Cooling Process Determines the Quality
Sheet and film can be cooled on a conventional polishing stack, in a water bath and through use of a calendering belt. Comparison of these three approaches shows that sheet and film produced using the calendering belt technique or the water bath approach exhibit mechanical characteristics that are favourable for thermoforming.
A conventional polishing stack produces good optical characteristics in the film or sheet as a result of the forces in the nip and replication of the highly polished roll surface on the web. The immediate cooling and long contact time on the centre roll yield a high-gloss surface. However, cooling of the underside of the web by the ambient air in the vicinity of the centre roll results in a matte surface. As a consequence, the web is usually glossy on only one side.
The polishing stack can be equipped with an additional calendering belt. At the beginning of the cooling stage, the melt is held between the first roll and calendering belt. The benefit of the calendering belt is the defined cooling of both sides of the web. The upper surface of the web is in contact with the first roll, while the lower surface is in contact with the calendering belt. It is possible with this approach to produce thinner film and sheet with good optical characteristics than with a polishing stack.
As a third alternative, a calibrating tool can direct the melt into a cooling bath, where the web is cooled. As a result of the direct contact with the cooling medium, the web cools uniformly. The mechanical properties of the web in the lengthwise and crosswise directions are almost identical, so that shee and film cooled in water baths are especially well-suited for thermoforming.
As part of a research project at the Insitute for Plastics Technology at the University of Paderborn (Institut für Kunststofftechnik / KTP) the three different approaches were systematically compared with one another. The comparison showed that the web produced with the calendering roll had higher gloss and better transparency The comparison showed that the sheet produced with the calendering belt had high gloss and better transparency that the sheet from the water bath. However, because of the surface roughness of the calendering belt, the optical properties are not as good as those for sheet from a polishing stack. The optical characteristics of the sheet from the calendering belt can be improved by increasing the quality of the belt through chrome plating and polishing. The benefit of the calendering belt approach is that almost identical mechanical properites are achieved in all axes. This makes sheet produced with a calendering belt ideal for thermoforming. This situation is a consequence of the more symmetrical and faster cooling of the extrudate compared with that achieved on the polishing stack.