The function and proper setup of size press sealing blades

May 14, 2019

On Valmet SymSizers/OptiSizers, starch or coating is metered onto the applicator roll with rotating rods – grooved for starch, smooth for coating. The starch/coating is applied to the roll in a small application chamber. This chamber is pressurized to ensure adequate coverage. The chamber is bordered by the coating supply chamber, the rotating rod, and the stationary sealing blade (as shown in the following illustration).

Design and operating principle of the sealing bladeFunction of the sealing blade

Probably the most important component is the sealing blade because it is stationary and runs against the applicator roll. It is important in three ways. First, it creates one of the three boundaries for the application chamber. Second, it allows some of the coating to pass through its slots, creating a barrier which prevents air from entering the application chamber. Finally, a portion of this recirculated starch/coating moves upward with the roll and acts as a lubricant for the tip of the sealing blade. Without this lubrication, the sealing blade tip would wear to a sharp point and start to wear the applicator roll cover prematurely.

Checking the sealing blade

When inspecting the sealing blade, first check how it moves in its slot. It should be able to easily move in the cross machine direction by hand. CAUTION: Use gloves to check this as blade tip may be sharp.

Along with the CD movement, the machine direction should also be free to move. The blade should not be stiff in the MD, which can cause extra force on the applicator roll. If it is stiff, the sealing blade should be replaced and its mounting slot cleaned out.

Next, the chamber end seals have to be set properly to allow thermal expansion of the sealing blade. The rule of thumb is to set the end seals so they are almost touching the foam end seals. The sealing blade has to be free to flex and not be influenced by any part of the end seals. It's important to have the foam sealing blade dimensions set correctly to match the shape of the application zone. It is also necessary that at least one of the sealing blade slot deckle strips be about 1/4” out from the blade to allow for thermal expansion.

Securing the blade ensures trouble-free use and prevents accidents.

Securing the blade ensures trouble-free use and prevents accidents

Another thing to keep in mind is the claws that lock the blade into its holder, as shown in the preceding illustration. An important safety issue is to produce and keep the claws at the correct angle and in good shape.

Packaging prevents sealing blade damage.Blade packaging

Valmet ships sealing blades in individual cardboard boxes to prevent any damage during transportation and handling.

Examples of the packaging are shown in this illustration. The blade is neatly coiled and ready for removal and installation into the size press.

The package is also handy at the mill for blade storage and inventory control.

Blade geometries vary based on application

Typical sealing blade geometriesAs seen in this illustration, the size and placement of the slots is critical for proper lubrication. Blade height and thickness are also critical for proper chamber application.

The type of steel used is important in order to guarantee that the sealing blade is straight rather than wavy. A wavy sealing blade will cause coat weight and roll wear issues in the cross machine direction.

The blade material should be corrosion resistant stainless steel with good straightness, surface finish and wear resistance.

Valmet Sealing Blade Bent

Valmet Sealing Blade Bent is a pre-bent sealing blade designed to increase the contact area between the blade tip and the roll surface (shown in the green circle in the following illustration). By bending the blade at an angle, the surface pressure between the tip area of the sealing blade and the roll decreases.

Valmet Sealing Blade Bent increases contact area between the blade tip and roll surface.

Valmet Sealing Blade Bent increases the contact area between the blade tip and roll surface.

Valmet Sealing Blade Bent is less prone to breakage, resulting in longer blade lifetime. Blade replacements can be synced with pre-planned machine shutdowns due to the blade's long lifetime. This results in improved machine efficiency and optimized production costs.

The blade has less bending and reduces the splashing tendency. The high quality stainless spring steel provides corrosion resistance and a long applicator roll cover grinding interval. There are no sharp edges in Valmet Sealing Blade Bent making it ideal for easy handling and improving work safety.

For more information on improving your size press operation, contact your Valmet representative.