Autoclave is mainly used to form high-performance composite products. Using programmed temperature and gas pressure inside autoclave, laminated blanks of composite materials can be solidified and formed under a certain temperature and pressure. Process is to place prepreg material on mold coated with release agent in a certain order, then lay release cloth and release film, lay absorbing air felt on release film, and then cover it. High-temperature-resistant vacuum bags, and seal periphery with sealing strips, as shown in Figure 1. Then air is continuously extracted from vacuum bag and heated to make interlayer of prepreg material reach a certain degree of vacuum. After heating to specified temperature, compressed air is filled into autoclave to pressurize molded product. General curing process is heating → pressurizing → heat preservation → cooling → depressurization, in which heat in tank is mainly transferred through flow of air. Main equipment of autoclave molding process is autoclave, heating and pressurizing system, vacuum system and control system.

 

Fig.1 Cured and molded structure of composite product
1. Flat mold 2. Connected to vacuum source 3. Product 4. Isolation layer 5. Release layer 6. Separation layer 7. Pressure equalizing plate 8. Air felt 9. Vacuum bag 10. Breathable layer 11. Flexible stopper 12. Exhaust Material 13. Sealing strip 14. Exhaust duct
When composite material product adopts autoclave curing molding process, mold is main process equipment for molded product, which is used to determine product shape, structural relationship and obtain good surface quality; composite material is polymerized and solidified on mold, and sometimes prepreg material is placed on forming mold. Design and manufacture of composite material forming mold has a great influence on quality of formed product.

Instrument cabin is load-bearing section of rocket, its main function is to accommodate and install various instruments and equipment. Instrument compartment is usually a cylindrical or frusto-conical shell structure. There are many types and quantities of composite material shells with this structure on rockets, so it is of great significance to summarize molding die design methods for such products.
Taking a cone shell made of carbon fiber reinforced/epoxy resin base (hereinafter referred to as carbon/epoxy) material as an example, design method and process of its forming mold are introduced. Cone shell adopts forming process of carbon/epoxy composite material winding and laying, and autoclave curing. Shell is in the shape of a truncated cone with inner flanges at both ends, as shown in Figure 2.

 

Figure 2 cone shell

Mold structure is shown in Figure 3. Composite material product (uncured carbon/epoxy cone shell) is wrapped in punch, die, upper pressure ring, and lower pressure ring, these mold parts are transferred to composite material product during curing and molding in autoclave temperature and pressure. When laying composite material products, upper and lower flanges and shaft are connected to mandrel, and the two ends of shaft are connected to winding machine to facilitate winding and laying of prepreg filaments and prepreg cloth; these parts are removed before being put into autoclave for curing, combination of composite product and punch, die, upper pressure ring, lower pressure ring and other parts is sealed with a vacuum bag. These parts will expand and move due to heat during curing process of composite product. If these variables cannot be controlled well, surface accuracy of final carbon/epoxy cone shell product will not be ideal. If it exceeds design tolerance, it will not be allowed to use.

 

Figure 3 Cone shell molding die
1. Die 2. Punch 3. Mandrel 4. Product 5. Upper pressure ring 6. Upper flange 7. Shaft 8. Lower pressure ring 9. Lower flange

Composite products formed by autoclave curing mostly use metal forming molds. Commonly used mold parts materials include aluminum, steel and INVAR steel, etc., and their main properties are shown in Table 1. Among them, density of aluminum mold is about 1/3 of that of steel mold or INVAR steel mold, weight is lighter, and processing cost is low, but its thermal expansion coefficient is large. INVAR steel has a small thermal expansion coefficient, high material and processing costs, and is mainly suitable for forming products with large curvature changes. Thermal expansion coefficient of steel mold is between aluminum and INVAR steel, material cost is lower, but processing cost is higher.

Table 1 Material performance parameters of commonly used mold parts

Upper and lower ends of carbon/epoxy cone shell are of inverted flange structure, and shape of inner cavity is determined by punch, which adopts a split structure. Upper and lower inner flanging structures of carbon/epoxy cone shell form a semi-enclosed shape to punch. After cone shell is formed, there is a certain "tight hoop" phenomenon on punch. In order to enable punch to be demolded from cone shell smoothly, mold design considers that punch must have a large amount of expansion during curing to facilitate demoulding. Aluminum has a high coefficient of linear expansion, so it is more suitable to use aluminum to make punches. Another advantage of aluminum punch is that density is small, and quality of processed mold is also light, which is convenient for workers to operate. Punch material of final carbon/epoxy cone shell is 5A06 aluminum. 5A06 aluminum is Al-Mg series anti-rust aluminum, which has high strength and corrosion stability, can be shaped well in annealed and extruded state.

Because punch is made of aluminum material, core mold is also made of aluminum material. Thermal expansion of same material will remain consistent during heating and curing process. This is more beneficial to surface accuracy of product after curing, and is also conducive to calculation of mold size during design (considering impact of thermal expansion). Because mandrel needs to be processed into an overall conical shape, ZL104 casting mandrel is selected. This aluminum alloy has good casting performance, no tendency of thermal cracking, and high air tightness. Die is made of 5A06 aluminum plate, which is rolled and bent before being processed; upper and lower pressure rings are also made of 5A06 aluminum plate. Upper and lower flanges and shafts are made of Q235-A carbon structural steel, raw material is cheap and meets requirements of use, machining and welding performance is good.

When carbon/epoxy cone shell is cured in autoclave, it needs to go through process of heating → cooling. When temperature rises to gel point of resin, cone shell is solidified and formed. At this time, size of cone shell is determined by size of punch. For example, if diameter of a certain inner cavity of cone shell is L1 during design, diameter of punch at corresponding position is L0 at room temperature (25℃), gel point temperature of resin is T during curing, rough corresponding relationship is: L1=L0× (1+(T-25)×α), punch material is aluminum alloy, and its thermal expansion coefficient α=24×picture. Assuming that L1=1 000 mm, T=120 ℃, substituting above formula, L0=997.73 mm can be calculated. When carbon/epoxy cone shell is cured and molded, due to winding angle of prepreg, applied pre-tightening force, etc., size of product after curing generally cannot reach theoretical size of expansion of mold part. Formula L1=L0×(1+( T-25)×α) also needs to increase correction coefficient β (β is slightly smaller than 1), that is, L1=L0×(1+(T-25)×α)×β. Size of correction coefficient β is related to winding angle of carbon/epoxy prepreg, whether to add preload force and magnitude of force when making product blank, etc. Determination of correction coefficient β depends on engineering practice experience. At present, there is no accurate calculation formula.

Because flanging of upper and lower ends of carbon/epoxy cone shell is inward flanging, if punch is processed into a whole, punch cannot be taken out after cone shell is formed, and punch must be processed into a combination of multiple pieces (punch is disassembled into multiple pieces and taken out after cone shell is formed), as shown in Figure 4. Convex mold is fixed with core mold by positioning pins and screws. After cone shell is solidified, remove positioning pins and screws, first take out core mold, then take out convex modules one by one. When manufacturing mold, joint surface of punch and core mold (concave surface of punch mold and convex surface of core mold) are processed to same size to ensure a tight fit. Use Arabic numerals and stamp marks on junction of each convex mold flap and core mold to ensure that corresponding relationship will not be wrong every time it is used.

 

Figure 4 split punch

Prepreg blank of carbon/epoxy cone shell is prefabricated and cured in autoclave. When curing and pressing, movable parts on mold tend to move centripetally (volume reduction caused by interlayer compaction of prepreg material). In order to ensure that mold parts transmit pressure to prepreg material, mold parts cannot interfere with each other when moving, so it is necessary to consider leaving an appropriate gap between mold parts during design. For example, die is divided into 6 lobes, and a gap of 1 mm should be left between each lobes; outer diameter of upper pressure ring should be 1~2 mm smaller than inner diameter of corresponding die.

Structural dimensions of main parts of mold are shown in Figures 5 to 7. These parts are in contact with prepreg blank of carbon/epoxy cone shell, and are closely related to size of molded product.

 

Figure 5 core mold

 

Figure 6 punch

 

Figure 7 Die
After mold design is completed, mold is manufactured, and carbon/epoxy cone shell is successfully formed through manufactured mold. As shown in Figure 8, molded product meets design requirements.

 

Figure 8 Actual solidified cone shell