Forming & Fabricating Plexiglass PLEXIGLAS is thermoplastic, it softens when Ssionalince heated and can then be bent to two or three-dimencurves. It is not practical to "cold form" PLEXIGLAS (i.e., to spring it into a curved frame without heating) unless the material is thin and the curve very slight. PLEXIGLAS is flexible, but if an outer fibre stress of over 1000 psi is developed by cold forming, tiny fissures called "crazing" may appear on the surface. The temperatures used should be sufficiently high so that a minimum of strain will be set up in the material during the forming operation. On the other hand, if acrylic plastics are heated too hot, the surface becomes too soft and will pick up minor mold imperfections called "mark-off". Complex shapes require somewhat higher temperatures than simple curves. The recommended procedure is therefore to heat the plastic sheet for at least 15 to 20 min. at the proper temperatures. As a general rule, the average temperature of the sheet should be at least 100° to 105° C. (212° to 220° F.) when it comes in contact with

the form. Care should be exercised while handling acrylic sheets to avoid finger printing and otherwise damaging the soft surface. Operators should wear soft cotton gloves to protect not only their hands but also the surface of the plastic. Forming should be carried out in a clean room and forms should be brushed free of dust and dirt before use. The acrylic plastic blank should be slightly larger than the required finished size to provide an edge which can be handled freely without danger of marring the surface of the material of the finished part. The pressures required to form PLEXIGLAS are extremely low; often the weight of the material itself is sufficient. The application of high or even moderately high pressure is neither necessary nor desirable since it tends to increase the extent of "mark-off". After the material has been formed, it should be allowed to cool slowly. If the formed parts show excessive "mark-off" or other imperfections on the surface except scratches, the pieces can be placed back in the forming oven. The material will resume its flat shape and original surface except for scratches and can be formed again. It is usually easier and better to reform several times than to sand and polish "mark-off". Since forms for acrylic plastics are subjected to no great pressure, they may be made of plaster, wood, plywood, impregnated wood or metal. Forms should be free of waviness and other variations in contour which might cause optical distortions in the formed part. The surface should be sanded smooth and covered with soft cloth such as billiard felt, outing flannel, flannelette or imitation chamois or flocked or suede rubber sheeting. Where cloth is used, folds on the formed surface should be avoided and an application of medium grease may be advisable. Some provision for holding the edges of the PLEXIGLAS sheet against the form should be made. In some cases, simply flat rubber bands are sufficient. In others, it is well to provide a wooden yoke or flange corresponding to the contour of the edges. Before forming, remove all masking paper and masking paper adhesive from the plastic sheet. Heat the sheet to the proper temperature, preferably in a hot-air oven. Hot oil may also be used but even boiling water is usual-

ly not sufficiently hot for forming. For simple bends, the hot sheet is merely placed over the form and held by the edges until cool. Cooling may require 10 to 30 min., depending on room temperature and the thickness of the sheet. Three-dimensional shapes may be formed by literally stretching the hot plastic across a form and clamping the sheet at the edges. It is also possible to form certain three-dimensional shapes by the use of air-pressure differentials, the so-called vacuum or blow forming technique. The use of male and female forms is not usually desirable since both surfaces of the PLEXIGLAS come in contact with the mold surfaces and chances for "markoff" are therefore doubled. PLEXIGLAS may be cut readily by the use of ordinary band, circular or jig saws. Especially in cutting thin sheets, it is preferable to use a saw which does not have much "set". Routing tools may be used to cut irregular shapes. Thin PLEXIGLAS, up to about .080 in., can be broken along a straight scribed line provided the line is scribed deeply enough. In cutting a regular shape, the desired design may be drawn on the masking paper as a guide to cutting. Drills such as are used for wood and metal can also be used for drilling PLEXIGLAS. To prevent clogging and burning of the PLEXIGLAS, lift the drill from the hole frequently and remove shavings. Use moderate speeds and light pressure to avoid "grabbing" when the drill penetrates the PLEXIGLAS. The use of drills with narrow lands and wide, preferably polished, flutes also helps to keep the hole cool and free of chips. End mills are excellent tools for drilling PLEXIGLAS. The usual rules of good machining practice apply to the machining of PLEXIGLAS, which has working qualities similar to those of brass or copper. Cutting edges of

the tools should have no rake and should have a scraping rather than a cutting action. Tools and work should be held firmly to prevent chattering. Use of a coolant may be desirable but usually is not necessary. Water or soap and water are preferred coolants. The recommended cements for PLEXIGLAS are solvent materials which soften the plastic surfaces. The two surfaces, when pressed together, intermingle, and as the solvent evaporates and penetrates, a hard, clear joint is obtained. The preferred cement for strong PLEXIGLAS joints is Cement 1-A, a mixture of equal parts of monomeric methyl methacrylate (inhibited with .006 percent hydroquinone) and methylene dichloride. Where quickdrying is important, ethylene dichloride is recommended. Where joint strength is not critical, Cement 1-C (1, 1, 2trichloroethane) is excellent. The two surfaces to be cemented should be polished

or sanded or machined smooth. They should fit accurately without forcing. To confine softening action of the cement to the area of the joint, it is desirable to mask the surrounding plastic with a tape impervious to the cement. Masking tape should be applied carefully so that the cement does not seep under the edges and attack the plastic where the action is not desired. For maximum strength, the cement should be baked for several hours at temperatures as high as 50 °C.

There are two methods of cementing PLEXIGLAS. In "soak cementing", the smaller piece, masked except for the area to be cemented, is placed in the cement for apMAY 1959

• •

trailing behind a car to and from the airport, but to remove the wings and rear end of the fuselage altogether for self-propelled travel into town at your destination?" Steve Ekbery, 2512 Meadowlark Dr., East Point, Ga.,

sends us a sketch of his idea. A two-place pusher, Steve describes his aircraft thus: "This is my idea, combined with another foreign

plane. Certain modifications with the Continental 75 engine and a boom tail that folds so the aircraft can be

R

andy Barnes, 816 W. Glen Ave., Peoria, 111., is our official photographer, but he also is very interested in aircraft design. He sent in this little sketch of an idea he had and calls it "Design Contest Doodling". Here's

trailered home are incorporated in this all-wood aerobatic plane. This idea is worth enlarging on. I hope you can ask the readers for ideas on it."

what he says about it:

stirt fxtfty

"Wouldn't it be fun to have time to build something like this? How would it be to fold the wings back for

proximately 15 min. until the surface is softened. The two pieces are then assembled quickly before the cement on the soaked piece evaporates. This wet cement forms a cushion on the surface of the other piece. The two pieces should be held together for 15 to 30 sec. before applying pressure to allow the second cushion to form.

The pieces should be clamped together with just

enough pressure to force out air bubbles. Pressure should be distributed evenly all along the joint. Scrape excess cement and extruded cushion onto the masking tape and remove the masking tape as soon as possible. Allow the assembly to stand in the jig at least four hours. To increase strength, allow the joints to stand another four hrs. and then heat-treat 48 hrs. at 50"C (122°F.).

From time to time we'll publish those aircraft design ideas sent in if they are properly prepared and offer some food for thought. This can supplement the informa-

tion offered in the reports on those designs already built and flown, and the excellent design studies presented

by Joe Kirk and any other that will follow. So get out your pencil and do a little sketching. We can't offer any prizes for those we print, but we do offer you an oppor-

tunity to get that idea out of your head and on paper. Who knows? Maybe you have just the idea that will solve someone else's problem.

A CHANGE IN STAFF

It is with deep regret that we announce the resigna-

In the other method of cementing, "glue cementing",

tion of our Advertising Manager and Art Director, S. H.

sistency of syrup. It can then be applied like glue to the

to devote the time to the magazine that he has in the past. His wife Julie, who has conducted the Chatting with the Chapters column, also is leaving the staff.

PLEXIGLAS Cement 1-A is thickened by adding clean PLEXIGLAS shavings and chips until it has the con-

areas to be joined, the excess solvent softening and forming a cushion on both surfaces. The coefficient of expansion of PLEXIGLAS is much greater than that of metal in which it is usually mounted, and allowance should be made for this difference if the installation is to be exposed to a wide range of temperatures. In installing PLEXIGLAS do not subject the material to stresses over 1000 Ibs./sq. in. In bolt installation, for example, nuts should not be tightened excessively. In general, however, simple channel and clamp installations are preferable to those involving bolting or riveting through holes drilled in the plastic. PLEXIGLAS should be mounted between rubber, cork or other gaskets to make the installation waterproof, to reduce vibrations,

and to help distribute possible stress concentrations. SPORT AVIATION

"Wes" Schmid. Because of other commitments connected with his new business venture, Wes will not be able

This husband-and-wife team has made an outstanding contribution to the advancement of EAA through their work for SPORT AVIATION. Wes did the layout work

for the magazine each month, and the Amateur Builder's

Manual and EAA Data Book as well. Julie did proofreading and rewrite in addition to the chapter work. The result of their effort is very evident in the high standard

throughout all of our publications for the past two years.

We extend to them both our greatest thanks for an outstanding job well done, and wish them well in their new venture.