CA2387165A1

CA2387165A1 - Bristle sub-assemblies having parallel pairs of bristles; and methods of making same

Info

Publication number: CA2387165A1
Application number: CA002387165A
Authority: CA
Inventors: Mark Stephen Edwards; Winship Stinson Reed, Iii; Robert Maurice Brown
Original assignee: Individual
Current assignee: EIDP Inc
Priority date: 1999-12-06
Filing date: 2000-12-06
Publication date: 2001-06-07
Also published as: BR0016094A; KR20020064923A; US6478907B2; JP2003515371A; CN1407864A; US6351868B1; US20020036048A1; US20020033234A1; US6478925B2; EP1235495A1; WO2001039630A1

Abstract

An apparatus for forming bristle sub-assemblies includes a vertically orient ed slot, which is formed by two endless belts (52, 54). A wheel (76) disposed above the slot pushes a bristle string into the slot. The bristle string has a plurality of bristles connected to a base string in two rows. The bristle ro ws can be horizontally oriented, so that all of the bristles are co-planar in a horizontal plane, or they can be angularly oriented with the base string at the vertex of the angle. When the bristles are pushed into the slot, they be nd to adopt a new, substantially parallel orientation. An ultrasonic heater (80 ) heats the bent bristles to re-set the thermoplastic material from which the bristles are made so that the cooling, the parallel orientation is maintaine d.

Description

w v BRISTLE SUB-ASSEMBLIES HAVING PARALLEL PAIRS
OF BRISTLES; AND METHODS OF MAI~NG SAME
BACKGROUND OF THE INVENTION
The present invention relates generally to the field of brush and bristle making, and more specifically, to a bristle sub-assembly having substantially parallel pairs of bristles that are attached to a base string. Bristle sub-assemblies of the type described herein are used in making bristle tufts in a variety of applications, especially toothbrushes.
DESCRIPTION OF THE RELATED ART
US 3, 216, 038 to Gould et al. describes the use of an adhesive strip which is laid down to hold the pre-cut bristles after the heat treatment step to reduce the springiness of the bristles. This invention relates to synthetic plastic broom 1 S bristles, especially for use in large rotary street sweeping brooms.
DE-2351482 to Halonen et al. describes a brush such as a car-wash brush having a brush base and bristles attached thereto. The base has at least one groove in which the bristles are affixed by means of a wire-like attachment element in such a way that the bristles pass under the attachment element that is anchored in the groove.
GB-A-0 384 909 to Epp describes an apparatus for making eylindicrical brushes, having U-Shaped bristle holders wound in helical form.
US 2,812,214 to Sandelin describes a method and device for making brush-strips of the kind, where the bristles are fastened within a groove-shaped 10698_1 Substitute Sheet (Rule 26) AMENDED SHEET

23'11-2001 CA 02387165 2002-04-25 US003299E
holder. A thread of metal or other material is fed together with a ribbon with bristles arranged transversally to the freed direction.
In US 6,096,151 to Edwards et al., there is described a method of forming bristle sub-assemblies which, in brief, involves wrapping a polymeric monofilament around a mandrel and bonding the individual wraps to one or more base strings by applying thermal energy. As seen in Fig. 1, an apparatus 10 for making bristle sub-assemblies includes a rectangularly shaped mandrel 12 having a central passageway and open, opposite axial ends. Four base strings 14, 16, and 20 are fed into one end of the mandrel from respective supply sources 22, 24, 26, and 28.
From the orientation of Fig.l, the base strings are fed dowawardly through the mandrel 12 and pass outwardly from the opposite end. Pulleys or wheels (not shown) are used to turn the base strings substantially 180 ° so that they can run along the respective four corners of the mandrel in the upward direction.
While the base strings are running upwardly, a monofilament 30, supplied from a supply 32, is supplied at a controlled feed rate to a wrapping mechanism 34. The wrapping mechanism 34 includes a motor 36 which drives a wrapper 38, through which the monofilament 30 is fed, so that the monofilament is caused to wrap around the mandrel to form a plurality of wraps 40. The wraps are transported upwardly by the upward movement of the base strings.
The wraps pass under heaters 42, one being disposed at each of the four corners of the mandrel 12 (and only two of the four heaters being illustrated in Fig. 1). As the result of application of thermal energy, such as by ultrasonic Substltute Sheet (Rule 2~
AMENDED SHEET

23-11-2001 CA 02387165 2002-04-25 US003299f welding, the monofilament wraps 40 bond to the four base strings, either by melting the base strings or the monofilament, or both. After bonding, the wraps are passed over rotating cutting knives 44, of which there are four in the embodiment of Fig. 1, thereby forming four bristle strings 46, of which only two are partially shown and one is shown being taken up on a spool 48.
Fig. 2 is an enlarged, end view of the bristle string 46, revealing the base string 20 and two bristles 30a and 30b, constituting a bristle pair, which are connected to the base string 20 as a result of the thermoplastic melting of the monofihnent 30 doting the bonding step. The bristle string 46 would be a continuous length of base string with two, complementary rows of bristles, which as viewed in Fig. 2, would constitute a left row and a right row disposed at complementary angles to each other. Each bristle of one row would be paired, and substantially co-linear with, a bristle from the other row.
The angled orientation of the bristles results from the bonding step involved in the illustrated embodiment. However, variations of the manufacturing process of Fig. 1 could result in a similar bristle string except with all the bristles being co-planar. An example is shown in Fig. 3, in which the bristle pair 30a' and 30b' of the bristle string 46' are not only co-linear, but co-planar as well.
While the bristle strings described above are advantageous in making toothbrushes, among other things, by cutting the bristle strings into lengths that become "bristle sub-assemblies" that can be used to form bristle tufts on a brush head, the bristle pairs of the bristle sub-assemblies may at times be preferred to be fixed in a parallel orientation with respect to each other, prior to secW ag the 10698_1 Substitute Sheet (Role 2~
AMENDED SHEET

.23-11-2001 CA 02387165 2002-04-25 US0032991:
bristle sub-assemblies in a brush.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a bristle sub-assembly in which bristle pairs are fixed in an orientation where the bristles of each pair are oriented substantially parallel to each other.
Another object of the present invention is to provide an apparatus for forming bristle sub-assemblies in which the bristles of each bristle pair are faced in a substantially parallel orientation with respect to each other.
Still another object of the present invention is to provide a method of forming bristle sub-assemblies in which the bristles of each bristle pair are fixed in a substantially parallel orientation with respect to each other.
Yet another object of the present invention is to provide a method of changing the directional orientation of monofilament bristles by heating to a temperature that is high enough to relieve the spring forces that are generated by bending the bristles to the new direction of orientation, yet low enough to avoid causing the molecular structure of the polymeric monofilament bristle material to change significantly. Partial melting at the interface between the bristles and base string is preferred to provide additional bonding between the bristles and base string.
These and other objects are met by providing a bristle sub-assembly which includes a base string having a defined, solid physical structure with a regular, geometric shape and an outer surface; a first row of bristles each having a 10698_1 Substitute Sheet (Rule 2~
AMENDED SHEET

23'~ 1-2001 CA 02387165 2002-04-25 US003299f defined, solid physical struchue with a regular, geometric shape and an outer surface; connected to and extending radially outwardly from the base string;
and a second row of bristles each having a defined, solid physical structure with a regular, geometric shape and an outer surface connected to and extending radialiy outwardly from the base string, and a partial melting contact area disposed at locations where the outer surfaces of the first and second mws of bristles contact the outer surface of the base string wherein the first and second rows of bristles are substantially parallel to each other, the partial melting contact area defining a bond between outer surfaces of the first and second rows of bristles and the outer surface of the base string.
In another aspect of the invention, a method of making bristle sub-assemblies includes the steps of: forming a bristle string having first and second rows of bristles extending outwardly from a base string haying a defined, solid physical structure with a regular, geometric shape and a base string outer surface, 1 S wherein the first and second rows of bristles are disposed in a first orientation with respect to each other, the bristles of each row having a defined, solid physical structure with a regular, geometric shape and a bristle outer surface connected to the base string outer surface by a partial melting contact area defining a bond where the bristle outer surfaces of the first and second rows of bristles contact the base string outer surface; bending the bristles of at least one of the first and second rows of bristles to achieve a second orientation with respect to each other, thereby developing a spring force in the bristles that tends to restore the bristles to the first orientation; heating at feast the bent ones of the bristles to a temperature sufficient 10698_1 Substitute Sheet (Rule 26) AMENDED SHEET

23-~ 1-2001 CA 02387165 2002-04-25 US003299a to relax the spring force while maintaining the bristles in the second orientation;
and cooling the heated bristles so that the bristles maintain the second orientation.
In another aspect of the present invention, an apparatus for malting bristle sub-assemblies comprising: means for feeding material comprising a bristle string having first and second rows of bristles connected to a base string having a defined, solid physical structure with a regular, geometric shape and a base string outer surface, the first and second rows of bristles being in a first orientation relative to each other, the bristles of each row having a defined, solid physical structure with a regular, geometric shape and a bristle outer surface connected to the base string outer surface by a partial melting contact area defining a bond where the bristle outer surfaces of the first and second rows of bristles contact the base string outer surface, means for bending the bristles of at least one of the first and second rows of bristles to achieve a second orientation of the bristle rows relative to each other, thereby developing a spring force in the bent bristles that biases the bent bristles~towards the first orientation; means for heating the bristles bent in the bending step at least for a time and temperature sufficient to relax the spring force; and means for cooling the heated bristles so that the bristles maintain the second orientation.
These and other objects of the invention wiU become more apparent from the following detailed description when taken in conjunction with the illustrative embodiments in the accompanying drawings.

~os9s_i Substitute Sheet (Rule 26) AMENDED SHEET

. 23-11-2001 CA 02387165 2002-04-25 US003299f BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a schematic view of an apparatus for forming bristle strings, in which a plurality of bristles are connto a base string in first and second rows;
Fig. 2 is an end view of a bristle string in which the first and second rows of bristles are disposed at an angle with respect to each other, with the base string disposed at the vertex of the angle;
Fig. 3 is an end view of a bristle string in which the first and second rows of bristles are horizontally disposed, extending in 180° opposite directions, in substantially the same plane;
Fig. 4 is a schematic, top plan view of an apparatus for making bristle sub-assemblies with substantially parallel pairs of bristles according to the present invention;
Fig. 5 is a schematic, side elevation view of the apparatus of Fig. 4;
Fig. 6 is an enlarged schematic, partial side elevational view of a portion of the apparatus of Fig. 4, and showing the bristles of the bristle string being folded into a parallel orientation;
Fig. 7 is an enlarged partial sectional view, substantially of the structure shown in Fig. 6, taken along line VII-VII;
Fig. 8 is an enlarged end view of a bristle string, showing the bristles of a bristle pair folded into the vertical, parallel orientation;
Fig. 9 is a top perspective view showing a particularly prefen~ed apparatus according to the present invention for making bristle sub-assemblies;
Fig. 10 is an end view of a bristle sub-assembly capable of being formed _7_ ios9a_~ Substitute Sheet (Rule 2~
AMENDED SHEET

23-11-2001 CA 02387165 2002-04-25 US003299f using the apparatuses of the present invention, in which two bristle strings are fed into the apparatus; and Fig. 11 is an end view of a bristle sub-assembly capable of being formed using the apparatuses of the present invention, in which the bristles of one row are longer than those of the other row.
DETAILED DESCRIPTION.OF THE PREFERRED EMBODIMENTS
Referring now to Figs. 4 and 5, an apparatus 50 for making bristle sub-assemblies includes a first endless belt 52 and a second endless belt 54. The first endless belt is supported on rotatable pulleys 56, 58, 60 and 62, while the second endless belt 54 is supported on rotatable pulleys 64, 66, 68 and 70. The belts and 54 are juxtaposed to define a nip 72 at a forward end where the belts are parallel and running in the same direction, which is indicated by the directional arrow "A"
A bristle string 74 having a base string and plural pairs of bristles, whether in the V-shaped orientation or the flat orientation described previously, is fed into the area of the nip 72. A "tucker" wheel 76 presses on the bristle string 74 to effectively bend the bristles from a generally outward orientation to a generally upward orientation. In the upward orientation, the bristles of the bristle pairs are substantially parallel to each other.
Now with the bristles in the upward, parallel orientation, the bristle string 74 is transported by the first and second belts 52 and 54, being held therebetween by friction, to a heating station 78. While any of a variety of heating devices can _8_ 10698_1 Substitute Sheet (Rule 26) AMENDED SHEET

be employed; a particularly preferred device is one that uses ultrasonic energy to heat the thermoplastic materials from which either the base string or the bristles;
or both, are made. In the illustrated embodiment, the heating station 78 includes an ultrasonic horn 80 and an anvil 82, which are positioned below the belts 52 and 54 at a height selected to allow the base string to pass through the gap between the hom and the anvil.
The various pulleys, the heating station, and the tucker wheel are mounted on a base plate 84. The various drive mechanisms are not shown for clarity of illustration. The first and second belts are synchronized to rotate at the same speed using any of a variety of known drive mechanisms. Preferably, the tucker wheel 76 rotates at a speed which is synchronized with the first and second belts, so that the tucker wheel 76 neither brakes nor accelerates the bristle string as it passes between the belts.
The height of the tucker wheel 76 is adjustable, as is the gap between the two belts 52 and 54, to accommodate various sizes of bristle strings. Also, a guide plate 86 is positioned beneath the tucker wheel 76 to limit the downward extent of movement of the bristle string as it is "tucked" between the two belts 52 and 54. The vertical, as well as horizontal, position of the guide plate 86 is adjustable. Any of a variety of well known structures can be used to render the guide plate 86 adjustable.
Fig. 6 shows an enlarged view of the process by which the bristles are folded from an initial flat disposition, in which the bristles of a pair are co-liner and extend substantially 180° in opposite directions, to a parallel disposition, in _9_ ~os9s_i Substitute Sheet {Rule 26) AMENDED SHEET

.23-11-2001 CA 02387165 2002-04-25 US003299f which the bristles of a pair are each folded substantially 90° to extend upwardly from the base string in the same direction. As the tucker wheel 76 rotates in the direction "B," and the belts transport the bristle string 74 in the direction "A," the outer circumferential surface of the tucker wheel 76, which is disposed between the bristle pairs, causes the bristle string to move downwardly until it abuts, or comes into proximity to, the guide plate 86.
Fig. 7 shows that the tucker wheel 76, in a preferred embodiment, can be formed from two sheet metal discs 76a and 76b which are mounted on a rotatable shaft 88. When a securing nut 90 is tightened, the discs 76a and 76b rotate in unison with each other and the shaft 88.
As seen in the drawing, the peripheral or circumferential edges of the two discs are spaced slightly apart so that, when the discs extend between the bristles 92 and 94 of a bristle pair, they engage opposite sides of the base string 96.
At the same time, belts 52 and 54 maintain the bristles 92 and 94 in the parallel, folded position as the bristle string is transported under the tucker wheel 76.
As seen in Fig. 8, after folding the bristles 92 and 94 are parallel to each other and extend substantially in the same direction, which is substantially perpendicular to the direction of the base string 96. Since the bristles 92 and 94 are integrally formed, a portion of the monofilament that forms the two bristles wraps around the base string; before wrapping, the base string 96 was essentially tangential to the bristles, so that the area of contact between the base string and the bristles was relatively small. Folding in essence increases the area of contact, or points of contact, between the bristles and the base string.

ios9s_~ Substitute Sheet (Rule 26) AMENDED SHEET

.23-11-2001 CA 02387165 2002-04-25 US003299~
To keep the bristles in the newly adopted vertical and substantially parallel orientation, the belts 52 and 54 transport the bristle string to the heating station 78.
In the embodiment that employs ultrasonic heating, the horn 80 and anvil 82 are preferably positioned so that they form a gap, as shown in Fig. 8, through which the base string 96 and bristles 92 and 94 pass. Note that the horn 80 and anvil 82 are shaped to assist the bending of the bristles around the sides of the base string, thus providing additional points of contact in the vertical direction. When ultrasonic energy is delivered to the bristles and base string at this area of contact between them, the energy has two effects: first, it causes both the base string and bristles to partially melt at their contacting surfaces which in taro causes them to further weld them together, thereby fixing the bristles in the second, parallel orientation. Secondly, it heats the bristles so that they relax, i.e., they no longer have the tendency to spring back to their unfolded, original orientation. Upon exiting the heating station, the bristles are cool and will stay in their new substantially parallel orientation, either by the stress relief imparted to the bent bristles, or by the additional bonding which occurs along the sides of the base string, or both.
More details of the preferred apparatus 50 are shown in Fig. 9. In particular, the anvil 82 is mounted on a support plate 98 which is adjustably mounted on a backing plate 100 that is mounted under a opening 102 formed in the base plate 84. The backing plate 100 is provided with parallel slots 104 which facilitate longitudinal adjustment of the anvil 82 along the length of the belt path which transports the bristle string. Lateral adjustments of the anvil 82 can be 10698_1 Substitute Sheet (Rule 26~
AMENDED SHEET

.23-11-2001 CA 02387165 2002-04-25 US003299f accomplished by using shims to mount the anvil 82 to the support plate, or by adjusting the position of the backing plate laterally inwardly or outwardly.
Similarly, the ultrasonic stack assembly 105 is mounted on a support plate 106 which is adjustably mounted on the backing plate 100. The stack assembly S 105 is pressed into the bristles, base string and anvil 82 by a pneumatic cylinder 107. The horn 80 is driven by a h~ansducer 108 with enough energy to deliver sufficient thermal energy to the bristles to cause partial melting. The transducer 108, horn 80 and anvil 82 are commercially available and their operation is well known. Using the preferred monofilament materials, which are basically nylons and polyesters, the ultrasonic energy preferably rapidly heats only the surfaces of the materials to their melting points before the energy is removed by the forward progress of the bristle string. However, virtually any thermoplastic monofzlament materials can be employed. Particularly preferred materials include 1'YNEXTM
and CHINEXT~"~, which are nylon monofilame~ats, and ORELT"t, which is a polyester monofilament, all of which are made by E.I. du Pont de Nemours & Co.
of Wilmington, Delaware USA.
The monofilament material which comprises the bristles can be made of other similar materials, including aliphatic polyamides, aromatic polyamides, polyesters, polyolefins, styrenes, fluoropolymers, polyvinylchloride (PVC), polyurethane, polyvinylidene chloride, and polystyrene and styrene copolymers.
A particularly suitable polymeric material for toothbrush applications include the following: 6,12 nylon; 4 nylon; 6 nylon; 11 nylon; 12 nylon; 6,6 nylon; 6,10 nylon; 6,14 nylon, 10,10 nylon; 12,12 nylon and other nylon co-polymers. The m9s 1 Substitute Sheet (Role 2~
AMENDED SHEET

z3-11-2001 CA 02387165 2002-04-25 US0032998 base string can be made of similar materials.
The monofilaments used for bristles and/or the base string can have shapes other than circular cross-sections, and may be hollow or have voids in their cross-section. Embodiments described herein show circular cross-sectional shapes for the base string and bristles, although virtually any cross-sectional shapes can be used, including oval, square, rectangular, etc. In any shape, the preferred thickness for the base string and monofilarnent bristles are selected to provide a level of functionality to the individual brush applications. Typically, the base string is slightly thicker than the bristles. For many brush applications, the range of thickness (assuming circular cross-section) ranges from 2 to 200 mils (.0508 mm-5.08 mm) and 6 to 12 mils (.1524 mm-.3048 mm), with the preferred range for toothbrush applications being in the range of 6 to 12 mils {.1524mm-.3048 mm.) Fig. 9 also shows details of the mechanism for adjusting the position of the tucker wheel 76. In particular, the wheel 76 is mounted on a support arm 110 which is adjustable in the lateral direction (shown by the directional arrow).
The tucker wheel 7b is further mounted on a slide mechanism 112 which facilitates vertical adjustment of the wheel 76. Once the proper height is selected, the slide mechanism 112 locks the wheel 76 in position. Various other mechanisms can be used to adjust the position of the wheel 76, horn 80 and anvil 82.
A tension pulley 114 is provided for belt 52 to maintain proper tension of the belt. A similar tension pulley, and supporting tensioning mechanism, are provided for belt 54, but not illustrated for the sake of simplicity. The belts are synchronized and driven by drive means (not shown) that could include a servo-~o69s_~ Substitute Sheet (Rule 2~
AMENDED SHEET

.23-11-2001 CA 02387165 2002-04-25 US003299f motor. One way of synchronizing the belts would be to provide timing pulleys and 118, which can be connected via a timing belt (not shown) and driven by a common motor.
A take-up mechanism (not shown) is located downstream of the exit nip between the two belts 52 and 54, so that the bristle string can be gathered on a spool for subsequent use andlor further processing. Alternatively, cutting mechanism (not shown) can be located downstream of the exit nip to cut the bristle string into lengths which can be used to make bristle tufts for a variety of brush shapes, sizes, and types. Once cut into lengths, the bristle string can be referred to as "bristle sub-assemblies," as they would then be used to make the bristle tufts. However, it is equally accurate to refer to the bristle string as a bristle sub-assembly, both before re-orienting the bristles according to the present invention, and afterwards.
The apparatus described herein for making bristle sub-assemblies can be used to make variations of the structures illustrated above. In particular, and with reference to Fig. 10, two bristle strings can be fed in tandem to the wheel 7b, so that the two base strings 120 and 122 are disposed one on top of the other.
The bristles of one can be slightly shorter than the other so that the ends of all of the bristles terminate substantially in the same plane, as shown in Fig. 10. They can also be of different diameters or cross section. Also, as seen in Fig. 11, the base string 124 can be offset from the middle of the bristles, so as to produce bristles in one row being longer than the bristles of the other row.
The base string used to make the bristle sub-assemblies described herein 10698_t Substitute Sheet (Role 2~
AMENDED SHEET

23-11-2001 CA 02387165 2002-04-25 US003299~
can be varied widely by selection of the material as well as the processing conditions used to make the oriented monofilament. Typical materials include one or more of the following: b nylon; 6,6 nylon; 6,9 nylon; 6,10 nylon; 6,12 nylon; 11 nylon; 12 nylon; 10,10 nylon, copolymers of 6 nylon and 6,6 nylon, or S mixtures of nylons. Other examples of materials are polyesters such as polybutylene terephthalate (PBT) or trimethyleneterephthalate (3GT), polyethylenephthalate (PET) or a polyurethane, polyvinylidine chloride or polystyrene.
The base string can be varied by selecting a material with a higher or lower modules to make the monofilament stiffer or more flexible. for example, a very stiffbase string could be made from polyethylene terephthalate resin from which a monofilament with tensile modules over 1,000,000 psi (6894.757 Mpa (megapascals)) can be made. A very flexible base string can be made from a thermoplastic elastomer such as a polyether block amide, like PEBAX or a polyester ether block copolymer like HiYTRELTM, the latter being a Dupont product. In the latter two cases the monofilament would have excellent stretch recovery properties as well as low modules. Base string properties can be further tailored either by blending materials or by coextrusion to combine materials.
Although the invention has been described with reference to a particular embodiment, it will be understood to those skilled in the art that the invention is capable of a variety of alternative embodiments within the spirit and scope of the appended claims.

~ob9s_~ Substitute Sheet (Rule 26) AMENDED SHEET

Claims

WHAT IS CLAIMED IS:

1. A bristle sub-assembly comprising:
a base string having a defined, solid physical structure with a regular, geometric shape and an outer surface;
a first row of bristles each having a defined, solid physical structure with a regular, geometric shape and an outer surface; connected to and extending radially outwardly from the base string; and a second row of bristles each having a defined, solid physical structure with a regular, geometric shape and an outer surface connected to and extending radially outwardly from the base string, and a partial melting contact area disposed at locations where the outer surfaces of the first and second rows of bristles contact the outer surface of the base string wherein the first and second rows of bristles are substantially parallel to each other, the partial melting contact area defining a bond between outer surfaces of the first and second rows of bristles and the outer surface of the base string.

2. A bristle sub-assembly according to claim 1, wherein the bristles of the first and second rows are made of a polymeric, thermoplastic material.

3. A bristle sub-assembly according to claim 2, wherein the polymeric, thermoplastic material is selected from the group consisting of aliphatic polyamides, aromatic polyamides, polyesters, polyolefins, styrenes, fluoropolymers, polyvinylchloride (PVC), polyurethane, polyvinylidene chloride, and polystyrene and styrene copolymers.

4. A bristle sub-assembly according to claim 2, wherein the polymeric, thermoplastic material is nylon.

5. A bristle sub-assembly according to claim 1, wherein the bristles of the first and second rows of bristles are substantially straight and flexible, and have proximal ends connects to the base string.

6. A bristle sub-assembly according to claim 1, wherein the bristles of the first and second rows of bristles are integrally formed bristle pairs, wherein each bristle pair comprises a single length of monofilament connected to the outer surface of the base string at about a medial point of the length of monofilament, so that the bristles of the first and second rows are substantially the same length.

7. A bristle sub-assembly according to claim 1, wherein the bristles of the first and second rows of bristles are integrally formed bristle pairs, wherein each bristle pair comprises a single length of monofilament wrapped around and connected to the base string at about a point spaced from a medial point of the length of monofilament, so that the bristles of the first row has a length different from that of the bristles of the second row.

8. A bristle sub-assembly according to claim 1, wherein the base string is made of a polymeric monofilament material.

9. A method for making bristle sub-assemblies, comprising the steps of:
forming a bristle string having first and second rows of bristles extending outwardly from a base string having a defined, solid physical structure with a regular, geometric shape and a base string outer surface, wherein the first and second rows of bristles are disposed in a first orientation with respect to each other, the bristles of each row having a defined, solid physical structure with a regular, geometric shape and a bristle outer surface connected to the base string outer surface by a partial melting contact area defining a bond where the bristle outer surfaces of the first and second rows of bristles contact the base string outer surface;
bending the bristles of at least one of the first and second rows of bristles to achieve a second orientation with respect to each other, thereby developing a spring force in the bristles that tends to restore the bristles to the first orientation;
heating at least the bent ones of the bristles to a temperature sufficient to relax the spring force while maintaining the bristles in the second orientation; and cooling the heated bristles so that the bristles maintain the second orientation.

10. A method according to claim 9, wherein the first and second rows of bristles extend in substantially 180° opposite directions in the first orientation, and wherein the bending step comprises bending at least one of the first and second rows until the rows are substantially parallel to each other in the second orientation.

11. A method according to claim 9, wherein the first and second rows of bristles form an angle with the base string at a vertex of the angle in the first orientation, and wherein the bending step includes bending at least one of the first and second rows of bristles until the rows are substantially parallel to each other in the second orientation.

12. A method according to claim 9, wherein the heating step includes applying ultrasonic energy to the bent bristles at least for a time and magnitude sufficient to relieve stresses induced by the spring force.

13. A method according to claim 9, wherein the heating step includes applying ultrasonic energy to the bent bristles at least for a time and magnitude sufficient to induce surface melting at points of contact between the bristles and the base string, thereby enhancing the bond between the bristles and base string and fixing the bristles in the second orientation.

14. A method according to claim 9, wherein the forming step comprises forming a plurality of monofilament bristles having opposite axial ends, aligning the monofilament bristles side-by-side so that all the bristles are substantially parallel, positioning the base string on the plurality of monofilament bristles along a line substantially perpendicular to the monofilament bristles, bonding the monofilament bristles to the base string, whereby each monofilament bristle forms a bristle pair comprising a first bristle disposed in the first row of bristles and a second bristle disposed in the second row of bristles.

15. A method according to claim 9, wherein the heating step includes applying at least one of radiant and convective heat energy to the bent bristles and base string.

16. A method according to claim 9, wherein the bristles are made of a polymeric, thermoplastic material selected from the group consisting of aliphatic polyamides, aromatic polyamides, polyesters, polyolefins, styrenes, fluoropolymers, polyvinylchloride (PVC), polyurethane, polyvinylidene chloride, and polystyrene and styrene copolymers.

17. A method according to claim 16, wherein the polymeric material is a nylon.

18. An apparatus for making bristle sub-assemblies comprising:
means for feeding material comprising a bristle string having first and second rows of bristles connected to a base string having a defined, solid physical structure with a regular, geometric shape and a base string outer surface, the first and second rows of bristles being in a first orientation relative to each other, the bristles of each row having a defined, solid physical structure with a regular, geometric shape and a bristle outer surface connected to the base string outer surface by a partial melting contact area defining a bond where the bristle outer surfaces of the first and second rows of bristles contact the base string outer surface, means for bending the bristles of at least one of the first and second rows of bristles to achieve a second orientation of the bristle rows relative to each other, thereby developing a spring force in the bent bristles that biases the bent bristles towards the first orientation;
means for heating the bristles bent in the bending step at least for a time and temperature sufficient to relax the spring force; and means for cooling the heated bristles so that the bristles maintain the second orientation.

19. An apparatus according to claim 18, wherein the bending means includes a vertically oriented slot having an open upper end, and means for pushing the bristle string into the slot.

20. An apparatus according to claim 19, wherein the slot is defined by first and second endless belts which retain the bristles in the second orientation prior to heating.

21. An apparatus according to claim 19, wherein the pushing means includes a wheel having an outer circumferential edge that extends at least into the slot.

22. An apparatus according to claim 21, wherein the wheel comprises at least two thin discs having spaced apart outer peripheral edges.

23. An apparatus according to claim 18, wherein the heating means includes an ultrasonic heater.

24. An apparatus according to claim 23, wherein the ultrasonic heater is energized at a magnitude and for a time sufficient to impart surface melting at points of contact between the bristles and the base string, whereby after cooling, the bristles and base string are bonded to each other at the points of contact.

25. An apparatus according to claim 23, wherein the ultrasonic heater includes an ultrasonic horn and an anvil juxtaposed the ultrasonic horn to define a gap through which the bristle string passes.

26. An apparatus according to claim 18, wherein cooling means includes ambient air at room temperature.

27. An apparatus according to claim 21, further comprising means for adjusting the vertical and horizontal positions of the wheel.

28. An apparatus according to claim 25, further comprising means for adjusting the gap between the anvil and the horn of the ultrasonic heater.