Liquid application is one of the key technical considerations in pull-out flushable wet toilet tissue production. The timing and method of liquid application directly affect moisture uniformity, liquid penetration, finished-product weight, machine design, maintenance requirements, and equipment investment.
Unlike ordinary wet wipes, wet toilet tissue often uses folded and perforated sheets that are stacked before packaging. This means the liquid must penetrate the complete stack while maintaining consistent moisture and finished-product quality.
Different wet toilet tissue production lines use different liquid application methods. The four common approaches include online liquid application during folding, liquid application before cutting, liquid application after cutting, and a combination of pre-wetting with secondary liquid application.
Each method has its own advantages and limitations.
Online Liquid Application During Folding
Online liquid application provides good material coverage because the liquid is applied continuously while the nonwoven material is being folded.
Since the material is wetted during the folding process, liquid can contact the material before the sheets form a complete stack. This reduces the risk of some inner or middle layers remaining completely dry.
However, when online wetting is used as the main liquid application process, a relatively large amount of liquid must be added during folding. This means all downstream equipment needs to handle wet material.
As a result, more stainless-steel construction and enclosed machine structures may be required.
For wet toilet tissue, the folding machine itself is also more complicated than conventional wet wipe equipment. A fully enclosed structure can make machine adjustment, operation, observation, and troubleshooting more difficult.
Long-term contact between wet material and components such as rollers and folding forks may also create concerns about product quality and machine durability.
Liquid dosing and final product weight must therefore be carefully controlled.
Overall, online liquid application can provide good wetting coverage, but it requires a more complex machine design and greater consideration of wet-material handling.
Liquid Application Before Cutting
Applying liquid after folding and stacking but before cutting reduces the amount of wet material handled inside the folding section, but uniform liquid penetration becomes more challenging.
With this method, the material is first folded, perforated, and stacked. Liquid is then applied to the complete stack before it is cut into individual packs.
Moving the main wetting process away from the folding section reduces the need for the folding machine to continuously process heavily wetted material.
The main challenge is liquid penetration.
A stack may contain many folded sheets. The outer layers are naturally easier to wet, while liquid needs more time and appropriate distribution to reach the middle layers.
If the liquid is mainly applied from the outside, some internal areas may receive less liquid than the outer areas.
Manufacturers therefore need to consider several factors:
- Spray arrangement
- Liquid volume
- Penetration time
- Stack thickness
- Material characteristics
- Stack structure
These factors need to work together to achieve consistent moisture throughout the product.
Liquid Application After Cutting
Applying liquid after cutting provides better access to the finished stack and can offer a practical balance between moisture distribution and equipment investment.
Once the folded material has been cut into individual packs, more surfaces and gaps are exposed.
Liquid can therefore penetrate the stack from multiple directions, generally making it easier to achieve more uniform wetting compared with applying liquid to a larger uncut stack.
Another important advantage is equipment cost.
Because the main wetting process begins after folding and cutting, these upstream machines do not necessarily need to be designed as fully stainless-steel wet-processing equipment.
This can reduce the overall investment for manufacturers that need to control the initial project budget.
The liquid application unit can then focus on controlled dosing and even spraying of the individual finished stacks.
For this reason, post-cut liquid application can be a practical solution for manufacturers looking for a balance between wetting performance, equipment complexity, and investment cost.
Pre-Wetting + Secondary Liquid Application
For higher-end wet toilet tissue production lines, combining pre-wetting during folding with secondary liquid application after cutting can provide better control over both wetting coverage and final moisture levels.
The two stages perform different functions:
Pre-wetting solves coverage. Secondary liquid application solves distribution and final liquid quantity.
Pre-Wetting During Folding
The purpose of pre-wetting is to ensure that the entire material receives initial moisture, including the inner and middle folded layers.
A relatively small amount of liquid is applied while the material passes through the folding process.
Unlike a system where the full liquid quantity is applied during folding, the objective is not to reach the final moisture level at this stage.
Instead, pre-wetting helps ensure that internal layers are not completely dry before the product enters the main liquid application process.
This reduces the liquid load handled by the folding section while retaining the coverage advantage of online wetting.
Secondary Liquid Application After Cutting
The main liquid application after cutting controls the final liquid quantity and helps achieve more consistent moisture and finished-product weight.
After the folded stack has been cut, the remaining liquid can be accurately dosed and distributed through the individual stack.
Metered dosing and multi-side spraying can be used to control the required liquid quantity and improve penetration from different directions.
This provides another opportunity to adjust the final moisture level and product weight.
The two-stage approach therefore combines the advantages of different liquid application methods while reducing some of their individual limitations.
However, it also requires a more complex liquid application system and higher equipment investment. It is generally more suitable for higher-end production projects with stricter requirements for moisture consistency, weight control, and finished-product quality.
How to Choose the Right Liquid Application System
There is no single liquid application method that is suitable for every wet toilet tissue production line. The right solution depends on the product, capacity, equipment design, quality requirements, and investment budget.
Manufacturers should consider the following factors before selecting a system:
- Product structure: Sheet dimensions, stack height, and sheets per pack can affect liquid penetration.
- Moisture requirements: Products requiring more precise moisture levels need better dosing and distribution control.
- Production capacity: High-speed production lines require stable liquid application without creating bottlenecks in conveying or packaging.
- Equipment design: Applying liquid earlier in the process means more upstream equipment must handle wet material.
- Maintenance: Stainless-steel and enclosed structures can improve resistance to moisture but may increase equipment complexity and maintenance requirements.
- Investment budget: Post-cut liquid application can reduce the need for full stainless-steel construction and help control initial equipment cost.
- Product consistency: For products with stricter requirements, two-stage liquid application can provide better overall process control.
Manufacturers should therefore evaluate the liquid application system as part of the complete production line rather than selecting it as an independent machine function.
Conclusion
Liquid application is more than simply spraying water or lotion onto wet toilet tissue. The timing and method of liquid application affect wetting coverage, penetration, product weight, machine construction, maintenance, and overall production-line investment.
Online liquid application during folding provides good material coverage but requires more extensive wet-material handling.
Applying liquid before cutting reduces the wet-processing requirements of the folding section, although achieving uniform penetration through a large stack can be more difficult.
Post-cut liquid application exposes more surfaces of the individual stack, making multi-directional penetration easier while potentially reducing the need for full stainless-steel upstream equipment.
For higher-end production requirements, combining a small amount of pre-wetting with secondary liquid application after cutting can provide a practical balance between initial coverage, final liquid distribution, equipment design, and finished-product consistency.
For manufacturers planning a new wet toilet tissue production line, the liquid application method should therefore be selected together with the folding, cutting, packaging, material, capacity, and finished-product requirements.
The best solution is not necessarily the most complex liquid application system. It is the configuration that can consistently achieve the required moisture level and product quality while matching the manufacturer’s production requirements and investment budget.
