Metal working – Method of mechanical manufacture – Roller making
Reexamination Certificate
2000-12-19
2003-02-25
Cuda-Rosenbaum, I (Department: 3726)
Metal working
Method of mechanical manufacture
Roller making
C492S056000, C492S058000
Reexamination Certificate
active
06523262
ABSTRACT:
TECHNICAL FIELD
The invention relates to the manufacture of elastomer-covered rollers.
DESCRIPTION OF THE BACKGROUND ART
Rollers with covers of elastomeric material are used in a wide variety of industrial applications. One process is a plastic film manufacturing process known as cast film extrusion. Films of thermoplastic polymers such as polyvinylchloride and polycarbonate are made by this process. The most popular polymers made by this process are polyethylene and polypropylene.
In a typical application, molten polymer from an extruder is dropped into a nip formed between a chrome-plated steel roller and an elastomer-covered steel roller. One common type of elastomer used in this application is silicone rubber, although Hypalon™, available from Du Pont, and other polymer materials can also be used. The chrome-plated steel roller and the elastomer-covered roller are normally water cooled, or otherwise chilled, since the molten plastic may be at a temperature of several hundred degrees Fahrenheit. The chrome-plated roller quickly chills the plastic below its melting point and the nip formed with the elastomer-covered roller determines the gauge (thickness) and the surface finish of the film. The flexibility of the rubber covering compensates for minor machine misalignment and other variables, and provides a wider nip than two hard surfaced rollers.
In some applications, the film manufacturer also runs the surface of the silicone roller through a water bath to provide a measure of release (non-stick) from the tacky semi-molten plastic, and to provide additional cooling of the rubber surface. The excess water is removed with a doctor blade or other device and most of the remainder quickly evaporates from the hot roller surface. A small amount of the water (or water vapor) does pass through the extrusion nip. Because of the temperature difference between the rubber roller surface and the core/rubber bond line, water vapor tends to migrate through the rubber covering and condense at the core surface. Although the interface between the core and other materials is an area, this will be represented by the term “bond line” which is the portion seen in the drawings.
The vapor pressure of water at the hot roller surface may be several pli (pounds per linear inch), while the vapor pressure at the core surface (typically 40 to 70° F.) is a fraction of one psi. This pressure difference drives the water vapor through the wall of the silicone rubber covering. Silicone rubber, in particular, is quite permeable to water vapor. Other polymers that may be used for the rubber covering, such as Hypalon, are either less permeable to water vapor or are more hydrophilic and absorb the water. These polymers are still susceptible to water vapor migration. The rate of migration is slower, however.
As used herein, the terms “permeable” and “impermeable” shall mean permeable and impermeable to moisture or to DOP-type plasticizers. “Moisture” shall mean water or water vapor, or both. Moisture and DOP-type plasticizers shall be included in the term “bond-degrading fluids”, and the term “fluids” shall include liquids and vapors.
Even in applications where the rubber nip roller is not run through a water bath, migration of water vapor to the core is still known to be a problem, especially if the core temperature is below room temperature or the dew point. Rollers used for the application of coatings frequently have problems with water accumulation at the rubber/core bond line. As a result, rollers applying water-based coatings are known to have corrosion and bond failure at the core. The water vapor migration problem is not limited to cast extrusion nip rollers.
Silicone rubber is a material that may be used as the cover in the elastomer-covered roller described for the above application. Silicone rubber is, however, a material that is difficult to bond to a metal roller core on a consistent basis. It is also difficult to maintain the bond in applications where the roller is used at high temperature and high pressure. In some cases, the silicone rubber may peel cleanly off the metal core without leaving any residue of rubber, indicating a loss of bonding. Loss of bonding in one area requires that the elastomer-covered roller be replaced.
Under high pressure, typically 100 pli or more, there is a stress concentration at the rubber-to-metal bond line causing the silicone to tear away from the core due to shearing forces. The best currently available method to achieve a consistent bond is to grit blast the surface of the core to a high roughness, about 500 R
a
, before applying the bonding agents or primers to the core. Bonding agents are applied in thin layers, typically less than 1 mil.
Grit blasting has certain limitations, however. It is difficult to achieve surface roughnesses greater than 500 R
a
. Even maintaining 500 R
a
generally requires the constant use of new grit and attention to the grit blasting process variables. The grit-blasted core surface has a high R
a
but it is not a complex, high profile, surface like Velcro which not only has a high surface area, but also has a lot of “hooks”. So called “hooks” improve the bond strength by trapping material within small surface features.
If a rougher surface were available, for example, in the 500 to 2000 R
a
range, not only would the surface area available for bonding be greatly increased, but the location of the rubber-to-metal bond line would be thicker and more diffused. This would in turn diffuse the shearing forces trying to tear the rubber from the core. Simply tooling a thread pattern into the core does not achieve this goal, because the direction of the thread is nearly parallel to the applied forces in the roller nip rather than perpendicular to them.
An improved bonding surface is needed for elastomers that are weakly or inconsistently bonded, especially if the covered rollers are exposed to high temperature or pressures, or high moisture conditions. The bonding surface must provide a high surface area and surface roughness and yet be easily and consistently produced. A very high surface roughness will also diffuse the stress at the bond line improving the longevity of the rubber to metal bond.
Polyurethane is another material that can be used for the elastomeric cover in the present invention. For bonding polyurethane to supporting layers, primary reliance has been placed on chemical bonding, to be assisted by mechanical bonding. As with silicone-based materials, the limit of surface roughness available with current methods of mechanical bonding is about 500 microinches R
a
.
In addition, water vapor easily permeates through either a silicone rubber layer or a polyurethane layer, and corrosion may occur at the bond line due to the collection of water vapor there. Moisture will migrate into the outer roller cover if the roller is either chilled or exposed to water in the application. It would be beneficial to provide a bond coat or layer that is resistant to such corrosion, as well as one providing a stronger mechanical bond.
Another application of rollers is in the embossing and calendering of polyvinylchloride (PVC) film. The PVC film is made from a rigid PVC resin and is plasticized with oils to produce a softer material. PVC film is typically plasticized with DOP (dioctyl phthalate, a synthetic ester type oil) or chemically similar materials. These forming type operations require that the film be heated in a range of from 350° F. to 400° F. The film can lose DOP at these temperatures due to migration and evaporation.
In either embossing of calendering, the arrangement of rollers is similar to cast film extrusion with a hard surface roller forming a nip with a rubber-covered roller. Due to high temperatures, the rubber-covered roller is typically internally water cooled. The temperature differential through the rubber cover creates the same type of migration problem with DOP as is seen with water vapor in the cast film process. DOP migrates through the rubber covering to the core where it can degrade the bond between the core and the ru
Butters Gary S.
Hyllberg Bruce E.
Kaprelian Paul J.
American Roller Company, LLC
Quarles & Brady LLP
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