Typical spunbonded processes utilize a series of spaced-apart spinneret assemblies to convey a fibrous material from a spinning orifice onto a foraminous collection belt. Multiple spinneret assemblies are often located downstream from one another in order to lay down a number of overlapping layers of the fibrous material. The fibrous material is conveyed to the collection belt in a stream of gas. A typical system is disclosed in Troth, Jr., U.S. Pat. No. 3,477,103, the contents of which are incorporated herein by reference. The fibrous material is separated from the gas stream and electrostatically pinned to the surface of the collection belt. The spent gas stream is exhausted away from the belt in some fashion. In many processes, this is done by sucking the gas stream through the foraminous belt.
However, if the fibrous material is relatively dense, so that it clogs the openings in the foraminous belt, or if the collection belt is impermeable to the flow of gas (e.g., rubber), the gas stream cannot be effectively exhausted by sucking it through the belt. If the spinneret assemblies are spaced far enough apart, the gas streams produced by the spin orifices will not interact nor interefere with each other and the gas will simply dissipate as it travels along the collection belt. However, if the spinneret assemblies are spaced too close together, the gas streams produced by the spin orifices will interact and interfere with each other and adversely affect laydown of fibrous material at adjacent positions along the collection belt. This latter condition greatly affects sheet uniformity.
A spunbonded fibrous sheet comprised of plexifilaments of flash-spun polyethylene is described in Lee, U.S. Pat. No. 3,504,076, the contents of which are incorporated by reference herein. The spin-cell apparatus used to form the plexifilaments (shown in FIG. 1 of Lee) utilizes a number of spin orifices spaced across the width of the apparatus and positioned downstream one from the other. In a subsequent improvement to Lee, the spin orifices are further equipped with rotating baffles and aerodynamic shields to direct the gas streams downwards toward the collection belt. The downwardly directed gas streams are often referred to as laydown jets. The aerodynamic shields are shown in Brethauer et al., U.S. Pat. No. 3,860,369, the contents of which are incorporated by reference herein.
When a gas stream conveying fibrous material is directed downward so that it impacts the belt, approximately half the flow is diverted in a generally upstream direction with respect to the moving belt and approximately half the flow is diverted in a generally downstream direction with respect to the moving belt. These flows are typically turbulent in nature and remain so until they slowly lose velocity as they travel along a sufficient length of the belt. When gas streams (i.e., laydown jets) are closely-spaced in the machine direction, so that one gas stream which travels along the belt collides with an adjacent gas stream, the flows are diverted in a more upward direction thereby generating a turbulent fountain or plume of exhaust gas. The resulting plume recirculates into the flow path of the downwardly directed laydown jets causing instabilities and disruptions in the uniform formation of the fibrous sheet. The closer the machine spacing between laydown jets, the more severe the disruptions caused by these uncontrolled turbulent flow patterns.
While the Lee-Brethauer apparatus works satisfactorily when the laydown jets are spaced far apart, it is not nearly so satisfactory when the laydown jets are close together as would be desired for several reasons. These reasons include: (1) investment is reduced when the spin-cell and enclosed spinneret assembly are made smaller in size; (2) sheet uniformity is improved by increasing the number of laydown positions and thereby the number of overlapping layers of fibrous material that make up the spunbonded sheet; and (3) spinneret assembly capacity is increased by increasing the number of laydown positions or the throughput per laydown position.
Clearly, what is needed is a gas management system which reduces or even prevents interferences or, interactions between the gas streams of adjacent, closely-spaced laydown jets. Other objects and advantages of the invention will become apparent to those skilled in the art upon reference to the attached drawings and to the detailed description of the invention which hereinafter follows.