viability. Indeed, if a company is to convert to, or supplement their existing MWF strategies with, CO2, there are a range of set-up and running costs that have to be weighed against the relative benefits of the strategy. With regard to set-up, at the moment, each machine must be retrofitted with a CO2 coolant delivery system and both volumetric and Coriolis flow meters (i.e., Fusion Coolant Systems Pure Cut ©); of course, the cost of which is, at this point, relatively significant. With that being said, it is almost always the case that the early adoption and implementation of an emerging technology is costly; however this is not likely to be restrictive in the long term for two primary reasons. Foremost, it seems likely that the initial capital outlay will reduce as machining centres begin to be built from stock with integrated CO2 coolant systems, or equivalently, as the option to incorporate a CO2 coolant system as a manufacturer add-on becomes available (in the same way that high pressure coolant is often available as an add-on). Secondly, and perhaps most importantly, it is clear that if marginal improvements can be made to the productivity of the manufacturing sector, i.e., by reducing lead time via the use of more aggressive feeds and speeds or, equally by reducing machine down time via fewer tool changes, the cost saving incurred by adopters of the technology would invariably far surpass this initial capital outlay.

In addition to the initial capital outlay associated with the purchase of the coolant system, it is also important to consider the operating costs associated with large-scale CO2 consumption and, additionally, the practicalities associated with the supply of suitable tooling. On the topic of CO2 consumption, the cost of small-scale CO2 usage is generally relatively high compared to the low cost of conventional MWFs. As such, it would likely be the case that the machine shop would be supported by a large cryogenic tanker retrofitted to the externalities of the premises. The tanker would thereafter be filled on site by a CO2 supplier. In adopting this strategy, the machine shop would be able to benefit from significant economies of scale relative to small scale purchase of CO2 pallets, wherein pallet rental, refill and delivery costs become significantly cost restrictive. With regard to the availability of tooling, in much of the cryogenic machining research, bespoke tool holders are employed to accommodate the CO2 nozzle. Whilst this is suitable for small scale research, it is unsurprisingly not cost effective. This, however, is again a consequence of the infancy of the technology and the limited demand placed upon tooling manufacturers to produce tool holders capable of CO2 cooling. It is thus likely that, as CO2 cooling strategies become more commonplace, new 'off the shelf' tooling will be developed in concert: a hypothesis supported by patents filed by both SECO [18] and Kennametal [19] to protect their prospective tool holders and the novel CO2 nozzle configurations that they will employ.

Although the initial capital outlay associated with the use of CO2 cooling strategies is non-trivial, it is worthwhile to consider the multitude of ways in which these costs can be recouped. As such, it is important to outline the ways in which CO2 MWF strategies can avoid the operational shortcomings of conventional MWFs and subsequently benefit from improved economic sustainability. With this in mind, it is noteworthy that one of the primary driving forces of cryogenic MWF research is an intent to find operationally superior MWFs, so that tool life can be prolonged. It thus follows that more aggressive feeds and speeds may be employed, and subsequently cycle time significantly reduced. In doing so, it is likely that less energy would be consumed in the machining process, and by proxy, both expenditure and carbon footprint should be reduced in kind.

Another benefit of CO2 MWF strategies is the avoidance of post-process cleaning. This has the potential to incur a significant cost saving as, in many safety critical applications, parts are unable to enter service with a cutting fluid residue. There are a range of reasons for this, primarily focused upon the altered component tribology as well as general cleanliness and ease of handling. These cleaning procedures generally add a significant amount of lead time to the (often) already sedate process of manufacturing safety critical components, and as such, have a clear downstream impact upon the supply chain, and subsequently the cost per component (and ultimate cost of the overall system). This is a factor that can be entirely subverted by the preferential use of cryogenic cooling strategies (in lieu of conventional MWFs), wherein no residue remains post machining, but rather the coolant dissolves and returns to the atmosphere. Moreover, this benefit of cryogenic machining strategies may persist even in cases of machining with CO2 + MQL, where, by design, MQL leaves little to no residue on the final machined component.

Evidently, the range of problems discussed makes for a compelling argument for the deracination of conventional MWFs (in favour of cryogenic fluids). Although the current widespread use of conventional cutting fluids is born of necessity, it is clear that such strategies are not only unsustainable, but equally, are potentially operationally sub-optimal across a range of performance indices (Section 5).
