Plastics

Plastics: They are a class of high polymers whcih can be moulded into any desired form by heat and pressure. Resins: Binders used for plastics.

These two terms are used synonymously.

Types #

1. Thermoplastic Resins
2. Thermosetting Resins

Compounding #

Polymers are generally mixed with a wide variety of substances which impart a certain functionality or a desired property to the polymers. The finished product after the molding process is known as plastic.

Additives:

• Impart properties to plastics
• Make the processing easy

Compounding: Mixing of additives to virgin plastics

1. Resins : it is the binder which holds the different constituents or polymeric chains together. 

2. Plasticizers: they are added to increase the flexibility of the material. It should be kept in mind that adding plasticerzs decreases the mechanical strength and chemical resistance of the polymer. Eg: vegetable oil, camphor, esters and organic phosphates 

3. Fillers: they increase the hardness, tensile strength and workability of the material. It also reduces the cost, shrinkage and brittleness of the material. Eg: carborundum, quartz, mica 

4. Catalysts: they are only added to therosetting plastics for their easy molding. 

5. Stabilizers: thermal and chemical stabilizers are also added so that the polymers do not chemically react when exposed to high temperatures during the molding process 

6. Lubricants: they are added to prevent the sticking of the polymers to the mold. They also impart a glossy finish to the end product. 

7. Coloring material.

Moulding #

1. Injection moulding:

   1. Applicable for only thermoplastics.
   2. in injection moulding, the polymers mixed with other additives are fed through a injection like syringe whose nossel is kept at high temperature.
   3. The polymer along with additives is fed into the syringe with the means of a hopper. 

2. Compression molding: In this, the molten plastic material (already mixed with additives) is taken in form of a lump and kept in the lower part of a mold. Then from the top, the other half of the mold compresses the lump at high pressure. Due to the pressure, the lump takes the shape of the mold.  

3. Transfer moulding: is similar to injection moulding. The only difference being that due to high friction near the nozzle, temperatures are increased tremendously which convert the molten plastic into liquid state. The liquid state allows easy flow and better packing along with better finishing of the final material.  

4. Extrusion moulding: this technique does not uses any mould. Hence it is also known as “extrusion process”.

   1. Material is introduced and electrically heated and pushed by a screw conveyer.
   2. Used for drawing long wires from a molten mass.
   3. Most of the times, cylindrical pipes, wires,  materials of uniform cross sections,  are made using extrusion moulding.

5. Blow moulding: Hot softened tube of thermoplastic is placed between two halves of mould and compressed air is blown where the plastic takes the shape of the mould

   1. Used to form hollow articles like bottles, cans etc.