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Manufacture of Plastics


The production of plastics can be roughly divided into four categories:

1. Acquiring the raw material or monomer.
2. Synthesizing the basic polymer.
3. Compounding the polymer into a material that can be used for fabrication.
4. Molding or shaping the plastic into its final form.

Raw Materials

Historically, resins derived from vegetable matter were used to produce most plastics. This included such materials as cellulose (from cotton), furfural (from oat hulls), oils (from seeds) and various starch derivatives. Today, most plastics are produced from petrochemicals which are widely available and tend to be cheaper than other raw materials. However, the global supply of oil is exhaustible, so researchers are investigating other sources of raw materials, such as coal gasification.

Synthesis of the Polymer

The first step in plastic manufacturing is polymerization. The two basic methods by which polymerization can occur are addition and condensation reactions. These can occur in the gaseous, liquid and occasionally solid phase. Sometimes the polymer synthesis can take place at the interface of two immiscible liquids in which the monomers are dissolved.

Additives

Chemical additives can be used in the production of plastics to achieve certain characteristics. These additives include:

Plastics are often manufactured as composites. This is achieved by adding reinforcements such as glass or carbon fibers to the plastics, increasing their strength and stability.  Plastic foam is a different type of composite which combines plastic and gas. An example of this can be seen in styrofoam cups which are made of foamed polystyrene.

Shaping and Finishing

Compression molding is one of the oldest methods used for converting polymers into useful materials. It uses pressure to force the plastic into a certain shape. One half of a two-piece mold is filled with plastic and then the two halves of the mold are brought together and the plastic is melted under high pressure as shown below:

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A common method used for shaping plastics is extrusion. A device, called an extruder, forces softened plastic through a shaped die from which it may emerge in almost any form, including a circular rod or tube, and a wide, flat sheet. The driving force is supplied by a screw which provides constant pressure. All extrusion products have a regular cross section. A variation on this method is extrusion blow molding, in which a plastic tube produced by extrusion is sealed around a blowing tube and expanded to the shape of a mold with compressed air. This technique is illustrated below:

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Injection molding involves one or more extruders which force melted plastic into a cold mold where it is allowed to set to the required shape. An adaptation of this method is injection blow molding which is used to make plastic pop bottles. A thick-walled plastic tube is initially injection-molded around a blowing stick and is then transferred to a blowing mold. The tube is reheated and expanded to the shape of the mold by passing air down the blowing stick. This method is shown below:

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Other methods also exist for shaping and finishing plastics including calendering which produces plastic sheets and transfer molding, in which softened plastic is forced by a ram into a mold.

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