Plastics cover a broad field of organic synthetic resin and may be divided into two main classifications - aerospace thermoplastics _ and aerospace thermosetting plastics. Thermoplastics may be softened by heat and can be dissolved in various organic solvents.
Thermoplastics may be softened by heat and can be dissolved in various organic solvents. Two kinds of transparent thermoplastic materials are commonly employed in windows, canopies, etc. These are known as acrylic plastics and cellulose acetate plastics. Cellulose acetate was used in the past but since it is dimensionally unstable and turns yellow after it has been installed for a time, it has just about passed from the scene and is not considered an acceptable substitute for acrylic. Acrylic plastics are known by the trade names of Lucite or Plexiglas and by the British as Perspex and meet the military specifications of MIL-P-5425 for regular acrylic, MIL-P-8184 -~ 184 for craze-resistant acrylic.
Aerospace Thermosetting Plastics.
Thermosetting plastics do not soften appreciably under heat but may char and blister at temperatures of 240 to 260 'C (400 to 500 °F). Most of the moulded products of synthetic resin composition, such as phenolic, urea-formaldehyde, and melamine formaldehyde resins, belong to the thermosetting group. Once the plastic becomes hard, additional heat will not change it back into a liquid as it would with a thermoplastic.
Storage and handling.
Because transparent thermoplastic sheets soften and deform when they are heated, they must be where the temperature will never be excessive.
Transparent acrylic plastics get soft and pliable when they are heated to their forming temperatures and can be formed to almost any shape. When they cool, they retain the shape to which they were formed. Acrylic plastic may be cold-bent into a single curvature if the material is thin and the bending radius is at least 180 times the thickness of the sheet. Cold bending beyond these limits will impose so much stress on the surface of the plastic that tiny fissures or cracks, called crazing, will form.
Simple Curve Forming. 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 ten minutes to one-half hour. Do not force-cool it.
This type of forming is normally used for such parts as 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.
Stretch forming. Preheated acrylic sheets are stretched mechanically over the form in much the same way as is done with the simple curved piece. Special care must be taken to preserve uniform thickness of the material, since some parts will have to stretch more than others.
Male And Female Die Forming. This requires expensive matching male and female dies. The heated plastic sheet is placed between the dies which are then mated. When the plastic cools, the dies are opened.
Aerospace Vacuum Forming Without Forms. Many aircraft canopies are formed by this method. In this process a clamp with an opening of the desired shape is placed over a vacuum box and the heated sheet of plastic is clamped in place. When the air in the box is evacuated, the outside air pressure will force the hot plastic through the opening and form the concave canopy. It is the surface tension of the plastic that shapes the canopy.
Aerospace Vacuum Forming With A Female Form. If the shape needed is other than that which would be formed by surface tension, a female mould, 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 will force the hot plastic sheet into the mould and fill it.
Ponds need Pond Liners like a building needs a foundation. Among all the materials used for Pond Liners, plastic is the most versatile. It comes in two forms- pre-formed and flexible.
Preformed or rigid plastic liners are strong and long-lasting. They are made of polythene and other recycled plastic material. They do not develop leaks. They are more cost-effective than concrete. An average gardener can set up a small pond within one day using this liner.
Some of these models are fitted above the ground and some below the ground. Rocks and stones may be placed around it to give a natural look. The liners may be both UV- as well as frost-resistant. But they may be difficult to fit because of their different shapes and sizes.
Among the preformed models, the plastic liners may be cheaper than fiberglass, but do not offer support for free-standing use of the pond in a raised water garden. It is difficult to build around a plastic pond and support it evenly. When support is not uniform there is a danger that the plastic liner will crack.
Flexible Pond Liners are also available, like PVC (Poly Vinyl Chloride) and HDPE (High Density Poly Ethylene).These liners are not as flexible as rubber. Plastic liners are often used on large holding ponds when economy is a bigger concern than flexibility.
Large ponds require a Pond Liner that is safe for fish and aquatic plant life, inexpensive, easy to install and durable. Polyethylene fits the bill. Medium-density Polyethylene contains up to 5 percent carbon black, which makes it highly UV stable and suited to outdoor life. Twenty and 30 mm polyethylene can be custom-made according to your specifications.
PVC, or Polyvinyl Chloride, has excellent chemical compatibility and puncture resistance, lending itself to membrane liner applications. Some PVC liners contain UV stabilizers which protect them from breaking down when exposed to sunlight. PVC is not a crystalline membrane liner material, so it can elongate in all directions. It is the most cost-effective buried membrane and it has the longest successful use of liner material.
Another plastic is Polypropylene. It has special properties, like outstanding dimension stability, low coefficient of expansion and contraction, wide temperature seaming range, good chemical resistance and no stress cracking.
Plastic has its plusses and minuses, and the choice depends on the individual's requirements.