By Dr. Charles A. Bishop
A: I regularly get asked advice about scaling up a process where there is a belief that all the research and development (R&D) work has been done. Samples of the desired coating have been produced, the decision has been made to scale-up, and often this means scale-up directly to the final production system. Usually this will be the questioner’s first opportunity to scale-up a process and so they do not have experience to help them.
The process of scale-up can be looked at as how to reduce the risk. There are several potential risks: 1) the risk the process does not work as desired; 2) the cost of scale-up is more than predicted; 3) the delivery of the system or product is late; or all of these.
Where does the risk come from?
In my experience, there is often a technology gap between the production of samples during R&D and the proposed production process. It is typical for samples to have been produced with very different process conditions than those specified for the production system, ignoring the fact that changes in process conditions can change the nature of the coating that might, in turn, affect the properties of the coating produced. With laboratory systems it is common that if there is a process failure or the product is out of specification, it is written off as being a problem of the laboratory system. If the failure is due to poor control, this might represent a small risk when scaling up, but if the failure is due to an unidentified variable, this will potentially be a big risk when scaling up.
When producing specifications for the production system, there is often pressure to increase the winding speed and deposition rate to reduce the cost per sq meter of coating. I have seen this include a change of source type from sputtering to evaporation because this meant an increase in productivity by a couple of orders of magnitude or more. If all that is required of the coating is that it needs to be a bright, shiny metallic coating, this might be regarded as low-risk, but if there are more specific technical attributes that are required such as optical and/or electrical properties, this risk will be much higher.