Patent Application
20080276488
The present invention relates to devices and methods for contactlessly drying and guiding traveling webs, and more particularly, an improved web air flotation device that minimizes, eliminates or removes web wrinkles.
In web coating, printing and drying operations, it is often desirable that the web have contactless support, in order to avoid damage to the web itself or to the coating (such as ink) previously applied to one or more surfaces of the web. One conventional arrangement for contactlessly supporting a web during drying includes horizontal upper and lower sets of air bars between which the web travels. Hot air issuing from the air bars both dries and supports the web as it travels through the dryer.
Important characteristics of any flotation system are the amount of cushioning provided by the flotation device, and the stability of the web as it passes over the device. Adequate support removes web wrinkles that typically are caused by higher tensions in lightweight webs. Airflow instabilities near the web can induce web flutter and subsequent web contact with mechanical parts of the dryer, resulting in coating disturbance or web damage. Web flutter can be manifested in a multitude of forms, ranging from a violent flapping of the web to a high frequency drumming.
Single slot air bars are known in the art as air foils. They differ from opposing double and triple slot air bars in that they have both a positive and negative pressure on the face of the air bar, whereas the double and triple slotted bars have only positive pressure. As a result, double and triple slotted air bars can be operated over a wider range of pressures and clearances; typical flotation clearances of air foils being about 2.3 mm compared to 6.3 mm for double and triple air bars. Air foils also have a dramatic decrease in both heat transfer and flotation stability as clearance is increased, whereas the heat transfers for double and triple air bars are relatively stable up to a clearance of 25 mm (single size bar). A typical application for single slot air bars is where flotation must be accomplished with air on only one side of the web.
Conventional air foils discharge air at about 45° to the web, which pushes the web up and relies on the flatness of the web to trap the air and force it to follow the air foil face. This creates a negative pressure to pull the web back down and hold it in place over the air foil. When floating lightweight webs under medium to high tensions, machine direction corrugations will form in the web. These corrugations allow the discharged air from the 45° slot to escape and not trap the air between the air foil face and the web, thereby reducing or eliminating the velocity created to draw the web down to the air foil face. This can result in poor flotation and can render the air foil ineffective.
The present invention relates to a device which discharges an air flow through a primary and secondary air slot or orifice for the purpose of one-sided flotation and stabilization of a moving web.
The problems of the prior art have been overcome by the present invention, which provides a step air foil web stabilizer having an integral blower for one-sided non-contact flotation of a running web, particularly suitable for supporting and/or stabilizing a moving web traveling from a printing press to a web dryer. The stabilizer design includes two discharge slots which allow for increased draw down force, which flattens machine direction wrinkles in a floating web. The design does not rely on a flat web to help create a cross-face velocity to draw the web to the face for proper flotation as in conventional air foil designs. Air discharged from the primary slot is gathered into the air stream of the secondary slot and creates an increased air cushion to provide greater support to the moving web and thereby remove machine direction web wrinkles caused by higher tension in light weight webs, and creates a higher flotation height for higher tensioned webs. The two air discharge slots blow gas (air) parallel to the web over a longer area than conventional designs thereby increasing the draw down force against the web. A constant or substantially constant air velocity is maintained across the flat face of the device for maximum draw down force. Incorporating a secondary slot that discharges air parallel to the web maintains a constant pull down force that is not dependent on a flat web on one-sided flotation applications. Two large flat face areas create a draw down force that is about twice that of conventional devices.
The stabilizer includes a primary discharge slot and a second discharge slot spaced from and stepped down from the primary discharge slot, a first web support surface between the primary discharge slot and the secondary discharge slot, and a second web support surface downstream of the secondary discharge slot in the direction of web travel. The stabilizer includes an integral air supply which provides a supply of air that is uniformly distributed to the primary and secondary slots.
The web stabilizer can be primarily used for one-sided flotation, but also can be used with two sided arrangements for enhancement of drying. It is particularly suited for placement between the last printing unit of a press and the entry slot of a web dryer.
The web stabilizer incorporates a secondary slot that discharges air parallel to the web in order to maintain a constant pull down force that is independent of a flat web. It is particularly useful for one-sided flotation applications (an opposing air bar is not necessary), particularly above the web, although placement below the web is also contemplated and within the scope of the present invention. Air discharged from the primary slot (the first slot encountered by the traveling web as it travels over the device) is reclaimed and gathered in the air stream of the downstream (in the direction of web travel) secondary slot, and an enhanced air cushion is created to provide greater support to the moving web, which in turn removes machine direction web wrinkles such as those caused by higher tensions in light weight webs. One-sided flotation and flattening of a wide range of web weights is achieved, from thin films to heavier paper and films. The working window of operation with different web weights is over twice that of prior art air foils. The device does not rely on a flat web to help create a cross face velocity to draw the web to the face for proper flotation of the web as in the prior art; the downstream secondary face discharging parallel to the web creates a draw down force without the presence of a web. The increased cushion pressure of the secondary slot and the reclaimed air from the primary upstream slot stretches or spreads the web, removing any machine direction wrinkles that may have formed in the web, creating a glass-like appearance.
Turning now to
The flange portions 12a, 12b and bent portion 13 of header 1, together with the trailing top plate 2 and trailing bottom plate 3, define the primary and secondary slots of the device 10. Specifically,
The plate 3 also includes a relatively long portion 33 that extends from the short portion 31 at an angle therefrom. The relatively long portion 33 forms the wing of the air foil, as best seen in
The distance between the primary slot P and the secondary slot S is important for proper air flow and web flotation. If the distance is too small, the air issuing from the primary slot P will not flow parallel to the web. If the distance is too great, the primary slot airflow will lose its velocity. Preferably the distance between the slots is from about 2.5 inches to about 6.5 inches, with 3.25 inches particularly preferred.
Turning now to
In order to adequately support the wing extension of the trailing bottom plate, a plurality of gussets 60 (
As can be seen in
The increased hold down force of the device creates a flat web for a stable transition into opposing air bar zones without web flutter, web billowing, or marking problems.
Thus, in operation, air flow from the integral blower 50 is discharged through primary and secondary slots or orifices. The design allows for the reclamation of the discharged air from the primary slot to be gathered into the air stream of the secondary slot and create an increased air cushion to give greater support to the moving web, which in turn removes the machine direction web wrinkles caused by higher tensions in lighter weight webs. A higher flotation height (e.g., a positive 0.125 inch flotation height off the air foil face regardless of line speed) is possible for higher tensioned webs. Because air is discharged below and parallel to the web, there is always a velocity across the air foil face to draw the corrugated web down to the face and hold it in place for controlled transport. The increased cushion pressure of the secondary slot stretches the web, removing any machine direction wrinkles that may have formed in the web, thereby creating a glass-like appearance to the web. By incorporating two discharge slots on two different face locations and thus providing two large flat face areas, the draw down force is doubled, which is a necessity when flattening machine direction corrugation wrinkles. Conventional air foils discharge air at about 45° to the web, which pushes the web up and relies on the flatness of the web to trap the air and force it to follow the air foil face, which in turn creates a negative pressure to pull the web back down and hold it in place over the air foil. When floating light weight webs under medium to high tensions, machine direction corrugations tend to form in the web, which allow the discharged air from the 45° slot to escape through the corrugations and not trap the air between the air foil face and the web. As a result, there is no velocity created to draw the web down to the air foil face. Since the instant web stabilizer discharges the air parallel to the web through a primary and secondary slot with the secondary slot being downstream of the primary slot in the direction of web travel, there is always a velocity across the step air foil web stabilizer face to draw the corrugated web down to the step air foil web stabilizer face and hold the web in place. By incorporating the primary and secondary discharge slots on two different step foil face heights and also increasing the length of the face compared to conventional air foil designs, the draw down force is approximately doubled and machine direction corrugations wrinkles can be flattened.
The range of web weights and tension conditions with which the present invention exhibits excellent flotation characteristics is more than twice that of conventional designs.
The device is particularly useful between the last printing unit of a printing press and the entry of the dryer. It is also useful downstream of the dryer in industrial units, and in optical film applications where the use of idler rolls is undesirable.
