Today's KNOWLEDGE Share:CRTM PROCESS

Today's KNOWLEDGE Share:

CRTM process:

Resin Transfer Moulding (RTM) processes provide for excellent part quality, the relatively large cost of tooling being justified for medium to high production numbers. However, for high fibre content parts from continuous fibre reinforcements, the high resistance to resin flow has proven to be a barrier to the rapid processing required in some industries. The Compression RTM (CRTM, a.k.a. I/CM, RTCM, RI/CM and CTM) process has evolved from RTM to fill this need, by enabling very short mould filling times.

To extend the application of high fibre volume fraction composites into high-level production runs, factors limiting the cycle times for RTM need to be addressed. An RTM cycle is primarily composed of performing, mould filling, resin cure, part demoulding and mould cleaning and preparation. Accurate and repeatable performance is required for the manufacture of quality components, and this stage can be very time-consuming. However, preforming can be carried out independently of moulds used for injection and cure, preform units being prepared in advance using separate equipment. Injection and cure must occur within the mould imparting the final component shape, mould heating and cooling being desirable for the reduction of cycle times.

By working in combination with methods to significantly reduce mould filling times, faster curing resins can be used with more confidence and large improvements made in total cycle time. The filling stage comprises a combination of injection and compression driven flows. Unlike RTM, the mould is not completely closed prior to the initiation of filling. The mould is closed to some predetermined position, which results in lower overall fibre volume fraction, and hence a lower global resin flow resistance offered by the preform. It is also possible to leave a small empty cavity on one side of the preform, further reducing flow resistance during injection. Once the required volume of resin is injected, injection gates are closed, and the mould is closed in a controlled manner. This secondary mould-closing phase drives the resin front through the preform to the vents, and advances the laminate to the final part dimensions and composition. With careful selection of processing parameters, CRTM can realise significant reductions in mould filling times, relative to RTM. However, faster mould filling is typically gained at the expense of larger required mould clamping forces, process design therefore being a complex optimisation problem.

The CRTM process is described in the picture shown, in which (a) is the placement of preform, (b) is the closure to cavity thickness for injection, (c) is injection, (d) is compression driven flow and (e) is cure and demoulding.

Bibliographical Reference:
Manufacturing Techniques for Polymer Matrix Composites - Page 350
Source:#managingcomposites #thenativelab