Apparatus and method for monitoring cover sheet webs used in the manufacture of diapers

[0001] This invention relates to an apparatus and method for detecting defects in cover sheets used in manufacturing diapers.

[0002] Non-woven cover sheets are used in the manufacture of disposable diapers in order to cover the absorbent pads of cellular material.

[0003] These cover sheets must not contain any holes through which the cellular material might fall. They must also not contain thickened areas which interfere with the rapid incorporation of the cover sheets into the diaper during manufacture. Also, foreign matter on the cover sheet such as flies or dirt are obviously undesirable since it would be disturbing to customers.

[0004] During the manufacture of diapers, the cover sheet is supplied to the diaper making machine in the form of a continuous web. In the past, the cover sheet web has been monitored by a line camera with a lens. Such cameras are susceptible to soiling; they are also difficult to align and must be mounted at a significant distance from the web.

[0005] The main object of the present invention is to provide an improved method and apparatus for monitoring cover sheet webs used in the manufacture of diapers.

[0006] A more specific object of the invention is to provide an apparatus and method for monitoring cover sheet webs used in the manufacture of diapers which overcome the abovementioned deficiencies of the known monitoring systems.

[0007] A further object of the invention is to provide a monitoring system of the type described wherein the resolution (number of available sensor points for a given width of the web) is greater than with current monitoring systems.

[0008] In accordance with the invention, a linear sensor is used to detect defects in cover sheet webs used in machines for manufacturing diapers. The sensor may be of the type used in scanners for personal computers and is arranged to scan the web prior to its incorporation into the diaper. When the sensor detects a defect in the diaper it generates a signal which is used, after a predetermined time delay, to eject the diaper containing the defective section of the cover sheet after it has passed through the diaper making machine.

[0011] FIG. 1 illustrates a diaper making machine at 10. The invention does not depend on a particular diaper making machine but, typically, such machines may be about sixty yards long and contain more than one hundred diapers in various stages of the manufacturing process. It is difficult to stop or interrupt the diaper making machine to eliminate a defective portion of the cover sheet and, in accordance with the invention, an entire diaper containing a cover sheet with a defective portion is eliminated after manufacture as explained below.

[0012] The diaper making machine includes a main motor 12 which drives the transport systems which move the diaper materials through the machine during the manufacturing process. The non-woven cover sheet is supplied in a roll 14 and is fed as a web by a series of rollers 18 into the machine 10 where it is joined with the absorbent material (not shown) to form diapers.

[0013] In accordance with the invention, a linear sensor 20 is provided near the point at which the cover sheet web 14 enters the machine 10. In the preferred embodiment, the sensor 20 is essentially the same as the sensor used in scanners designed for use with personal computers and extends across the entire width of web 14 (e.g. about 20 cm). When a defect occurs, it is sensed by the sensor 20 which then transmits a defect signal to processor 22. The processor 22 is also responsive to line speed signals which represent the line speed of the diaper making machine 10. The purpose of the line speed signals is to enable the system to determine precisely when a diaper manufactured with a defective cover sheet (as sensed by sensor 20) is exactly in position at an ejection gate so that it can be withdrawn from the line before packaging. Thus, after the processor 22 receives a defect signal from sensor 20, it counts the number of line speed pulses and, when the processor determines that the diaper manufactured with the defective cover sheet is at the ejector gate, it actuates a valve 24 to remove the defective diaper from the line.

[0014] The line speed pulses may be generated mechanically or electronically as long as they accurately represent the speed of the line so that if the line speed changes for any reason, the time delay provided by the processor before actuating the ejector gate is always the time required for the defective portion of the cover sheet to reach the ejector gate. The line speed pulses may be taken from the main motor or a driving shaft for the transport system, or they can be generated electronically, for example, by the computer control for the machine. Ejection is always triggered after a predetermined number of pulses which ensures that ejection occurs precisely as the diaper with the defective cover sheet reaches the ejection gate.

[0015] The invention does not require a specific sensor but, in the preferred embodiment, a linear sensor of the type used in scanners for personal computers is preferred. FIG. 2 illustrates diagrammatically the construction of such a scanner.

[0016] The scanner includes a housing 30 which may be an aluminum extrusion. The housing is open at its top and covered by a glass plate 32 which includes a non-reflective coating. A line of LED's 34 is supported within the housing although other light sources may be used as well. The sensor is arranged so that the cover sheet web 14 passes over the glass plate 32. Light from the LED's is reflected from the cover sheet through a lens 36 onto an array of photosensitive elements 38. The photosensitive elements 38 are preferably CMOS elements lined up on a plastic sheet with a density of about 8,000 pixels per meter. If desired, several sensor lines of different colors can be provided.

[0017] The signals from the photosensitive elements 38 are preprocessed in an analog-to-digital converter 40. The output of converter 40 is the defect signal which is passed to the processor 22 of FIG. 1. The entire unit can be sealed with only the wires for the power supply and defect signals leaving the housing. Because the cover sheet web is guided directly over the glass plate 32, the sensor unit which is compact and easy to handle, is also dust resistant.