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Temperature Control for Composite Material Curing in Hot Plate Machines

The first application of ADEX technology in the field of composite materials was carried out with ICSA (Internacional de Composites S.A.) within the project SINTONIA financed by CDTI. ICSA is owned by AERNNOVA AEROSPACE S.A. based in Toledo which is a Spanish aeronautical company founded in 2006 with headquarters in Vitoria (Alava), with more than 4,000 employees, and with facilities for fabricating aerospace components in Spain, Mexico and Brazil.

The ADEX application consisted of implementing an Optimised Adaptive Control System (OAS) for the control of one temperature only inside the mould used for curing parts used in the fabrication of the Airbus A350. The installation of the OACS was carried out without altering the existing control structure. The only modification consisted of establishing an additional line of communication between the local PLC and the ADEX COP platform which executed the OACS. In this way it was possible to maximise control precision of all the critical variables of the process with a minimum of intrusion to the Local Control System (LCS) and without affecting its operation.

Once implemented, the OACS demonstrated ADEX’s superior capacity over the LCS by guiding the temperature along a desired thermal profile while precisely establishing the appropriate values for the curing process and hence the requirements established by the quality standards. The OACS also demonstrated a high level of consistency in performance over several days and under different conditions. The analysis of the results obtained during the curing stage demonstrated a reduction in the root mean square of the temperatures with respect to the set point of approximately 70.6 % while completely avoiding non-conformities in the fabrication due to temperature deviations. In addition, there was a reduction in the fabrication cycle time of 13.5%. The success in this first experimental application provided the basis for further projects with AERNNOVA. 


Hot plate machine used to heat the mould



The left hand part of the diagram shows the hot plate machine used to heat the inside of the mould which is compressed between two flat plates using pneumatic cylinders. The right hand side of the diagram shows a representation of the same machine divided into three zones which are used in the heating process.

The curing process is carried out using a technique called Resin Transfer Moulding (RTM) which heats the flat surfaces of the plates which are in contact with the upper and lower parts of the mould. During this curing process, the hot, flat surfaces transfer heat to the mould. As can be seen from the diagram, in both surfaces, three similar zones are considered, each one with two temperature measurements giving rise to four temperature measurements per zone, two on the top surface and two on the bottom. The upper and lower surfaces of each of the zones store an electrical resistance inside which will transfer heat to the zones. This heat transfer is regulated via three relays located inside the control box, one for each zone, which allow or prevent flow of current to the respective resistances. Finally, there are three temperature measurements inside the mould, one of which is used for control while the remainder are recorded.

There also exists a resin deposit (DAR) from where resin is transferred or injected. The DAR also incorporates a thermocouple whose temperature must be under control during the whole curing cycle. 


The ideal injection and Curing Cycle



The figure shows the temperature control profile of the mould which must be followed during the curing cycle. The changes that have to be carried out during the cycle are described next. Firstly, the mould and the DAR (which contains the liquid resin) are heated concurrently to 120 ºC and 80ºC respectively. Once the temperature is stable at these values, a vacuum is created inside the mould. At that moment, the resin flows from the DAR, which is at 2.5 bar towards the mould which is at 3mBar. Once the resin has filled the mould, the curing process commences. The temperature in the mould is increased at a rate of between 0.5ºC and 5 ºC/min until it reaches around 180 ± 5 ºC which is the curing temperature to be maintained for 90 minutes thereafter. Once the curing cycle is complete, the mould is left to cool and can be opened at 60ºC. Finally, the temperature of the DAR must be maintained at 160º to cure the resin which has remained inside the DAR in order to extract it.

It is important to stabilize the temperature of the resin at 180 ± 5 ºC so that it solidifies enough to acquire the properties needed for application to the fabrication of aerospace components. This is extremely difficult since the curing process is very exothermic. Heating the mould too quickly can result in degradation of the resin due to excessive heat and give rise to defects in the parts while at the same time, heating the mould too slowly can result in high fabrication times and even nonconformities as a result of not achieving a minimum rate of temperature increase.


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