Composite Stiffener Curing via Inflatable Bag
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Solution Overview
Problem
Existing methods for manufacturing composite material stiffened panels, such as those used in aeronautics, face issues with achieving uniform compactness and quality finish due to the use of mandrels which are costly and prone to wear, and are not effective for producing structures with circular stiffening elements.
Innovation Solution
A method involving a trapezium-shaped inner support wrapped with impermeable and breather layers, a tubular separator, and a caul plate to maintain shape and apply pressure evenly during curing in an autoclave, eliminating the need for intermediate mandrels and ensuring compacted surfaces and desired geometries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If a hollow mandrel with trapezium-shaped cross section is used to maintain the shape of the wall to be cured, then the shape of the wall is maintained, but the produced surface is porous and not very compacted
Solution Approach 1:
The patent introduces an intermediary compressible body (inner bag) between the hollow mandrel and the composite material to be cured. This inner bag is inflated during curing to apply uniform compressive pressure on the composite material, ensuring compact surfaces while the hollow mandrel maintains the overall shape. The intermediary body transfers and distributes the pressure evenly, preventing porous surfaces.
Solution Approach 2:
The patent separates the shape-maintaining function (performed by the hollow mandrel) from the compaction function (performed by the inflatable inner bag). By extracting the compaction function into a separate element that can be independently controlled through inflation, the system achieves both shape maintenance and surface compactness without the trade-off present in using a rigid mandrel alone.
2Shape
If a hollow mandrel is used in the autoclave, then the shape is maintained, but the mandrel is greatly stressed by the pressures and has to be replaced frequently
Solution Approach 1:
The inflatable inner bag acts as a mediator that absorbs and distributes the autoclave pressure, preventing direct stress concentration on the hollow mandrel. The compressible body deforms under pressure to accommodate the curing process while protecting the mandrel structure, significantly extending its service life and reliability.
Solution Approach 2:
The patent changes the pressure application method by using an inflatable bag that can be pressurized independently. This allows controlled pressure application during curing while preventing excessive stress on the mandrel, changing the pressure parameter distribution to protect the mandrel from damaging stresses.
3Shape
If traditional mandrels are used, then shape maintenance is achieved, but production costs increase due to frequent replacement
Solution Approach 1:
The inflatable inner bag serves as a disposable or reusable intermediary element that protects the expensive hollow mandrel from wear. By absorbing the stress and strain during curing, the bag extends mandrel life and reduces replacement frequency, thereby lowering production costs while maintaining shape accuracy.
Solution Approach 2:
The patent employs an inflatable inner bag that can be designed as a disposable or easily replaceable component. This cheap short-living element protects the expensive mandrel from wear, and when the bag wears out, it can be replaced without replacing the entire mandrel system, significantly reducing production costs.
4Ease of manufacture
If existing methods are used, then manufacturing is possible, but uniform compactness and quality finish are not achieved
Solution Approach 1:
The inflatable inner bag acts as a pressure-distributing intermediary that ensures uniform compaction of the composite material during curing. It contacts the material directly and transfers pressure evenly across the surface, achieving uniform compactness and high-quality finish that cannot be obtained with traditional mandrel-only methods.
Solution Approach 2:
The patent introduces a dynamic element (the inflatable bag) that can adapt its shape and pressure distribution during the curing process. The bag can be inflated to match the contours of the composite material, ensuring uniform contact and compaction across irregular surfaces, thereby achieving consistent manufacturing quality.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This method achieves high-quality, uniformly compacted composite structures with improved surface finish and reduced production costs by avoiding mandrel wear and enabling the production of structures with circular stiffening elements.
Implementation Method 1
during the curing step in an autoclave, pressure is applied to the outer surfaces of the skins (upper and lower) and to the flanges of the spars
Implementation Method 2
pressure is applied to the outer surfaces of the skins (upper and lower) and to the flanges of the spars so as to compact them against the relative plates of the tool
Implementation Method 3
The plugs thus give shape to the inner tubular bags so that it is as close as possible to the final shape of the cavity that is aimed to be obtained
Data Source
Figure 1~4
Figure 5~7
AI summary
A stringer (16) of fiber reinforced polymerizable thermosetting composite material is provided, the stringer having a cross section of closed or concave shape with two opposite side flanges (16a) that are brought into contact with a skin panel (14) of fiber reinforced polymerizable or polymerized thermosetting composite material. An elongate support (10) coated with a film bag (11) is positioned in a cavity defined between the stringer and the skin panel. The bottom of this assembly rests on a stiff forming tool (15) while the top is covered by a caul plate (17) having a profile congruent with a part of the assembly to be cured. In an autoclave, pressure is applied to the composite material by the tubular inner bag (11) and through the caul plate (17). The pressure is transmitted without the interposition of intermediate mandrels.