Calendering. Technology and Process

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

Compared to extrusion blow molding and flat die extrusion processes, which also 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 throughput rates: from 2 t/h to 6 t/h depending on whether rigid PVC 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 by extrusion blow molding.

• large widths: 5,000 mm without stretching for sheets, or with stretching for films. These widths are comparable to those obtained by extrusion blow molding;

• very good quality films and sheets, i.e., flatness, profile, surface finish, dimensional stability;

• a narrower range of thicknesses (50 µm for rigid PVC to 1.3 mm for filled plasticized PVC) than in extrusion (10 µm in extrusion blow molding or flat die extrusion plus stretching to 2 mm for flat die extrusion).

It can therefore be said that calendering and extrusion 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 the size of the production run, is particularly sought after for its versatility (virtually all thermoplastic polymers can be extruded).

Annual PVC consumption in Western Europe is approximately 4.3 million tons (according to IHS Markit 2022). Calendering accounts for approximately 17% of all PVC conversion processes, with the main markets for calendered products being floor and roof coverings, conveyor belts, films for leather goods, automotive, packaging, textile coatings, etc.

In this article, we will focus primarily on PVC calendering. We will show how the process can...