Method of manufacturing a filtration bag

The method of manufacturing a filtration bag for a liquid comprises placing two flexible thermoplastic sheets adapted to filter a liquid upon one another, and welding the sheets together along a desired seam line by vibration welding using tool means which forcibly engage the superposed sheets along the desired seam line.

BACKGROUND OF THE INVENTION 
This invention relates to the manufacture of filtration bags for filtering 
liquids, such as water. Bags of this type are used in various industries 
and are made by seaming together two flexible sheets of a material 
suitable for filtering purposes. 
In the past, seaming has been accomplished by stitching or sewing the 
sheets together. Sewing uses consumables, i.e. thread, and leaves pin 
holes in the material through which unfiltered liquid can escape. The use 
of adhesives is slow, is expensive, and produces inconsistent results. 
Therefore, there has been the need to manufacture filtration bags in a more 
reliable and inexpensive manner, minimizing labor costs, yet providing 
highly consistent and reliable results. Of particular importance are 
leak-proof joints, uniform and repeatable results, elimination of 
consumables and retention of pliability of the manufactured bag. 
These and other objects have been achieved by the present invention using a 
welding process in which sheets of thermoplastic material are vibration 
welded together along a desired seam line for forming a bag. 
BRIEF SUMMARY OF THE INVENTION 
This invention discloses a method of manufacturing a filtration bag wherein 
two thermoplastic sheets adapted to filter a liquid are welded together 
along a desired seam line for forming a bag using reciprocating 
translating motion for causing, responsive to the dissipation of 
frictional energy, melted thermoplastic material at the desired seam line. 
Upon the cessation of the motion, the melted material cools and solidifies 
to form a welded seam. The process is carried out by a commercially 
available vibration welding apparatus which is fitted with appropriate 
tool means for forcibly engaging the sheets along the desired seam line. 
One of the principal objects of this invention, therefore, is the provision 
of a new and improved method of manufacturing filtration bags for liquids. 
Another important object of this invention is the provision of an improved 
method for manufacturing filtration bags for liquids in a relatively 
simple manner and without the use of consumable materials, and without the 
creation of pin holes. 
A further important object of this invention is the provision of a new and 
improved method for manufacturing filtration bags by providing leak-proof 
welded seam lines which join two superposed flexible sheets. 
Further and still other objects of this invention will become more clearly 
apparent from the following description when taken in conjunction with the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the figures and FIGS. 1 and 2 in particular, numerals 10 
and 12 refer to two flexible sheets of thermoplastic material disposed 
upon one another, suitable for filtering liquids, such as water. The 
sheets most suitably comprise, as what is termed, needle punch felt made 
from polypropylene or polyester material. In a typical example and without 
implying any limitation, each sheet has a thickness of about 2.3 mm and 
the finished bag is 84 cm long by 26 cm wide. In order to manufacture a 
filtration bag from the superposed sheets 10 and 12, the sheets need to be 
welded together along a continuous seam line designated generally by 
numeral 14, comprising lines 16 and 18, forming the sides of the bag, and 
a "V"-shaped seam line 20, forming the bottom portion of the bag. In an 
alternative design, the bottom may be rounded. 
Welding the sheets along the desired seam lines 16, 18 and 20 is 
accomplished most suitably by a vibration welding apparatus as shown in 
U.S. Pat. No. 3,920,504 issued to A. Shoh et al, dated Nov. 18, 1975, 
entitled "Friction Welding Apparatus". Apparatus of this type are 
available from Branson Ultrasonics Corporation, Danbury, Conn. 06813, 
Model No. VW4L or WV6L. The apparatus uses reciprocating translating 
motion (vibrations) between the workpieces to be welded, thereby producing 
frictional heat, which causes thermoplastic material to melt and 
subsequently to solidify responsive to the cessation of the vibratory 
motion. 
A friction or vibration welding apparatus of this type includes, as shown 
in FIG. 3, a lower member 22, movable in the vertical direction as noted 
by arrow 26, and an oppositely disposed upper member 24, movable in the 
longitudinal direction as noted by arrow 28. For inserting a workpiece 
between the members, the member 22 is moved away from the upper member 24, 
thus causing an opening. For welding, force means, not shown, provide a 
force upon the member 22 to cause a forced engagement between the sheets 
10 and 12 at the desired seam line. The member 24 is movable 
longitudinally, arrow 28, by being coupled to electromagnetic means as 
shown in the patent to Shoh et al supra, for undergoing reciprocating 
translating motion. This motion causes the required friction between the 
sheets for producing heating and melting of the thermoplastic material 
starting at the interface surface 33 between the sheets. 
The member 22 is provided with tool means 30 engaging the exposed side of 
sheet 12, and the member 24 is provided with tool means 32 for engaging 
the exposed side of sheet 10 along the seam lines. Additionally, as 
clearly shown, each tool means is provided with a rough textured surface, 
such as small serrations or knurls, for grippingly engaging the respective 
sheets with which the tool means are in contact. The tool means 30 and 32 
have a width substantially equal to the width of the weld seam 14 and a 
length commensurate with the seam line. 
For achieving a weld, the sheets 10 and 12 are inserted between the members 
22 and 24. Member 22 is brought into engagement with the sheet 12 and a 
force is applied to the member 22, thereby causing each sheet to be in 
forced engagement with its respective member. As the forced engagement 
prevails, the upper member 24 is subjected to a longitudinal reciprocating 
motion, arrow 28, causing relative translating motion between the sheets 
along the plane of the interface surface 33 of the sheets. Responsive to 
the dissipation of frictional energy, melted thermoplastic material is 
produced and upon the cessation of the motion the melted thermoplastic 
material solidifies to produce a weld between the sheets 10 and 12 at the 
area engaged by the tool means, i.e. the seam lines 16, 18 and 20. The 
lower member 22 then is lowered, arrow 26, and the welded sheets are 
removed from the apparatus. Finally the excess material is trimmed to 
provide a bag 34, FIG. 4, which is welded along three sides, but has an 
open top 36. FIG. 5 shows a section along line 5--5 in FIG. 4, wherein 
numeral 38 indicates the welded seam. 
In a typical example and using material as stated above, the apparatus is 
adjusted for a clamp force of ten to fifteen kN, frequency of the 
vibrations is 220 Hz, peak-to-peak amplitude of the vibrations is 1.8 to 
2.0 mm, and weld time ranges from 4.5 to 8 seconds. It will be apparent 
that a certain amount of experimentation is required to optimize the 
parameters for a particular material, size of the bag and the vibration 
machine used. The heretofore stated values are provided merely for 
illustrative purposes and to better understand the present invention. 
While there has been described and illustrated a preferred embodiment of 
our invention, it will be readily apparent to those skilled in the art 
that certain changes and modifications may be made without departing from 
the principle of this invention, which shall be limited only by the scope 
of the appended claims.