How to Heat Composites
With CoTexx® Electrical Surface Heating
Heated Moulds and Lightweight Components Made of Fiber Composites
CoTexx® Knitted Heating Fabric is preferably integrated into glass fiber or carbon laminates using infusion processes. This offers the following advantages:
- Placing reinforcing fiber textiles and heating fabric without time pressure, because the pot life of the resin is irrelevant.
- Reduced risk of damaging wires.
- Increased laminate quality due to low void content. This improves the heat flow from the heating layer to the surface.
Nevertheless, processing by hand lamination has also proven itself. At first, the heating fabric is electrically contacted by solder connections and positioned on the base laminate. The heating element is then covered with glass fiber fabric or fleece, impregnated with resin and compacted by means of vacuum extraction. Applications for this process are temperature-controlled moulds made of epoxy or polyester resin with heating temperatures up to approx. 100 °C.
The production of heated moulds from tooling prepreg has also been successfully proven. The heating layer is therefore separated from the carbon fiber prepreg layers by glass fiber prepreg. In the autoclave at e.g. 120 °C or 160 °C the layers harden under pressure to a stiff and lightweight composite mould. The processing of tooling prepreg in the autoclave promises a lower porosity of the laminate compared to pure vacuum consolidation (see e.g. “Tooling Prepregs from Toray”).
Flexible Silicone Heater Mats
Another possibility of processing is the casting method. Applications are flexible heater mats made of silicone or heated silicone hoods for prepreg processing and repair of fiber composite structures. Heated membranes and blankets are also used for producing preforms. Depending on the material used, the silicone membrane has a temperature resistance of around 180 °C.
Surface Heating for Moulds:
The Basic Procedure
The following sections focus on the construction of heated moulds for the production of fiber composite components and preforms.
The individual sheets of heated knitted fabric are laid down on a master model with a prepared covering layer in accordance with the design. The segments are soldered together and subjected to a functional test.
Then additional layers of glass fabric can be laid down. After that, the vacuum infusion is carried out.
- Prepare the surface (link layer) for later connection with the matrix (e.g. sanding, degreasing)
- Cut with scissors and align the heating fabric segments afterwards
- Attach the strips using hot glue nonwoven or spray adhesive
- Set the soldering points
- Measure and log the resistance of each strip
- Interconnect the strips to heating circuits (depending on the layout parallel or series connection)
- Prepare the electric supply
- Check the total resistance
- Install the temperature sensors or thermocouples
- Lay-up of the remaining fiber fabric layers
- Infusion / injection / infiltration as usual
After the resin has hardened, the stiffness of the mould can be increased if necessary. For example a core and a fiber top layer may be applied to form a sandwich structure.
Reliable Contacts by Soldering
In order to let electricity flow through the heating fabric, it has to be electrically wired to the voltage source. A great advantage of CoTexx surface heating is that connections can be made easily and reliably by soldering.
The soldering temperature must be at least 350 °C to remove the insulating varnish at the contact points. You can determine whether the coating has been completely removed by measuring the resistance of the individual strips.
We also have optional accessories for processing CoTexx® Knitted Heating Fabric. These accessories have been specially designed for the use of heating elements in fiber composite systems
PT-100 Temperature Sensors
Our temperature sensors are characterized by their small dimensions and low thermal mass for a quick temperature response. They can be embedded close to the heating wires, which is important for measuring temperature directly at the heat source. Depending on the measuring system, the PT-100 sensors are equipped with two or three test leads made of high-frequency litz wire.
- PT-100 sensor with 1,0 m leads, 2-wire technique
- PT-100 sensor with 1,0 m leads, 3-wire technique
Contacting elements such as our threaded sleeves are particularly suitable for the processing of CoTexx® Knitted Heating Fabric in combination with infusion processes. The internal wires that connect the heating are soldered onto the copper foil of the contacting elements. The sleeves remain within the vacuum bag during infusion. Thus, neither air can penetrate, nor matrix flow into the cables. The connecting cables are simply screwed to the connection elements after curement.
- M4 threaded sleeve, 15 mm, welded on copper foil
- M5 threaded sleeve, 15 mm, welded on copper foil
- M6 threaded sleeve, 15 mm, welded on copper foil
The threaded studs work on the same principle. They are welded onto a 0.3 mm thick copper sheet to which the wires leading to the heating elements are soldered. A shrink tube protects the thread from resin during infusion. All reinforcing layers placed above the heating layer are cut in such a way that the threaded bolts pierce the surface.
- M4 threaded stud with 10 mm or 15 mm length, welded on copper foil
- M5 threaded stud with 10 mm or 15 mm length, welded on copper foil
- M6 threaded stud with 10 mm or 15 mm length, welded on copper foil
If the heating element is operated with voltages above low voltage such as 60 VDC, the contacts must be enclosed to ensure an electric shock protection. This is the purpose of the special frames made of GFRP, in which cable entries may be inserted as required. Afterwards the frames are glued to the laminate and the cover plate is screwed on.
- GFRP frame of 60 mm edge length and 30 mm height, with cover plate
- GFRP frame of 75 mm edge length and 40 mm height, with cover plate
With this flat strand made of bare copper fine wires, the individual segments of CoTexx® Knitted Heating Fabric can be electrically interconnected, to form the complete heating resistance.
- Flat strand with 1,5 mm² cross section sold per meter
Tips and Tricks for the Processing of CoTexx® Knitted Heating Fabrics
Which solder is suitable?
Standard electronic solder with a certain amount of copper is suitable as solder. We can e.g. recommend SN99Cu1 with 3.5% flux. In combination with a soldering iron with an approx. 3 mm wide chisel tip (somewhat flattened for a better heat transfer) and a set temperature of 400 ° C, the high-frequency strands can be contacted reliably.
How can the heating fabric segments be fixed on the base laminate or glas fiber fabric?
Three methods are common:
- The Knitted Heating Fabric and, if necessary, the mating partner are covered with spray adhesive. After a short flash-off time, the heating fabric is placed on the lower layer below and pressed on by hand.
- Superglue and, if necessary, an activator for accelerated curing are also suitable.
- Alternatively, the surface can be covered with a thermoplastic binder fleece. Then the segment of heating fabric is positioned. Afterwards the binder fleece is melted locally by an air gun and the heating fabric is fixed by hand using a light pressure. Subsequent corrections are possible by heating the areas again.
In general, areas where soldering is still required (e.g. the ends of the heating fabric strips) should initially not be glued to the layer below. Thus a thermal insulation can be temporarily layed under the soldering spot in order to protect the layers already deposited on the original model from the influence of heat. After the contacting is completed, the remaining areas of the heating fabric are also fixed on the substrate as described above.
What should be considered when processing Heizgestrick in combination with carbon fibers?
Carbon fibers are electrically conductive. The wires of the heating fabric are provided with a thin insulation layer, but in particular all soldering points must be subsequently insulated in order to prevent electricity from taking the shortcut through carbon fibers.
Therefore, ideally three layers of glass fiber fabric are placed between the carbon fabric and the heating fabric. In areas that are particularly exposed to short circuits (contacting pins or sockets, bare copper strands for connecting several segments of heating fabrics, …), the insulation should be carried out with particular care. This also applies if voltages exceed the limit of a electric shock-safe extra-low voltage of 60 VDC.
Which cable cross-section do the supply leads need?
Cable cross-section of the supply lines depend on the current in the operating state of the heating. For example, ampacity tables recommend a cable cross-section of at least 2.5 mm² for a current of 30 A.
The calculated resistance of a heating fabric segment does not match the measured resistance
Electrical contacting or wiring errors can usually be corrected as long as the electrical surface heating is not yet embedded in a matrix. If you determine an increased resistance of a segment of heating fabric or a heating field when measuring with the ohmmeter, then one of the following causes is usually to blame:
- The inherent resistance of the ohmmeter or the resistance of the measuring lines was not taken into account.
- The soldering point does not contact all strands of the wire, because e.g. the soldering temperature was too low and the insulation varnish was not removed from all the wires as a result. Corrective: Solder again and check by measuring.
- Multiple strands are broken. Corrective: Exchange the strip of Knitted Heating Fabric.