JEC COMPOSITES MAGAZINE - Issue #99 - September 2015 - 51

The Tibgrid calculator below (see Figure
4, which can be downloaded from the
company's website) serves to determine
the maximum heat capacity per unit
of surface of the heating mat: e.g. for a
100-mm-wide Tibgrid mat, with a 20-mm
pitch and the Ti 3.6 Ω/m heating yarn
(limit amperage: 3.6A). In 230 volts,
the maximum equivalent power per
square metre will be 1450 watts/m² for a
relatively short mat length of 4.10 m. The
Ti 3.6 will be able to deliver a maximum
power of RI² = 3.6 X (3.6)² per linear
metre. The Pmax in open air at ambient
temperature for Ti 3.6 is 46.6 watts per
metre of filament.
For a filament based on higher conductivity alloys, e.g. Ti 0.48Ω/m, as shown
by the green line in the above diagram,
the limit amperage is 5.8 A. The advantage here is a greater useful length,
which makes it possible in the preceding
example to cover a greater useful area
using only two connections (giving a mat
length of 13.8m); on the other hand, the
filament's maximum heat capacity will
be 0.48 x (5.8)² = 16.1 watts/m, which
is relatively low. This can be explained by
the incapacity of the filament to dissipate
enough heat per unit of length.
To obtain the same resulting output, one
would have to add more circuits in parallel or shorten the pitch. Using parallel
networks complicates the connections,
and especially the layout on 3D complexshape parts, because each filament must
have the same resistance and therefore the
same length (even if the paths are different). One also have to be careful to avoid
potential delamination of the composite
or the cure pouch when inserting a mat
with a very dense network of conducting
filaments.
In short, a filament with greater conductivity makes it possible to manage larger
surfaces, but with lower heat exchange
capacities than for a more resistive filament, and often with lower mechanical
or fatigue resistance. You have to find a
trade-off between these aspects based on

Fig. 4: TIBGRID heating grids calculator , loadable from TIBTECH web site

the specific application, which is why it is
important to have a broad range of yarns
in terms of different resistivities, geometries and alloys.

Micromesh for safety and to
regulate the thermal gradient
For building composite heating moulds,
Tibtech recommends adding a flexible
conductive mesh like Tibmesh, or even
better, an elastic one like Stretchomesh.
This should be inserted between the heating mat and the internal mould surface,
separated by one or two plies of reinforcement. The mesh fulfils three purposes:
- safety: by earthing the mesh, the power
circuit can be broken immediately in case
the pouch or the heating mould is accidently damaged;
- regulation of the (small) thermal gradient between each circuit pitch during
heating;
- increasing heat diffusion within the
material by improving the heat exchange
between heating mat and inner mould surface, which makes it possible to use a higher effective power (this purpose can also
be fulfilled by doped conductive resins,
although there is always the risk of lowering the global dielectric, and therefore of
limited input voltage, especially when the
filament diameter must be limited).

Tibgrid® circuit for
high-capacity requirements
For large composite structures, it is difficult to bring in enough power without also
increasing the maximum heat capacity,
and therefore, the wire diameter (and this
is often impossible due to the risk of delamination or losing structural resistance)
or, as an alternative, increasing the number
of circuit connections in parallel, which
complicates matters and greatly increases
the cost.
To open up possibilities and eliminate
the need to handle and place the micromeshes, Tibtech developed the Tibgrid®
Interface mesh concept. These are Tibgrid®
mats to which are added very thin, but
fairly dense networks of heat transfer filaments that enable the heating yarns within
the mat to maximize their heat exchange
capacity without sacrificing their elasticity. Maintaining their elasticity means
that they are able to cover complex-shape
or 3D surfaces. And since no conductive
filaments are used in the warp direction,
the risk of a hot spot developing in case
of partial rupture is minimized. In certain
conditions, the Tibgrid® Interface concept
can be used to more than double the maximum heat dissipated by the same yarn and
pitch on a given surface. n

No99 September 2015 / jec

More information:
www.tibtech.com

composites magazine 51

http://www.tibtech.com

Table of Contents for the Digital Edition of JEC COMPOSITES MAGAZINE - Issue #99 - September 2015