Investigation of Infrared Curing as an Alternative to Autoclave in Composite Manufacturing

Composite materials are increasingly used across numerous sectors, from maritime shipping to aviation, automotive to defense industries. In particular, as production process costs and raw material prices have decreased and different manufacturing methods have been developed, the market share and application areas of composite products have begun to expand. One of the most frequently used types of composite materials is polymer matrix composites. Today, fiberglass is the most widely used fiber type in polymer matrix composites. The most important reason for this is the low cost of raw materials. Carbon fiber reinforced composite materials, despite having relatively limited application areas, are frequently used where advanced mechanical properties are required. Currently, autoclave curing is the most advanced production method for obtaining polymer matrix composites of quality that would satisfy the aerospace sector. The autoclave – a temperature and pressure oven – has quite high initial investment costs and operating expenses, particularly as dimensions increase. For this reason, academic and industrial research has begun to focus on developing non-autoclave production methods. For example, research continues on production methods using microwaves [1], laser methods [2], electron beam [3], ultraviolet radiation [4], induction [5], and resistance [6]. Among the major problems encountered in applying alternative methods to the autoclave, one is non-uniform heating, and another is that the method may not be sustainable – that is, consistently producing the same quality product. In this study, the feasibility of infrared composite curing process, which could be an alternative to the autoclave, was investigated. The wavelengths of infrared rays vary between 0.7 microns and 1 mm, and wavelengths between 1 micron and 15 microns are frequently used in industry for heating purposes. In this study, a halogen infrared lamp with short wavelengths between 1 micron and 3 microns was used. The main reason for proposing infrared curing as an alternative production method to the autoclave is its potential to shorten curing time. In a study conducted in France [7], it was determined that infrared curing could provide an improvement of 40% to 70% in curing time.

Figure 1. Infrared curing oven test apparatus

Reducing curing times, which is one of the significant cost factors in autoclave curing, will reduce the total operating costs of the process. In addition, the much easier adjustment of heating rates is an important advantage. One of the major issues in infrared curing is uniform heating. How temperature distribution is observed across the material surface and through thickness must be tested and process parameters must be evaluated accordingly. Another objective of the study was to perform cure cycle experiments at a selected thickness taking into account the determined temperature distributions. For this purpose, a Programmable Logic Controller (PLC) system for monitoring temperature distributions was integrated into the infrared curing unit, and temperatures were recorded at regular intervals using a thermal camera. As shown in Figure 1, the temperature distribution exhibited at full power by the carbon fiber prepreg material divided into a total of 9 separate zones, without any intervention in lamp power, was monitored both across the surface and through the thickness. In the experiments, twill weave carbon fiber reinforced prepreg materials with OM11 resin system produced by Kordsa were used. The recommended curing cycle for the autoclave of the material is as shown in Figure 2. Based on data obtained from the studies, it was observed that a lamp operating at the relevant wavelengths heated the carbon fiber prepreg material to provide temperature distribution up to a certain thickness. This means that in parts thicker than this thickness, heat transfer occurs through conduction rather than radiation. For this reason, the layers in the inner regions will heat more slowly, and this will lead to residual stresses causing warping in the material. Observations made across the material surface revealed that temperatures in the middle section of the material were approximately 15% higher on average. At this point, the selection of the region that best represents the overall temperature of the material is important.

Figure 2. Recommended cure cycle for prepreg material for autoclave [8]

This is because the PLC system needs a single temperature value to perform power control to provide the desired cure parameters for the cure cycle tests. Therefore, a thermocouple placed at an incorrect location will not correctly represent the overall temperature distribution on the material, making it impossible to apply the cure cycle correctly. In the second phase of the study, productions were carried out at different cure cycles and these products were subjected to tensile testing. Three separate process parameters were used in the tests: heating rate, maximum temperature, and dwell time (duration at maximum temperature). By assigning 5 different values to each parameter, a total of 25 cure cycles were determined using the Taguchi experimental design method. As a result of these production and tests, the most suitable cure cycle for the infrared curing process was obtained according to tensile test results. The cycle is completed in approximately 80 minutes. This time is 38% shorter than the recommended curing cycle time of approximately 130 minutes for the prepreg material used for the autoclave. Looking at the tensile test results, while the tensile strength value of the material after autoclave curing was 835 MPa, the tensile strength values of the specimens subjected to the above-mentioned cure cycle with infrared averaged 800 MPa. The above results demonstrate that infrared composite curing process has the potential to be a successful alternative to the autoclave. Especially in the production of thin laminated sheets and sandwich panels where infrared curing can be used successfully, with scientific studies to model the entire process and ensure uniform heating, it is evaluated that aerospace-quality products could be produced at considerably lower costs compared to the autoclave. Mert Kılınçel - General Manager - Numerics Composites - Müh. Mak. San. Tic. Ltd. Şti. Yakup Okan Alpay - R&D Manager - Numerics Composites - Müh. Mak. San. Tic. Ltd. Şti.

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