Evaluation and Characterization of Polymers



3. Evaluation of Polymers

Molding Casting Extrusion

i) Compression molding

Compression molding utilizes heat and pressure to force molten polymer, introduced between the mating surfaces of a movable mold, into the shape of the mold. The sample is then cooled before pressure is released to allow the solidified article to be removed.

ii) Injection molding

In injection molding, molded polymer is forced by hydraulic pressure from a heater chamber into a closed mold cavity. The two methods are shown diagrammatically in below.

iii) Blow molding

Another type of molding, particularly useful for manufacturing bottles, is blow molding. In this process, polymer tubing(called parison) is blown by compressed air or stream or drawn by vacuum into the shape of the mold

iv) Casting

Casting, a much simpler process, involved pouring molden polymer into a mold and allowed it to cool.

v) Extrusion

With extrusion, molden polymer is compacted and forced by a screw thorough a die shaped to give the desired objects, such as rods and pipe

Fibers are made by the process of spinning which resembles extrusion in that polymer is forced under pressure through small holes in a plate, called a spinneret. The resultant filaments are then drawn together to form a thread. Spinning can be done either with molten polymer(melt spinning) or with polymer solution. In the letter case, solvent can be removed from the filament by evaporation(dry spinning) or by leaching with another liquid(wet spinning).

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Thermoplastic polymers are most commonly shaped by one of three basic techniques:

Compression molding

Blow molding

Injection molding

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