Calendering

Calendering is a process for manufacturing polymer sheets (> 100 µm) or films (< 100 µm) by squeezing the plastic melt between several heated, mechanically driven cylinders. This technology is very old: a US patent (no. 16) from 1836 claims the calendering of rubber. Calendering was developed in conjunction with PVC (polyvinyl chloride) from the 1940s onwards. Even today, PVC remains the most calendered polymer, although other polymers can also be calendered: elastomers, elastomer alloys (e.g. : Santoprene, Alcryn), chlorinated polyethylene, chlorosulfonated polyethylene (e.g. Hypalon); and certain polyolefin polymers [low-density polyethylene (LDPE), ethylenepropylene copolymers, ethylene vinylacrylate (EVA), polypropylene (PP) ...].

Compared with the extrusion blow-molding and flat die extrusion processes, which can also be used to produce thermoplastic films and sheets, calendering differs in the following ways:

• a much higher level of investment, typically by a factor of 20 ;

• very high throughputs: from 2 t/h to 6 t/h, depending on whether rigid or filled plasticized PVC is being processed. These figures are 3 to 4 times higher than those obtained by flat die extrusion, and 2 to 3 times higher than those obtained by blow extrusion;

• large widths: 4,000 mm without stretching for sheets, or with stretching for films. These widths are comparable to those obtained by extrusion blow-molding;

• high film and sheet quality, i.e. flatness, profile, surface finish, dimensional stability;

• a more restricted thickness range (50 µm for rigid PVC to 1.3 mm for filled plasticized PVC) than for extrusion (10 µm for blow-molding or flat die extrusion plus stretching to 2 mm for flat die extrusion).

Calendering and extrusion can therefore be said to complement each other: calendering is well-suited to large-scale production runs where both quality and throughput are required; extrusion, which can be easily scaled to run size, is especially sought-after for its versatility (virtually all thermoplastic polymers can be extruded).

In Western Europe, annual PVC consumption is around six million tonnes (according to the European Council of Vinyl Manufacturers: 1997). Calendering accounts for around 18% of all PVC conversion processes, with the main markets for calendered products being rigid packaging (thermoformed films), floor and roof coverings, leather goods and automotive films, textile coverings, etc.

In the following, we will only discuss PVC calendering.