Plexiglas is used extensively in light aircraft; it can be found everywhere from windshields to side windows to landing-light and position-light lenses. This is understandable. The material is easy to form, lightweight and available in a wide variety of colors; it has a long life because it’s relatively simple to restore to like-new clarity.
Before I get into the uses for, and care of, Plexiglas, I want to take a minute to explain the differences between Plexiglas, which is an acrylic, and Lexan, which is a polycarbonate. They are both clear, can be purchased from aircraft parts and supply stores and your local Home Depot or Lowe’s, and at first glance, may seem interchangeable. Yet Lexan is rarely used in the airplane world.
A bandsaw is an ideal tool for cutting Plexiglas.
Here are the primary differences between the two. Plexiglas transmits more light than Lexan but only by a little (92% versus 88%). Both materials are easy to scratch, but scratches in Plexiglas can be polished or buffed out; Lexan scratches are there to stay. Lexan is stronger. Plexiglas cracks more easily and is less resistant to shattering on impact; it’s very difficult to shatter Lexan. Lexan costs slightly (15%) more than Plexiglas and is 10% heavier. Finally, Lexan will discolor when exposed to sunlight; Plexiglas will not.
In the certified-airplane world, Plexiglas windshields, side windows, landing- and position-light lenses are supplied by three companies: Great Lakes Aero Products, Cee Bailey’s and L. P. Aero Plastics. Each of these companies also sells a wide variety of cleaning and polishing products and tools (such as twist drills ground for drilling Plexiglas). Each company’s web site is chock-full of information about caring for and working with its products.
The Unibit eliminates the need to purchase special drill bits for drilling holes in Plexiglas.
Cut it, Drill it, Form it
A bandsaw is a very effective tool for cutting Plexiglas. Thin sheets can also be scratch-scored and broken on the scratch-score line. Disk or belt sanders work well for final trimming.
Since Plexiglas expands and contracts faster than aluminum during temperature changes, holes used for securing Plexiglas parts must be larger than the shaft of the fastener. Twist drills that have been modified for drilling Plexiglas can be purchased or ground locally, but I’ve found that the Unibit tool does a fine job of drilling holes in Plexiglas. High speed and light to moderate pressure is the rule.
Molding Plexiglas is relatively easy. Parts that aren’t vision-critical, or one-off parts such as landing-light lenses, are easy to fabricate with common tools. Cut the Plexiglas to size, heat it up and then form it by hand around a plug. The ASA Aviation Maintenance Technician textbook recommends heating the Plexiglas to 230° in an oil bath, but many small forming operations can be done using a household oven.
If there’s a need to produce many like parts, it’s easy to put together a vacuum-forming system. Vacuum forming requires a pattern or plug, which can be made of wood or plaster. The plug is positioned on a vacuum box or table. This box or table consists of nothing more than the appropriately sized box that is airtight except for the top, which is perforated with hundreds of holes. A frame holds the Plexiglas sheet while it’s heated, and then the frame and heated Plexiglas is lowered onto the plug and a vacuum is applied to the box. Fish-tank pumps and vacuum cleaners are just two of the readily available ways to provide the vacuum. Bubble-type windows can be produced by introducing a slight positive air pressure into softened Plexiglas in a frame. Steve Mahoney details this process using his wife’s kitchen oven (he had to wait until she left for the evening for band practice) in “The Kitchen Window,” published in the December 2010 issue of KITPLANES®. There are many videos on YouTube of home-brewed vacuum-forming contraptions that work. You’ll also find hints on how to build vacuum-forming tools on the Internet.
Each hole size is marked on newer Unibits. When the author wants to drill a series of like-sized holes, he puts a wrap of tape around the next larger hole as a reminder of where to stop drilling.
Cell Cast and Tinted or Clear
L. P. Aero Plastics of Jeannette, Pennsylvania, uses different types of cell-cast acrylics for its windshields. Cell-cast acrylic is used for its optical clarity and surface hardness. Although it’s harder than the acrylics such as continuous cast or extruded acrylics, it’s still soft and scratches easily. Even surface-hardened Plexiglas is so easy to scratch that the softest paper towel will damage the surface. Never use a paper towel to clean or polish a windshield.
Visible light transmission numbers for acrylic windows are 92% for clear windows, and about 77-78% for tinted windows. However, the percent of solar energy (heat) allowed to pass through the windows is highest (85%) for the clear windows, between 75-78% for green and gray tints, and only 52% for solar-blocking windows that were first introduced by L. P. Aero.
L. P. Aero began selling side windows (and windshields) that are much more efficient at blocking ultraviolet (UV) rays and infrared (IR) radiation than existing windows. They called their products UV-SG (solar gray) and UV-GT (green tint).
According to George Mesiarik at L. P. Aero, these windows block 100% of UVA and 99% of UVB and are 30% more effective than previous windows at reducing near infrared radiation (heat). UV- and IR-radiation blocking is significant because while older windows are pretty effective at blocking radiation at sea level, this blocking effect decreases by approximately 5% for each 1000 feet above sea level. These windows not only reduce the effects of UV and IR on skin and eyes, they reduce temperatures inside the cabin by up to 30°.
The Unibit in action.
Mesiarik also said the UV-blocking windows end sun-related degradation of non-organic materials used in interiors and avionics, and the lowered temperatures also reduce wear and tear on air-conditioning systems. The UV window systems are about 30% more expensive than standard tinted windows, but Mesiarik said customers are starting to understand that while these windows may cost a bit more up front, they more than pay for themselves in the long run with increased comfort and reduced maintenance costs.
Great Lakes Aero also sells windows with the same UV- and IR-blocking advantages. It markets its blocking windshields and windows under the Solar Control (SC) name and offers SC products in gray tint.
The most common windshield thicknesses are 3/16 inch (0.1875 inch) and ¼ inch (0.250 inch). Thicker windows are heavier but do reduce the amount of wind and engine noise in the cabin. The most common side-window thicknesses are 1/8 inch (0.125 inch) and 3/16 inch (0.1875 inch).
Safe and Sane Window and Windshield Cleaning
First, a list of don’ts. Chemicals and cleaners that are commonly used during aircraft maintenance should never be in proximity to, or be allowed to contact, airplane windshields or windows. Keep glass cleaners and household cleaners containing ammonia, such as Windex and 409, and aromatic solvents such as MEK, acetone, lacquer thinner and paint stripper, and rags that have been dampened with these liquids, a long way from your airplane windows. Mineral spirits (100%)—also called Stoddard solvent—and kerosene are safe to use for removing tape residue or oils.
Don’t ever wipe any rag or towel across a dry windshield, and, as mentioned, never use a paper towel on a windshield.
The ideal cleaning process starts by flooding the window from the top down with lots of clean water to flush loose dirt and dust off the window. Keep the surface wet for a few minutes to soften bug splats. Next, those who want to be hyper-vigilant will remove their rings and wristwatch and unbutton and roll up the sleeves of long-sleeved shirts. Continue to flood the window with water while softly scrubbing and loosening bug debris and stubborn dirt with the palm of your hand. This may sound weird and messy, but nothing works as well as the human hand for this task.
I make sure I always have water at hand by keeping a refillable spray bottle of water with my supply of acrylic-window cleaner-polish and polishing cloths. Adding a couple of drops of liquid dishwashing detergent to the water aids in softening up contaminants during this all-important initial windshield-care step.
After it appears that all of the dirt and debris has been flushed off the windshield, do a second flush. This step may be eliminated if the windshield has already been cleaned earlier in the day, but it should be considered mandatory if there was visible dirt on the window prior to the first cleaning. It’s critical to get every speck of dust and every bit of grime off the surface before the next step. If all the dirt is not flushed off the windshield during this first step, scratches are inevitable during the second step.
Once the windshield is dried, clean and polish the windshield with any of the wide variety of commercially available acrylic window cleaner-polishes on the market. One web site tells owners to always use linear motions—never circular motions—when applying and removing cleaner-polishes. The theory here is that “flashes” from sunlight reflecting from the tiny scratches in the windshield will only be seen in vertical and horizontal orientations. All of the commercial cleaner-polishes do a good job of cleaning; some contain anti-static coatings and also have scratch-filling properties. Experiment until you find the one you like.
Sanding by using a stationary shop sander as shown or by using a portable or hand sander are the most common methods of removing small amounts of Plexiglas.
Chamois skins, cotton flannel cloth from a yardage store, soft cotton terry cloth, cheesecloth or non-printed T-shirts round out the stable of wipes that won’t damage windows. A recent addition to the OK-to-use wipes list are microfiber cleaning towels, which are available at auto-parts stores, most big-box stores and aviation-supply houses. Buy the softest ones you can find and keep all wiping cloths sparkling clean. I seal all of my freshly washed windshield-wiping towels and rags in Ziploc plastic bags before stowing them in the plastic tub that lives in my airplane baggage compartment. Don’t use liquid or sheet-type fabric softeners when drying these rags.
Windshield manufacturers and generic parts supply houses all recommend Dupont Sontara AC aircraft-windshield cleaning cloths if a commercially available product is needed. These towels are manufactured without adhesives or additives using a hydro-entanglement process. They work great and are relatively inexpensive—especially because they can be washed and reused. Make sure you use only the Sontara window wipes—Sontara wipes made for other applications can scratch acrylic windows.
A number of pilots I know use nothing but Pledge spray-on furniture polish on their windows, so I asked a windshield manufacturer for an opinion. The response: “We don’t have any evidence that it’s harmful to windshields, but we do know that an oily residue is left behind.”
Scratch Repair
In spite of our best intentions, windshield scratches are inevitable. The first remedy is the use of a non-abrasive product that has scratch-filling properties. However, at some point this won’t be enough. It’s easy to tell when more work is needed—flying into the sun will reveal all. Fortunately, there are solutions. Meguiar’s, 3M and Micro-Surface (Micro-Mesh) all sell scratch-removal products. Great Lakes Aero Products recommends the Scratch Off Optica products. All scratch-removal kits consist of a series of increasingly fine abrasive papers, a sanding block, some towels and a tube of crack-filling cream. Relatively coarse grits of abrasive are first used to sand through the crack layer of the window, and then the area is sanded with progressively finer grits until the surface of the window is scratch-free. Small kits such as the ones from Micro-Mesh do a good job of removing scratches from small areas. Three additional kits—for maintenance, light damage removal and heavy damage removal—are sold for use with power tools. Power tools speed up the repair process and are a good option for restoring an entire windshield. These restoration kits do remove material, so always check with the kit manufacturer before using them. The kits retail for about $100; each kit comes with directions. These kits do require a measure of patience. Do not surrender to the temptation to see rapid results—trying to go too fast can generate temperatures in excess of 240°, which spells disaster. Unfortunately, it’s difficult to completely restore like-new clarity without inducing distortion unless you’re very adept at the process. It’s a good idea to practice on a scrap windshield. You can find one at the local airplane maintenance shop.
I never let a gas-pump line boy wash my airplane windshield. This is because I know that in Plexiglas windows, an ounce of prevention is worth pounds of cure. Preventing scratches, while a pain, is much easier than polishing them out.
