CoTexx Knitted Heating Fabric Produces:
Heat for Composites
Whether hand laminate, prepreg, RTM or infusion – all manufacturing processes for components made of fiber composite materials require heat. Only a few resin systems cure at room temperature. And heat resistance and strength are increased by subsequent tempering in an oven.
More than Hot Air
We bring Heat in Shape
The idea behind CoTexx® Knitted Heating Fabric: generate heat where it is needed. The “flexible heating element from the roll” is embedded into the GFRP or CFRP laminate close to the surface. If current flows through the heating element, heat is generated evenly across the whole area. It heats the fiber composite component to be hardened or activates the binder of the preform. The result is direct tool heating that does not require water, oil or air as heat transfer medium.
CoTexx® Knitted Heating Fabric. Our answer to minimizing cycle time and maximizing energy efficiency. Since 2010.
HEATED AREAS IN Composite COMPONENTs
Heat does not only support manufacturing processes. There are applications where the final composite parts need to have heating elements integrated in the laminate. CoTexx® heated knitted fabric removes snow and ice from GFRP and CFRP parts. Large components for satellite communication systems or aerodynamic parts in the aerospace industry must function in all weather conditions and thus have to be kept free from ice. Some areas of wind turbines do also benefit from anti-icing or de-icing systems.
For CoTexx® Knitted Heating Fabric high-frequency litz wires are processed into a textile. These wires consist of strands, which are individually insulated (enamelled wire). These special wires are also the raw material for vehicle seat heaters. The automotive industry appreciates this technology for safety reasons. After breakage of individual wires, a local overheating (hot spot) is excluded. Therefore the material of the wires for CoTexx® Knitted Heating Fabric is also either 99.9% copper or a resistance alloy, depending on the application.
From Seat Heating to Knitted Heating Fabric
Proven Technology Adapted to the Fiber Composite Industry
Copper wire, which is coated with a temperature-resistant lacquer layer to insulate it electrically, is called enamelled wire. A bundle of such enamelled wires (strands) is a so-called high-frequency litz wire.
The automotive industry has been using high-frequency strands as heating element in seat heaters for years. For CoTexx® Knitted Heating Fabric, the same raw material is processed into a textile during a knitting process. Afterwards, our customers mainly in the field of composites integrate the heating fabric in laminates for heating purposes.
The advantages compared to a heating made of carbon fiber as heating element are obvious:
✔ local overheating is impossible after the breakage of individual wires
✔ reliable contacting of the wires by soldering
✔ Copper and copper alloys have a high ductility and do not break during processing
✔ High deformability and drapability of the heating fabric due to the mesh structure
✔ By varying the alloy, wire diameter and number of strands, the resistance can be matched to the voltage source (usually 12-230 V DC or AC voltage)
In one word: Flexibility
Mould Heating, Temperature Control of Tools, Anti-Icing
Our aim is to offer an “electrical surface heating from the roll”. Flexible and suitable for various applications. Because we are aware that there are numerous processes of manufacturing a component from fiber composite materials. And quantity, component geometry and complexity have a direct influence on the tooling concept.
CoTexx® Knitted Heating Fabric was developed especially for heating tools and moulds made of CFRP / GFRP. Both are utilized e.g. in vacuum infusion methods or hand lamination techniques. Another field of application is preforming technical textiles if binders needed to be activated by heat.
The requirements regarding the process temperatures are as varied as the production techniques.
CoTexx® Knitted Heating Fabric is suitable e.g.
- for accelerated curing of a gel coat layer at 50 ° C
- for activating a binder during preform production at 120 ° C
- for curing a 180 ° C matrix system
TEXTILE HEATING STRUCTURES FOR EVERY APPLICATION
Versions of Knitted Heating Fabric
We manufacture CoTexx® Knitted Heated Fabric both for individual toolmaking and for series production. It is possible for us to provide you with the optimal heating element for your application by varying the following properties:
- Knit width (25 – 120 mm)
- Alloy (usually Cu 99.9% or CuNi or CuNiMn alloys)
- Number of strands of the high frequency litz wire (usually 10 – 48)
- Strand diameter (usually 0.05 – 0.2 mm)
- Thickness of the lacquer layer (grade 1 – 3; breakdown voltage according to IEC depending on the strand diameter 150 – 1000 V)
HEATING SYSTEMS FOR FIBER COMPOSITes
Form of Delivery
CoTexx® Knitted Heated Fabric is supplied as standard on spools. Lengths of up to 35 m per spool are possible. The heated knitted fabric can be cut to length directly on the spot and processed precisely.
Alternatively, we can solder several strips of Knitted Heating Fabric together to form a heating element according to your specifications. The heating element is fixed, e.g. on a glass fiber fabric, in order to maintain the position of the webs during shipment. The prefabricated heating element can then be integrated into the fiber structure at the processor.
180 °C at 5000 W/m²?
The maximum possible operating temperature of a mould heated with Knitted Heating Fabric depends on many factors. The temperature resistance of the wire insulation coating is 190 °C for standard applications. This temperature may only be slightly exceeded for a short time.
However, the temperature resistance of the resin system of the laminate is usually lower and thus represents the maximum for operation.
It should also be noted that the temperature on the tool surface is always below the temperature of the heating wires. This is where the heat is generated. Therefore, a sufficient distance between design temperature and temperature resistance of the matrix should be maintained. For example, if the temperature resistance of the resin system is 180 °C, the tool surface can possibly only be heated up to 160 °C.
Similarly, statements about surface heating wattage per area cannot be generalized. The higher the difference between heating temperature and temperature resistance of the components, the more heat can be released without the risk of overheating. Components to be taken into consideration are e.g. the matrix (epoxy resin, silicone, …), the gel coat and the insulation coating of the heating wires.
Just to demonstrate the possible wattage per area for systems with CoTexx® Knitted Heating Fabric: De-icing applications with 5000 W/m² have already been realized. This power can also heat up heated tools to moderate temperatures of around 100 °C in a short period of time. If, however, the focus is not on minimal heating-up times, then an output of only 500 W/m² for a surface temperature of 50 °C can be regarded as a guideline.
THE PTC BEHAVIOR OF COPPER
The electrical resistance of copper increases linearly with increasing temperature. This should be taken into account when designing a heating for higher heating temperatures. If the resistance R of a heating field is e.g. 1.0 Ω at a temperature T of 20 °C, it increases due to the positive temperature coefficient (αCu=0,0039 [1/°C]) at 150 °C to 1.5 Ω. In general:
In contrast, a temperature-dependent resistance change of the version “resistance alloy” can be neglected, because the temperature coefficient is approximately 0.
We therefore recommend CoTexx® Knitted Heating Fabric “resistance alloy” for the following applications:
- Heat dissipation is not evenly distributed over the heating area (e.g. varying thicknesses of the thermal insulation)
- Non-uniform laminate thickness or heat sinks such as reinforcing ribs in the mould, which act like cooling ribs
- High heating temperatures above approx. 120 °C
THE IDEAL: GENERATE HEAT where it is needed
The Advantages of Surface Heating
Up to now it is customary generating heat in heating cartridges, if moulds need to be directly heated. With this technique heat is generated in one spot or one line. In order to achieve a uniform temperature distribution on the mould surface, there needs to be a thick layer of thermally conductive metal between the heating cartridge and the surface. The entire mass of the tool must be heated in each heating cycle. Results are a sluggish regulation characteristic and a high energy consumption.
This contrasts with the electric surface heating. CoTexx® Knitted Heating Fabric generates heat in a whole layer, which is why the heating can be integrated into the mould close to the surface. From a thermal point of view, the mould thickness can be significantly reduced, which saves mass compared to thick metal tools – and significantly increases the heating rate. Several reasons, why CoTexx® Knitted Heating Fabric can be found in electrically heated GFRP moulds for producing large components such as rotor blades.
Heated production equipment with CoTexx® heating fabric – heat in the right place at the right time. Thanks to near-contour temperature control, high energy efficiency and short heating-up times.