Design and Research of Molding Die for Composite Material Autoclave

With the gradual increase in the amount of composite materials used in aircraft structural parts, the parts become larger and more complex, and are gradually used in the main bearing parts, which puts forward higher requirements for the quality of composite parts. Due to the solidification and molding characteristics of composite parts, its quality depends to a large extent on the quality of the molding mold, and high-quality molds come from scientific and reasonable design, especially for large molds, except that mold quality affects the quality of parts. In addition to the influence of the mold, the size and weight of the mold have a great impact on the cost of the mold and the total manufacturing cost of the composite part.

    Through engineering research and analysis of the design, manufacturing, transportation and use verification of composite material autoclave molding molds, combined with the empirical method of composite mold design, the following mold design principles are summarized.

Meet the requirements of part structure and process

    Before designing a composite material forming mold, the design input of the part must be fully analyzed to produce a preliminary concept of the mold structure.

    (1) Analyze the engineering structure of the parts. Usually there are structural forms such as wall panels, beams, ribs, long trusses, joints, and integral box sections. According to the structure of the part, a general concept of the mold can be given. Wall panels are often large-scale frame structures; beams are generally longer, often in the form of female and male molds (Figure 1 and Figure 2); long trusses are generally slender structures ; The overall box section generally needs to be closed up and down.

Figure 1 Female mold Enlarged picture

Figure 1 Female mold

Figure 2 Male model Enlarged image

Figure 2 Male model

    (2) Analyze the engineering interface of the part. Whether there are aerodynamic surfaces, assembly surfaces, glued surfaces, etc., these surfaces can be determined to be film-coated surfaces in general; but if the structure of these surfaces is complex, the design can consider adding a compensation layer on the engineering interface side, and the film-coating surface can be designed at this time On the back of the engineering interface.

    (3) Analyze the quality requirements of the parts. The shape and size accuracy of the part directly affects the quality requirements and cost of the mold. The accuracy requirements can be achieved by designing a reasonable mold structure, positioning method and processing method.

    (4) Analyze the molding process method of the part, whether it is co-curing, co-bonding or secondary handover (Figure 3). In co-curing, all layers are wet-laid into the tank at one time, which requires more molds to be combined and used at the same time. Usually the whole set of molds is more complicated; co-bonding is for dry and wet parts to enter the tank for curing, which requires a part of the forming mold, and Cured parts and wet layup are cured in the second tank; all parts have been cured during the second bonding, and they are cured together through the adhesive film, which requires a molding mold for all parts and a positioning mold for the second bonding.

Figure 3 Enlarged image of curing process

Selection of mold material

    The materials used for composite material forming molds mainly include ordinary steel, INVAR steel, composite materials (double horse and epoxy resin), aluminum and so on. It is usually selected according to the performance of the material (mainly the coefficient of thermal expansion at high temperature), cost, cycle and use times, see Table 1.

    For large-sized and large-curvature molds such as fuselage, wing surface, rudder surface, INVAR steel is usually selected, and INVAR steel is often selected for some beams, ribs, long trusses, etc. with high matching requirements; for tooling that requires rotation, Considering the weight factor, composite molds are a good choice; for parts that are not too complex in shape, small in curvature, or uniform in cross-section, ordinary steel or aluminum is usually selected to reduce costs, but expansion and deformation factors need to be designed Get compensated. The performance characteristics and application range of different mold materials are shown in Table 1.

Table 1 Comparison of characteristics of mold materials

Table 1 Comparison of characteristics of mold materials


Compensation of mold thermal expansion

    When the mold and the prepreg are cured in the autoclave, as the temperature rises, the mold will expand to increase the size, reaching the maximum when the heat preservation state is reached. At this time, the prepreg reacts and solidifies, which is consistent with the mold size. In the process of cooling after curing, the mold and the cured part will shrink, but if the thermal expansion coefficient of the mold material and the composite part are inconsistent, the shrinkage will be inconsistent, resulting in deviations in the part size and the mold size. According to the thermal expansion coefficient of the materials in Table 1, the thermal expansion of INVAR steel and composite material molds has little effect on the parts, and can be ignored; but ordinary steel and aluminum have an effect, especially when the size is large, the expansion must be considered , Otherwise the product size will be too large with the expansion of the mold.

    The thermal expansion of the mold can be compensated by certain methods. According to the empirical formula and test verification, the entire part is reduced by the following correction coefficients based on the center of mass of the part, and the reduced part is used as the engineering input during the mold design.

formula

 
    In the formula, F is the thermal expansion correction coefficient; T is the thermal expansion coefficient of the mold; P is the thermal expansion coefficient of the composite material; △T is the difference between the curing temperature and room temperature.

    On the other hand, the thermal expansion of the mold will also affect the deformation and demolding of the part. These factors should be taken into consideration when designing the mold. For example, whether the shrinkage deformation can be offset by the symmetry of the mold structure, or eliminated by post-processing; the shrinkage of the male mold may help demolding, especially for parts with small closed angles, the shrinkage of the female mold will increase the demolding The difficulty. By considering these factors, combined with the product requirements of composite parts, choose the appropriate mold material and mold structure. At the same time, it is also possible to optimize the mold design through digital simulation and verification of test pieces.

Compensation of mold rebound angle

    In addition to the influence of mold expansion on the curing deformation of the part, the composite parts due to the asymmetry of the layer and structure, and the curing characteristics of the material itself, the internal stress generated during curing will also cause the part to deform. For parts with large included angles for beams and long trusses, the most significant curing deformation is the rebound angle, that is, after the part is cured and demolded, the included angle is smaller than the mold angle due to shrinkage, and this difference is the rebound angle. As shown in Figure 4, A1 is the included angle of the mold, A2 is the included angle of the part after demolding, and θ is the difference between the two, that is, the rebound angle. In the design, regardless of the rebound angle or improper consideration, it will have a greater impact on the quality of the part, especially the impact on the assembly.

Figure 4 Springback angle compensation

Figure 4 Springback angle compensation

    The rebound angle is related to the mold material, layering direction and structural symmetry, and can be obtained through digital simulation and experimental testing. At present, the size of the rebound angle is obtained through the combination of empirical values and experimental values. Digital simulation needs to improve the accurate analysis of the temperature field of the autoclave and the reaction mechanism of composite materials. In the mold design, the rebound angle is taken into consideration in advance, that is, the part angle plus the rebound angle equals the mold angle, so that the part meets the engineering digital model requirements after the part is released from the mold and rebounds.

Stress Analysis Criteria

    At present, domestic composite material forming molds are generally designed by conservative estimation methods and not optimized by strength analysis. The molds designed in this way, especially the molds for large parts, are usually very bulky, not only consumes a lot of materials, but also increases the heat capacity. High may also affect the curing reaction. At the same time, the tonnage of related equipment in the transportation and use process is very high, which greatly increases the cost and even affects the manufacturing capacity. Therefore, weight reduction is an important aspect of mold design optimization. The method of weight reduction must pass reasonable calculations, otherwise it will directly affect the strength and rigidity of the mold, resulting in deformation during use and affecting the quality of the part. At the same time, the strength of the rings, wheels, and supporting feet needs to be checked to ensure that the mold can be lifted and transported safely.

    The optimization of weight reduction can use finite element analysis to analyze the strain and stress of the mold under different working conditions (Figure 5 and Figure 6), and adjust and optimize the mold structure according to the calculation results, and finally make the stress and strain values under the most severe working conditions smaller than the allowable material. Multiply the value by a certain safety factor (this safety factor is generally an empirical value, for safety reasons, it can be considered greater than 1.5), and as far as possible to make the safety factor deviations everywhere, so as to effectively reduce weight. The working conditions considered in mold stress analysis usually include the following aspects:

    (1) In the case of vertical lifting. The weight of the whole set of mold plus parts acts on all the hanging points of the mold.

    (2) Emergency braking situation during vertical lifting. The weight of the whole set of mold plus parts and the impact load generated by sudden braking act on all the hanging points of the mold.

    (3) The use of manual ply. The weight of the paving mold overlay layer acts on the support points of the mold base.

    (4) The use of automatic layering. The weight of the paving mold and the pressure of the paving head act on the support points of the mold base.

    (5) Tooling transfer process. The weight of the whole set of mold additions acts on the mold wheels.

    (6) The condition of the tooling in the autoclave. At 177°C (the allowable value of the material at this time is less than the allowable value at room temperature), the weight of the whole set of mold additions acts on the support point of the mold base.

Figure 5 Strain analysis

Figure 5 Strain analysis

Figure 6 Stress analysis

Figure 6 Stress analysis

Detailed design requirements

    The detailed design in composite material mold design is very important, which directly affects the operability, convenience and automation degree of mold use, and even affects the quality of composite material parts. The detailed design on the mold (Figure 7) includes but is not limited to the following:

    (1) Product line: the contour line of the product, which is used for non-CNC trimming, determines the accuracy of the product's external dimensions; the expansion factor of the mold material should be considered in the design for proper scaling, and the position accuracy of the product line And width accuracy needs to be defined in the mold design according to the requirements of the composite parts' shape and size accuracy.

    It is not completely neat, and the quality of the product edge after curing is not high due to factors such as flow glue, rubber strips, and vacuum bags. It is necessary to start paving at a certain distance outside the product line, and there will be a certain margin on the part to be processed . At the same time, the paving line is used for manual paving and positioning when there is no laser projection to control the paving margin, not to waste too much material, but also to ensure product quality. Usually, 20-30mm can be left from the laying line to the product line.

    (3) Benchmark hole: used as a benchmark for mold machining and testing, with high accuracy and reusability. The coordinate value of the reference hole is usually engraved on the mold for easy use.

    (4) Target hole: used to place laser projection targets during manual paving to locate the paving area. The target hole can be measured according to the actual value, and the coordinate value is engraved on the mold.

    (5) Automatic tape-laying cross-line: the center of the cross-line is used for target positioning during automatic tape-laying, and the coordinates of the center point of the cross-line are engraved on the mold.

    (6) Positioning holes for composite parts: also called part craft lug holes, which are used as positioning benchmarks for composite parts after demolding, testing, machining and assembly, and require high precision. This hole needs to have a corresponding drilling template for accurate positioning, the drilling needs to be carried out before demolding, and the hole should be in the normal direction of the mold.

    (7) Margin area: A certain distance should be left between the paving line and the edge of the mold for vacuum bagging and automatic tape unwinding. Usually the margin area of the manual paving mold is 100-200mm, while the automatic tape paving needs 200-300mm.

    (8) Humanized design: The safety and convenience of tooling should be considered when designing the mold. For example, the height of the tooling must be suitable for paving and inspection; the mold must be stable when paving and transporting, and there will be no danger of smashing; when the weight exceeds When manually carrying the weight, it is necessary to design auxiliary tooling to assist the transfer of the mold.

Constraints of extra-large molds

    Composite forming molds are usually very heavy, such as tail, wing, fuselage and other parts forming molds, moving more than ten tons, or even dozens of tons. Such a large and heavy mold will greatly increase the difficulty in the process of manufacturing, transportation and use.

    Design process: As the size increases, it will become more difficult to ensure mold processing accuracy, thermal uniformity, and deformation control. In the design, these factors should be considered, such as the setting of benchmarks, the distribution of tolerances, and the calculation of stress, as well as the influence of the temperature field of the autoclave on the heat distribution of the mold that is currently being studied.

    Manufacturing process: It is necessary to ensure the design requirements of large-scale mold manufacturing accuracy, air tightness and other design requirements. High requirements are put forward for welding, heat treatment, CNC machining and testing technology. At present, new processes such as laser welding and helium leak detection are required. It is gradually being applied to the manufacture of large molds.

    Transportation process: Excessive weight will have higher requirements for bridges and roadbeds; increased size will require the width of checkpoints, toll stations, etc., usually when the length and width are more than 3.5m, it will be restricted, and when it is more than 5m, it will be The entire transportation road is inspected and even transformed. In this case, you need to consider dividing the mold into pieces and transporting it to the manufacturing site for welding.

Figure 7 Mould detail design

Figure 7 Mould detail design

    Use process: For large-size and large-tonnage molds, it is usually necessary to consider factors such as transfer space, crane tonnage, and equipment stroke during plant planning and equipment procurement.

Concluding remarks

    In summary, mold design not only needs to consider engineering and process inputs, but also factors such as manufacturing and processing capabilities, transportation, use, and related equipment capabilities, so that the designed mold can not only produce qualified composite parts, but also Achieve the higher goal of reducing costs and facilitating use. At present, weight reduction, deformation control, humanized design, etc. are the development directions of mold optimization. Using continuously upgraded design tools, combined with the accumulation of practical experience, composite molding molds will be better optimized, thereby promoting the use of composite materials in aviation Field development.