After proper high temperature treatment, PMI foam can withstand the high temperature composite material curing process requirements, so that PMI foam has been widely used in the aviation field. Medium-density PMI foam has good compressive creep properties and can be autoclaved at a temperature of 120oC -180oC and a pressure of 0.3-0.5MPa. PMI foam can meet the creep performance requirements of the usual prepreg curing process, and can realize the co-curing of the sandwich structure.
As aerospace material, PMI foam is a uniform rigid closed-cell foam with basically the same pore size. PMI foam can also meet FST requirements. Another feature of the foam sandwich structure compared with the NOMEX® honeycomb sandwich structure is that its moisture resistance is much better. Because the foam is closed-cell, it is difficult for moisture and moisture to enter the sandwich core. Although the NOMEX® honeycomb sandwich structure can also be co-cured, it will reduce the strength of the composite panel.
In order to avoid the core material collapse or side shift during the co-curing process, the curing pressure is usually 0.28-0.35 MPa instead of 0.69 MPa for laminates. This will result in higher porosity of the composite panel. In addition, because the pore diameter of the honeycomb structure is relatively large, the skin is only supported at the honeycomb wall, which will cause the fibers to bend and reduce the strength of the composite skin laminate.
As the core material of sandwich structure foam, PMI foam has been successfully used in various aircraft structures. One of the most prominent applications is the engine air intake side panel at the rear of Boeing MD 11 aircraft. The CNC precision machining and thermoforming of the foam greatly reduces the cost of the layup. The high-performance PMI foam core material has good compression and creep resistance during the curing process, so that the panel is compacted and the surface is uneven.
Compared with the honeycomb core, the isotropic pore structure of PMI foam can also meet the requirements of dimensional stability under lateral pressure during the curing process of the autoclave. Unlike the honeycomb structure, it does not need to be filled with foam glue. In addition, the foam can evenly transfer the pressure of the autoclave to the ply of the panel under the foam to make it compact without surface defects such as indentation. The foam-filled A-type stiffened strip structure can be applied to components such as radar launch surfaces, nacelle walls, fuselage skins, and vertical stabilizers.