Experiment for improving the manufacturing process of composite material made serpentine pipe parts using ceramic lost foam mold

Nowadays, composite materials are widely used in high-strength and low-weight parts, such as airborne structures, military equipment, and sports devices. Since most of the serpentine pipe parts have the requirements of one-piece forming and high strength, the material of serpentine pipe parts is still mainly metal. However, the parts cannot be easily separated from the metal molds after autoclave curing is always a difficult problem in the manufacturing process, which results in failed products. The autoclave process is often long and expensive. Therefore, improving its manufacturing yield rate is very important. To solve this problem, this study employs a ceramic lost foam mold to replace the metal mold used in the autoclave process. By adjusting parameters of autoclave and modifying the curing processes in different experiments and experienced several failed experiments, workable manufacturing processes and parameters are finally obtained such that the composite material made serpentine pipe parts are successfully produced. This study investigates the causes of failed products and corresponding solutions. Moreover, the current study executed process reproducibility experiments to verify the reproducibility of the proposed method. Experimental results determine workable manufacturing processes and parameters. The results not only facilitate the production of composite material made serpentine pipe parts but also significantly reduce the production cost and increase the throughput.


Introduction
Energy saving is a basic requirement in the aerospace industry, and the most direct and simple way to achieve this goal is to reduce weight. Replacing structures and components made of traditional metal materials with composite materials is one of the effective ways to reduce weight. Composite materials have many advantages [1] such as lightweight, high rigidity, high strength, easy design, and corrosion resistance such that they have been widely used to make various aerospace structural parts. Compared with steel, carbon fiber composite material is only one-fifth of the weight of steel under the same strength condition, which is the most commonly used composite material in the aerospace [1,2] industry nowadays. The autoclave [3,4] process has been adopted to manufacture the secondary structure of aircraft for several decades. Among many processes, the autoclave process has the merits of producing better quality parts without the common micro-void phenomenon [5,6] and insufficient fiber wetting and can produce complex shaped parts, which is unmatched in comparison with other processes. However, the autoclave process requires high temperature and pressure. Therefore, except for the thermal expansion coefficient [7,8] of aerospace composite materials, the separation of parts from the mold after curing also has to be considered, thus greatly limited the diversity of aerospace composite material parts.
Chang [9] developed an easy-to-assemble, lightweight, and high-strength composite truss, which is made of wound composite linkage and ball joint. He described the process and method of manufacturing the truss, including the fabrication and assembly of the connecting rod and ball joint. The specially designed end connections of the winding composite rod and the ball joint can reduce the assembly tolerance and make the assembly easy. To reduce the weight, a composite material ball joint was developed, including the design and manufacture of the joint, as well as the design of the mold and the mechanical analysis of the truss. Olofsson et al. [10] developed a model for wet-winding thick-walled circular tubes for calculating the fiber content, the level of resin curing, the temperature and stress distribution during the winding process, and the stress distribution of the finished circular tube under load after curing and demolding. Rizzo and Vicario [11] analyzed the distribution of stresses in tubes to loads using the finite element method. They found that when the thickness/diameter ratio is less than 0.1, the distribution of stresses in the cross-section of the circular tube is linear.
If the thickness/diameter ratio is greater than 0.1, the distribution is nonlinear; if the thickness/diameter ratio is less than 0.02, the tube can be considered a thin shell composite tube. Pagano and Whitney [12] used plain strain and plate and shell mechanics to analyze the distribution of stresses in entangled composite tubing. The results showed that even under unidirectional loading, the distribution of stresses in composite fittings was still very uneven. Tseng [3] tried to solve the vacuum leakage problem of cast metal molding molds under high temperature and pressure by plating or coating the surface of metal molding molds in a hot autoclave. It is found that the high phosphorus electro-less nickel-plate layer has excellent performance for vacuum leak prevention under high temperature and pressure. His experiments proved that the further application of this technology to autoclave forming and casting molds is effective. Furthermore, quite a few scholars continued contributing their efforts in the past years. Zhao and Pang [13] studied the stress strain of a composite pipe under torsion. They did the failure analysis for the pipe at the same time. Jarvela et al. [14] presented a new ceramic technology named Greencast-process for plastics processing. In the 2000s, Zhang et al. [15] proposed the processing of foam core sandwich structures by thermal expansion molding. Yang and Huang [16] investigated the plasma treatment for enhancing the mechanical and thermal properties of biodegradable PVA/ Starch blends. Jiang et al. [17] studied the preparation and properties of a novel water-soluble core material for molds. Weight reduction resulting from serpentine pipe parts made of composite materials brings limited effect on ground vehicles such as cars, whereas it is a considerable issue for aircrafts. Weight reduction of an aircraft leads to the advantages of less fuel consumption, cost reduction, and increased flying range. Because the composite material has to perform a lay-up process in the hot autoclave process, the product quality is related to the mold closely. In order to achieve the precision and stability required for mass production, molds are made of metal. Furthermore, since most of the serpentine pipe parts have the requirements of one-piece forming and high strength, the material of the serpentine pipe is still mainly metal. Therefore, producing serpentine pipes made of composite materials to replace those pipes made of metal is the motivation of this study. In the manufacturing process of serpentine pipe parts made of composite materials, the parts cannot be easily separated from the metal mold after autoclave curing. This problem results in failed products. The autoclave process is often long and expensive [18]. Therefore, improving its manufacturing yield rate is very important. To solve this problem, this study employed a ceramic lost foam mold to replace the metal mold used in the autoclave process. Parameters of the autoclave are adjusted and curing processes are modified in different experiments. Having experienced several failed experiments, workable manufacturing processes and parameters are finally obtained such that the composite material-made serpentine pipes is successfully produced. Accordingly, the manufacturing process and correlated executing parameters have to be carefully designed to match the new mold [4,19]. For successfully manufacturing the required products, this study designed and performed several experiments for achieving the following objectives: (1) Obtain the best curing parameters for the autoclave process. The mold may break or fracture due to improper temperature or pressure settings during the process; therefore, it is necessary to find out the optimal curing parameters of temperature, pressure, duration, and vacuum in the execution of the autoclave process to meet the requirements of curing composite materials for aerospace applications.
(2) Find out the best production mode for the autoclave process. The materials of ceramic lost foam molds and

Classifications and properties of composite materials
The classifications of composite materials are according to the types of matrix. The main consideration in choosing a matrix is the ambient temperature of the material. More than 99% of composite materials matrix is polymer, which is divided into two major categories: Thermosetting (TS) and thermoplastic (TP) resins. The working temperature for most of the two categories is below 200 °C. Metal-based composites with aluminum or titanium matrix can operate under temperatures up to 500 °C. For higher temperatures, ceramic-based composite is one possible choice, which can withstand up to 1200 °C. The carbon-carbon (C/C) composite material has the highest temperature resistance, which can withstand up to 3000 °C under the anaerobic condition. The carbon-carbon (C/C) composite material refers to the composite material made of carbon fiber and carbon matrix, but the carbon structures of both are different.
Although there are various types of composite materials, polymer composite is the most popular one used in industries. In addition to low price and lightweight, the most important reason is that the manufacturing method is simpler than other composite materials. The advantages of composite materials mainly result from the high specific modulus and high specific strength. These two items are defined as follows: where E represents modulus, strength, and density. The main reason why these two items of composites are higher than those of metals is that composites are much lighter than metals. The specific modulus and strength of some common materials are listed in Table 1.
Composite materials made airframe structures and engine cowlings have the following merits: (1) Higher corrosion resistance. Aircrafts always experience various atmospheric conditions during flights, so it is necessary to use high corrosion-resistant materials to reduce maintenance cost and increase service life. (2) Lighter weight. Lighter materials allow the aircraft to carry more payloads under the same thrust that results in fuel saving and easier operation. (3) Simpler design. Composites can be used to produce a variety of complex parts in aerospace applications. Normally, one composite part can replace dozens of metal fastener assemblies, which facilitates the structure design and installations. (4) Higher humidity resistance. Since aerospace used composite is more resistant to humidity than metal, a composite material made airframe can increase the (c) View screen of the hot autoclave monitoring system (d) Status monitoring interface of the hot autoclave humidity level in the cabin and improve the physical comfortableness for passengers.

Pre-preg
There are two types of pre-preg materials, namely unidirectional fiber and woven fiber [21,22], as shown in Figs. 1 and 2, respectively. Unidirectional fiber pre-preg has the maximum strength in the fiber direction and is usually used in unidirectional laminates. On the other hand, woven fiber pre-preg has different weaving methods and the strength is approximately equal in all directions, which can be used in laminates with different combinations of directions.
Pre-preg is a composite material made by laminating epoxy resin on fiber, consisting of fiber yarn, epoxy resin, release paper, and other materials, through processing procedures such as coating, heat pressing, cooling, laminating, and crimping. The fiber impregnation of resin and fiber is only the initial impregnation, and the final impregnation is at the forming process of the product.
When making composite parts, the angle and sequence of the pre-preg laminations should be designed according to the force pattern and mode of the finished product. There is an important principle in the structural design of airborne used composites, which is that the angles of the laminations should be symmetrical [23]. In other words, if a fiber is in the positive 30° direction, there should another fiber be in the negative 30° direction. In the aerospace industry, there is a very popular practice of folding, i.e., folding the pre-preg material on the pre-preg forming machine into a symmetrical form as needed, and then cutting it to the required length and width according to the design of the lays structure for application in the manufacturing process [24].
In order to improve the efficiency, manual cutting is no longer able to meet the production demand. Accordingly, the automatic fiber-cutting machine, as shown in Fig. 3, is necessary. Nowadays, intelligent automation, for example automatic material arrangement, automatic material feeding, automatic profile recognition, and one-click cutting by importing data, has been introduced into an automatic fiber-cutting machine which makes production faster and cost lower.

Experiment planning
In order to solve the problem that composite-made serpentine pipe workpieces are not easy to demold after autoclave curing, this study employs ceramic lost foam molds to replace the metal molds in the hot autoclave process. Therefore, this paper designed and executed experiments to verify this trial.

Experiment process
At first, check the appearance of ceramic lost foam mold. If it passes the inspection, then the mold is stabilized and inspected again. Next, performing the lay-up process on the ceramic lost foam mold with pre-preg fabrics made of carbon fiber epoxy composite material, and installing a set of thermocouples between the layers of laminations to measure the temperatures of the workpiece. Then put the product into a sealed bag [25] and send the bag to the autoclave for curing conducted by a predetermined computer program. After curing, unpack and demold the finished workpiece for inspection and recording. Figure 4 shows the flowchart.

Hot autoclave
The basic structure of the hot autoclave is a closed pressure chamber where the environmental parameters such as temperature, pressure, vacuum, and temperature changing rate can be set in the executed curing program. The time from the beginning to the end required for the process is called the curing cycle, and the conditions of the workpiece inside the chamber can be accurately monitored during the entire curing cycle by internal sensors of the hot autoclave. At the beginning of the curing cycle, it needs a vacuum pumping to suck the air from the gaps between the layers of laminations to make the pre-preg dense. Therefore, the pre-preg-wrapped mold should first be placed inside the hot autoclave and be connected to the internal vacuum pipes. In the early period of the process, the viscosity of the resin decreases due to the increasing temperature of heating. When the resin viscosity lowers to make it flows, the flow of resin will block the air passageway such that the vacuum pumping cannot evacuate air anymore. Therefore, terminate the vacuum pumping right after the resin flows. After that, inject air into the sealed bag to compress the layers by the air pressure. Because air pressure applies to all directions, it is possible to compress the uneven surface of the workpiece such that the layers completely fit to each other. Meanwhile, the air pressure squeezes out the excess resin and air bubbles. Since the viscosity of the resin is gradually increasing, the resin will no longer flow, so the time of compression should be during the viscosity of the resin is low. However, the external temperature should not be too high; otherwise, the heat accumulation inside the workpiece will lead the   material to be fragile. Therefore, the curing temperature setting must take into account both the reaction of the resin and the thickness of the layers that affect heat transfer. Figure 5a ~ d show the hot autoclave assembly and its peripheries.

Hot autoclave manufacturing process
For producing high-quality and high-precision parts, the orientation of fibers and the distribution of the matrix of composites need to be specially designed. Similarly, the shape-cutting and the lay-up process of the prepreg also have to be planned and controlled and then executed by using a hot autoclave to produce the required products. Figure 6 shows the manufacturing process of the hot autoclave.

The first experiment
All experiments in this study have been performed in a real workshop. The prepreg used is carbon fiber-epoxy resin produced by Hexcel Corporation, USA [26]. The molds are ceramic lost foam molds supplied by Ching-Huei Ceramics, Taiwan [27], and the hot autoclave is manufactured by ASC, Canada [28]. After checking the ceramic lost foam mold, this study sticks MG2 tape made by Airtech, USA [29], around the ceramic lost foam mold to isolate the mold from the carbon fiber prepreg. Then, this study performs the lay-up process on the ceramic lost foam mold using the prepreg that was prepared according to the required shape and angle. Figure 7 shows the overlaid workpiece. Next, this study installs three thermocouple wires at each end of the workpiece, numbered T/C1, T/C2,…, and T/C6, respectively, as shown in Fig. 8. Two of the three thermocouples at each end of the workpiece are the main sensors, while the other two are backups. Six thermocouple signals are fed to the hot autoclave to monitor the temperature of the workpiece. The equipped workpiece is then wrapped in a sealed bag provided by Airtech, as shown in Fig. 9. After the seal tightness test, if there is no leak, the sealed bag with the workpiece is placed into the hot autoclave for curing. At this moment,   Table 2 to the hot autoclave program.
When the curing cycle was completed, after unwrapping the sealed bag, it is found that the workpiece was deformed, as shown in Fig. 10. Through thorough inspections and discussions, two possible causes were determined: (1) the temperature of the workpiece was too low and (2) the pressure difference between the inside and outside of the sealed bag was insufficient. These two problems are depicted as follows.
(1) The workpiece temperature is too low   Fig. 11, where the temperatures should be between 347 and 365°F (175~185°C) and the curing cycle should be between 180 and 210min. However, the curing temperature curves of the first experiment, as shown in Fig. 12, are quite different. Figure 12 implies three problems:    The presumed cause of the problem is that the ceramic lost foam mold absorbs most of the heat during the curing process. As a consequence, the prepreg cannot obtain enough heat energy. To solve this problem, this study proposes three methods: (1) Shift the starting time of the curing cycle to the moment when the overall temperature of the mold has reached the required level. (2) Modify the curing process to a two-phase mode; i.e., phase one satisfies the necessary heat absorption for the ceramic lost foam mold, and then phase two curing process follows. (3) Add a new process, namely the mold stabilization process, which means baking the ceramic lost foam mold in the hot autoclave to reduce and stabilize its moisture content [30,31]. Because ceramic lost foam mold is made of clay through a biscuit firing, to maintain the strength and solubility of the mold, the biscuit firing temperature is set within 150 ~ 180 °C such that there is still crystallized water in the mold. This fact affects the temperature  Consequently, in the mold stabilization process, it needs not only high-temperature baking but also compression to force the crystallized water out of the mold. Moreover, the temperature rising rate should not be too high during the curing process, because a high temperature rising rate will induce a violent reaction to the ceramic lost foam mold, and the violent reaction results in air passageways and deformation on the workpiece.
(2) Insufficient pressure difference between the inside and outside of the bag After the workpiece was packaged and sealed, in the curing process, the layers are compressed by the pressure difference between the air pressures inside the chamber and the vacuum inside the bag, as shown in Fig. 13. Insufficient pressure difference between inside and outside of the bag is due to the large amount of gas generated by the mold during the heating process, which decreased the vacuum level of the bag. Figure 14 shows the vacuum curve inside the sealed bag of the first experiment. Because the gas inside the sealed bag cannot evacuate quickly through the airway, the pressure inside the bag offsets the pressure of the hot autoclave such that the vacuum level inside the bag cannot meet the − 20inch-Hg (− 9.826psi) requirement during the curing cycle. Thus, the workpiece deforms.
To improve this problem, three actions are prepared to take: (1) For avoiding the gas generation due to vacuum pumping, change the vacuum level inside the bag to 0 ~ 5 psi and increase the hot autoclave air pressure to 58 psi to compress the layers. (2) Install air conduction ropes on the workpiece and replace the original non-perforate isolation tape with the perforate type tape on the mold to improve the air   conductivity of the sealed bag. Figure 15 shows the modified product. (3) Perform the mold stabilization process before the curing process.

The second experiment
According to the experiences learned from the first experiment and the predetermined plans, the first step of the second experiment is the mold stabilization process. This is a three-phase baking process in that every phase endures 180 min and the maximum temperature of every phase increases from 140 to 248°F. Parameters in the process are listed in Table 3.
The weight of the mold is 7.0 kg before the process, while it reduces 0.5 kg to be 6.5 kg after the process.
All the steps next to the stabilization process are the same as in the first experiment. Table 4 depicts parameters used in the second experiment that are adjusted based on Table 3. The experiment has two phases, i.e., phase one satisfies the necessary heat absorption for the ceramic lost foam mold, and then phase two curing process follows. For the purpose of comparing the curing conditions, an extra ceramic lost foam mold without prepreg layers was cured at the same time, as shown in Fig. 16.
After the curing cycle is completed, it is found that the workpiece still deforms when it is taken out from the sealed bag, as shown in Fig. 17. Figure 18 shows the curing temperature curves of this experiment, which reveals two problems: (1) The temperature-rising process is not stable.
(2) The duration of the curing cycle is too long. The normal value is 9-10 h whereas this time is about 14 h.
The possible cause of the problems is that the ceramic lost foam mold still affects the temperature of the hot autoclave. In phase one, for satisfying the necessary heat  Figure 19 shows the fracture of the extra ceramic lost foam mold without prepreg layers curing together in this experiment. Concerning the effect of gas generated by the mold, the vacuum curve of this experiment, i.e., Fig. 20, shows that the bag pressure rapidly increases in phase two due to the heating effect. This shows that the ceramic lost foam mold is still affected by the environmental [32] humidity or room temperature change after stabilization and thereby fracture occurs.
Although the product still has deformation, however, the second experiment improves the unstable situations in temperature-rising processes compared with Figs. 12 and 18. Accordingly, the ceramic lost foam mold does not obviously affect the temperature during curing this time, which is a resultant effect of the mold stabilization process.

The third experiment
According to the above analysis, four adjustments are made in the third experiment: (1) Change the mold stabilization process from three phases back to one phase to shorten the baking time to 180 min. Executing parameters set to the hot autoclave for the stabilization process are listed in Table 5. (2) Increase the pressure and temperature in the hot autoclave to 58psi and 356°F, respectively. (3) Change the two-phase curing process back to one phase to shorten the curing cycle to 180 min. (4) Reduce the temperature and pressure changing rates from 8 to 3 °F/min, and 4 to 3 psi/min, respectively. Table 6 depicts hot autoclave parameters in the third experiment.
The stabilization process of the ceramic lost foam mold was still performed first, and the mold mass is 7.0 kg before the process, while it was reduced by 1.0 kg to become 6.0 kg after the process.
Based on the experience learned from the past two experiments, signals of the two backup thermocouples are not necessary this time. The temperatures measured from the four working thermocouples lie within 347 ~ 365°F for 180 ~ 210 min in the third experiment, as shown in Fig. 21. The vacuum levels are between 0 and 5 psi, as shown in Fig. 22, which means that the effect of the moisture [33,34] content in the mold is almost zero. After unpacking the workpiece that is shown in Fig. 23, no deformation was found. This study demolded the workpiece in warm water and inspected the finished product by using instruments, as shown in Fig. 24. The product passed the inspection.

Verification of reproducibility
Although the finished product of the third experiment meets the requirements, the reproducibility of the successful process still needs to be verified to show that the experiment does not work by chance. This verification uses the same procedures and the same parameters as used in the third experiment, i.e., parameters listed in Tables 5 and  6 to deal with four ceramic lost foam molds (no. 6 ~ 9) at the same time. Each weight of the four molds is all 7.0 kg before stabilization and 6.0 kg after stabilization.
The temperatures sensed by the four thermocouples are all also between 347 and 365°F for 180 ~ 210 min in this verification. The temperature curves are shown in Fig. 25. The vacuum levels are 0 ~ 5 psi, as shown in Fig. 26, which means that the effect of the moisture content in the mold is almost none. After unpacking the workpieces, there is no deformation. As a result, all four products passed the inspection.

Conclusions
This study has carried out experiments to improve the manufacturing process of composite material-made serpentine pipe parts using ceramic lost foam mold. Because the mold material for producing serpentine pipe parts is still metal, the parts cannot be easily separated from the metal molds after autoclave curing and remain a difficult problem in the manufacturing process, which results in failed products. The autoclave process is often long and expensive. Therefore, improving its manufacturing yield rate is very important. To solve this problem, this study employed a ceramic lost foam mold to replace the metal mold used in the autoclave process. By adjusting the parameters of the autoclave and modifying the curing processes in different experiments and experiencing several failed experiments, workable manufacturing processes and parameters were finally obtained, such that the composite material-made serpentine pipes were successfully produced. This paper has found out key factors in processes that may lead to failed products: (1) Maximum temperature and temperature changing rate of the process (2) Cycle time of the process (3) Vacuum level of seal bag due to the moisture content of the mold (4) Pressure difference between inside and outside of the sealed bag (5) Gas ventilation for the sealed bag (6) Maximum pressure and pressure changing rate of the process Accordingly, solutions are proposed: (1) Added the mold stabilization process to reduce and stabilize the moisture content of the mold (2) Installed air conduction ropes on the product and use perforate isolation tapes on the mold to improve the air conductivity of the seal bag (3) Adjusted executing parameters of the hot autoclave to meet the requirements of the manufacturing.
In addition, the current study has verified the reproducibility of the proposed method. All experiments have been performed in a real workshop. The results solve the problem of mold separation in the manufacturing process of composite material-made serpentine pipe parts and hence significantly reduce the production cost and increase the throughput.
Author contribution Experimental results determine workable manufacturing processes and parameters. The results not only facilitate the production of composite material made serpentine pipe parts, but also significantly reduce the production cost and increase the throughput.
Data availability All data and materials are available.
Code availability All codes are available.

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Conflict of interest
The authors declare no competing interests.