Process Planning II – Exit Signs

Manufacturing Process Planning Report II

Radioluminescent Exit Sign

For our manufacturing process planning report, we decided on radioluminescent exit signs because we feel they are a very innovative and fascinating product. They provide a very necessary function to both the public who need directions out of a building, and to building owners, who must by law mark all emergency exits. Radioluminescent technology is also very interesting as it has existed for over 50 years, but is still in its infancy as it has been used very sparsely over that time. It can be used for many great things, but presents some significant challenges.

Radioluminescence was discovered shortly after Marie and Pierre Curie first discovered Radium. They found that their newly discovered element was capable of spontaneously creating light through its own radioactive decay. Over time, it was learned that mixing radium with a phosphorescent helped to increase the light output caused by the isotopes decay, and became a popular paint for watches, gages, and many other items. In the 1930s when is became known of the effects of radiation on the human body, radium use began to quickly decline, and it was banned from most uses by the 1950s.

Sometime in the 1940’s, the United States government began looking for a replacement for the then declining Radium, and found Tritium. Tritium is an isotope of hydrogen, but having 2 neutrons and 1 proton, as opposed to the normal 1 proton only. Tritium is also radioactive, but gives off only Beta radiation instead of Alpha, Beta, and Gamma like radium. Also, the radiation given off by tritium is of lower power than radium. As tritium decays, like all radioactive materials do, one of the excess neutrons splits apart, forming a proton and a high-speed electron. This new element is a form of Helium, He3, which is quite safe. The high-speed electron is the beta particle.

The tritium gas is kept within a glass tube that is capable of blocking the beta particle’s escape. There is also a coating of phosphorescent material inside of the glass tube. When the electron strikes the phosphorescent material, it creates a small flash of light. As thousands of these flashes occur, it produces a faint glow, The total light can be increased by adding more tritium, and thus creating higher beta emission. This is much the same as what occurs in your television set, but in a much less controlled manner. By using sufficiently thick and break-resistant glass, and keeping strict quality control guidelines, radiation exposure outside the glass vessel can be kept at safe levels, and leaks to an absolute minimum. In the even of a breakage or leak the gas would, being lighter than air, float away and quickly dissipate. It is still recommended that the area be evacuated and well ventilated after a breakage though, just as is recommended with many household chemical spills.

By using different phosphorescent coatings within the glass, different colors can be produced. For example, Zinc Sulfide with Copper produces a green glow, while Yttrium Oxide-Sulfide produces a red. These are both accepted colors for emergency lighting signs. For the sake of safety, rather low amounts of tritium would be used. This means lower light output, as fewer beta particles are produced. As a result, radioluminescent lighting is not visible in daylight, and glows sometimes rather faintly even in the dark. This is something that must be monitored, as emergency exit signs have prescribed minimum illumination requirements, as set by the National Fire Safety Act.

The primary benefit of this type of lighting are that since the reactions rely only on the tritium and phosphorescent inside the glass tube, no connection to an external power supply is needed. This means no wiring to a power system, no light bulbs to change, and no batteries for backup in the event of a power failure. This allows you to mount a radioluminescent sign once, and provide near zero maintenance to it for several years. This is very helpful in the even that the sign needs to be mounted in a hard to access location or an area prone to flooding or other wet conditions where electrical connection could be hazardous, or in locations such as aircraft or sea vessels where power cannot be spared. The average life of such signs is approximately 10 to 20 years. This is due to the decay of tritium into helium. Using increased amounts of tritium during manufacturing can provide a longer life, but this presents possible risks in event of tube failure.

For materials, the glass tubes are most often made of borosilicate glass, also known by the brand name Pyrex. Borosilicate glass has a typical composition is about 70% silica, 10% boric oxide, 8% sodium oxide, 8% potassium oxide, and 1 calcium oxide. This produces a glass that is highly resistant to heat, chemicals, and breakage. Tempering further increases its resistance to breakage. The chemical resistance, in particular, is why the glass is so well suited. The high-speed of the emitted beta particles as well as the phosphorescent coating applied to the glass tubes could degrade other types of glassware over time. For the casing of the exit sign, we chose sheet metal, because it is fairly easy to work with, can easily be painted whatever color is needed, and will help protect the tubes inside from breakage. A polymer casing could also be used.

There will also be a polymer diffraction sheet, made of a thin, semi-transparent plastic. This sheet will diffuse the light into various directions creating the effect of a single, large light source within the sign, rather than the multiple smaller ones actually used. This would be made of a polystyrene polymer with one surface etched, most likely my mechanical means such as light sanding or grinding.

To manufacture the signs, we would first receive all required materials: paint, sheet metal, polystyrene sheeting, borosilicate glass tubing, phosphorescent powders, and tritium gas. The metal would be stamped out into the correct size and shape, a rectangular piece with the word “EXIT” removed. These sheets would then be pressed into a form to bend up the 4 sides. The back panels would be stamped out at the same time. Both the box and its back would then be spray painted inside and out with an epoxy base paint to provide environmental resistance. The most likely color would be white, as is traditional among emergency signs as it blends in well with the walls and ceilings of most environments, but could be pained to any color.

At the same time, long tubes of borosilicate glass are coated with a phosphorescent. This is accomplished by pouring a liquid solution containing the desired phosphorescent into the tube, and draining out any excess. Once the remaining residue has set, the ends are sealed at both ends by heating them and pressing the ends down to crimp them. When the glass cools, an airtight seal is formed. While one end is still soft though, a probe is inserted and the tube is filled with tritium gas. Next, the glass is heated and crimped every 6”, producing smaller tubes that are all of the same length and sealed at both ends. The process provides a means of filling the tubes with zero leakage of tritium gas, which is very expensive as well as a potential health risk in industrial-scale quantities.

Also occurring at the same time is the cutting of the polystyrene sheets into the correct dimensions. These would have been obtained with one side already roughed, and require only a small amount of cutting with a shear. At last, all parts come together. Metal clips are used to fasten the tubes to the back plate of the housing, the diffraction sheet is inserted into the housing box and fastened with a bead of glue, and finally the two parts of the housing are fastened together. Although not required for water protection, as it is in other exit signs, the housing would still be fully sealed to help protect against leakage in the event of a tube failure within the housing.

Quality control would be exercised at many different points. Most importantly, in leak testing the glass tubes after they have been filled with tritium. This could be accomplished by passing the tubes through a pool of water and watching for bubbles that signify leakage, or by passing them under a Geiger counter or other beta particle detecting sensor. Quality control would also me exercised in making sire that the paint coating is correct, the cutout of “EXIT” is correct, and that the light is correctly assembled and working before leaving the plant. Making sure the light properly functions could be done by passing the assembly line through a dark room, where it could be easily observed whether the sign was emitting light or not.

Our factory layout was chosen to be as efficient as possible. By organizing into secondary lines to produce components, and those leading to a primary production line to assemble the components, we can make the best use of floor space with minimal waste to over-movement. We could have separated the lines and put each component onto trays, to be brought over to an assembly line, but that would have created wasted movement and could lead to excess inventory.