Patent Application
20050206038
The present invention relates to a die for knurling sheet materials and in particular, but not limited to, a roller die for knurling sheet materials in the manufacture of an article.
Knurling is a process by which materials are subjected to a compressive force at a plurality of locations over their surface through the application of knurls, which are projections that extend from an otherwise relatively smooth surface. Knurling allows a material to be embossed or otherwise deformed at discrete points. Knurling also allows a material to be sealed by the application of force, and optionally heat.
The apparatus further comprises a cylindrical anvil roller 11 having a generally smooth cylindrical surface and an axis of rotation 13 that is generally parallel to the axis of rotation 7 of the die roller 3. The anvil roller 11 and the die roller 3 are arranged such that their cylindrical surfaces lie opposite each other and are spaced to form a narrow gap 15 therebetween for the passage of the sheets of material to be sealed. The die roller 3 and anvil roller 11 are spaced apart such that as they are rotated and the sheets of material pass through the narrow gap 15, the region of projections on the die roller 3 closest to the cylindrical surface of the anvil roller 11 simultaneously engage the upper surface of the top sheet and apply a downward force at discrete locations on the material's surface in accordance with the pattern of projections.
In addition to the material being subjected to localized compression by the pattern of projections, the projections may be heated to assist in sealing the sheets of material together.
One of the industrial applications where knurling is commonly employed is in the commercial production of disposable sanitary absorbent articles. Disposable sanitary absorbent articles are articles designed to be placed against the body of a wearer in order to absorb and retain fluids. Examples include, among others, sanitary napkins, panty liners, adult incontinence briefs, infant diapers, and wound dressings. Typically, these articles are of laminate construction comprising two or more layers of material united together to form an integral structure. For example, sanitary napkins commonly comprise a fluid-permeable cover layer intended to face the body of a wearer when the sanitary napkin is in use, a liquid-impervious barrier layer intended to face the undergarment of the wearer when the sanitary napkin is in use, and an absorbent system intermediate the fluid-permeable cover layer and the liquid-impervious barrier layer. Many other layers or structures may also be present. The fluid-permeable cover layer and the liquid-impervious barrier layer are united together around the periphery of the absorbent system to form a peripheral seal.
The commercial-mass production of these articles typically proceeds in the following manner. A web comprising the component materials of the article is formed. This web will have at least two and possibly more sheets of continuous material. It will also include discrete (discontinuous materials), for example, those which form the absorbent system. The continuous sheets are repeatedly united together around the absorbent systems to form seals. Final discrete articles are then severed from the web by cutting around or partially through the seals. The seal around each absorbent system thus forms the peripheral seal in the final article.
Given the speed at which it is desired to manufacture these articles, the aforementioned seals are formed via a conventional knurling process. The knurling process is carried out at a sealing station comprising a die roller and an anvil roller as described above. The projections are arranged on the die roller so as to project in the pattern of the peripheral seal to be formed about the absorbent systems. Typically, there are several of these patterns of projections about the cylindrical surface 5 of the die roller 3, each pattern capable of registering with a successive absorbent system in the web.
In practice, it has been found that conventional knurling pattern designs produce unsatisfactory seals, either because the seals are not strong enough and fail to hold, or because the material has been pierced by the projections such that the seal contains pin holes.
The problem of poor quality sealing is believed to reside in the material selection and the force exerted between rollers. In this respect, the seal strength is adjusted by varying the force exerted between the die roller and the anvil roller or the type and/or thickness of the material used. Attempts have been made to find the correct force and choice of material that forms a proper seal without piercing the material. Despite these efforts the problem of poor quality seals continues to exist.
Under a first broad aspect, the present invention provides a die for impressing a material between the die and an anvil such as to apply a compressive force thereto. The die comprises a plurality of defined spatial fields. Each of the fields includes at least two projections arranged within the spatial field, the projections within a field being structured and arranged to engage the material substantially simultaneously. The projections within each field are structured and arranged such that the pressure on the material in each field is maintained within a specified range. In particular, the pressure on the material within each field is not more than double the pressure on the material in any other individual field.
Under a second broad aspect, the present invention provides a method of making a die for impressing a material at a plurality of discrete locations. The die comprises a plurality of defined spatial fields that each include a plurality of projections. Each field has at least two projections arranged on the die to engage the material substantially simultaneously. The method comprises defining a maximum pressure to be applied to the material by any one of the fields of projections and determining a minimum total contact area of projections within any one of the fields, based at least in part on the maximum pressure. The method also involves arranging the projections within the fields of the die based at least in part on the determination. The method further includes structuring and arranging the projections such that the pressure on the material within each field is not more than double the pressure on the material in any other field.
Under a third broad aspect, the present invention provides a method of impressing a material at a plurality of discrete locations in the manufacture of an article including the material. The method comprises providing a die having a plurality of defined spatial fields. Each of the fields has at least two projections arranged to engage the material substantially simultaneously. The method further comprises applying in succession each of the plurality of fields of projections to the surface of the material such as to apply a compressive force thereto. The method further includes structuring and arranging the projections such that the pressure on the material in each field is not more than double the pressure on the material in any other field.
Non-limiting examples of implementation of the present invention will now be described with reference to the drawings in which:
a shows a plan view of a sanitary napkin in accordance with the prior art;
b shows a cross-section through the sanitary napkin of
In the drawings, preferred embodiments of the invention are illustrated by way of examples. It is to be expressly understood that the description and the drawings are only for the purpose of illustration and as an aid to understanding. They are not intended to be a definition of the limits of the invention.
The apparatus and method for knurling according to the present invention will be described, for exemplary purposes, in the context of the manufacture of a sanitary napkin. However, prior to describing the apparatus and method for knurling according to the present invention, a conventional knurling process used in the manufacture of a sanitary napkin will be described with reference to
Referring to
In a conventional manufacturing process, the peripheral seal 315 is formed by, for example, impressing a pattern of knurling projections in the material of the top sheet 303 and the back sheet 307 with the knurling apparatus shown in
In a conventional knurling process, the quality of the peripheral seal 315 is optimised by selecting the appropriate materials for the top sheet 303 and back sheet 307 and setting the force applied to the material through the die roller at a value which is sufficient to bond the top sheet 303 to the back sheet 307, to thereby form the peripheral seal 315. However, the knurling projections that form conventional knurling patterns are uniform. As such, depending on the region of the napkin 301 that is being engaged by the die roller, the contact area over which the die roller applies compressive force to the napkin 301 is different. For example, when the die roller engages the napkin 301 along its sides, it engages the napkin 301 over a larger contact area than when it engages the napkin 301 at ends 333 and 335. As such, since the compressive force applied by the die roller to the material is constant, the portion of the material that forms the peripheral seal 315 along the sides of the sanitary napkin will have experienced less pressure than the material that forms the peripheral seal at the ends 333 and 335. The inventors have discovered that a lower pressure tends to form a weaker peripheral seal 315. As such, if the force applied to the die roller is set optimally for the portion of the material that forms the peripheral seal 315 at ends 333, 335 of the napkin 301, a lower pressure will be exerted on the material that forms the peripheral seal 315 along the sides of the napkin 301, thereby forming a weaker peripheral seal 315 at the sides. However, if the compressive force applied to the die roller is set optimally for the portion of the material that forms the peripheral seal 315 along the sides of the napkin 301, the pressure applied to the material along the ends 333, 335 of the napkin 301 will be considerably higher and may possibly puncture the material in that region. The present invention attempts to alleviate these drawbacks.
Briefly, the apparatus for knurling according to a preferred embodiment of the present invention overcomes the drawbacks of the prior art by providing a die roller having a plurality of spatial fields, each spatial field having a plurality of projections arranged therein. The projections in each field are structured and arranged so that the total contact area between the projections and the material in each field is such that the pressure applied to the material in each field is maintained in a specified range. In particular, it is desirable that the total contact area in each field is such that the pressure applied to the material in any individual field is not more than double the pressure applied to the material in any other individual field. More particularly, it is desirable that the pressure applied to the material in any individual field does not exceed the pressure applied to the material in any other field by more than 50%. More preferably, it is desirable that the pressure applied to the material by any individual field does not exceed the pressure applied to the material by any other field by more that 30%, and most preferably it is desirable that the pressure applied to the material in each of the fields is substantially equal.
To achieve the above objectives, at least one of the size of the individual contact area for each projection, the number of projections within each field and the spacing between projections in each field is selected so that the total contact area between the projections and the material in each field is such that the pressure experienced by the material in each of the fields is maintained within the specified range, as set forth above.
The die roller, as described in greater detail below, may optionally further include “islands” of additional projections that are adapted to reduce the pressure applied to the material in selected fields.
The sealing pattern 401 shown in
As shown in
The fields 435, 437, 439, 441, 443 contain zones of projections, which are capable of engaging a material to be impressed substantially simultaneously. The projections within different fields are arranged on the die to engage the material at different times. The fields of projections are progressively spaced from and run generally parallel to a longitudinal axis 445 of the sealing pattern 401, the longitudinal axis 445 being parallel to the rotation axis of the die. In the case where the sealing pattern 401 is positioned end to end, as described above, the fields of projections run parallel to the axis of rotation of the die.
Each of the fields 443, 441, 439, 437, and 435 shown in
Field 443, whose projections form a seal near an extreme lateral edge 447 of a flap portion of the sealing pattern 401, includes zone 451 of projections which extends beyond the end of the flap portion of the sealing pattern 401. The field 443 also includes two discrete zones 459, 461 of projections contained within islands 415 and 417, that are discontinuous and remote from zone 451. A zone is said to be discontinuous and remote from another zone when there is a space located between the two zones that does not contain any projections.
The zones 459, 461 of projections are located towards each end of the sealing pattern 401 and contained within optional islands 415, 417, respectively. The projections of the field 443 that are located outside of the flap portion of the sealing pattern 401, such as the projections within the islands 415, 417, serve to provide additional area over which the force of the die roller is distributed. This reduces the pressure applied to the material in field 443. In this manner, the optional islands 415, 417 can be used to selectively reduce the pressure on the material within selected fields by increasing the contact area between the die and the material within a given field. Optional islands 419 and 421 may be used in a similar fashion.
Referring again to
The field 437 contains two discrete zones 483, 485 of projections each within the imaginary boundary 411 and four further zones 475, 477, 487, 489 of projections. The zones 475, 477 of projections are within the imaginary boundary 411 and they are continuous with the zones 487, 489 of projections, respectively, that are outside the imaginary boundary 411. The zones 483, 485 of projections are discontinuous and remote from the zones 475, 477, 487, 489 of projections.
The field 435, which is located near the longitudinal axis 445 of the sealing pattern 401, contains two zones 495, 497 of projections within the imaginary boundary 411. The field 435 further includes zones 499, 501 of projections which extend outside the imaginary boundary 411 and which are continuous with the zones 495, 497 of projections, respectively. The sum of geometric areas defined by zones 495, 497, 499, 501 is substantially less than the sum of the geometric areas defined by the other fields 437, 439, 441 and 443.
Since the sum of the area of zones 495, 497, 499 and 501 in field 435 is significantly less than the sum of the area of the zones in the other fields, the pressure on the material in field 435 would be significantly higher if the projections were uniform in all of the fields. To prevent this phenomena at least one of the size individual contact area for each projection, spacing and number of projections in each field is selected so that the total contact area in each of the fields is such that the pressure experienced by the material in each of the fields is maintained within a specified range. In particular, the projections are structured and arranged such that the pressure applied to the material by any individual field is not more than double the pressure applied to the material by any other field.
For example, the projections in the zones 495, 497, 499, 501 of field 435 may be arranged such that they have larger individual contact areas than the projections in fields 443, 441, 439 and 437 to thereby increase the total contact area in field 435 and thereby reduce the pressure on the material in this field. Alternatively, the projections in the zones 495, 497, 499, 501 of projections may be spaced more closely to one another than the projections in the fields 443, 441, 439 and 437 to thereby increase the total contact area in field 435. Yet another alternative is to include a greater number of projections in zones 495, 497, 499 and 501 of field 435 to thereby increase the total contact area in field 435. By using one or more of the above techniques, although the total geometric area of field 435 is significantly less than the total geometric area of any one of fields 443, 441, 439 and 437, the total contact area in field 435 may be increased so that it is similar to the contact area of fields 443, 441, 439 and 437. As such, the pressure applied to the material in field 435 will be maintained within the specified range of pressures.
Obviously, the approaches described above may be combined, if necessary. For example, the projections in a given zone may be spaced more closely together and the size of the individual contact area of the projections within the field may be increased to thereby increase the total contact area in the selected field.
As seen in
However, as best seen in
In a non-limiting example of implementation, the shape of the contact surfaces of the projections 503 and 505 is generally rhomboidal. However, it should be understood that other shapes may be used such as squares, circles, triangles, ellipses or any other suitable shape, without departing from the spirit of the invention. Furthermore, the contact area of one or more projections 503, 505 within a given field may be different from that of one or more other projections 503, 505 within the same field. Furthermore, the spacing between two or more immediately adjacent projections 503, 505 within a given field may be different from the spacing between two or more other projections within that same field. It should be understood that the spacing between each projection in a zone can vary. As such, the average spacing between projections is determined by adding up all the spacings and dividing that sum by the number of spacings that were added together.
In a non-limiting embodiment, the width of the field “a” is approximately 2 mm, the width of each projection “b” is approximately 0.8 mm, the overall length of each projection “c” is approximately 1.16 mm, and the length of each projection “d” is approximately 0.46 mm. The contact area 517 of the projection 503 is a rhombus and thus its geometric area can be calculated as follows A=½ (b) (d). As such, the overall projection contact area for each projection 503 is approximately 0.184 mm2.
In the non-limiting embodiment shown in
Referring back to
Increasing the total contact area of projections within each of the other fields may be achieved in any one of the techniques described above, individually or in combination. For example, one or more additional projections may be added to each field external of the imaginary boundary corresponding to the peripheral edge of the article. The contact area of some or all of the projections within the other fields may be increased or the spacing between immediately adjacent projections may be decreased.
The pattern of projections for producing a seal around the peripheral edge of a sanitary napkin, shown in
An example of a die roller 600, otherwise known as a rotary die, containing the seal pattern of
The die roller 600 may be incorporated in any conventional knurling apparatus, such as that shown in
The design and manufacture of a die, according to an example of implementation of the present invention will now be described in more detail. For the purposes of the non-limiting example described below, the knurling process is used to create a peripheral seal between a top sheet made of a polypropylene fiber blend having approximately a 2% level of TiO2, and a back sheet made of polyethylene homopolymers (metallocene catalysed film). It should, however, be understood that the knurling process can also be used to form a peripheral seal in other suitable materials used for forming sanitary napkins.
In a first step of designing a die, the regions within the sealing pattern, which are to be subjected to a knurling process, are identified and then each region is divided into a plurality of fields each having a selected width. In a non-limiting embodiment, the width of each field is in the order of 2-3 mm.
The total geometric area defined by each of the fields can then be calculated by multiplying the width of the field by the length of the field. The total geometric area defined by each field is important to know when distributing the projections within the field. In a non-limiting embodiment, the length of a field can be defined as the sum of the length of the zones within that field. As such, the length of each field will depend on where along the sealing arrangement the field lies. For example, it will be appreciated that the length of field 441 will be greater than the length of the field 435, as shown in
Another preliminary step when designing a die roller is to establish a pressure range that can be applied to the material. The pressure range may be defined by a maximum and minimum pressure that can be applied to the material being sealed.
In a specific example of implementation, the maximum pressure can be defined as the pressure above which the material is likely to be punctured by the projection. Alternatively, when embossing the material, the maximum pressure can be defined as the pressure required to form an impression in the material of a predetermined depth.
The minimum pressure can be defined as the pressure required to form a seal in the material having a predetermined minimum tensile strength. When the material includes two or more sheets sealed to one another the tensile strength of the seal is determined by pulling the sheets away from one another at the seal and noting the level of force being applied when the seal breaks. In general, for the material described above, it is desirable for the finished napkin to have a seal strength of approximately 59 g/cm. A critical minimum seal strength is generally considered to be in the order of 39 g/cm.
In an optional embodiment, a possible variation in pressure between different fields can also be established.
In a non-limiting embodiment wherein the die roller is used to knurl the materials described above, the minimum and maximum pressure that can be exerted on the material are in the order of 41000 psi and 68000 psi.
Once the pressure range has been established, the compressive force applied by the die roller is determined. In a non-limiting embodiment wherein the die roller is used to knurl the materials described above, the die roller can have a set point pressure of between 60 psi to 80 psi. If the pressure is set at 80 psi with an air cylinder of 6 inches, the force applied by the die roller will be in the order of 2262 lb. It should be understood that a different force can be set depending on multiple different parameters of the knurling process, such as the materials being sealed together, the weight of the die roller, the mechanical force applied to the die roller via an external element, etc.
Given that the force applied to the material by the die roller is known, the pressure applied to the material can be maintained within the desired pressure range by controlling the contact area within each field of the die roller. Increasing and decreasing the total contact area of projections within each of the fields may be achieved by any one of the techniques described above, individually or in combination. For example, one or more projections may be added or subtracted to each field, the contact area of some or all of the projections within the fields may be increased or decreased, or the spacing between immediately adjacent projections may be increased or decreased. As such, the contact area within each field can be controlled by adjusting the size, spacing and quantity of projections within that field.
The amount of contact area required in each field, such that the pressure exerted on the material is within the established pressure range, can be calculated using the formula of Area=Force/Pressure. It should be understood that it is desirable to keep the pressure (in psi) applied to the material within each of the fields at no more than double the pressure (in psi) applied to the material within any other one of the plurality of fields.
Once the contact area has been calculated, the number of contact projections can be calculated by dividing the total contact area by the contact area of each projection. In a non-limiting example of implementation, each projection can have a contact area of between 0.13 and 0.49 mm2.
In general, only a single row of projections will lie in each field. As such, the manner in which the projections are distributed within a field can be determined based on the length of the field and the length of the projections. Based on this information, a uniform amount of spacing between each projection can be calculated. As such, the projections can be equally spaced within each field. In an alternative embodiment, it should be understood that it is not necessary that the projections within a field be equally spaced. For example, the projections may be equally spaced within a first zone of a field by a first spacing, and may be equally spaced within a second zone of the field by a second spacing that is different from the first.
In addition, the spacing can be of a certain length in the largest field, and that certain spacing can be reduced proportionally in the other fields on the basis of the length decrease of the field. As such, the same number of projections can be fit within each field, regardless of the size of the field.
In other possible variants, projections may be arranged to form recesses in the surface of the material as, for example, in an embossing pattern. In contrast to a sealing operation, an embossing operation does not aim primarily to join sheets of material, but mainly to create alternating peaks and valleys in the material. The embossing pattern may be formed across portions of an article such as the sanitary napkin 409 of
Other embodiments and further modifications to the embodiments described above will be apparent to those. skilled in the art.
