Plastic chairs are made in a number of ways, all of which involve metal molds carved into the desired shape. One form of furniture molding is known as rotational molding, in which a large metal mold is injected with plastic polymer powder. This is then heated to the melting point, and the mold is mechanically turned three-dimensionally to evenly distribute the polymers to the inside of the mold. After several minutes of spinning, the mold is cooled off and the rotation stops. The mold is opened and the new, completely hollow chair is removed.
Injection molding is another method of plastic furniture making. Again, a metal mold is used, but this time the result is not hollow but solid plastic. This type of mold does not spin, but the polymers are heated at a predetermined rate and temperature. They're then poured into the mold and cooled. A certain amount of precision must be used with this type of molding process, because if the plastic is poured too slowly, it will dry too soon. If it is poured too quickly, the result will not be a uniform configuration of plastic. If done properly, the result is a plastic chair with no seams, molded all in one piece.
Gas Assisted Injection Molding
Injection molding can also be done with the help of gas that is blown at high velocity inside the mold, pushing the molten plastic out of certain parts of the mold. This creates channels in the plastic form that make the chair much more lightweight while at the same time maintaining its strength. All of the other aspects of injection molding remain the same, but the result is a chair with some parts that are hollow and other parts that are solid.
The plastics used in the chair-making process are usually a composite of several different types of plastics all blended together during the heating part of the molding process. This makes the chair much stronger than it might be if only one type of plastic was involved.