The main advantages of resin transfer moulding are:- Highly integrated and complex design which results in a lower number of pieces and reduced assembly cost;

- Multi-material solutions may be produced by incorporation of metallic parts (inserts);

- Low-investment production facilities;- Uncomplicated material handling (reinforcing fibres & resin systems).

As the air transport industry is under increasing pressure to cutcosts, the costly material logistics for prepregs and the highinvestment cost of autoclaves favour substituting the simpler RTMtechnology – with almost equal product quality.

RTM processes are closed processes, i.e. during injection, the mouldcavity is open only to the vacuum vent and the inlet where liquidresin mix is flowing in. For this reason, it is difficult to follow themould filling process or to control it in any way. At best, sensors candeliver a signal at the instant the filling front reaches them. Suitableprocess control is decisive in ensuring process repeatability and thequality of the finished component. If either the injection points or

myRTM, a new tool for RTM process design

60 jec composites magazine / No58 June - July 2010

esin Transfer Moulding (RTM) is a manufacturing techniquefor high-quality fibre reinforced composites. Figure 1 showsa schematic diagram of the process sequence. First, a textile

with a thermoplastic binder is softened by infrared heating. This softmat is transferred rapidly to a forming tool that is closed and cooled.A mechanically-stable preform is then ejected. Alternatively, othertextile processes such as sewing, knitting or stitching may be appliedto preform the textile structure. After any necessary trimming, thepreform is placed in the heated RTM tool for the actual injectionprocess, where a liquid thermosetting resin is injected into the dryperform. The mould cavity fills and the resin system undergoeschemical curing. When the part has become strong enough, it canbe de-moulded and tempered. Following any necessary finishingsteps, the component is ready for use.

RTM

PROF. DR. MARKUS HENNE

DEPUTY HEAD

DR. GION A. BARANDUN

SENIOR RESEARCHER

INSTITUTE FOR MATERIALS TECHNOLOGY

AND PLASTICS PROCESSING, UNIVERSITY OF

APPLIED SCIENCES RAPPERSWIL

R

KNOW-HOW

The Institute for Materials Technology and Plastics Processing (IWK) has developed auser-friendly software program to simulate RTM processes. Thanks to a newapproach based on cellular automats, fast and reliable process design is just a fewclicks away. myRTM is free and available for immediate download.

Fig. 1: RTM process withpre- and post-processing

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No58 June - July 2010 / jec composites magazine 61

The areas of different permeability in the fibre reinforcement can bemodified during the simulation. Additional gates may be added at anypoint in time to implement concepts such as cascade injection. Theinjection pressure can be altered and the effect of race trackingobserved, considering different injection strategies. Such features makeit possible to obtain the highest possible reliability for a RTM process.Fig. 3 shows a case study for a whitewater paddle, where the influenceof race tracking is visualised.

The input parametersfrom simulation arethen transferred to themanufacturing process.In the case of thewhitewater paddle, atwo-point cascadeinjection strategy usinggates at the top of theshaft and at the root ofthe blade isimplemented. Thanksto the preliminaryprocess design with myRTM, the paddle is produced without any dryspots in a very efficient and reliable injection process (Figure 4). �

More information: www.myrtm.ch

the mould evacuation points are incorrectly chosen, then air pocketsmay appear, impairing product quality. Apart from this, the processshould be laid out so as to achieve the shortest possible cycle timeand hence the highest productivity. Process simulations based onFEM allow the accurate numerical calculation of mould fillingprocesses. In real life, liquid moulding may be influenced by manyadditional effects, which are hard to consider in numericalsimulations. Especially, race tracking effects in areas of low porosityare dominating the propagation of the flow front. Because of thesereasons, it is crucial that the software user has deep knowledge ofliquid moulding processes, that will help him program a simulationprocess and interpret the results. Additionally, many features ofcommercial software, such as thermal simulation and cure kinetics,may have negligible influence on most infusion processes and aretherefore not useful. For these reasons, process simulation is seldomused in practice. In particular, process simulation is not applied insmall- and medium-sized enterprises because of the high softwarelicensing costs, missing material data and the fact that engineers lackthe necessary knowledge to apply simulation tools.On top of all this, the algorithms currently available for efficientiterative optimisation of the process require too much computingpower. Depending on the number of elements and the geometricalcomplexity of the component, a single run can take from a fewminutes to several hours. Consequently, an optimisation that mayrequire several hundreds to some thousands of iterations is time-consuming and may prove unfeasible due to limited computingcapacity.

Cellular automatsmyRTM takes a new approach to the simulation of resin injectionprocesses, which is based on the concept of cellular automats.Numerical simulations using cellular automats are easy to implementand generally require fewer calculations. Therefore, they cansubstantially reduce the computing time required. In developing thepackage, the least frequently used features of the commerciallyavailable software were consciously set aside, priority being given touser-friendliness in general and clarity of visualising the process inparticular. The digitised description of the component is importeddirectly from a CAD system or via finite-element (FE) software. Basicvalidation of the nodes – to ensure a more or less even distribution –is done in the pre-processing routine. Additionally, gates, injectionpressure and permeability distribution are applied.Resin viscosity and vacuum pressure should be provided beforestarting the simulation. In fact, this is the only data needed bymyRTM. With just a few clicks of the mouse, myRTM displays theinjection process (Figure 2).

Fig. 3: The influenceof different injectionstrategies or heavyrace tracking iseasily implementedand visualized inmyRTM

Fig. 2: The myRTM userinterface – afterapplying boundaryconditions suchas gates, injectionpressure andpermeabilityzones, theinjection processis simulated inreal time and canbe changed at anystage within thesimulation

Further details, including a user tutorial anddownload of the myRTM software, are available onthe website www.myrtm.ch

More informationMore information

No race tracking Heavy race tracking at mould corners

Fig. 4: Comparison of experimental and numerical tensile curves in the warp direction

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