Luminance, chromaticity and tight specs demand unique technology.
Coating and plating are often underestimated by the design engineering community; the assumption is that painting is a very simple process and not worth worrying about in the design phase.
However, the process is not as simple as you might think. Consider illuminated components like cockpit panel displays.
Instrument and control panels need to be illuminated if they’re going to be used in a dark environment. Backlit speedometers or gearshift indicators in the auto industry might be the first examples that spring to mind, but the aerospace industry also makes extensive use of illuminated instrument and control panels.
The main difference between the two is volume: millions of panels in the case of automobiles, tens or hundreds in the case of aircraft. In either case, an assembly’s design will have a significant impact on the available options for illuminating the panel.
It’s easy to overlook the additional design constraints introduced by requiring the component to be illuminated, but doing so costs time and money if the design turns out to be unbuildable.
According to J.J. McKenzie, president of Prime Products Inc., “People developing drawings often don’t understand our manufacturing process, so they ask for things that aren’t possible. For example, they might call for a white paint mil thickness that doesn’t properly control the light transmission, or a black paint mil thickness that isn’t completely opaque, or that is too thick for removal via laser etching.”
In order to avoid mistakes like these, it helps to understand what goes into the process of manufacturing illuminated components.
Manufacturing Illuminated Components
The two most important metrics for illuminated components are luminance (brightness) and chromaticity (color). The former will be affected by the thickness of paint on the illuminated surface, while the latter is adjusted with filters or tints.
“We develop in-house formulas to tint our white paint to adjust the chromaticity of the illumination,” said McKenzie. “Then we put a very light coat of un-tinted white on top so you don’t see the color in daylight.”
In addition to these two layers, Prime Products, Inc. almost always adds a clear matte polyurethane topcoat — the exception being when all the painting is done on the backside of an overlay. “Anytime the pilot can touch the paint job, there’s a clear topcoat for protection and solvent resistance,” said McKenzie.
The most interesting aspect of this particular manufacturing process is the laser etching, which is used to remove the topcoat from an illuminated component.
The manufacturer starts with a clear resin part, which is then painted white at the thickness necessary to achieve target luminance and chromaticity. After that, an opaque topcoat is applied over the entire part. Finally, a laser is used to etch away the opaque topcoat, exposing the white graphics underneath.
Prime Products, Inc. normally uses epoxy or multicomponent catalysed paint for laser-etching in order to get good grip on the surface. Roughing the surface through liquid-honing with white aluminum oxide also improves paint adhesion.
Although the focal point of the laser is relatively small, it can still tolerate curved surfaces to a certain degree. However, multiple depths in a surface require multiple setups, which impact cost and production time.
Designing Illuminated Components
Based on the above, we can infer some advice for design engineers working on illuminated components.
For one, it’s better to specify light transmission, or luminance and chromaticity, rather than a specific paint and paint thickness. This gives the contract manufacturer more leeway to choose the best paint and application method for the job.
Another important piece of advice concerns burred or sharp edges. Allowing the manufacturer to break sharp edges on plastic parts is key to avoiding paint chipping. Liquid honing helps to some degree, but it’s still worth ensuring that parts have smooth edges before sending them for painting.
When it comes to selecting an aerospace contract manufacturer, be sure to confirm that the company has the proper accreditations in place.
Prime Products, Inc. is both AS9100 and Nadcap (the National Aerospace and Defense Contractors Accreditation Program) certified, but not all contract manufacturers are. Nadcap is an additional level of ultra-tight process control for special processes over and above AS9100.
“Our customer’s paint application specifications are very detailed, covering what paint to use, how it should be stored, how the surface should be prepared, how the paint should be applied and what testing must be done to approve the finished product,” McKenzie commented. “Our Nadcap accreditation gives them confidence that we are in full compliance with their specifications.”
Open communication with contract manufacturers is the key to avoiding lengthy revision periods in the design process.
“We itemize technical exceptions on our quote,” said McKenzie. “Customers may come back and ask questions, or they might realize that they need to revise their drawing.”
Of course, some designs just aren’t feasible. McKenzie recalls a customer who wanted an aluminum part that could also be illuminated.
“We were able to machine the aluminum frame or panel and then pocket-out areas for illuminated graphics. Then we glued in a clear acrylic insert and painted and sanded it so it would look like a single integrated piece,” he said.
For the best and timeliest results, it’s important to start communicating with contract manufacturers early – ideally before the design engineers have started developing their drawings.
When asked what advice he would give to a design engineer approaching a contract manufacturer like Prime Products, Inc., McKenzie said, “I would encourage them to understand what we do before releasing their drawings. How we get from A to B in the toning process. An understanding of that process would help them make decisions in their design.”
Prime Products, Inc. has been in the aerospace industry for over 40 years, servicing companies that supply major aerospace players like Boeing, Lockheed-Martin and General Dynamics.
For more information, check out their illuminated components design guide.
Prime Products, Inc. has sponsored this post. It has no editorial input into this post. All opinions are mine. –Ian Wright
