38. 4. Polymer Processing under pressure

 

Sometimes the molding process is carried under high pressure in which the solid thermoplastic materials are melted, molded and cooled subsequently. Some of the commonly used techniques for processing under pressure are –

 

• 1.  Compression molding

2.  Rolling and Calendering

3.  Extrusion

4.  Blow Molding

5.  Injection Molding

 

1. Compression Molding : For the manufacture of thick – walled components and those that cannot be plasticized in extruders, compression molding is used. It plays an important role in the processing of elastomers and thermosets.

A press consisting of fixed and moving platens is used. The plunger and cavity components are mounted on these plates, and the parting plane is kept horizontal. The cavity is filled with the cold plastic and is melted by heating the mold under low pressure. When the melting is complete, the mold is closed and pressure applied. The molding is removed by releasing the press once the ejection temperature is reached.

Compression molding is uneconomical due to the fact that the process has a long cycle time ; as the material in the mold is heated and cooled by conduction via the mold surface.

 

2. Rolling and Calendering (Figure 38.4): The rolling mills consist of two rolls that are used exclusively for the compounding of thermoplastics. The product that results from rolling is a sheet that can be granulated after cooling or fed in plasticated form directly into the processing equipment. PVC and rubber are processed using this process.

When the rolls are more than to in number, and the polymer is to be shaped using these rolls; then the machine is called a calendar. The number of rolls in a calendar can be upto seven ; however, most commonly , four rolls are used. The rolls are made of polished cast iron, and are driven individually and heated. The heating medium is usually oil or water. The temperature and speed increases from one roll to the next. The roll mill must be able to withstand the extreme forces to which it is exposed, and hence, be made extremely rigid.

The calenders are one of the most expensive types of processing equipments and are employed for the manufacture of a few special products like floor coverings and films made of plasticised PVC.

Figure 38.4 .Calendering line

a) Winder and Edge Cutter; b)Cooling Rolls; c)Four-roll calendar; d)Extruder; e)Mixing Roll Mill.