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.
Thermoforming is the process of heating huge plastic sheets into a very high temperature and cooling them in the required format. Vacuum forming is a sub process or a type of thermoforming where vacuum is applied in between the molds and the plastic to create various shapes. Thermoforming has several advantages and disadvantages compared to the other plastic molding methods like injection molding. In addition to vacuum forming, several other methods like drape forming, twin sheet forming and pressure forming can be incorporated along with thermoforming to produce different kind of products. Several types of plastic can be molded in the method.
Disadvantages of thermoforming
1. Thermoforming is quite costly. Compared to other methods like injection molding, this method can cost up to 50% more.
2. Large plastic sheets are used in the method. They have to be fed into the rollers again and again. This leads to lots of extrusion.
3. The plastic street is stretched in the under pressure in this method. Hence the products formed using thermoforming break at a certain temperature.
4. Thermoforming uses more plastic than other methods. Nearly 10 to 20% more plastic is used to make any product in this method. This adds to cost and creates extensive wastage too.
5. Vacuum forming process done along with thermoforming does not give the required shapes sometimes. The whole process has to be repeated which is expensive as well as time-consuming.
Advantages of thermoforming
There are several advantages in using this method in spite of all these disadvantages.
1. Thermoforming is the only method in which huge plastic sheets can be molded into huge objects. All other methods are used for making small objects only.
2. Though the process is costly, the tools required for the trade or quite cheap. Injection molding machinery is nearly 33% costlier than the Thermoforming machinery tools.
3. Several different products can be prepared in the thermoforming machines. Only certain products can be prepared in other machines. Nearly all types of plastic can be used for the plastic. Other methods use only specific plastic for production.
4. Skilled labor to operate the thermoforming machineries are available in abundant. But, laborers capable of operating machineries of injection molding or vacuum forming are quite less.
5. Superior quality products using more plastic are formed in the method. Hence, they are more durable than the objects created through other local methods.
There are many more advantages in the method. Though costly and a bit time-consuming thermoformed products are preferred by most of the packaging industries due to their high quality.