Research on the manufactured Technology of High-precision Load Connecting Frame Structure with Special-shaped Variable Shaped Cross-section

Aiming at the research and development technology of high-precision load connecting frame structure with special-shaped variable cross-section reinforced by internal cross bars, the two-step weaving and stitching fiber preform forming process scheme and the general RTM product forming process scheme of load connecting frame are discussed by using the comparison method; The virtual simulation of RTM glue injection process is studied by using ESI Group software, and the results show that the completion time of the glue injection process with four sprues is about 29.4% of that of the glue injection process with only one sprue, which guides the design of RTM molding design; The key process of product manufacturing, such as the glue injection and curing process of the product and the quality control process during the technological process, are analyzed, and the performance of the final product is evaluated. Through the research on the manufacturing process, we have overcomed the major constraints on several molding technologies, such as the two-step fiber preform molding technology of weaving and stitching, the RTM molding design and processing technology of complex and special-shaped composite products, the split molding technology, and combination finishing processing technology after secondary bonding. The results prove that the formed products have better forming quality, dimensional accuracy and thermal cycle dimensional stability. The nondestructive testing results of composite parts show that the internal structure of the product is good. The fiber volume fraction is controlled at (55 ± 3)%, the flatness of the important surface of the product is 0.05 mm and 0.03mm respectively, and the dimensional accuracy of the important interface is controlled within ± 0.02 mm. The dimensional stability after the thermal cycle test is good, and all indicators of the product meet the user’s requirements, which achieved expected goals. The success on the manufacturing of these products provides a technical basis for the design and manufacture of high-precision and high stiffness composite structures for deep space exploration and manned spaceflight.


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The antenna deployment arm is an important part of the satellite antenna.When the satellite is launched, the antenna is folded and pressed to the satellite.After the satellite enters the predetermined orbit, the antenna deployment arm drives the whole antenna to deploy stably and accurately to the correct working position [1][2][3][4].The joint-load-connection frame is an important structural member for the load connection and load force transmission of the rotary joint in the antenna deployment arm.It is exposed to the space when it is in service, and must undergo the cold and hot alternating environment, as well as the influence of space environmental factors such as electrons, protons and ultraviolet radiation.Therefore, it has higher requirements for its structural accuracy, mechanical performance and space environment resistance.In order to improve the stiffness of the unfolding arm joint-loadconnection frame, reduce the structural weight, make full use of the high modulus and high strength characteristics of m55j fiber, and supplement the in-plane performance, the special-shaped composite parts are formed by the braiding stitching RTM process, which has unique advantages.The stitching reinforced structure of braided stitched composite structure in the thickness direction can not only achieve structural integrity, but also overcome the shortcomings of traditional laminated structure with low inter laminar strength.It also has good fatigue resistance, damping and shock absorption, high impact toughness, high damage resistance, and nearly zero thermal expansion coefficient.It is suitable for structures with high dimensional stability requirements in high and low temperature alternating environments [5][6][7].RTM molding process is a process of molding in a closed mold, which can manucature products that has the advantages of high dimensional accuracy, integrated molding of complex structures, high fiber content, and low porosity.In foreign countries, many RTM composite parts have been applied in V-22 "Osprey" and F-22 "Raptor" fighter projects, Airbus R & D program, aerospace Commission ATR program and missile program [8][9].Compared with the development of RTM molding technology in foreign countries, China started relatively late.In recent years, China has also made certain progress in studies of RTM resin, resin filling flow process, numerical simulation in the mold cavity [10][11][12][13][14], preform manufacturing technology, RTM equipment and glue injection process control, and RTM Mold Design and manufacturing technology [15][16][17][18][19][20][21][22][23][24].With the development of RTM technology, materials, processes and equipment, the ultimate goal is to develop products that meet various performance requirements with low manufacturing costs.
Based on the research and development task of the joint-load-connection frame, this paper studies the feasibility of the joint-load-connection frame using the braiding stitching RTM molding process, and expounds the braiding stitching RTM molding technology of the joint-load-connection frame from the aspects of the braiding stitching process scheme of the product, the design of the molding mold, the quality control of the process, and completes the compound processing of the mechanism structural parts.Finally, the structural parts meeting the design requirements are developed.

Technological process 2.2.1 Weaving and sewing of products
It is easy to mistakenly change the direction and position of the original reinforced material design by manual laying of fiber reinforced composite in RTM cavities.At present, with the development of composite braiding technology, more and more complex fiber preforms can be pre braided by braiding machines, which greatly reduces the cost of manual laying and improves the forming accuracy.However, due to the poor weaving process of m55j high modulus carbon fiber, a two-step weaving stitching scheme is selected, which can not only increase the inter laminar mechanical properties of the final composite products, but also lower the level of difficulty on the weaving process.First, m55j carbon fiber is woven into two-dimensional plain weave cloth, and then it is layered in a quasi isotropic layered manner (0 / + 45 / -45 / 90) ns.Then, high-performance sewing thread with high strength, low density, heat resistance, radiation resistance and other excellent properties is used to sew the layers through the control of sewing process parameters.The number of fiber bundles needs to be properly supplemented at the corners to avoid stress concentration in the molding and bearing process, so as to ensure the performance required by the design.As shown in Fig. 1, a physical view of the preform is presented.
Fig. 1 a real picture of the preform

Injection molding
BS-2 resin is selected.The viscosity of the resin is less than 1 pa•s at 80 ℃, which has good reactivity, low shrinkage, good compatibility with preform reinforcement materials, and the toughness and fracture elongation can meet the design requirements.The injection equipment can achieve tasks like heating and maintaining constant temperature, mixing and metering, and accurate control of injection pressure, which can meet the requirements of RTM injection system.The curing scheme shown in Fig. 2 is implemented to cure the RTM parts after the glue injection.
Fig. 2 Curing scheme of product

Test method
The inspection of composite products is the core content of evaluating product forming technology.Through the evaluation of the physical effect of the molded product, the best molding process parameters are determined to further verify the feasibility of the molding technology.The inspection of composite structures is mainly carried out from non-destructive inspection, fiber volume fraction measuring, dimensional accuracy measuring, dimensional stability testing and so on.

Nondestructive testing
Low frequency ultrasonic nondestructive testing instrument and X-ray testing instrument are used to test the internal molding quality of the composite joint-load-connection frame with reference to the 20℃ sheshi 90℃ sheshi 165℃ sheshi 50℃ sheshi class B requirements of GJB2895-97 《General Specification for Carbon Fiber Composite Laminates and Laminated Parts》.

Fiber volume fraction
The fiber volume fraction of the product is measured by weighing method, which record the weight of the molded fiber preform and the weight of the product after RTM glue injection and curing respectively.After calculation, the fiber volume fraction of the product is obtained.The calculation formula is as follows: Where, ρ r is the density of epoxy resin and ρ f is the density of domestic carbon fiber.

Product dimensional accuracy detection
After the machining process, a CMM is used to test the flatness of assembly surface 1 and assembly surface 2, the parallelism of assembly surface 1 and assembly surface 2 on both sides, and the coordinate dimension accuracy of important installation interface points on both sides to verify whether they meet the design requirements.

Evaluation of thermal cycle test
High and low temperature thermal cycle tests shall be carried out for the joint-load-connection frame products according to the design requirements.At least 3 temperature measuring points shall be used to track the thermal cycle test of the joint-load-connection frame products.After the thermal cycle test is completed, the flatness of the assembly surface 1 and the assembly surface 2 shall be re measured with a coordinate measuring instrument to detect whether there are changes before and after the thermal cycle test, so as to evaluate the dimensional stability after the thermal cycle test.

Analysis of product structure characteristics
As an important part of the deployable arm joint, the joint-load-connection frame is an important connecting piece for the connection, bearing and force transmission of the composite deployable arm rod and the high-precision rotary joint, and is a kind of high-precision composite structural member with internal cross reinforcement and variable cross-section.The overall envelope size is 295 mmx220 mmx232 mm, the overall design thickness is 8 mm, and the thickness of the central cross stiffened part is 4 mm.The bottom flange mounting surface is the assembly surface 1, and the upper special-shaped opening mounting surface is the assembly surface 2. See Fig. 3 for the specific product.
The joint-load-connection frame products mainly have the following requirements: (1) The flatness of assembly surface 1 is ≤ 0.06 mm, and the dimensional accuracy tolerance of installation interface is required to be ± 0.05 mm; (2) Flatness of assembly surface 2 ≤ 0.03 mm, parallelism of both sides ≤ 0.03 mm, and dimensional accuracy tolerance of installation interface is required to be ± 0.02 mm; (3) Perpendicularity between assembly surface 1 and assembly surface 2 ≤ 0.03 mm; (4) The fiber volume fraction shall meet 55 ± 3%; (5) Ultrasonic nondestructive testing was carried out on the product, and the defects met the requirements of GJB2895-97B; (6) The product is subject to high and low temperature thermal cycle test to ensure that the assembly plane still meets the above flatness requirements after the thermal cycle test

Selection of process plan
The dimensional accuracy, form and position accuracy and mechanical properties of the formed products shall be considered in priority in the selection of composite product forming process plan.On this basis, it is also necessary to consider the braiding stitching processability, RTM mold closing processability, demoulding operability, etc.For the joint-load-connection frame described in this paper, the bottom flange mounting surface is assembly surface 1, the upper special-shaped mounting surface is assembly surface 2, and the middle cross reinforcement is a stiffening structure.Assembly surface 1 and assembly surface 2 have very high requirements for dimensional accuracy.Therefore, the overall dimensions of the blank are guaranteed by the net size molding of the mold design, whereas the key interfaces are designed to have machining allowance in the composite blank state.The flatness and parallelism are guaranteed by fine grinding and fine machining, and the mechanical properties are guaranteed by the weaving stitching RTM molding process.There are two main process schemes, one is the overall molding scheme, and the other is the split molding secondary bonding scheme.Table 1 shows the comparison of the two molding schemes.It can be seen from the table that both the overall forming scheme and the split forming secondary bonding scheme can meet the user's requirements for ensuring the mechanical properties and interface accuracy.However, the overall forming sewing mold design is particularly complex, the sewing process and the design of the sewing layer are particularly complex, and the RTM forming mold design is particularly complex, with poor processability, difficult forming, and difficult demoulding.However, bubbles, dry spots, and resin rich zone defects are more likely to occur.Therefore, in this paper, the product is divided into two parts: the joint-loadconnection frame body and the cross-rib flange support.The blanks are formed separately, and then the key interfaces are combined after the secondary bonding scheme as the main process scheme to ensure the mechanical requirements and interface accuracy requirements of the product.

Design of Forming Mold
The mold design of composite structure is closely related to the process plan.The process plan of the joint-load-connection frame of this product is a secondary bonding plan for the split forming of two parts, so RTM forming mold is also divided into two sets of mold processing.
The design of RTM process mould for composite products is mainly affected by the decision of process plan and the product model of net size forming of product blanks.The design of die directly affects the shape, size, surface quality and porosity of composite structure.The material, structural form, sealing method, position and quantity of glue injection port and glue outlet, clamping device of the RTM mold directly affect the performance and quality of the final part.The structural form of the mold is related to the mold lifting, gas discharge and mold clamping.Reasonable design is the key to ensure the molding quality.
RTM mold design mainly includes the following contents: (1) Selection of mold materials: RTM mold materials include steel, aluminum, cast iron, organic glass and some composite materials.As the size accuracy of the product is required to be high, and there are multiple size accuracy requirements, all parts of the product mold are made of 45 steel, and the quenching and tempering treatment is HRC28-32.
(2) Determination of net size forming three-dimensional model: according to the control of product blank allowance, the three-dimensional model is processed.At this time, the dimension of the inner cavity of the tooling is determined by the overall dimension of the product blank model.The inner cavity formed after the assembly of the whole set of tooling is the real size of the blank formed by the product RTM.The threaded holes and pin holes of the blank parts of the joint-load-connection frame body are not machined at this point, and 10-15 mm machining allowance is left on one side of the flange flanging profile at the bottom, and 1 mm machining allowance is left on the thickness direction of the bottom flange.The opening position is reduced by 10-15 mm on one side, and the circular opening and slot are rounded to form the same plane, with 5 mm allowance at the highest point.The schematic diagram of products and blanks are shown in Figure 4,.(3) Block processing of complex molds: In order to meet the requirements of RTM molding and demoulding of products, the molds should be reasonably divided to ensure that the products can be loaded smoothly during mold installation and demoulded smoothly during the demoulding process.The body mold is divided into four parts, which are upper, middle and lower core molds.To facilitate demoulding, the middle mold is divided into two parts, left and right molds.The cross rib mold is divided into three parts , which are upper, middle and lower parts, excluding the core mold.In order to facilitate demoulding, the middle mold is divided into four parts, front, rear, left and right molds.
(4) Treatment of sealing channel: The sealing method of the mold in RTM molding process plays a very important role in the entire molding process.Usually, different types of sealing strips are used to seal the mold.The sealing strips used include O-shaped, rectangular and other shapes, and the material is usually silicone rubber.When designing the sealing structure of the mold, in order to ensure the airtightness of the RTM molding tooling, the tooling needs to be provided with a sealant groove.The width and depth of the sealant groove are determined according to the specification of the sealant strip.The sealant groove layout needs to ensure the overall sealing of the tooling cavity.The distance between the sealant tank and the product area is more than 7mm~10mm, which can neither be too far nor too close.If the distance is too close, the structural connection strength and sealing effect will be affected , and if the distance is too far, resin flash will be easily caused .Ensuring sufficient safety distance to enable it to bear the injection pressure of glue injection is important.
(5) Treatment of repeated disassembly and positioning: In order to meet the requirements of repeated disassembly and positioning of the mold, sufficient threaded holes shall be arranged everywhere on the tooling.Considering the size envelope of the product, M8 threads and Φ 10 pins are chosen.The positioning pins shall be positioned to ensure that the form and position accuracy of the mold and the assembly clearance are less than 0.1 mm. ( 6) Handling of convenient demoulding: To facilitate the demoulding of tooling, the segmented tooling needs to be provided with a demoulding ejection threaded hole.Because the product is a hollow special-shaped variable angle section structure, the demoulding design is more complex.Therefore, after the mold design is completed, CATIA software should be used to simulate the curing process.The CATIA software should be used to analyze the movement path of the mold during demoulding, to reasonably select the separation surface, and to help design a scheme to disassemble the entire molding mold to ensure that the product can demould smoothly after curing.The metal core mold plays a supporting role.In order to ensure the demoulding, the mold is designed into a combined structure, and the metal blocks are connected with the main structure through screw connection, glue connection and other connection forms.When the product is demoulded, the metal blocks are taken out according to the preset demoulding track.
(7) Treatment of glue flow channel: RTM molding tooling shall be reserved with glue injection port, glue outlet and glue flow channel.The specific quantity and position can be adjusted according to the actual product.The design of resin flow channel includes the setting of glue injection port and glue outlet, which is the core of RTM mold design.The design of glue injection port and glue outlet on the mold is related to the flow direction of resin flow front in the mold cavity.The injected resin enters the mold cavity through the glue injection port on the RTM mold, and the gas in the mold cavity is discharged through the glue outlet on the mold.In this paper, ESI Group, special process simulation software, is used to carry out the process simulation of the glue injection process.First, for the product model, single point glue injection mode is selected.The glue injection port is designed at the center of the obtuse flange flanging at the bottom of the product, and the glue outlet is located at the center of the abnormal opening on the upper part of the product.In this glue injection mode, the glue injection completion time is about 5684s.In order to save the glue injection time, improve the production efficiency, and improve the design of the glue injection port, the bottom four point glue injection method are selected.The glue outlet is still designed as a single-point outlet, and the product molding margin are considered.Under this glue injection mode, the glue injection time is rapidly reduced to 1673.9s.Therefore, in order to save time, improve the resin service life, reduce costs, and improve efficiency, the improved glue injection runner design is selected.

Quality control of process
Quality and process control is a key to the successful application of composite RTM process.The quality control of RTM process mainly includes dimensional accuracy control and thermal deformation control.
(1) Fiber volume fraction control: RTM is a closed grinding process for net size forming, and the inner cavity of the mold is the actual size of the product blank.The fiber volume fraction of the final product can be controlled by strictly controlling the quality of the braided stitched fiber preform.Therefore, the fiber volume fraction requirement of the formed product can be guaranteed by controlling the fiber preform quality within ±30 g of the theoretical quality.
(2) Dimension and precision control: the main technical difficulty of joint composite parts is that the dimensional tolerance and form and position tolerance required by the product are very high, and it is difficult to achieve one-time molding of composite parts.Therefore, it must be realized through the forming of blank parts, numerical controled machining of the parts, secondary bonding and combined machining of components.In this paper, the joint-load-connection frame product is formed by weaving stitching RTM process, the shape envelope is near net size forming, and the shape dimension accuracy of the blank is guaranteed by the mold; However, the high-precision key interface dimensions are guaranteed by numerical control machining, including: the flatness of bottom assembly surface 1, the flatness and parallelism of upper assembly surface 2, the position of two large openings on the assembly surface and each connecting hole are guaranteed by the processing accuracy of the numerical control machine tool.
(3) Thermal deformation control: the machining tool shall be selected strictly during the composite material machining process, the precision grinding and finishing speed shall be controlled, and the machining stress during the machining process shall be reduced.After the machining is completed, the CMM shall be used to measure and record the flatness of assembly surface 1 and assembly surface 2. According to the design requirements, the thermal cycle test shall be carried out to remove the machining stress.After the thermal cycle test, the flatness of the assembly surface 1 and 2 are measured with the CMM and recorded again.If it is not suitable, the machine tool shall be used for fine grinding and finishing again to ensure that it meets the requirements.

Final performance of the product
Joint-load-connection frame products made by weaving and stitching techniques after knitting, sewing, glue injection, curing, demoulding, correction, bonding and machining are shown in Figure 7.The results of nondestructive testing on the internal forming quality of composite joint-loadconnection frame show that there are no surface bubbles, dry spots, resin rich areas, surface depressions, folds, scratches, cracks, inclusions and other defects inside the parts, and the internal quality meets the requirements of Class B in GJB2895-97 General Specification for Carbon Fiber Composite Laminates and Laminates.After accurate weighing and calculation, the fiber volume fraction of the structural member meets 55±3%, which is within the reasonable range specified in the design.Through the inspection of the forming accuracy of the braided stitched RTM physical product of the joint-load-connection frame, the dimensional accuracy of the product meets the design requirements, including the flatness of the assembly surface 1 and the transfer surface 2 are 0.05 mm and 0.03 mm respectively, the parallelism of the two sides of the assembly surface 2 is 0.03 mm, and the dimensional accuracy of the important installation interface is ±0.02 mm.All of its indicators meet the design requirements, further verifying the rationality of the process plan.The high and low temperature thermal cycle test was carried out for the joint-load-connection frame products according to the design requirements.During the test, the joint-load-connection frame products did not show any water condensation or dew condensation.After the thermal cycle test, the CMM was used to detect the flatness of the arm assembly and mounting surfaces and the rotary joint assembly and mounting surfaces, which were 0.05 and 0.03 respectively.During the thermal cycle test process, the flatness did not change, and the products met the design and use requirements.

Conclusion
The joint-load-connection frame can realize the molding of complex composite structural parts by using the weaving suture RTM molding process and the split molding and secondary bonding scheme.The results show that this set of process plan can solve the molding problem of composite structural parts, with variable angle section and high-precision multi-faceted interface, that are complex, specialshaped, hollow, stiffened mainly in the following aspects: (1) The weaving stitching two-step scheme, which can not only increase the interlaminar mechanical properties of the final composite products, but also lower the requirements for the weaving process, is used to realize the molding of fiber preforms.It can not only make full use of the high modulus and high strength characteristics of M55J fiber, but also supplement the in-plane performance, and ensure the molding stiffness and mechanical properties of the molded products; (2) The key to ensure the successful development of the joint-load-connection frame is to adopt the block design of the combined mold, the optimization design of the sealing channel and the glue flow channel, the design of repeated disassembly and positioning, and the demoulding design; (3) The split forming secondary bonding process scheme is adopted, and then the combination of key finishing interfaces as the main process scheme can ensure the mechanical requirements and interface accuracy requirements of the product, and all technical indicators of the final product meet the designer's requirements; (4) ESI Group digital simulation software can be used to guide the design of mold injection runner and optimize the design of RTM molding mold, but the mold still needs to be designed manually.Later, a rapid design system for mold structure based on 3D entities can be established, which can automatically determine or manually determine the parting surface, automate the addition of standard components, parametric modification, interference inspection and other functions, reduce design errors from the design source and reduce the number of mold trials, and therfore improve the automatic production efficiency of RTM products.
Fig.3 Diagram of joint-load-connection frame structure (a) Body blank (b) Cross rib blank (c) Final product Fig.4 Diagram of product and preform

As shown in Figure 5 ,
it is the schematic diagram of the forming tooling of the joint connecting frame.(a) Single point glue injection simulation of products (b) Four point glue injection simulation of product blanks Fig.5 Diagram of ESI Group process simulation (a)Mould of joint-load-connection frame body (b) Mould of cross stiffened flange support Fig.6 Diagram of forming mould of joint-load-connection frame structure

Fig. 7
Fig.7 Diagram of real part of joint-load-connection frame structureThe results of nondestructive testing on the internal forming quality of composite joint-loadconnection frame show that there are no surface bubbles, dry spots, resin rich areas, surface depressions, folds, scratches, cracks, inclusions and other defects inside the parts, and the internal quality meets the requirements of Class B in GJB2895-97 General Specification for Carbon Fiber Composite Laminates and Laminates.After accurate weighing and calculation, the fiber volume fraction of the structural member meets 55±3%, which is within the reasonable range specified in the design.Through the inspection of the forming accuracy of the braided stitched RTM physical product of the joint-load-connection frame, the dimensional accuracy of the product meets the design requirements, including the flatness of the assembly surface 1 and the transfer surface 2 are 0.05 mm and 0.03 mm respectively, the parallelism of the two sides of the assembly surface 2 is 0.03 mm, and the dimensional accuracy of the important installation interface is ±0.02 mm.All of its indicators meet the design requirements, further verifying the rationality of the process plan.The high and low temperature thermal cycle test was carried out for the joint-load-connection frame products according to the design requirements.During the test, the joint-load-connection frame products did not show any water condensation or dew condensation.After the thermal cycle test, the CMM was used to detect the flatness of the arm assembly and mounting surfaces and the rotary joint assembly and mounting surfaces, which were 0.05 and 0.03 respectively.During the thermal cycle test process, the flatness did not change, and the products met the design and use requirements.

1 2 Table 1
Comparison of two molding methods