1. Field of the Invention
The present invention relates to a coating machine having a nozzle for discharging a liquid coating material under pressure toward a web on a backing roll.
2. Description of the Related Art
This type of coating machine is known as a die coater, which comprises a nozzle extending parallel to a backing roll for discharging a liquid coating material under pressure toward a web on the backing roll, and a doctor edge arranged on the nozzle adjacent the outlet at a narrow gap between the backing roll and the doctor edge for obtaining a layer of a coated material with a uniform thickness. The liquid coating material under pressure is supplied to the nozzle by a pump. The nozzle comprises a nozzle body having a chamber connected to the pump and a generally planar narrow passage with a width generally corresponding to the width of the backing roll; the upper end of the passage being an outlet for discharging the liquid coating material. The doctor edge is located at the upper surface of the nozzle body on the rear side of the outlet.
The liquid coating material is injected from the outlet and a gathering of the liquid coating material is formed between the nozzle and the backing roll. The gathering liquid covers the outlet and a portion of the backing roll facing the outlet so that the liquid coating material is continuously carried by the web on the backing roll to be coated thereon. However, the amount gathering liquid is relatively small in the conventional die coater since the outlet is at the upper end of the narrow passage. Thus, there is a tendency for air to be entrained in the liquid coating material and the layer of the liquid coating material is not uniformly formed on the web.
Japanese Unexamined Patent Publication (Kokai) No. 2-152574 filed by the assignee of the present application on Dec. 2, 1988 discloses a die coater called a lip coater including a nozzle comprising a nozzle body having a first chamber connected to an inlet, a passage with a lower end connected to the first chamber and an upper end, and an outlet connected to the upper end of the passage, the outlet comprising a recess in the upper wall extending toward the rear of the frame so as to form a second chamber between the upper wall of the nozzle body and the backing roll. The outlet of the nozzle is large in size compared with the upper end of the passage and thus a large gathering of the liquid coating material is formed. Accordingly, the above described problem of the conventional die coater can be overcome by the lip coater. In addition, a sensor is arranged in the second chamber for detecting a pressure of the gathering of the liquid coating material and a clearance between the doctor edge and the backing roll can be adjusted in response to an output of the sensor so as to stabilize the gathering of the liquid coating material.
However, there is a desire to further improve the die coater so that the layer of the liquid coating material is uniformly formed on the web even if there is a variation in the conveying speed of the web and if there is a variation in the web itself, such as a partially sagging portion.