Needled felt filter bags and method for forming same

Multiple filter bags are formed from a layered composite sheet of two, essentially identical fibrous or felt layers by needling a plurality of transverse paths each 0.75 to 1.5 inches wide on the composite sheet, the paths being spaced 4-18 inches apart so as to form a plurality of tubular shaped channels. The needled transverse paths are then slit longitudinally along a central line so as to form separate elongated tubes, which are then cross cut apart transversely to produce segments of any desired length such as 12-36 inches. These segments are each final closed at one end of each segment by needling or stitching to form multiple filter bag structures. The invention also includes the felt filter bags produced by this method of manufacture.

BACKGROUND OF INVENTION 
The invention relates to a method for forming filter bags by joining 
together two layers of superimposed fibrous felt sheets by needling at 
selective areas, and also relates to felt filter bags formed by the 
needling method. 
Forming filter bags from flat felt filter media has been conventionally 
done by cutting and folding, then machine stitching the edge seams 
together. One process comprises the steps of cutting out bag blanks by 
using templates, folding and sewing together the side seam, then sewing 
the bottom end closed and adding any finishing steps as desired. However, 
such felt filter bag production methods are relatively expensive, as 
stitching the side seam together can amount to between 25 and 75% of the 
total labor component in felt filter bag manufacture, depending on the 
size and shape of the bag. Also, undesired leakage of fluids can occur 
through the stitched seams of the filter bag. 
A process which is used in the fabric product manufacturing industry for 
joining together fabric sheets is called mechanical interlocking, needle 
punching or needling, and consists basically of tucking a small bundle of 
individual fibers down through a carded batt of fibers in such large 
numbers of penetrations that a cohesive textile structure is formed. For 
example, U.S. Pat. No. 3,431,611 to Rentz has disclosed use of needling to 
produce a plurality of tubular channels on a layered substrate to form 
nonwoven shells for electric blankets. A pattern of longitudinal parallel 
lines is needled utilizing an appropriately patterned board, the resulting 
needled shells are then cut transversely into large articles, and the 
plurality of channels thus formed between the needles lines in each 
article receive the electrical cords of the blanket. Also, U.S. Pat. No. 
4,955,116 to Hayamizu et al has disclosed a method for producing a tubular 
felt body which is needle punched along overlapping edges, in which at 
least a long sheet of felt material is overlapped by dislocating the 
sheets in width direction to form thin layer portions at both side edges 
of the overlapped sheets, and the overlapped thin layer portions are then 
needles together to form one tubular body. However, adapting such a method 
to the manufacture of felt filter bags would be complicated and 
uneconomic. 
The method for forming filter bags from a fibrous or felt media material 
according to the present invention eliminates the folding step and the 
side seam stitching procedure of the prior art constructions, by utilizing 
a spatially oriented needling procedure for making seams which is similar 
to that used for making filter media sheets to replace the conventional 
stitching step for the seams. The present invention represents an 
important technical advance over the known prior art conditions, and will 
be clearly defined hereinafter. 
SUMMARY OF INVENTION 
One object of the present invention is to provide an improved method for 
reliably producing fibrous or felt filter bags which have improved 
filtering effectiveness and strength. 
Another object is to reduce the unit cost of filter bags by employing a 
more efficient cutting method which reduces the loss of material due to 
any defects in the flat filtering media from which the bags are formed, 
and by reducing the sewing labor and the amount of sewing threads required 
in the manufacture of the filter bags. A further object is to provide an 
improved elongated filter bag which has needled seams along opposite sides 
and the lower end of the bag. 
These objects are achieved according to the method of the present 
invention, whereby an elongated composite formed by two layers of a 
fibrous filtering media such as needled felt material is joined together 
by multiple needled transverse paths, the adjacent paths being spaced 
apart so as to provide a plurality of transverse tubular portions in the 
composite. These tubular portions are then separated by cutting them along 
the control portion of each needled path so as to thereby form individual 
tubes each having two needled side seams, rather than one stitched side 
seam. The tubes are then cross cut and divided into segments and then 
finished by either further needling or stitching across one end as desired 
to form multiple filter bags of suitable length. 
Accordingly, the method for forming multiple fibrous felt filter bags 
according to the present invention comprises the steps of: 
(a) superimposing two elongated layers of a flat fibrous filtering media 
each having substantially identical weight and composition so as to form a 
layered composite sheet; 
(b) joining the two flat media layers of the layered composite sheet 
together by needling a series of transverse needled paths, so as to form 
multiple transverse tubular channels which are spaced apart from each 
other along the length of the elongated layered media composite sheet; 
(c) slitting each needled transverse path along the central portion of the 
needled path and thereby forming separate tubes of the fibrous filtering 
media each having two needled side seams; 
(d) segmenting the tubes of fibrous media to any desired length, so as to 
provide multiple tubular segments; and 
(e) closing one end of each the tubular segments so as to form multiple 
filter bag structures. 
To provide a suitable needled path or seam for the filter bags, the 
transverse needling paths should be made at least about 0.75 inch wide, 
and need not exceed 1.5 inch wide, and have a needling density of 50-200 
punctures/square inch. The needled paths can be spaced apart by 4-18 
inches depending upon the filter bag width or diameter desired. If the bag 
lower end is closed by needling, the needled path or seam should be 
0.40-0.75 inches wide for adequate joining strength. Other conventional 
filter bag finishing steps may be added as desired, such as adding a 
collar and bail to the bag open end. 
There are numerous functional and economic advantages provided by this 
method for forming multiple needled fibrous felt filter bags, and for the 
needled filter bags produced according to the invention. The needled seam 
filter bags are stronger and unit cost per filter bag is lowered, because 
the amount of sewing labor and thread are reduced by at least half because 
the seams are formed by needling rather than by sewing or stitching. Also, 
the cutting time is reduced by making simple multiple straight line cuts 
along the needled paths, rather than by using a template to cut out the 
individual bag blanks. Moreover, any defective filtering media material 
does not result in the loss of an entire bag, because the defect can be 
removed from the preformed tube and a correct bag made from the 
immediately following good filter media material. 
These advantages may be achieved without requiring new needling equipment, 
excepting only suitable adaptation of the needle-holding boards to produce 
the desired pattern of transverse needle paths. Moreover, the method is 
applicable to sheets of any fibrous material or blend of fibers and/or 
deniers currently used in the production of felt filter bags.

DETAILED DESCRIPTION OF INVENTION 
FIG. 1A-1C generally illustrate the usual prior art method for making 
filter bags by cutting, folding and machine stitching together the edges 
of a sheet of filtering material. FIG. 1A shows a bag blank 10 which has 
been cut out from flat filter media material such as felt using a 
template. FIG. 1B shows the blank 10 after being folded in half along the 
fold line 11. FIG. 1C shows the folded blank for which the side edges have 
been stitched together to form a seam 13, and the lower conical bottom 14 
has been stitched together to form the completed filter bag. 
Various other shapes of filter bags can be produced by cutting and folding 
the filter media material and stitching it together along the seams. FIG. 
2A shows a rectangular shaped bag having a stitched side seam 15 and a 
straight bottom seam 16; FIG. 2B shows a sewn bag having a side seam 17 
and a generally circular bottom seam 18, and FIG. 2C shows a sewn bag 
having a side seam 15 and a curved bottom seam 19, and FIG. 2D shows a 
sewn bag unit having a side seam 15, a conical bottom seam 14, a metal or 
plastic ring sewn into a collar 20 and a lifting handle 21. 
The method for forming multiple needled fibrous filter bags according to 
this invention will now be described. Two layers or sheets of a suitable 
fibrous filter media material such as felt, each having substantially 
identical composition and thickness, are first superimposed on each other 
to form a layered composite. Then a plurality of transverse needled paths 
or seams are formed in the superimposed layers, by utilizing a needle 
punching or needling procedure. As shown schematically by FIG. 3A, such 
needling procedure can be achieved by passing an elongated layered 
composite sheet 30 formed by two superimposed layers 30a and 30b of a 
filtering media material over a roller 31 and then across a special 
needling board 32. The needle board is adapted for producing a selective 
needling pattern utilizing needle punchings having a density of 50-200 
punchings/in.sup.2 in the filter media composite sheet 30. One useful 
needled pattern is shown by FIG. 3B, in which a pattern of transverse 
parallel needling areas or paths A have been formed separated by sheet 
portions B. After allowing for an unused end portion 33, the multiple 
needled paths 34 are formed transversely across the elongated composite 
sheet 30, leaving a plurality of intervening portions 35 therebetween. As 
shown by FIG. 3A, these intervening portions 35 are later cut apart along 
and within needled path 34 at cutting step 38 by suitable cutters such as 
rotary cutter blade(s) 39 to form a plurality of elongated tubes, as 
described hereinafter. The needled paths A are each made 0.75-1.5 wide, 
and are spaced apart by varying distance B equal to the desired width of a 
filter bag element, such as 4-18 inches wide. 
As an alternative needling procedure as shown in FIG. 3C, intervening 
spaces having two different width B and C can be provided between 
alternate needled paths A. For this alternative needling pattern, after 
allowing for the unused end portion 33, multiple transverse needled paths 
34 are formed across the elongated composite sheet 30, leaving intervening 
portions 35 and 36 therebetween which form tubular portions each having a 
different width. Typically the width of the needling paths A is between 
0.75 and 1.5 inches, the width of the intervening spaces B which provide 
tubular channels is between 6 and 18 inches, and the width of the 
intervening spaces C is 4-12 inches. Although for the FIG. 3B and 3C 
constructions the needled transverse paths A are all substantially 
parallel with each other, these needled path A can be made non-parallel, 
so that the intervening portions 35 and 36 are tapered relative to each 
other at an included angle of 5.degree.-15.degree.. 
As another alternative needling procedure for the composite sheet 30 as 
shown by FIG. 3C, two or more cross needled paths D can be provided in the 
composite sheet 30 as it is moved across or through the special needling 
board 32. These additional cross needled paths 37 are each made 0.40-0.75 
inches wide and serve to provide a bottom seam for each filter bag after a 
segment cutting step as described further below. 
FIG. 4A shows schematically a perspective end view of two filter media 
layers 41a and 41b which are superimposed so as to form a composite media 
sheet 40 illustrating the first step according to the method of the 
invention. FIG. 4B shows a perspective end view of the composite sheet 40 
after strip needling along multiple spaced apart paths 42 to form tubular 
portions. FIG. 4C shows two of the separated tubular portions 45 and 46 
which have been formed by cutting along a central line 42a of the needled 
paths 42. These tubular portions 45 and 46 have also been cross severed 
along line 43 so as to provide the segmented tubular portions of any 
desired length. If composite sheet 40 also has cross needled paths 44, the 
severance line 43 is located adjacent to the needled path 44 so as to 
provide one open end and one closed end for each tubular segment 45 and 
46, which may each have length of 12-36 inches. 
FIG. 4D shows a perspective view of a segmented portion 46 of the needled 
tubular structure of FIG. 4C, which has been cut at one end so as to have 
a conical shaped end portion 47. The media layers of end 47, which can be 
straight or conical, can be sewn together at seam 48 in the conventional 
manner to form a filter bag 50. Alternatively, the end of the bag can be 
joined together by needling at seam 49. The bag may be further finished, 
e.g. to provide a lifting handle and metal or plastic ring sewn into a 
collar, as is generally shown in FIG. 2D, to provide a filter bag 50 in 
which both side end seams are formed by the needling method of this 
invention. 
The method according to the invention is applicable to layers of any fiber 
or blend of fibers and/or deniers currently used in the production of 
filter bags, including but not limited to acrylics, aramids, nylon, and 
olefinic polyester and Viscose rayon fibers. For filter media material 
used for forming filter bags, the fiber denier or size and the medial 
thickness used will depend on the desired particle micron size rating of 
the filter media. For example, filter bags having low micron particle size 
rating such as capable of removing particles larger than 1-10 microns 
would use a mixture of 1.5 and 3 denier plastic fibers and have media 
thickness of 0.055-0.090 inch. Filter media rated 15-50 microns would 
utilize a mixture of 5.0-15.0 denier fibers and have thickness of 
0.060-0.125 inch, and filter media rated 75-200 microns would utilize a 
mixture of 15-35 denier fibers and have media thickness of 0.70-0.250 
inch. 
For filter bags formed according to this invention, the needled paths or 
seams 42 and 44 serve mainly as an effective media layer attachment means, 
so that very little fluid flow occurs outwardly through the needled seams 
and they have very little filtering function. However, it is emphasized 
that attaching the edges of a filter bag together by needling according to 
this invention instead of by conventional stitching is very advantageous, 
because it eliminates any holes formed by the stitching needle and thread. 
Such stitching holes are usually up to 50% larger than any needling 
punctures or openings, so that a filter bag having needled seams can have 
a precise micron particle size removal rating. Conventional filter bags 
having stitched side and end seams usually have particle size removal 
rating only 75-97% that of the filter media itself. Furthermore, the 
filter bag having needled seams will be stronger and can withstand greater 
pressure differential than a bag having stitched seams, because the 
needled seams will have substantially the same strength as the media 
material itself. 
Approximately 75% of the total annual felt filter bag production consists 
of bags identified in the industry as sizes No. 1 and 2, each being 
approximately 11.5 inch flat width and either 18 inches long for No. 1 
size or 31 inches long for No. 2 size bag. When the process according to 
this invention is run at a normal 3 linear yards/minute rate, using a 
layered felt material 72 inches wide, side seam labor is reduced to about 
three seconds for a No. 1 bag and six seconds for a No. 2 bag. This 
contrasts to the sewing labor for filter bags of the prior art design, 
which can be from 30 seconds to one minute per bag depending on its size. 
The invention will be further described by a typical example for 
manufacture of felt filter bags having needled side seams and conical 
ends, which example should not be construed as limiting in scope. 
EXAMPLE 
Two continuous flat layers of needled felt filter media material, each 72 
inches wide, made from acrylic viscose fibers and having a denier of 3-8, 
are provided and superimposed to form a composite sheet. The composite 
layers are passed over rollers and under a needling board at a rate of 3 
linear yards per minute. The needling board is patterned so as to produce 
a series of transverse needled paths each 1.0 inches wide and spaced six 
inches apart. The needling for each path takes about 15 seconds elapsed 
time. The patterned layered composite then passes through a cutting step 
in which a slit is made along a central portion of each needled path, 
thereby forming a plurality of tubular pieces each having length equal to 
the composite sheet width. These long tubular pieces are then segmented to 
a length of 12 inches with a conical shape at one end. The conical edges 
are then stitched together on a sewing machine to form the individual 
filter bags similar to that shown by FIG. 4D. 
Other modifications and variations of the method of manufacture for 
multiple filter bags and the filter bag products can be made without 
departing from the scope of the invention, which is defined by the 
following claims.