Enhancing performance through effective sizing – Valmet’s soft rubber covers
Sizing plays a critical role in paper and board manufacturing by improving key product properties. Starch-based sizing increases both surface and bulk strength, while alternative sizing or pigmenting agents – such as PVA or silicone – can transform base paper into specialty products like release papers. Sizing can also densify and seal the sheet surface, reducing absorption during subsequent coating processes.
Valmet Sizer Roll Cover SH is a rubber cover giving solid performance in the sizer. During 20 years of appearance, more than 1,000 covers have been delivered around the world.
Several sizing and coating methods are used in the industry, with film transfer sizers and pond sizers being the most common for starch application. While effective, pond sizing has limitations in speed and application control, which has driven the strong shift toward metered film sizing over recent decades.
In the metered film sizing process, starch is applied to a soft-covered roll and metered to the correct film thickness using a grooved rod. The size is highly fluid and contains few particles, enabling nearly complete transfer to the sheet as it passes through the nip between two soft applicator rolls. To ensure efficient starch penetration, the size must be warm, and the nip load sufficiently high.
Soft sizer roll covers – the foundation of efficient film transfer
Starch efficiency – the degree to which starch penetration improves paper or board strength – is strongly influenced by nip pressure, making the sizer rolls a critical part of the process. Both nip load and its cross‑directional profile affect sizing performance and sizer runnability.
The applicator roll cover must maintain reliable contact with both the metering rod and the sheet. Traditionally, film sizers have used rubber covers in the 15–25 P&J hardness range. To further improve performance, Valmet has also introduced modern polyurethane covers, but soft rubber covers remain essential for many applications.
Valmet’s soft sizer covers are engineered to provide an optimal balance of flexibility, strength, and wear resistance. They ensure stable contact with the grooved rod, withstand high nip loads, and deliver long running intervals. Harder covers promote deeper starch penetration – ideal for liner and fluting grades – while softer covers leave more starch on the surface, preferred for copy paper and uncoated woodfree grades. As always, Valmet experts support mills in selecting the optimal cover design for each application.
The rubber compounds used for the covers are resistant to chemicals and perform reliably under the hot, wet conditions of the sizer nip. To meet the full range of sizing needs, Valmet offers a comprehensive portfolio of soft rubber covers at various performance and cost levels.
Valmet Sizer Roll Cover SH product family
Valmet offers several soft rubber covers options for various sizing needs.
Valmet Sizer Roll Cover SH
Introduced more than 20 years ago, Valmet Sizer Roll Cover SH has become a trusted workhorse for many sizing applications. It delivers long running times, stable film formation, and consistent paper quality. With more than 1,000 deliveries worldwide across grades ranging from printing papers to containerboard, SH remains a benchmark for reliability.
Valmet Sizer Roll Cover SH‑W
Valmet Sizer Roll Cover SH‑W builds on the proven SH platform with an enhanced rubber compound that significantly improves abrasion resistance. This cover bridges the gap between the cost‑efficiency of rubber and the durability of polyurethane, offering longer running intervals without compromising film quality.
Valmet Sizer Roll Cover SH‑EC
The newest member of the SH family, Valmet Sizer Roll Cover SH‑EC, combines excellent profile wear resistance and impact strength with a stable, elevated surface roughness. This surface design ensures robust film transfer even under harsh conditions, such as recycled furnish contamination, aggressive sizing chemicals, and heavy rod wear. SH‑EC is engineered for mills requiring maximum reliability and film quality in demanding environments.
Valmet Sizer Roll Cover SH-EC is based on the latest rubber cover technology to offer maximal film quality and robustness in extreme conditions.
Hard roll covers for pond sizers and hard nip sizers
In addition to soft rubber covers, Valmet provides a full range of hard covers for pond sizers and hard nip sizers. These include bone‑hard rubber covers (Valmet Press Roll Cover GG and Valmet Press Roll Cover RR2), composite covers (Valmet Sizer Roll Cover SG), and ceramic covers (Valmet Sizer Roll Cover SG‑W).
These covers have a long track record of performance and reliability in hundreds of installations worldwide. The SG and SG‑W covers, featuring updated material formulations, are the preferred technologies for Valmet Hard Nip Sizers. They enable higher nip loads and peak pressures, improving starch efficiency and runnability while maintaining long cover lifetimes.
Winning solutions for improving overall sizer performance
Valmet’s expertise in sizing processes, combined with its comprehensive range of advanced roll covers, provides mills with a powerful foundation for cost‑efficient production of high‑quality paper and board. Beyond roll covers, Valmet offers full‑scope optimization solutions with fast payback potential.
Technologies such as iRoll Sizer, iRoll portable, and optimized crowning help mills address challenges including sheet wrinkling, the need to reduce nip load over roll cover running interval, web breaks, and uneven sizing application. Working closely with Valmet specialists ensures that mills can achieve measurable improvements in runnability, application quality, and overall sizer performance.
