Sizing adds value to the paper quality by enhancing its properties. Sizing paper with starch can increase both the surface strength of the paper, as well as the bulk strength. Sizing or pigmenting can also be applied with PVA or silicone which transforms the base paper into release paper. Additionally, sizing can be used to densify and seal the paper surface, thus reducing the absorption, for instance, from the coating process.
There are a number of sizing and coating processes, but in this article, we mainly concentrate on the ﬁlm sizing process. In this process, the starch is typically applied onto a soft cover surface and metered to the correct ﬁlm thickness using a grooved rod. The size is very liquid and does not consist of many particles. Almost all of the size is transmitted to the paper as it is running through the nip between two soft applicator rolls. To ensure the eﬀective absorption of starch into the paper, the starch needs to be quite warm and the nip load between the applicator rolls should be high.
The applicator roll cover plays a crucial role in the successful ﬁlm sizing process. It maintains contact with both the rod and the paper. Traditionally, the ﬁlm sizer is equipped with rubber covers, in a hardness range of 10-25 P&J. To achieve the best sizer performance, however, Valmet has introduced a polyurethane cover with notable results.
The hardness range of the Valmet soft sizer covers is optimal to ensure good wear resistance that extends machine running times. The covers are ﬂexible enough to ensure good contact with the grooved rod and they are strong enough to withstand substantial nip loads.
The rubber used for the covers is also resistant to chemicals, and it withstands the wet and hot sizer nip conditions proﬁciently. To cover all sizer processes, Valmet oﬀers an extensive range of soft rubber sizer covers, available at various levels of performance and cost.
This cover has been in the Valmet range for many years already. It provides good basic performance, for instance, for suitably long grinding intervals. The Valmet Sizer Roll Cover RF2 is a cost-eﬀective solution for normal sizing conditions, running typically with a surface roughness of Ra 0.6-0.8 μm.
With a long and proven track record, this special rubber cover is needed when the sizer process suﬀers from web breaks. The Valmet Sizer Roll Cover RF5 is impact resistant and it is predominantly used in pond sizers.
With this cover, Valmet has introduced a highly engineered rubber-based cover to the sizing process. The cover not only aims to achieve longer running times, but also better paper quality development through improved ﬁlm formation on the cover surface. As a relatively rough cover at Ra > 1.0 μm, the Valmet Sizer Roll Cover SH can easily transfer the starch ﬁlm from the metering rod to the paper in the nip. Its high performance is proven by more than 800 deliveries worldwide.
A newcomer on the market, the Valmet Sizer Roll Cover SH-W bridges the gap between the economic advantages of the rubber covers and the wear resistance of the polyurethane covers for the beneﬁt of the customer. This covers builds on the reliable high performance Valmet Sizer Roll Cover SH, but further development takes the running time in the sizer
to new heights.
Valmet’s knowhow in the sizing process together with the range of dedicated state-of-the-art rubber sizer covers is a winning combination for today’s cost-eﬃcient manufacture of high-quality paper.
Published in Valmet Roll Services catalog 04/2019