« AnteriorContinuar »
METHOD AND APPARATUS FOR CREATING A PULSED STREAM OF PARTICLES
FIELD OF THE INVENTION
 The present invention provides a method and an apparatus for forming a pulsed stream of a particulate material, allowing high pulsing frequencies, and being particularly suitable for the production of disposable absorbent articles, such as baby diapers and the like.
 The invention is further directed to a kit or pack of individual absorbent articles which can be produced using the method and the apparatus.
 Creating constantly and quickly repeating pulses of particulate material suspended in a carrier means such as air has been a long lasting desire for many applications, in particular for pulses which are well controlled with regard to their shape, to their frequency, and to the amount of material transferred during these pulses. A particularly useful application is during the manufacture of disposable absorbent articles, such as baby diapers, adult incontinence or feminine hygiene pads, and the like, where the manufacturing aims at high production speed and low variability.
 In U.S. Pat. No. 4,800,102 (Takada), an apparatus and method for spraying or scattering solid particulate powders onto a substrate is described. The powder is scattered onto a rotatable disc member, which has at least one opening through which a portion of the powder can pass through to reach an underlying substrate, while the nonpassing powder is recycled to the powder feeder. Another masking process is described in PCT publication WO-A-92/ 19198 (Perneborn). Thereby, a device for depositing particles on a moving web of material has an apertured belt which moves over a material web and has a particle dispenser to dispense particles in a uniform pattern in the shape of the apertures of the belt. The particles not dispensed through the apertures are recycled back to the particle feeder.
 Both of these systems use the gravity for accelerating the powder particles, and are limited in pulse frequency and hence overall production speed. Further, as part of the powder delivered to the device is recycled, there is only limited control of the amount of powder disposed on the substrate, and hence in the produced article.
 U.S. Pat. No. 5,213,817 (Pelley) describes a powder spray ejector oscillating over a flow separator, which separates a portion of the powder being deposited on a web, and the other portion being recycled.
 Other approaches use pulsing of an air stream to create a pulsed particle stream, such as described in U.S. Pat. No. 4,927,346 (Kaiser), U.S. Pat. No. 6,033,199 (Vonderhaar). In U.S. Pat. No. 5,028,224 (Pieper) an apparatus and process for providing a pulsed particle stream is described, wherein a continuous gas entrained stream of particles is centrifugally diverted into an accumulation region, from where it is selectively discharged, such as by the use of a pulsed air stream.
 U.S. Pat. No. 4,543,274 (Mulder) discloses a powder spray gun wherein high velocity air is said to impact powder entrained air contained in the bore of the gun. U.S.
Pat. No. 4,600,603 (Mulder) discloses a powder spray gun apparatus wherein an inverted flow amplifier is located adjacent to the inlet of the gun to enhance blending of powder within the gun. From the inverted flow amplifier, the blended powder is supplied to a downstream air flow amplifier which is operable to impact air entrained powder with a high velocity stream of compressed air. A powder control system controls powder supply from powder supply pumps to the spray gun. The powder pumps are said to be conventional venturi powder pumps.
 U.S. Pat. No. 4,770,344 (Kaiser) discloses a powder spraying system including a volumetric or gravimetric material feeding device for metering a quantity of powder into a manifold, and air flow amplifiers connected to passageways formed in the manifold. Kaiser '344 teaches that a problem associated with venturi powder pumps is the difficulty in obtaining a consistently accurate feed rate of powder material, especially when a spray gun is operated intermittently. U.S. Pat. No. 4,927,346 and U.S. Pat. No. 5,017,324 (Kaiser) disclose additional embodiments for depositing particulate material into a pad with a spray gun, including an embodiment having an inverted flow amplifier and an embodiment having a rotating screw for providing a metered quantity of absorbent particles. U.S. Pat. No. 5,037, 247 (Kaiser) discloses a powder pumping apparatus having a venturi passageway and an air ejector including a valve mechanism. Kaiser '247 teaches that it is desirable to include a valve in the air ejector to eliminate the "dead zone" in the air supply tube extending between the valve and the inlet to the pump body, and thereby eliminate the powder pulse "tailing effect" experienced in other powder pump designs. However, such an arrangement has the disadvantage of a requiring a valve assembly adjacent to or within the ejector, which may not be practical or even possible in every installation due to space or geometry constraints. These approaches have in common, that they primarily create a pulsed gas/air stream, which accelerates the particles to create a pulsed particle stream. However, such air pulses are difficult to control in stable manner, in particular for higher pulse frequencies and higher particle flow rates.
 Henceforth, the present invention aims at overcoming limitations of the known systems, in particular with regard to pulse frequency so as to allow for higher production speeds, as well as with regard to higher throughput on a per pad basis, so as to satisfy the requirements of modern absorbent article design.
 As a further objective the invention provides a kit or pack of individual absorbent articles in a cost effective manner.
 The present invention is a method of creating a pulsed stream of particles in a carrier means, which includes the steps of suspending a first metered stream of particles in a carrier means, guiding this first stream to a pulsing means, accumulating a portion of these particles in a pulsing chamber of the pulsing means, which further includes a separator means, and emptying the particles out of the pulsing means by a suction means, whereby the accumulation is performed by interrupting the stream of particles as flowing from an inlet of the pulsing means to an outlet of the pulsing means by the separator means for not less than 95%, preferably not
less than 90%, more preferably not less than 75% and even more preferably not less than 50% of said time of a pulse.
 Preferably, the separator means rotates in the pulsing means. It is also preferred, that the suction means is an venturi-type ejector, or a ring-jet-type coaxial ejector, preferably positioned in proximity to the outlet of the pulsing means, and that the suction means is positioned in proximity to the outlet of the pulsing means. The present invention is particularly suitable for creating pulses at a frequency of at least 10 Hz preferably more than 15 Hz, even more preferably more than 20 Hz.
 In a further aspect, the present invention is an apparatus for pulsing a metered stream of particular material in a carrier means comprising, a metering means, a pulsing means having an inlet, an outlet, a pulsing chamber located there between and comprising a separator means, and a suction means arranged in proximity of the outlet. The separator means is arranged to interrupt said flow of particles between the inlet and the outlet for not less than 95%, preferably not less than 90%, more preferably not less than 75% and even more preferably not less than 50% of said time of a pulse. The separator means may be designed to not interrupt the flow of the carrier means, which preferably is a gas, such as air.
 It is also preferred, that the suction means is an venturi- type ejector, or a ring-jet-type coaxial ejector, preferably positioned in proximity to the outlet of said pulsing means, and that the suction means is positioned in proximity to the outlet of said pulsing means.
 In a further aspect the invention relates to a kit or pack of individual absorbent articles, the absorbent articles being produced by a method of low standard deviation manufacturing, the kit or pack of absorbent articles comprising at least 10 individual absorbent articles which have been produced consecutively by the method of low standard deviation manufacturing, the absorbent articles each comprising a topsheet and a backsheet and an absorbent core encased between the topsheet and the backsheet, the absorbent core comprising a first material providing for a first absorbent capacity and a second absorbent material providing for a second absorbent capacity, the absorbent core having a longitudinal direction, the absorbent core comprising a front half and a rear half, the halves having equal length as measured in the longitudinal direction, the front half of the absorbent core comprising more than 60% of the second absorbent capacity, the second absorbent material comprised by the absorbent material of each of the absorbent articles having a total weight, the kit or pack of absorbent articles having a average total weight taken as the average of the total weights of the particulate absorbent material of individual articles, the kit or pack of absorbent articles having a standard deviation of total weight calculated based on the deviation of the total weight of the particulate absorbent material of individual articles from the average total weight, wherein the standard deviation of total weight is less than 8%.
BRIEF DESCRIPTION OF THE FIGURES
 FIGS. 1A, B, C, and D show schematically diagrams of pulse patterns.
 FIGS. 2A and 2B show schematic presentation of an uninterrupted, continuous particle flow path (FIG. 2A), and an interrupted flow path (FIG. 2B).
 FIGS. 3A and 3B show schematic presentations of an exemplary pulsing means according to the invention.
 FIGS. 4A through 4C show exemplary embodiments for separator means useful in such a pulsing means.
DETAILED DESCRIPTION OF THE
 Within the context of the present description, the term "pulse" is used to describe the time dependency of a particle flow in a certain, repeating pattern. This pattern can be described via the local flow of material per time interval (in units of g/sec) and a repeating frequency defining a time interval for the pulse.
 Thus, in FIG. 1A, a typical pulse pattern 100 is depicted, showing an example for a repeating particle flow pulse. The pulse has a pulse duration 110, a pulse repeating time period 120 (defining a pulse frequency), and a peak pulse flow rate 130. If there is no particle flow between two pulses, the minimum pulse flow rate 140 is equal to zero. The particle flow can be further described by the average flow rate 135. The particle flow can also be expressed by the particle density, defined by the volumetric flow of particles divided by volumetric flow of air.
 In particular cases, the pulse can have two (or even more) plateaus with a second plateau flow rate 150 for a second plateau duration time 155 (see FIG. IB), which even further may be interrupted (see FIG. 1C), whereby a first pulse duration and frequency (145, 147) and a second pulse duration and frequency (157, 159) can be distinguished.
 The shown rectangular "pulse shape" is certainly often desired, but generally the shape will differ to a certain extent, and in the extreme, a can be formed by gradually increasing and decreasing flanks 170, 180 (see fig. ID).
 Within the context of the present description, the term "flow path" is used to describe the path of a moving object, such as a particle. A flow path between two locations (such as cross-sectional areas 210 and 220 of a tube 200 as shown in the schematic cross-sectional view in FIG. 2) is called uninterrupted, or continuous, if a particle can move from such a location 210 (inlet) to another location 220 (outlet) without encountering a physical barrier, as indicated by the continuous arrows 240. It is called interrupted, if a particle is hindered by physical barrier, such as schematically and exemplarily indicated by a rotary valve element 230. For this instance, there will be separated flow paths on both sides of the barrier, as indicated by the flow path arrows 245 and 247 respectively. While, of course, also movements of fluids like gases can be described flow paths (and also both continuous as well as interrupted ones), the term "particle" is used herein to describe discrete solid particles, for example in the context of disposable absorbent articles it can be absorbent particles, or superabsorbent particles, which are essentially dry particles having a particle size which can range from several microns to several millimetres. Such particles can be suspended in a "carrier", such as a gas such as air.
 Description of the Features of the Process and Apparatus
 The present invention is not limited to a particular application, and flow rates, pulse frequencies can be varied
in a broad range without departing from the essence of the present invention. However, the following explanations will refer in certain aspects to specific examples, which will be—without limiting the present invention to this field—the manufacture of disposable absorbent articles, such as baby diapers and the like.
 Particle Flow Metering
 Metering devices to provide well defined particle mass flow rates, in particular constant predetermined flow rates, are well known in the art. Such a metering apparatus can include a hopper with, for example a screw feeder and a scale or "loss-in-weight control". A suitable metering apparatus particularly suitable in the manufacture of absorbent articles is an Acrison Volumetric Feeder, Model No. 405-105X-F, available from Acrison, Inc. of Moonachie, N.J. Such a metering apparatus can be operated to provide a mass flow rate of up to about 1500 kg/hr or more, preferably between 30 kg/hr and 1200 kg/hr.
 The particle metering apparatus can be connected for further conducting the metered particle stream to a connecting means. A typical example for such a connecting means is a tube having an inner diameter of about 2.5 cm (about 1 inch). Preferably, the connector means does not have sharp edges or bends, as this might influence the stability of the particle stream.
 If the metering apparatus and the pulsing means are appropriately arranged with regard to their relative positioning, there is no need for a carrier means to carry the particle from the metering apparatus to the pulsing means, but gravity would suffice to let the particles fall from the first to the second. However, often it can be advantageous to have some carrier flow, such as air flow. If an additional carrier stream is used, this is preferably done at moderate carrier speed, and in a preferred embodiment as described hereinafter, carrier velocities of between 1 and 20 m/sec have been found to be suitable. This carrier stream is further preferably steady to maintain a constant particle stream. In case of carrier flow fluctuations, these are preferably in phase with the pulsing frequency so as maintain stable conditions. For the described exemplary application in the manufacture of absorbent articles, such a carrier flow can be created by having an opening to the ambient in the connecting means, positioned close to the metering apparatus. Suction as applied on the other side of the pulsing means (and discussed hereinafter) can suffice to provide stable particle flow conditions.
 An important element of the present invention is the pulsing means, arranged (in following the flow path direction of particles) after the connection means, and operated so as to create the pulsed particle flow.
 The pulsing means is designed to allow interrupting the particle flow in a repeating manner, whereby the particles are accumulated during this interruption period and released thereafter. The pulsing means comprises an inlet, through which the particles can enter the pulsing means, an outlet, through which the particles can exit the pulsing means, a pulsing chamber positioned between the inlet and the outlet providing sufficient space to allow accumulation of at least some of the particles, and a separator means, positioned in this pulsing chamber.
 While it may interrupt the carrier flow for a part of a cycle time, there has to be a certain time, during which the
carrier flow path and a particle flow path are connected from the inlet of the pulsing means to the outlet of the pulsing means. Without wishing to be bound by the explanation, it is believed, that this period is important to stabilize the flow properties of the carrier.
 A pulsing means suitable for applications such as in the production of absorbent articles can be designed to pulse a stream of absorbent particles, with typical sizes in the range of several micron to few millimetres, and with particle flow rates in the range of 1500 kg/hr or more. For such an application, pulse frequencies can range from about 3 to about 35 Hz or even more.
 A suitable pulsing means in the context of the present invention impacts on the particles directly in a valve-type function. This is to be seen in contrast to other approaches, wherein a pulse of a carrier means, such as a pulsed air stream, impacts on the particles. The valve type-operation can be realized by various designs, such as oscillating slide valves, iris-type valves, diaphragm-type valves, rotating, apertured disks similar to the design as described in U.S. Pat. No. 4,800,102 (Takada).
 A further exemplary and preferred pulsing means builds on the principles of a rotary valve, as is well known in the art as a closure element, such as for a storage container for particulate material. Therein, however, they are designed to hermetically separate the storage container from the subsequent system, such as a pneumatic transport system, without providing a certain period of the cycle time with a continuous particle flow path—see as one of various exemplary disclosures U.S. Pat. No. 3,974,411 (Miller). Alternatively, rotary valves are known to provide for an "openclose" functionality (i.e. no accumulation functionality as in the present case), such as described in U.S. Pat. No. 4,393, 892 (Di Rosa).
 One particular benefit of such rotary designs is the avoidance of oscillatory movements, which, in particular for higher frequencies, would create either undesirably heavy (and hence difficult to accelerate) elements, or designs with a non-satisfactory reliability. In contrast to these, a rotary design can keep the separator means operating at a constant speed, thus allowing a much more stable operation even for high pulse frequencies.
 As depicted in a schematic, cross-sectional view— see FIG. 3A—such a preferred rotary pulsing means 310 can comprise a rotating separator means 330, rotatably mounted in a pulsing chamber 320, having a cylindrical shape with a diameter and a height, of the pulsing means 310. Further indicated is a particle flow path 370, freely connecting the inlet 340 and the outlet 350, without being obstructed by a separator means 330. FIG. 3B schematically shows the same equipment (with equal numerals indicating same elements), now at a different rotational position of the separator means 330, such that there is no free particle path connection between the inlet 340 and the outlet 350, but there is a filling flow path 372 disconnected from the emptying flow path 374.
 When, during the operation, the separator means 330, as it rotates at a predetermined frequency, it takes the position of interrupting the particle flow path, the particles, arriving at the inlet 340 at an essentially constant stream will accumulate in that part of the pulsing chamber 320, which