1. A tool for RF welding of a tube to two films forming a bag, with each film having a central region for surrounding the tube from above and from below, respectively, and two flat regions extending at both sides from each central region in opposite directions, said tool comprising an upper tool, a lower tool, and a central electrode disposed between the upper and lower tools and including multiple electrodes, wherein the central electrode is constructed of a divided rod to define the multiple electrodes, and a separator arranged between the multiple electrodes, wherein each of the upper and lower tools has a concave surface bounding a central cavity and two flat side surfaces extending from the central cavity in opposite directions, and wherein the multiple electrodes include an upper electrode received in the central cavity of the upper tool and having an upper convex surface facing the concave surface of the upper tool and a lower flat surface and a lower electrode received in the central cavity of the lower tool and having a lower convex surface facing the concave surface of the lower tool and an upper flat surface facing the lower flat surface of the upper electrode, so that the tube and the central regions of the films are receivable in the central cavities of the upper and lower tools below the concave surface of the upper tool and above the convex surface of the upper electrode and above the concave surface of the lower tool and below the convex surface of the lower electrode, while the flat regions of the films are receivable between the side surfaces of the upper and lower tools.

2. The tool of claim 1, wherein the central electrode includes two of said electrodes to polarize the tool in two different ways.

3. The tool of claim 1, wherein the separator electrically isolates the electrodes from each other.

4. The tool of claim 1, wherein the central electrode is constructed symmetrically with respect to a separation plane.

5. The tool of claim 1, further comprising a further electrode configured for arrangement on at least one of a bag perimeter, a region of a bag neck, and a bag base.

6. A method for RF welding of a tube to two films forming a bag, comprising the steps of providing each film with a central region for surrounding the tube from above and from below, respectively, and with two flat regions extending at both sides from each central region in opposite directions; using a tool having an upper tool, a lower tool, and a central electrode disposed between the upper and lower tools and including multiple electrodes; constructing the central electrode of a divided rod to define the multiple electrodes, and a separator arranged between the multiple electrodes; forming in each of the upper and lower tools a concave surface and a central cavity and two flat side surfaces extending from the central cavity in opposite directions; including in the multiple electrodes an upper electrode received in the central cavity of the upper tool and having an upper convex surface facing the concave surface of the upper tool and a lower flat surface and a lower electrode received in the central cavity of the lower tool and having a lower convex surface facing the concave surface of the lower tool and an upper flat surface facing the lower flat surface of the upper electrode, so that the tube and the central regions of the films are received in the central cavities of the upper and lower tools below the concave surface of the upper tool and above the convex surface of the upper electrode and above the convex surface of the lower tool and below the convex surface of the lower electrode, while the flat regions of the films are received between the side surfaces of the upper and lower tools.

7. The method of claim 6, further comprising forming a welded joint between the films and with the tube, and along a bag perimeter.

8. The method of claim 6, wherein welding is realized by first step a welding joint is formed on a region of the tube, and by a second step in which a welded joint is formed on a bag perimeter.