Non-marking web conveyance roller

A non-marking web conveyance roller has a generally irregular surface with a plurality of rounded up features that promotes superior venting of entrained air. The surface finish of the roller, formed by a thermal spray process, prevents the formation of impression and honing defects on the conveyed web.

FIELD OF THE INVENTION

The invention relates generally to the field of web conveyance rollers. More particularly, the invention concerns a web conveyance roller and method of making same characterized by a surface finish that eliminates the occurrence of objectionable impression and honing defects in the conveyed web.

BACKGROUND OF THE INVENTION

In the art of web conveyance, venting of air entrained between a web and a conveyance roller is needed to maintain contact between the web and the roller. Referring toFIG. 1A, a typical conveyance roller1with a web2trained around a portion of the roller surface4is illustrated. As the speed of the web2increases, the entrained air partially or wholly lifts the web2off the roller surface4which allows relative motion between the web2and the roller surface4to develop. This relative motion often results in damage (such as scratching) to the web2and associated machine conveyance problems (such as poor tracking).

Referring again toFIG. 1A, it is well known that air entrainment occurs when the boundary layer of air that travels with both the web2and roller1is drawn into the web-to-roller interface5. There are several existing roller surface technologies that have addressed, to some extent, venting of entrained air and the associated surface defects that arise if the air is not sufficiently vented. These roller surface technologies include: (1) grooved rollers described, for instance, in U.S. Pat. No. 3,405,855 by Daley et al., Oct. 15, 1968, titled “Paper Guide and Drive Roll Assemblies”; (2) grit-blasted rollers, disclosed, for instance, in U.S. Pat. No. 4,426,757 by Hourticolon et al., Jan. 24, 1984, titled “Web Guide Roller For Use At High Speeds And Process For Producing The Same;” and, (3) shot blasted rollers described, for instance, in U.S. Pat. No. 4,910,843 by Lioy et al, Mar. 27, 1990, titled “A Process For Finishing The Surface Of A Roller.” Experience has shown that each of these technologies can and do cause surface defects such as surface impressions in the web that mirror the roller surface venting pattern and/or roller surface roughness. These problems are particularly worrisome on highly sensitive webs, such as photosensitive film and paper, in areas of the web conveyance machine where the web or coatings are sensitive to marking. Particular examples include surface impression defects that can occur in the manufacture of soft webs like acetate, having a high solvent content or Poly(ethylene terephthalate) (PET), near or above the glass transition temperature (Tg).

Referring toFIGS. 1B and 2B, a highly polished roller surface4used in most web conveyance devices (not shown) has a typical surface roughness characterized by an Ravalue of less than 7 micro-inches (FIGS.3A and3B). This level of surface roughness minimizes the sharp up features that can damage the web2or coatings due to impressions or minute scratches. Commonly referred to as honing, this defect is caused by web dimensional changes during contact with a roller surface4.FIG. 9illustrates an image of a web conveyed on a typical prior art roller which produced a honing defect6typically caused by a smooth, grit-blasted or shot-blasted roller.

Referring again toFIGS. 1B and 2B, experience has shown that the polished roller4(described above) can cause a larger scratch defect in the conveyed web at tensions less than 175 N/m and at web speeds above 15 m/min. These larger scratches are caused by large relative motion between the web2and roller surface4due to loss of traction between the web2and roller surface4caused by air entrainment. Experience has also shown that using conventional roller surface technology (e.g. grooved rollers, grit-blasted rollers, and plateau honed shot-blasted rollers) to vent the entrained air prevents the larger scratches caused by loss of traction. However, the minute defects (e.g. impressions and honing) are worse than they were on the highly polished rollers.

With reference toFIG. 1A, grit blasted rollers provide air venting because the rough surface provides areas for the entrained air to go without lifting the web2away from the roller surface4. However, the grit blasting process produces sharp up features that cause impressions and honing. Polishing the surface4still leaves sharp up features and impressions, and honing problems remain. Shot-blasted rollers also have sharp up features due to the nature of the manufacturing process. Again, it is our experience that grinding the roller surface4still leaves sharp up features and impressions, and honing problems remain. Moreover, it is our experience that grooved rollers, even ones with well-rounded groove comers and lands with surface roughness characterized by an Raless than 5 micro-inches, cause impressions and honing.

Referring again toFIG. 1A, another problem common to typical high-traction conveyance rollers is the collection of debris that adheres tenaciously to the roller surface4. This debris may come from the web2or from the environment and cause the same defects as sharp up features on the roller surface4as described previously.

A commonly accepted solution to the aforementioned problem is to run higher tensions on smooth rollers (tensions up to 500 N/m) to prevent loss of traction due to air entrainment at speeds up to 150 m/min. However, higher tensions are not generally desirable because they can make impression defects worse. Moreover, high tension can also cause orientations of the polymer chains that are undesirable (e.g. triacetate retardation, polyester heat relax). Furthermore, the aforementioned solution cannot be used everywhere in web conveyance devices because rollers with different surface finishes are used depending on the process conditions in a particular machine section.

Therefore, a need persists in the art for a web conveyance roller that maximizes venting of entrained air and that has a surface finish that resists the formation of markings on the conveyed web, such as impression and honing defects.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide a web conveyance roller that provides exceptional traction and that eliminates objectionable surface defects, particularly impression and honing defects, in the conveyed web.

It is another object of the invention to provide a roller that has a surface that resists collecting surface debris in a web conveyance application.

Yet another object of the invention is to provide a method of making a web conveyance roller having a textured surface finish that resists defects in conveyed web.

The conveyance roller of the invention has a generally irregular surface with generally rounded up features, many of which overlap to form venting pathways randomly in the surface. Rounded up features have a roughness coefficient Rain the range of about 20 micro-inches to about 500 micro-inches to eliminate scratching a web conveyed by the roller. The height of the rounded up features is greater than about 0.00005 inches to about 0.002 inches.

The present invention is directed to achieving one or more of the objects, features, and advantages set forth herein. Briefly summarized, in one aspect of the present invention, a web conveyance roller has a generally irregular surface, said generally irregular surface comprising a plurality of generally rounded up features, many of said generally rounded up features overlapping to form venting pathways randomly arranged along said generally irregular surface of said web conveyance roller, the height of said generally rounded up features being greater than about 0.00005 inches but less than about 0.002 inches, said generally irregular surface comprising said generally rounded up features having an Rain the range from about 20 micro-inches to about 500 micro-inches to avoid scratching a web conveyed by said web conveyance roller.

In another aspect of the invention, a method of fabricating a web conveyance roller having rounded up features has a height ranging from about 0.00005 inches to about 0.002 inches, made by a process comprising thermal spraying a finely dispersed molten powder selected from the group consisting of WC—Co, WC—Co—Cr, WC—NiCr, and WC—NiCr—C onto a select substrate.

The textured surface of the conveyance roller of the invention has numerous advantageous effects: it eliminates the occurrence of honing and impression surface defects; it inhibits the collection of surface debris; the non-marking and high release surface characteristics enable the roller to be used in multiple locations in the conveyance machine environment; and, the roller surface has much improved traction properties which significantly reduces incidences of web scratches.

DETAILED DESCRIPTION OF THE INVENTION

Turning now to the drawings, and in particular toFIGS. 2A,5,6, and10, the irregular textured roller surface12of the web conveyance roller10(shown here in cross-section) of the invention is illustrated. Best illustrated inFIG. 10, irregular textured surface12bearing web2is characterized by a plurality of generally rounded up features14, formed by thermally sprayed molten resin (described below), on a substrate13. Many of the rounded up features14overlap to form a plurality of venting pathways16whose configuration conforms to the spacing between adjacent rounded up features14. It is well known that venting pathways16enable air entrained between the interface5of the roller surface12and web2to be vented away from the web2thereby enabling more consistent web-to-roller contact. In the roller surface12of the invention, venting pathways16are randomly formed on the surface12and constitute at least about 25% of the surface area of the roller10.

Referring toFIGS. 2A,5, and6, the finish of roller surface12and the process for producing such a finish (described more fully below) are important factors in venting air entrained between the web2and roller surface12. These factors also contribute to the elimination of objectionable impression and honing defects (described above) in the conveyed web2. Moreover, these factors enable web tensions less than 40 N/m at web speeds greater than 150 m/min.

Another important feature of the novel and unobvious roller surface12of conveyance roller10of the invention is that surface12has a high release surface property that inhibits the collection of debris. The high release surface property is measured by attaching a piece of stripping tape to a portion of the roller surface12and then operating the roller at a speed that ultimately produces separation from the roller surface12. The non-marking and high release features of the roller surface12of the invention allow its use throughout a web conveyance machine (not shown).

Referring toFIG. 10, the rounded up features14of roller surface12have average heights (h) in the range from 0.00005 inches to 0.002 inches. These average heights (h) are associated with the specific groups or types of sprayed surface finishes. For instance, one type of sprayed surface finish has a base material, e.g., acetate, which is minimally cured. Skilled artisans will appreciate that acetate goes through a range of cured states. When acetate is stripped from a casting surface, the acetate is at a 50% solvent concentration. By the time that the acetate is wound into a roll to form a finished product, the solvent concentration, for instance for photographic products, is less than 3.5%.

Referring again toFIG. 10, another sprayed surface finish used in our experiments is one in which the base material is partially or completely cured. According to this surface finish, heights (h) of rounded up features14are produced in the range of about 0.0001 inches to 0.004 inches.

Referring toFIGS. 5,7A and7B, surface roughness Raof the generally irregular surface12having the rounded up features14ranges preferably between 20 micro-inches to about 500 micro-inches. It has been demonstrated that the aforementioned roughness range of roller surface12avoid scratching a web2conveyed by the roller surface12of the invention. According toFIG. 5, experience has shown webs to be conveyed that are very sensitive to impression defects, e.g., freshly cast acetate, require a conveyance roller surface roughness of less than 80 micro-inches to avoid the occurrence of such defects.

Referring toFIGS. 5 and 6, the finished roller surface12of the invention is made using preferably a thermal spray process. High velocity oxy-fuel, high velocity air fuel, and a detonation gun are a few examples of pertinent thermal spray processes that may be used to produce the surface finish of the invention. Moreover, there is a wide variety of thermal spray powders such as WC—Co, WC—Co—Cr, WC—NiCr, WC—NiCr—C. We have found that the preferred powder, for instance, for an idler roller is WC—Co that results in a very unique surface morphology. Depending on the materials used, roller surface12made by the thermal spray process generally possess a large range of advantageous features, such as wear resistance, high release properties, corrosion resistance, etc. The roller surface12, once sprayed, has rounded up features14, illustrated inFIGS. 5,6,10, and11. As illustrated inFIG. 11, as a molten spray particle (arrow40is the particle impingement direction) impinges substrate surface30, a molten deposit32is formed on the substrate surface30. Molten deposit32generally has a rounded up portion36adjoining lateral portions38that are uniquely produced by the thermal spray process. These features (as previously discussed) impart good traction as a roller surface while not marking sensitive webs2(FIG.10).

Those skilled in the art will appreciate that thermal spray coating processes are commonly used to deposit metals, oxides, cermets, and inter-metallic compounds onto roller surfaces to improve performance characteristics. The thermal spray processes (as shown inFIG. 11) form coatings by depositing finely dispersed molten and semi-molten particles onto metallic or organic substrates. The particles impinge onto the surface and flatten forming a series of platelets that adhere to the surface and each other. The coating thickness increases as more particles strike the surface and cool forming a laminar structure. The resulting surface morphology consists of rounded, raised asperities or rounded up features. The height, spatial frequency, and curvature of the rounded up features are a function of the substrate roughness, coating material, specific thermal spray process, process parameters, techniques deployed, particle size, and post coating processes.

Referring toFIGS. 6 and 10, the extremely rough irregular surface features (Ra>100 micro-inches) provide venting of entrained air between the conveyed web2and roller surface12. Illustrated inFIG. 10, the smooth rounded up features14of roller surface12are substantially in continuous contact with the conveyed web2. As a result, impression or honing defects6in the conveyed web2are eliminated. Depending on the speed and tension required and web sensitivity to impressions, a different surface roughness can be used.

Referring toFIG. 8, web-to-roller traction of the roller surface12of the invention is illustrated. Comparing the curve ofFIG. 8to curve ofFIG. 4, the traction of the roller surface12is clearly shown to be far superior to the traction of the polished roller surface4of the prior art. More particularly, the roller surface12of the invention has a traction range (represented by the coefficient of friction) between 0.32 to about 0.12 corresponding to a roller speed of about 20 m/min to about 330 m/min, respectively. In contradistinction, the polished roller surface4of the prior art has a traction range (FIG. 4) of about 0.08 to about 0, corresponding to a roller speed of 20 m/min to a top speed of about 50 m/min.

In another embodiment of the invention, a method of fabricating a roller10for conveying a web2includes the step of thermal spraying a dispersed molten powder on a substrate to achieve the desired finish roller textured surface12. Preferred molten powders are selected from the group consisting of WC—Co, WC—Co—Cr, WC—NiCr, and WC—NiCr—C. As described above, the resultant textured surface12is characterized with rounded up features14protruding from the surface12, as illustrated in FIG.10. The average heights (h) of the rounded-up features14range from about 0.00005 inches to about 0.002 inches.

Examples of rollers10made by the aforementioned process include stripping and ladder rollers. Stripping and ladder rollers are typically located close to the casting surface and require a surface that has a lower Ra. These rollers are manufactured using the HVAF process and propitiatory brushing techniques and the use of WC—Co powder. Idler rollers in the balance of the machine can use higher Rasurfaces that are created by using the HVOF process with WC—Co powder with propitiatory brushing techniques.

PARTS LIST

1prior art conveyance roller2web being conveyed4polished roller surface of the prior art5interface between web and roller surface6honing defect10web conveyance roller of the invention12surface finish of roller1013substrate14rounded up features of roller1016venting pathways30substrate surface32molten deposit36rounded up portion of molten deposit3238lateral portions of molten deposit3240molten particle impingement direction