sheets on shelves or racks covered with soft felt or flannel. Be  sure there is enough open space to allow the air to circulate  around the sheet and heat it evenly. piece. Take special care to preserve uniform thickness of the  material, since some parts must stretch more than others. Many aircraft canopies are formed by this method. In this  process, a panel, which has cut into it the outline of the desired  shape, is attached to the top of a vacuum box. The heated  and softened sheet of plastic is then clamped on top of the  panel. When the air in the box is evacuated, the outside air  pressure forces the hot plastic through the opening and forms  the concave canopy. It is the surface tension of the plastic  that shapes the canopy. Male and female die forming requires expensive matching  male and female dies. The heated plastic sheet is placed  between the dies that are then mated. When the plastic cools,  the dies are opened. Small forming jobs, such as landing light covers, may be  heated in a kitchen baking oven. Infrared heat lamps may be  used if they are arranged on 7- to 8-inch centers and enough  of them are used in a bank to heat the sheet evenly. Place the  lamps about 18-inches from the material. Never use hot water or steam directly on the plastic to heat  it because this likely causes the acrylic to become milky or  cloudy. Heated acrylic plastic molds with almost no pressure, so the  forms used can be of very simple construction. Forms made  of pressed wood, plywood, or plaster are adequate to form  simple curves, but reinforced plastic or plaster may be needed  to shape complex or compound curves. Since hot plastic  conforms to any waviness or unevenness, the form used must  be completely smooth. To ensure this, sand the form and  cover it with soft cloth, such as outing flannel or billiard felt.  The mold should be large enough to extend beyond the trim  line of the part, and provisions should be made for holding  the hot plastic snug against the mold as it cools. A mold can be made for a complex part by using the damaged  part itself. If the part is broken, tape the pieces together, wax  or grease the inside so the plaster does not stick to it, and  support the entire part in sand. Fill the part with plaster and  allow it to harden, and then remove it from the mold. Smooth  out any roughness and cover it with soft cloth. It is now ready  to use to form the new part. Heat the plastic material to the recommended temperature,  remove it from the heat source, and carefully drape it over  the prepared form. Carefully press the hot plastic to the form  and either hold or clamp the sheet in place until it cools. This  process may take from 10–30 minutes. Do not force cool it. Compound curve forming is normally used for canopies or  complex wingtip light covers, and it requires a great deal  of specialized equipment. There are four commonly used  methods, each having its advantages and disadvantages. Preheated acrylic sheets are stretched mechanically over the  form in much the same way as is done with the simple curved  The drill used on acrylics must be carefully ground and free  from nicks and burrs that would affect the surface finish.  Grind the drill with a greater included angle than would be  used for soft metal. The rake angle should be zero in order  to scrape, and not cut. The length of the cutting edge (and  Unlike soft metal, acrylic plastic is a very poor conductor of  heat. Make provisions for removing the heat when drilling.  Deep holes need cooling, and water-soluble cutting oil is a  satisfactory coolant since it has no tendency to attack the  plastic. Several types of saws can be used with transparent plastics;  however, circular saws are the best for straight cuts.  The blades should be hollow ground or have some set to  prevent binding. After the teeth are set, they should be side  dressed to produce a smooth edge on the cut. Band saws are  recommended for cutting flat acrylic sheets when the cuts  must be curved or where the sheet is cut to a rough dimension  to be trimmed later. Close control of size and shape may be  obtained by band sawing a piece to within  1 ⁄ 16 -inch of the  desired size, as marked by a scribed line on the plastic, and  then sanding it to the correct size with a drum or belt sander. If the shape needed is other than that which would be formed  by surface tension, a female mold, or form must be used. It  is placed below the plastic sheet and the vacuum pump is  connected. When air from the form is evacuated, the outside  air pressure forces the hot plastic sheet into the mold and fills it. Male and Female Die Forming Vacuum Forming Without Forms Forms Vacuum Forming With a Female Form Sawing and Drilling Sawing Forming Methods Simple Curve Forming Compound Curve Forming Drilling Stretch Forming 7-55