Electrothermal heater mat

1. A method of manufacturing an electrothermal heater mat, comprising:

providing a preform that includes a laminated stack of dielectric layers made of thermoplastic material, the stack including a central group of one or more layers, and first and second outer groups of layers;

wherein the central group includes reinforcement and the outer groups do not include reinforcement;

wherein the reinforcement is concentrated at a neutral axis of the preform;

wherein the preform includes a heater element and the preform has a first configuration;

heating the preform to a temperature between a glass-transition temperature of the thermoplastic material and a melting point of the thermoplastic material; and

forming the heated preform into a second configuration which is different from the first configuration so as to produce the heater mat;

wherein, in the forming step, the preform is positioned on a tool which has a tool surface with a third configuration which is more curved than the second configuration and the preform is formed down onto the tool surface; and

after the heater mat has been formed on the tool, the heater mat relaxes from the third configuration to the second configuration.

2. The method of claim 1 , wherein the first configuration is a substantially planar configuration.

3. The method of claim 1 , wherein the second configuration is a curved configuration in which the heater mat is non-planar.

4. The method of claim 1 , wherein, in the second configuration, the heater mat is U-shaped in cross-section.

5. The method of claim 1 , wherein in the heating step the preform is heated to and held at a temperature in a range of 150° C. to 210° C.

6. The method of claim 1 , wherein in the forming step the heated preform is held at the temperature for a period of time from 15 to 60 minutes.

7. The method of claim 1 , wherein the reinforcement is woven e-glass.

8. The method of claim 1 , wherein in the forming step the heated preform is positioned on a tool having a tool surface with a ridge and the preform is formed around the ridge.

9. The method of claim 8 , wherein the ridge includes positioning pins and the preform has apertures which are fitted onto the pins.

10. The method of claim 1 , wherein in the forming step the heated preform is positioned on a tool having a convex tool surface and a vacuum bag is used to hold the preform down onto the convex tool surface.

11. The method of claim 1 , wherein:

in the forming step the heated preform is positioned on a tool having a convex tool surface; and

after the forming step the heater mat produced by forming the preform is returned to an ambient temperature before the heater mat is removed from the tool surface.

12. The method of claim 1 , wherein the second configuration is U-shaped in a transverse direction of the heater mat and tapers in a longitudinal direction of the heater mat.

13. The method of claim 12 , wherein the preform is formed against a tool which has a tool surface which is convex in a transverse direction and which becomes more convex in a longitudinal direction.

14. The method of claim 1 , wherein, in the heating and forming steps, the method further comprises positioning the preform against a component including thermosetting material and using heating of the heating step to bond the preform to the component.

15. The method of claim 9 , wherein the pins are spaced longitudinally along the ridge.

16. The method of claim 1 , wherein the second configuration is U-shaped in a transverse direction of the heater mat and twists in a longitudinal direction, the twist being transverse to the U-shape.

17. The method of claim 1 , wherein the tool surface is aluminum.