Some years ago it was suggested to me that a matte black cooling drum would be better than a highly reflective drum. The thought being that heat passing through the transparent film would be absorbed by the black surface and not be reflected back through the polymer film as it would with a mirror surface. This does lead to a number of other questions that I have never yet bothered to find out the answers to. How much of the heat passing through the polymer film before it gets coated does get reflected and has a second chance of being absorbed by the polymer film and what difference does this make to the film temperature. The discussions at the time suggested that this was a very minor part of the film heating process and that it was the condensation heat load and the radiant heat load once the film was coated that were the more significant cause for concern when considering managing the heat load.
What is known is that the rate limiting step of most systems is the heat transfer coefficient and so anything that improves the adsorption of heat by the drum has to be beneficial even if only marginally.
So having never chased an answer to that question I now have a new question that I may never find the answer to: "What would be the effect of using a carbon fiber hairy drum surface?"
There was some interesting work done at the Fraunhofer Institute on the use of a large area metal brush as the cooling surface. The brush being metal was quite stiff and with little deformation. This same work showed that using gas cooling was the more beneficial.
In some earlier modelling work done by Dr. Mike McCann it was shown that the gas leaks away from the edges and so a continuous feed of gas is essential for slow winding webs or narrow webs even at higher speeds. I then got to wondering if the brush was made of a more deformable material and also had a gas input there might be an additional benefit. It can be quite complex and expensive to build a deposition drum that has both a cooling liquid passing through as well as a gas feed that leaks gas through the drum surface to form a gas layer between the drum and web. It is easier to inject the gas from a gas bar as the web contacts the drum but the loss og gas at the edges means that the best cooling is close to the gas injection point and the worst cooling is as the web leaves the drum. So my thoughts were along the lines that if a densely packed carbon fibre brush has gas injected into it the more tortuous path could reduce the gas loss from the edges whilst the gas would still improve the heat transfer coefficient. In addition the carbon brushes would be more heat adsorbing and also would enable a higher direct contact area to the substrate and so could result in a further improvement in the heat transfer coefficient.
Even if this were to work there is then the question of how practical this would be. The aim of any coating system is to never coat the deposition drum but if the unthinkable happens it is relatively easy to clean the deposition drum. This could become a much more difficult problem if the surface was a hairy carbon brush surface. It could be much more damaging meaning that although it might be desirable to use such a drum for increasing the speed of production the potential downtime due to the coating of the fibers could make the risk too high to justify.
So that was the thought all it now needs is someone with enough time and a method of producing a carbon fibre hairy drum surface to try this out and prove me either right or a complete idiot. My suspicion is that absolutely nothing will happen and I will be left wondering. Still this just means that everyone may only wonder if I am an idiot as I will not have been proven to be an idiot.