GB2326647A

GB2326647A - Vacuum metallizing using a gas cushion and an attractive force

Info

Publication number: GB2326647A
Application number: GB9713311A
Authority: GB
Inventors: Frank Casey; Nicholas John Copeland; John Downs
Original assignee: General Vacuum Equipment Ltd
Current assignee: General Vacuum Equipment Ltd
Priority date: 1997-06-25
Filing date: 1997-06-25
Publication date: 1998-12-30
Anticipated expiration: 2017-06-25
Also published as: GB2326647B; GB9713311D0

Description

APPARATUS FOR AND METHOD OF VACUUM METALLIZING The present invention relates to apparatus for and method of vacuum metallizing.

In a vacuum metallizer, flexible film to be metallized is supplied from round a process drum to a rewind roll in a vacuum chamber. Metal is vaporised from a source of metal under conditions of high vacuum and deposited on the film. More specifically, the apparatus and metallizing techniques are used to provide very high quality metallized thin polypropylene, polyester and other thin plastic films. Typically 3.5 micron polypropylene can be coated with a thickness of 1.5 ohms/sq of aluminium at metallizing speeds at 600 m/min.

When metallizing such very thin and heat sensitive film plastic films, there can be damage or induced defects to the film due to the heat imparted into the film in the metallizing process.

This damage is such that "tramlines", "snake skin", Zpinholes", buckling and other stress related or heat related distortion occurs in the metallized product.

Any tendency for the film to move from or on the drum may result in damage to the film and/or non-uniform disposition. It has been proposed to provide an electrostatic attraction between film and drum to hold the film on the drum. However, this can lead to buckling of the film if the film expands during the metallizing process. It has also been proposed in EP-A0311302 to provide a cushion of air between the film and the drum. This enables any expansion of the film to be effectively absorbed but can lead to lack of control of the film or billowing of the film as it passes around the drum.

It is an object of embodiments of the invention to eliminate or mitigate these disadvantages.

According to one aspect of the present invention, there is provided apparatus for vacuum metallizing comprising a chamber, means for creating a vacuum in the chamber, a metallizing station in the chamber, a surface for supporting a film to be metallized in the chamber adjacent the station, means for supplying film to be metallized to the metallizing station, means for receiving metallized film from the metallizing station, means for providing a gas layer between the surface and the element and means for providing an attractive force between the treatment surface and the element.

According to another aspect of the present invention, there is provided a method of vacuum metallizing including the steps of supplying a film to be metallized to a metallizing station, supporting a film to be metallized on a surface adjacent the station providing an attractive force between the film and the surface, supplying a layer of gas between the film and the surface and receiving the element from the treatment surface.

In a preferred embodiment of the invention, the metallizing station comprises a metallizing source disposed adjacent a process drum. The means for supplying the gas layer advantageously comprises a source of gas and means for conducting the gas between the drum surface and the film to be metallized. The surface is preferably the surface of a water cooled drum. The means for conducting a gas is advantageously a gas manifold which preferably is wedge shaped and is disposed between the drum and the film on the supply side of the film such that the wedge dimension decreases in the direction of movement of the film. The leading axial ends of the manifold are shaped to prevent or restrict the escape of gas from those ends. Without this gas may be lost at those ends which could result in the film being stressed as mentioned earlier. This special shaping may also reduce the amount of gas used so that high vacuum conditions can be more readily maintained. The attractive force between element and drum surface is advantageously electrostatic and may be provided by an electrical circuit operative to apply a potential difference between the metal on the film and the drum surface. The gas manifold may be electrically isolated, a negative or positive voltage applied to it and the drum grounded to create a plasma discharge. In this configuration either the manifold or the drum would act as a cathode for the electrical discharge.

As a result of the plasma discharge gas injected through the manifold and trapped between the moving film and drum can be ionized to produce ions which can charge up the moving film. This in turn would attract the film to the water cooled drum and would eliminate the need to apply a negative bias to the drum. As an alternative to this arrangement, an ion or electron beam gun could be used to create a flow of ions or electrons.

Advantageously, the surface of the drum is polished and has a uniform electrical conductivity.

In order that the invention may be more clearly understood one embodiment thereof will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 diagrammatically shows a conventional vacuum metallizer, Figure 2 diagrammatically shows an end view of the drum of the metallizer of figure I modified in accordance with the invention, Figure 3 is a cross section through a part of the drum of figure 2.

and Figure 4 diagrammatically shows a side elevational view in partial section of the drum of Figures 2 and 3.

Referring to figure 1, the vacuum metallizer comprises a vacuum chamber 1, in which are disposed a metallizing source 2, an unwind roll 3, a rewind roll 4, a process drum 5 and guide rolls 6 and 7. The vacuum chamber 1 is provided with a door (not shown) and means for evacuating the chamber 1 (also not shown). With the chamber 1 at atmospheric pressure and the door open, film 8 to be metallized is supplied on the unwind roll 3 to the chamber threaded through the guide rolls 6 and 7 around the drum 5 and onto the rewind roll 4. Metal is supplied to the source 2, the door closed and the chamber 1 evacuated.

In normal operation, the film to be metallized is drawn from the unwind roll 3 round guide roll 6, drum 5, guide roll 7 and onto rewind roll 4 while metal is vaporised from source 2 which is heated under conditions of high vacuum. The heating of the source 2 produces a cloud of metal particles which are deposited on the film 8 as it passes by supported on the surface of the drum 5.

In order to hold the film 8 against the drum 5 to promote uniform deposition, the drum is electrically charged from an electrical circuit connected between the drum 5 and the metal 10 on the metallized film 8.

Whilst this attraction will control and hold the film 8 on the drum, heat entering the film 8 from the vaporised metal and source 2 will cause expansion of the film 8 and consequent buckling of the film resulting in unacceptable stress lines on the film. In order to avoid this, a cushion of a gas is provided between the film 8 and the surface of drum 5. This gas is injected between the film 8 and the drum 5 and forms a uniform layer 9 which is uniformly compressed between them. The gas layer 9 improves conduction of heat from the film 8 to the drum 5 which promotes uniform cooling tends to reduce any tendency to expand. However, if expansion should occur this can be absorbed as the film 8 floats on the drum 5. If the film 8 is simply supported on the gas layer 9 on the drum 5 without any electrostatic attraction, there could be a tendency for the film to billow on the drum out of control.

The gas is introduced via a gas manifold 11 disposed between the surface of drum 5 and the film 8. This manifold 11 is wedge shaped in cross-section in a plane at right angles to the axis of the drum 5. The wedge tapers in the direction of movement of the film 8. The manifold comprises a single gas feed pipe 12 which leads via a branch 13 to a number of subsidiary pipes 14 which in turn supplies a manifold box 15 which is shaped at opposite axial ends 16 thereof to prevent or restrict escape of gas from those ends. If gas is allowed to escape the film may be stressed as mentioned earlier. Also, the special shaping may also reduce the amount of gas used so that high vacuum conditions in the chamber 1 can be more readily maintained. The manifold 11 may be electrically isolated, a negative or positive voltage applied to it and the drum grounded to create a plasma discharge. In this configuration either the manifold or the drum would act as a cathode for the electrical discharge. As a result of the plasma discharge gas injected through the manifold and trapped between the moving film and drum can be ionized to produce ions which can charge up the moving film. This in turn would attract the film to the water cooled drum and would eliminate the need to apply a bias (in this case negative) to the drum. As an alternative the manifold could be replaced by an ion or electron beam gun operative to produce a flow of ions or electrons. In this arrangement, the resultant plasma would charge up the moving film 8 to attract it to the drum while the flow of gas would lift the film slightly from the drum to eliminate the effect of defects and other imperfections which may exist on the drum surface. The trapped gas may further cool the moving film 8 by convection. This would remove or prevent the formation of tramlines, pinholes and blow holes from the metallized film. Thermal contact to the drum 5 can be further improved by using a drum with a mirror like or polished surface finish. This would also remove any surface defects in the drum which could influence the breakdown of the metal film.

Also, the drum surface should have uniform electrical conductivity to achieve the same applied electrical bias particularly at the two axial ends of the drum. Apart from securing the above advantages, the above described arrangement may also be used for improving cooling after metallizing.

It will be appreciated that the above embodiment has been described by way of example only and that many variations are possible without departing from the scope of the invention.

Claims

1. Apparatus for vacuum metallizing comprising a chamber, means for creating a vacuum in the chamber, a metallizing station in the chamber, a surface for supporting a film to be metallized in the chamber adjacent the station, means for supplying film to be metallized to the metallizing station, means for receiving metallized film from the metallizing station, means for providing a gas layer between the surface and the element and means for providing an attractive force between the treatment surface and the element.

2. Apparatus as claimed in claim 1, in which the metallizing station comprises a metallizing source.

3. Apparatus as claimed in claim 1 or 2, in which the means for providing the gas layer comprises a source of gas and means for conducting gas between the treatment surface and film to be metallized.

4. Apparatus as claimed in claim 3, in which the means for conducting gas comprises a gas manifold.

5. Apparatus as claimed in claim 4, in which the manifold is wedge shaped, the wedge dimension decreasing in the operational direction of movement of the film.

6. Apparatus as claimed in claim 4 or 5, in which the leading axial ends of the manifold are shaped to prevent or restrict the escape of gas from those ends.

7. Apparatus as claimed in any preceding claim, in which the means for providing an attractive force between the treatment surface and the film is electrostatic.

8. Apparatus as claimed in claim 7, in which an electrical circuit is provided operative to provide a potential difference between metal on the film and the treatment surface to provide the electrostatic force.

9. Apparatus as claimed in any of claims 1 to 6, in which means are provided for ionizing gas supplied to provide the gas layer to provide the attractive force between the treatment surface and the film.

10. Apparatus as claimed in any of claims 1 to 6, in which an electron beam gun is provided operative to create a flow of electrons to provide the attractive force between the treatment surface and the film.

11. Apparatus as claimed in any of claims 1 to 6, in which an ion beam gun is provided operative to create a flow of ions to provide the attractive force between the treatment surface and the film.

12. Apparatus as claimed in any preceding claim in which the treatment surface is the surface of a drum.

13. Apparatus as claimed in claim 12, in which the drum is water cooled.

14. A method of vacuum metallizing including the steps of supplying a film to be metallized to a metallizing station, supporting a film to be metallized on a surface adjacent the station providing an attractive force between the film and the surface, supplying a layer of gas between the film and the surface and receiving the element from the treatment surface.

15. A method as claimed in claim 14, in which gas is conducted via a gas manifold to the surface to supply the gas layer.

16. A method as claimed in claim 15, in which escape of gas from axial ends of the manifold is prevented or restricted.

17. A method as claimed in any of claims 14 to 16, in which the attractive force is provided by applying a potential difference between metal on the film and the surface.

18. A method as claimed in claims 15 or 16, in which the gas manifold is electrically isolated, a negative or positive voltage applied to it and the surface grounded to create a plasma discharge, thus ionizing the gas to charge up the film.

19. A method as claimed in claim 18, in which the manifold or surface acts as a cathode for the discharge.

20. A method as claimed in claim 14, 15 or 16, in which an ion gum is used to create a flow of ions to charge up the film to produce the attractive force.

21. A method as claimed in claim 14, 15, or 16, in which an electron beam gun is used to create a flow of electrons to charge up the film to produce the attractive force.

22. Apparatus for vacuum metallizing substantially as hereinbefore described with reference to figures 2 to 4 of the accompanying drawings.

23. A method of vacuum metallizing substantially as hereinbefore described with reference to figures 2 to 4 of the accompanying drawings.