Gelation in UPVC Profile Extrusion.

Plastic processing involves -

a. Preparation of raw material,
b. Heating & Homogenizing
c. Shaping,
d. Cooling, &
e. Finishing.

 But in case of PVC, an additional activity of "Fusion" takes place during processing.

Fusion provides mechanical strength to PVC product. It has to be optimum for best mech. strength, neither low nor high. During processing, the secondary particle of 90 - 250 microns enters from the hopper & has to breakdown to 1 micron size.

 The "melt" exiting the die actually consists of three dimensional network of one micron (fused) primary particles.

The terms like Gelation (Europe) & Fusion (USA) had same meaning in early days. However, as per the modern thinking, the process where PVC sub grains, agglomerates, are loosely attached together during initial processing is referred to as gelation (Like consistency at vacuum port).

Gelation takes place up to vacuum port. Whereas, when primary particles are attached together during processing, it is known as fusion.

 Fusion takes place between the vacuum port & the die. This means that at higher extrusion speed, optimum fusion must take place rapidly between the vacuum port & the die exit.

This also means that for the same extruder, higher speed demands different formulation.

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