Glass Fiber Wetlaid - fiberglass nonwovens
Applications of fiberglass nonwovens «wetlaid»
- Non-flammable upholstery materials;
- Bases for gypsum-fiberglass panels for interior decoration;
- Bases for roofing materials impregnated with bitumen;
- Bases for non-combustible, lightweight composite structures for use in the manufacture of vehicles with a high degree of fire safety;
- Filtration of hot media;
- Base for linoleum and other composite materials for flooring;
- Insulation panels, etc.
Everything starts with fibers
In the production of nonwoven fabrics using wetlaid technology, regardless of which fibers are used, everything starts with the correct preparation of a water-fiber suspension. What does it mean – «to properly» prepare the suspension? To do this, many technological and technical factors must be taken into account, discussed below.
- Fibers dosage accuracy in the pulper.
- Correct mass balance of a water-fiber suspension:
- When producing a product on an industrial scale, you must know exactly the final proportion of water/fiber in the suspension, which should be at the entrance to the diffuser;
- It is necessary to understand if you mix several types of raw materials, whether they can be mixed in one tank and then transferred to an averaging container, or whether initial stirring (hydraulic fracturing) in different containers is necessary;
- It is necessary to understand, depending on the type of fiber (fibers), whether one averaging capacity will be enough for you, or a larger number of containers is needed to achieve a stable required mass balance.
- Absence of lumps of fibers, which is determined by the quality of raw materials, the presence of the effect of tortuosity in the fibers or oiling agents, as well as the correct setting of technological and technical parameters of the hydraulic breaker and averaging tanks with agitators, and, of course, the type of centrifuge paddle pump.
- Uniformity of the dispersion of fibers on the inclined mesh of the molding machine, which depends on the uniformity and constancy of the mass balance in the water-fiber suspension, as well as on the number and mood of diffusers.
- Cleanliness of the equipment during operation.
- Minimum possible air ingress into the elements of the technological system.
- Pipelines of the system must exclude the formation of dead zones in them (in order to exclude the accumulation of fibers in them that do not fall into the flow), as well as the formation of abnormal insufficient or excessive turbulence.
- Flow sensors (flow meters) and control valves must be in the right places.
Preparation of suspension (stock preparation)
Initial stages of the technological process are the opening of bales, openning and dry mixing (if necessary) of fibers. This is followed by the transportation and precise dosing of fibers into the pulper and further into the averaging tanks. Mixing of fibers in order to evenly distribute them in suspension and achieve a constant fiber-water proportion. Mixing should be carried out carefully, on previously confirmed settings of the process equipment, as excessive mixing can damage the fibers, attract air into the system and generate foaming. On the other hand, if there is insufficient mixing, lumps of fibers may be present in the suspension, and this also threatens the instability of the mass balance of the suspension (proportion of "fiber-water"), and, accordingly, unacceptable variation of the final product in thickness and surface density. An important role in achieving the required characteristics of the suspension is played by shapes of vessels and agitators, as well as the mixing speed.
When using fiberglass as a raw material, it is very important to choose the correct entry/exit points of pipelines to/from averaging tanks to improve mixing, reduce air ingress into the system and obtain a uniform suspension at the outlet. Blade deposits should be specially selected for fiberglass (type of impeller, flow rate). A closed water circuit should be used for the separation of dirt and substandard fibers, filtration of water and its return to the process. The presence of several types of filtration systems in the technological process (rotary, scraper, etc.) is mandatory.
When forming the primary web on an inclined grid, the following factors will have maximum impact on quality of the final product:
- Size and design of the blade pump (pulsation, etc.);
- Uniformity of supply and distribution of water-fiber suspension on the molding grid;
- Size, design and number of diffusers;
- Characteristics of the hydraulic pressure chamber (adjustable gap, lips, etc.);
- Characteristics of water removal chambers (vacuum, direction of flow of the removed water, orientation of fibers);
- Condition and features of the installation of doctor blades;
- Type and characteristics of the forming mesh, the system of transfer of the web from the forming mesh to the mesh for applying binders and to the conveyor passing through the drying chamber;
- Frequency of cleaning of the forming mesh;
- Edge removal and recycling.
Design specifics of the forming part
- A very important point for forming a uniform web is the presence of sufficient space in the tanks and working chambers of the process equipment, allowing the fiber to move freely in a water-fiber suspension.
- A sufficient degree of turbulence of the water-fiber suspension must be provided, allowing the fibers to be constantly in motion.
- Optimal orientation of the fibers during laying.
- Single or double-layer hydraulic pressure chamber;
- Use of suspension viscosity modifiers, dispersants and surfactants to avoid clumping in the suspension. However, the uncontrolled use of these substances has a negative impact on the molding of the web at different stages of the technological process and requires changes in technological settings.
Continuous vs. periodic production
The reasons that may prompt the manufacturer to switch from periodic to continuous production are:
- Periodic production limits the productivity of the line;
- There is no complete automation of the technological process;
- A small number of types of simultaneously used raw materials (3 or less);
- Difference in the proportion of water-fiber suspension in different recepies is not significant.
The article pays the main attention to such basic stages of the wetlaid process as mass preparation and web forming. The remaining stages include application of additives, drying, control of thickness and density of the web and winding/slitting. With regard to fiberglass, it cannot be said that standard solutions will suit you according to the data of uncovered stages of the technological process. To determine the most optimal layout of the line you will still need to work very closely with machine builders offering the described equipment and technology. Only on the basis of the experience of the machine builder and test results on pilot lines, as well as based on your requirements for the final product, you will be able to determine whether you should install single or double-sided additive application, full impregnation, additional infrared dryer, flat or Omega drying chamber, etc.
One of the world leaders in mechanical engineering offering wetlaid equipment and technology is, of course, ANDRITZ Küsters GmbH, Germany, which specialists will always be happy to clarify any points further if you have a desire to assess the prospects for launching wetlaid projects or the ones related to other available technologies.