US3641605A

US3641605A - Web cleaning apparatus

Info

Publication number: US3641605A
Application number: US34129A
Authority: US
Inventors: Thomas W Lindsay
Original assignee: Minnesota Mining and Manufacturing Co
Current assignee: 3M Co
Priority date: 1970-05-04
Filing date: 1970-05-04
Publication date: 1972-02-15
Anticipated expiration: 1989-02-15

Abstract

A web cleaning apparatus for thoroughly cleaning continuous webs moved through the apparatus. The apparatus comprises a lint free wiping cloth for wiping dust and foreign objects from a web, a means for using a vacuum to move the web to be cleaned into intimate contact with a section of the wiping cloth so that the web will be thoroughly wiped clean, and nuclear means for removing static electrical charges from the surface of the web before and after it is cleaned so that cleaning will be facilitated and so that the web will not tend to attract dust after being cleaned.

Description

United States Patent [151 3,641,605 Lindsay 1 Feb. 15, 1972 [54] WEB CLEANING APPARATUS 2,048,490 7/1936 Bilstein.... ..l5/ 1.5 X [72] Inventor: Thomas w. Lindsay Minneapolis, Minn. 2,91 1,330 11/1959 Clark ..15/1.5 X [73] Assignee: Minnesota Mining and Manufacturing Primary Examiner-Leon G. Machlin Company, St. Paul, Minn- Attorney-Kinney, Alexander, Sell, Steldt and Delahunt [22] Filed: May 4, 1970 3,128,492 4/1964 l-lanscom etal. 3,510,903

Appl. No.2 34,129

References Cited UNITED STATES PATENTS ....l5/ 1.5 X 5/1970 Stoever et a1. ..l5/256.5

[57] ABSTRACT A web cleaning apparatus for thoroughly cleaning continuous webs moved through the apparatus. The apparatus comprises a lint free wiping cloth for wiping dust and foreign objects from a web, a means for using a vacuum to move the web to be cleaned into intimate contact with a section of the wiping cloth so that the web will be thoroughly wiped clean, and nuclear means for removing static electrical charges from the surface of the web before and after it is cleaned so that cleaning will be facilitated and so that the web will not tend to attract dust after being cleaned.

11 Claims, 3 Drawing Figures memsorms I912 3.641.605

- INVENTOR.

7/10MA5fl/Z/N05AY WEB CLEANING APPARATUS This invention relates to the cleaning of webs and in one aspect to a cleaning apparatus for the removal of fine dust and other foreign material from moving webs such as are used in the manufacture of photographic film and food packaging film.

Dust attraction and accumulation on plastic webs may be a severe problem, especially where the webs are used in the production of photographic film. Because the conductivity of plastic webs is low, static electrical charges developed on the surface of the web duringconveying or handling are not easily neutralized or grounded. These static electrical charges tend to attract dust to the web from the ambient atmosphere or surrounding surfaces, and this dust must be removed before the web is coated to produce a finished product. Even very fine specks of dust incorporated in the coating applied to the web to produce photographic film will be magnified on the order of 100 times visually, and thus will affect the quality of the finished product. The emulsion coat of photographic film will be sensitized by the presence of metallic dust as small as l micrometer in diameter. Quality problems exist with the presence of 15 to 50 such metallic particles on a 240 square inch area of emulsified film. Attracting this quantity of metallic dust to a plastic web is not uncommon, even from the relatively clean surroundings of a film production area. Thus, production of high quality photographic film requires thorough cleaning of the plastic web before and after it is coated. I Dust creates a similar problem when it is attracted to a photographic negative to be printed. This is especially true where the print is to be an enlargement of a negative, as the defects in the print caused by dust on the negative will also be enlarged.

Prior art methods of removing dust particles from plastic webs include brushes and vacuum pickup systems. Both of these types of systems work reasonably well for removing large particles of dust in the 100 micrometer range or for partial removal of large concentrations of smaller dust particles, but both are almost totally ineffective for total removal of particles of one-half to micrometers in diameter. Particles in this size range are particularly difficult to remove because-the intermolecular forces which attract them to the web are large compared to their size. Thus,.actual contact with these small .dust particles by the cleaning means is required for their complete removal. This contact is not provided by vacuum removal systems and is only partially provided by brush removal systems.

The use of brushes to remove dust from aweb has additional drawbacks. The brush may cause abrasion on a sensitive web surface. Also, the typical brush used for removal of dust particles in such devices does not retain the dust removed from the web, but merely moves it off the web into the air where it may again be attracted onto the web by virtue of static electrical charges generated on the cleaned web by the brushing action itself. Thus, a brushing system can be self-defeating.

To prevent the collection of dust on plastic webs after the web has been cleaned by a brush or by a vacuum pickup system, prior art devices have attempted to eliminate static electrical charges from the cleaned web. One method of removing static electrical charges from the web is by ionizing the air around the web so that the charges on the web will be neutralized through ion exchange.

One type of equipment which ionizes air uses a high voltage corona discharge. While sufficient ionization can be produced through the use of corona discharge devices to completely neutralize charges on a plastic web, the equipment involved in producing the corona discharge is quite complex. Additionally, the high voltage used in producing a corona discharge can present a danger to operating personnel and the potential arcing from such equipment prevents its use in explosive atmospheres. I

In some cleaning devices radio isotopes which emit alpha particles have been used to ionize the air around the web so that static charges on the web would be effectively neutralized. Generally, however, the radio isotopes with the longest half lives (i.e., radium, half-life 1,690 years) emit quantities of gamma and beta particles which are injurious to operating personnel in addition to emitting the relatively harmless alpha particles needed for ionization. Thus, if a quantity of an isotope with a long half life sufficient to efi'ectively neutralize a web is used, serious radiation danger to operating personnel may-exist. Additionally, the decay of radio isotopes causes a deposit to be formed on most materials (typically metal) used to house them. This deposit can absorb the alpha particles emitted from the source used .while allowing the gamma and beta particles to pass, thus decreasing the useful life of a given quantity of an isotope, and causing a requirement for a larger quantity of the isotope for a given amount of ionization, thus increasing the danger to personnel from gamma and beta particle emission.

U.S. Pat. No. 2,943,059 teaches the production of a particle having a thickness no greater than 2 mm. and consisting essentially of a weather resistant refractory matrix in which a radioisotope may be distributed and encapsulated. The particle offers low resistance to the passage of alpha particles and does not tend to form a deposit over the isotope which would absorb alpha particles. The use of such particles make practical the use of radioisotope Polonium (Po as an alpha particle source. Po emits generally only alpha particles and so is safe even when used in large concentrations around operating personnel, but its half-life is relatively short, 138.40 days), so that it is not economical to use where deposits which will absorb alpha particles will develop. Through use of the particles described, the alpha particle output of a given quantity of Po can be maximized while the isotope is encapsulated to prevent direct contact with operating personnel. Thus distributing particles containing Po on a bar extending laterally across a web to be neutralized provides a source of alpha particles for ionization of the air around the web which is safe for operating personnel, and which is reasonably economical to use.

The present invention provides a method for thorough cleaning of a plastic web of dust particles and a safe and economical means for leaving the web electrically neutralized so that dust will not be attracted to the cleaned web after the cleaning process.

The apparatus of the present invention comprises a soft, lint free cloth for wiping foreign particles from the web without damaging the web and vacuum means for providing intimate contact between the web and the cloth to effect essentially complete removal of the dust from the web, including essentially total removal of dust of one-half to 10 micrometers in diameter. The wiping action of the cloth against the web in the present invention is essentially accomplished by the passage of the web over a substantially stationary section of the cleaning cloth. Means for changing the section of the cloth brought in contact with the web are also provided.

The apparatus further includes safe, efficient ionizing means for efiectively removing electric static charges from the web before and after the web is wiped clean by the cloth so that dust particles will be freed of a physical attraction and so that airborne dust particles will not be attracted to a cleaned web.

Static electrical charges on a web present invention are removed by the use source, comprising ceramic particles as described in the previously mentioned US. Pat. No. 2,943,059. The particles disclosed in this patent contain Po and provide a uniform and relatively safe'source of alpha particles, which source may be mounted on a bar of any desired length extending laterally across the width of a web being cleaned. The present invention is at least equipped with one ionizing bar following the web cleaning operation to insure that no static electrical charges will be present on the web after the wiping of the web by the cleaning cloth.

passed through the of a nuclear ionizing The invention will further be described with reference to the accompanying drawings wherein like numbers refer to like parts in the several views and wherein:

FIG. 1 is a schematic sectional view of a web wiping apparatus according to the present invention;

FIG. 2 is a schematic view in perspective showing the apparatus according to the present invention which is shown in FIG. 1; and

FIG. 3 is a sectional view in perspective taken approximately along the line 33 of FIG. 2.

Referring now to FIGS. 1 and 2, there is schematically shown an apparatus 9 according to the present invention which comprises a wiping means for wiping dust from one side of a web 10, which is moved through the apparatus 9 in the direction indicated typically at speeds between 50 to 600 feet per minute. The web 10 may be intended for use in the manufacture of photographic film and/or may be made of polystyrene, polyvinylchloride, acetate, polyester, or other plastic material.

The wiping means in the apparatus 9 comprises a soft, thick, lint free cloth for wiping foreign particles or dust from the web 10. Air pressure means are provided to achieve intimate contact between the web and the cloth to effect the wiping and removal of dust from the web 10. The web cleaning apparatus 9 shown has a first roller 11 and a second roller 12 upon which rollers a supply of wiping cloth 13 is wound with a length of the cloth 13 extending therebetween. A section 15 of the wiping cloth 13 extending between the

rollers

11 and 12 is brought into contact with a continuous web 10 moving through the apparatus 9 in the area where both the web 10 and the cloth 13 pass over a support member or tubular member 20. The tubular member 20 has closed ends and is formed with an axially extendingslot 22 in the uppermost portion of its wall. A vacuum source 24 is connected to the tubular member 20 so that the pressure within the tube 20 will be reduced, (typically by one-half to 20 inches of water) thus causing atmospheric pressure to push the nonporous plastic web 10 down upon the section 15 of porous wiping cloth 13 extending over the tube 20 and supported by a supporting surface 16 of the tubular member 20 on each side of the slot 22. Thus intimate contact is caused between the wiping cloth l3 and a web being drawn across the section 15 of the wiping cloth 13 and the tubular member 20 so that dust particles on the surface of the web l will be removed.

In addition to causing intimate contact between the cloth 13 x and the web 10, the air flow caused by the vacuum in the tubular member 20 will draw loose fibers from the wiping cloth 13 into the tubular member 20, and will also carry off dust particles removed from the web by the cloth 13 into the tubular member 20.

A soft, lint free, dust retaining cloth 13 is used to effect complete transfer of dust particles from the web 10 to the wiping cloth 13 without abrading the surface of the web 10. Good results have been obtained by the use of a soft polypropylene random woven cloth with long fibers of fine denier. This type of cloth has very good dust retention characteristics and is lint free. The binder in the cloth has been especially selected to eliminate adhesive transfer from the wiping cloth 13 to the surface of the web 10. I

An example of specifications on a wiping cloth which has been found very satisfactory in removing dust particles from a plastic web is as follows: The cloth is made by a random weave process from polypropylene fibers about I 1% inches long and of 1.5 to 1.8 denier. The cloth is made 0.0] inches thick and to weigh approximately 28 pounds/ream. The binder used to assemble the random weave cloth is a commercial acrylic emulsion made up in a concentration of to 30 percent solids.

Means are provided for changing the section 15 of wiping cloth l3 brought in contact with the web 10. In the apparatus 10 illustrated in FIGS. 1 and 2 the takeup roll 12 is driven by a 7 the movement of the web 10 at a speed in the order of H50 to l/2000 of the speed of the web 10 being cleaned. The selection of the advancing speed of the wiping cloth 13 will depend upon the anticipated amount and character of dust to be removed, the dust containing ability of the wiping cloth 13 used, and/or the abrasiveness of the dust being removed from the web 10. In moving in a direction contra to the movement of the web 10 the edge of the section 15 of wiping cloth 13 which first contacts the dirt particles on the web 10, and thus removes the largest percentage of the dust particles, is constantly being separated from contact with the web 10 to minimize possible scratching from particles or dust collected on the cloth 13.

An ionizing means or static eliminating bar 26 is mounted in a position adjacent and generally parallel to the tubular member 20 on the side of the tubular member 20 toward which the web 10 is moving. The bar 26 is sized to extend laterally across the entire width of the web 10 and is positioned with the side of the bar from which alpha particles are emitted adjacent to and spaced one to three inches from the web 10 to be neutralized. With this spacing the web 10 surface will be in the most effective range of the alpha particles emitted.

The static eliminating bar 26 provides a housing for containing the radioisotope emitting the alpha particles and provides a means for controlling the direction in which alpha particles will be emitted from the bar 26.

' As can best be seen in FIG. 3 the static eliminating bar 26 comprises a bar-shaped housing 27 in which is formed a slot 30 which runs the length of the housing 27. A quantity of polonium is encapsulated in particles or spheres 28 as described in U.S. Pat. No. 2,943,059 The spheres 28 are attached by a suitable adhesive to one side of a thin metal strip 32 which is mounted at the bottom of the slot 30 with the spheres 28 exposed exteriorly to the bar 26. The housing 28 will absorb alpha particles striking it, and thus will only permit alpha particles to escape from the bar 26 in a direction in which they will not strike the housing 27. For this reason the area which will be ionized by the alpha particles escaping from the bar 26 is controlled by the configuration of the slot 30 in the bar 26. A steel screen 34 and a retaining sleeve 35 for the screen 34 are provided so that the spheres 28 will be protected from damage while permitting passage of the majority of the alpha particles to ionize the air.

v Thus, with the bar 26 properly positioned with respect to the web 10, alpha particles emitted from the bar 26 will ionize the air adjacent the web 10, effectively neutralizing static electrical charges upon the web 10 through an ion exchange process so that dust will not be attracted to the cleaned web 10.

As illustrated in FIGS. 1 and 2, means for effectively removing static electrical charges from the web 10 prior to cleaning the web 10 are also provided to facilitate ease of cleaning of the web 10. A static eliminating bar 37 essentially identical to the static eliminating bar 26 is mounted generally parallel to the bar 26 with the side from which alpha particles are emitted spaced one to three inches from the path of the web 10 and in position just prior to the point at which the section 15 of cleaning cloth l3 wipes the web 10 free of dust. The effective elimination of static electric charges on the web 10 caused as the web 10 passes the ionizing bar 37 tends to condition the web 10 so that wiping of dust particles from the web 10 by the wiping cloth 13 may be easily accomplished. As the web 10 passes over the nuclear ionizing bar 37, the static electrical charges on the surface of the web 10 are reduced except for charges on the web 10 beneath individual dust particles. The dust will remain attached to the web 10 until the cleaning cloth section 15 is contacted at the tubular member 20. It is theorized that when the cleaning cloth 13 contacts the individual dust particles, the particles are moved from their original position on the web 10 to a position over an area on by providing two means for wiping dust the web on which static electrical charges have been dissipated by the bar 37. From this position the dust particles may be easily picked up by the section of cloth 13 as they are no longer attached to the web 10 by as large an attracting force as they were in their original positions. Thus, the presence of the ionizing bar 37 neutralizes the surface of the web 10 prior to the wiping action of the cloth l3 and prepares the web 10 for a more thorough cleaning action by the wiping cloth 13.

When large metallic particles are anticipated on the surface of a plastic web, a magnetic removal means for metal particles may be used in conjunction with the cleaning apparatus 9. As illustrated in FIGS. 1 and 2, a magnetic bar 40 extends laterally across the path of the web 17 in a position in advance of the ionizing bar 37. The poles of the magnetic bar 40 are oriented in the direction of travel of the web as shown. The magnet is of sufficient strength and is spaced an appropriate distance from the web 10 to produce a field strength preferably in the range of 300 to 400 gauss at the surface of the web 10. The bar 40 illustrated includes a magnetic core comprising particles of a permanent magnet material contained in a workable material, which core is made according to the method described in U.S. Pat. No. 2,999,275 and is commercially referred to as Plastiform. The core has a three-eighths inch wide by 1 inch high vertical cross sectional area and is sandwiched between two metal plates to shape the field. The bar 40 is spaced preferably one-eighth inch or less from the web 10 to produce the desired field strength at the web 10. The magnetic bar 40 will serve the function of collecting metallic particles which might cause abrasion of the web during the wiping process by the section 15 of wiping cloth 13 if the metal particles are collected by cloth 13.

It will be noted that a cleaning apparatus similar to the ones previously described can be build to clean both sides of a web from the web with a soft cloth similar to the wiping means previously described, with one wiping means disposed to each side of the web path. The two tubular members utilized in the wiping means for cleaning both sides of a web cannot be placed directly opposite each other on opposing sides of the web path, but should be spaced from each other a short distance along the direction of travel of a web so that the web may move in a direction normal to its surface to be pressed into contact with the wiping cloth across the tubular members. Even when an apparatus for cleaning both sides of the web is used, only two static eliminating bars are needed as the static eliminating bars will reduce electrical charges from both sides of the web regardless of the side of the web on which they are positioned. Additionally, where two magnetic bars are utilized to remove metallic particles from opposite sides of a web, the location of the magnetic bars should be spaced from each other in the direction of travel of the web so that the magnetic flux lines of the magnetic bars will not interfere with each other.

It should be recognized that with some types of webs and/or with various requirements in the level of dust removal required, adequate cleaning of the web may sometimes be achieved by using other means to bring the web and the wiping cloth into intimate contact. Such a means might be a curved shoe over which the wiping cloth extends and over which the web passes, or it might be a pair of opposing rollers between which the cleaning cloth extends and between which the web passes.

Having thus described the invention with reference to preferred embodiment what is claimed is:

l. A device for thoroughly cleaning at least one surface of a web moving through said device, said device comprising:

a support member, said support member having a supporting surface defining a portion of a path through said device for a said web and extending transversely to the direction of movement of a said web;

a lint free porous cloth for wiping foreign particles from a said web, said cloth having a section thereof positioned over said supporting surface between said support 6 member and said ath; air pressure means or providing a differential air pressure on opposite sides of a said web moving along said path for urging the portion of a said web opposite said' support member into even, intimate contact with said section of cloth positioned over said supporting surface to afford thorough wiping contact between a said web moving through said device and said section of cloth positioned on said supporting surface; and ionizing means for neutralizing electrostatic charges on a said web after said web has been wiped clean by said section of cloth. 2. A device according to claim 1 wherein said support member is a hollow tubular member mounted on said device with the axis of said tubular member extending normal to said being formed with an axially extending slot through the wall of said tubular member, said slot extending across said path and being generally centrally located in said supporting surface adjacent said path with said section of cloth positioned over said supporting surface and said slot, and wherein said differential air pressure means comprises vacuum means for reducing the pressure in said tubular member so that a said web passing over said tubular member will be urged by atmospheric pressure into even intimate contact with said section of cloth over said supporting surface.

3. A device according to claim 1 wherein said ionizing means comprises nuclear means for neutralizing static electrical charges on a said web.

4. A device according to claim 3 wherein said ionizing means comprises encapsulated Polonium positioned along the path of a said web for neutralizing static electrical charges on a said web.

5. A device according to claim 3 further comprising nuclear means positioned along the path of a said web for essentially removing static electrical charges from a said web before a said web is wiped clean by said cloth.

6. A device according to claim 1 wherein said lint free cloth is a random woven cloth comprising long fine denier nonlinting fibers.

7. A device according to claim 2 wherein said lint free cloth is a random woven cloth comprising fine denier, nonlinting polypropylene fibers and means which will not transfer to a said web for binding said fibers together.

8. A device according to claim 1 further comprising magnetic means positioned along the path of a said web for removing metallic particles from a said web prior to the cleaning of a said web by said cloth.

9. A device according to claim 7 further comprising magnetic means positioned along the path of a said web for removing metallic particles from a said web prior to the cleaning of a said web by said cloth.

10. A device according to claim 9 which further comprises means for changing the section of said cloth positioned on said ,support member.

11. A method for thoroughly removing dust and foreign material from at least one side of a nonconductive web which comprises the steps of:

passing said one side over an ionizing means to neutralize essentially all of the static electrical charges on the web to be cleaned;

urging said one side of the moving web into intimate contact with a wiping cloth on a transverse supporting surface by air pressure means acting differentially on opposite sides of said web adjacent said supporting surface so that essentially all of the dust and foreign particles will be removed by even intimate contact between the wiping cloth and the web; and

passing said one side over an ionizing means to neutralize essentially all of the static electrical charges on the cleaned web.

l i l

Claims

1. A device for thoroughly cleaning at least one surface of a web moving through said device, said device comprising: a support member, said support member having a supporting surface defining a portion of a path through said device for a said web and extending transversely to the direction of movement of a said web; a lint free porous cloth for wiping foreign particles from a said web, said cloth having a section thereof positioned over said supporting surface between said support member and said path; air pressure means for providing a differential air pressure on opposite sides of a said web moving along said path for urging the portion of a said web opposite said support member into even, intimate contact with said section of cloth positioned over said supporting surface to afford thorough wiping contact between a said web moving through said device and said section of cloth positioned on said supporting surface; and ionizing means for neutralizing electrostatic charges on a said web after said web has been wiped clean by said section of cloth.

2. A device according to claim 1 wherein said support member is a hollow tubular member mounted on said device with the axis of said tubular member extending normal to said path, said supporting surface being the peripheral surface of said tubular member positioned adjacent said path, said tubular support member being formed with an axially extending slot through the wall of said tubular member, said slot extending across said path and being generally centrally located in said supporting surface adjacent said path with said section of cloth positioned over said supporting surface and said slot, and wherein said differential air pressure means comprises vacuum means for reducing the pressure in said tubular member so that a said web passing over said tubular member will be urged by atmospheric pressure into even intimate contact with said section of cloth over said supporting surface.

10. A device according to claim 9 which further comprises means for changing the section of said cloth positioned on said support member.

11. A method for thoroughly removing dust and foreign material from at least one side of a nonconductive web which comprises the steps of: passing said one side over an ionizing means to neutralize essentially all of the static electrical charges on the web to be cleaned; urging said one side of the moving web into intimate contact with a wiping cloth on a transverse supporting surface by air pressure means acting differentially on opposite sides of said web adjacent said supporting surface so that essentially all of the dust and foreign particles will be removed by even intimate contact between the wiping cloth and the web; and passing said one side over an ionizing means to neutralize essentially all of the static electrical charges on the cleaned web.