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Autoclave Processing Essay

2078 words - 8 pages

Autoclaves can be an essential part for the curing process of both thermoset and thermoplastic composites. An autoclave is defined in the aerospace industry as large pressure vessel that also has a heating ability and can be very large in order to accommodate parts that would need to be cured within it. The material used to make an autoclave would need to be able to withstand high pressures and temperatures that can be in high range of 300-400 °C and over 1 MPa. The purpose of the autoclave is to reach a high enough temperature to lower the viscosity of the resin and start the chemical curing reaction as well as reach a pressure to eliminate voids, remove excess resin, and press the plies of composite together. Any type of composite material can be cured in an autoclave as long as the polymer’s cure cycle falls within the temperature and pressure limits of the autoclave. The autoclave also allows the production of complex shaped parts due to the two different ways pressure can be applied to a part in an autoclave. Pressure can be increased inside the autoclave, pressing the bagged part into a tool and vacuum can be applied within the bag for added pressure on the part. A couple of major disadvantages for autoclaves are that they have a slow with temperature and pressure changes and that they have low temperature control which is mostly due the autoclave’s size and the methods to heat and pressurize the autoclave. For materials where the cure cycle is long and require a few hours, slow heating and cooling are actually not a problem at all. Furthermore, temperature control can be improved by using internal gas circulation along with typical temperature sensing and control. A simple equation can be used to determine the performance limit of an autoclave. The amount of time (Δt) to reach an adiabatic temperature rise (ΔT) is equal to the product of the mass of the autoclave (M), the heat capacity for the autoclave at a constant pressure (Cp), and the ΔT over a given amount of power. For large autoclaves, the product of M and Cp can be quite large and therefore not very useful for rapid processing of composites with short curing cycles. For most cases, the materials used in an autoclave should be similar or slower than Δt. Typical autoclaves used in the aerospace industry use natural gas burners that can heat up at a rate around 2 to 3 °C and are pressurized using nitrogen. Pressurization using air is still used, but is not recommended for high temperature applications due to the possibility of autoignition. There are other safety precautions when using an autoclave. A heated, pressurized vessel is essentially a bomb and pressure release valves and regular inspections are required for an autoclave’s operation. Also, the bulk of the material for an autoclave is usually housed outside while only the opening of the autoclave is indoors. The use of an autoclave in relation to the processing of a polymeric composite including the steps...

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