High-damping hybrid composite material wet molding process and molding die thereof

Abstract

The invention discloses a high-damping hybrid composite material wet molding process and a molding die thereof. The process comprises the following steps: preparing a hybrid laminated preform; transferring the mixed laminated preform into a mold, covering resin containing damping filler on the surface of the mixed laminated preform, and performing wet-process mold pressing to cure the resin; the forming die comprises an upper die and a lower die; a plurality of cushion block containing grooves are distributed in the outer edge of the inner surface of the lower die, cushion blocks are placed in the cushion block containing grooves, an annular flow guide groove and a sealing groove are formed in the working face of the lower die, the sealing groove is formed in the outer edge of the annular flow guide groove, and a sealing strip is arranged in the sealing groove. And four ejection devices for ejecting a product are arranged in the lower mold. The method effectively solves the problems of uneven resin infiltration, poor interlayer interface bonding, insufficient mold adaptability and the like when the high-damping composite material is prepared by the traditional wet molding process, the prepared composite material product has high damping characteristic and high mechanical property, and the universalization of the mold and the high efficiency of production are realized at the same time.

Description

Technical Field

[0001] This invention relates to a wet compression molding process for hybrid composite material components with high damping and high strength requirements, as well as a molding die specifically designed to implement this process, belonging to the field of high-performance composite material preparation technology. Background Technology

[0002] Fiber-reinforced composite materials, such as carbon fiber and glass fiber composites, have been widely used in aerospace, high-end equipment, and transportation due to their excellent properties such as high specific strength and high specific modulus. However, these traditional composite materials have a significant drawback: poor damping performance, resulting in insufficient vibration reduction and noise reduction capabilities of the structure, which constitutes a bottleneck in many applications with stringent dynamic performance requirements.

[0003] To improve the damping performance of composite materials, the art typically employs methods that introduce viscoelastic damping materials (such as polyurethane, rubber, etc.) into the laminated structure to form a constrained layer damping structure. However, current mainstream processes for preparing such "rigid-flexible" hybrid structures, such as the prepreg-autoclave method, while producing high-quality products, suffer from limitations such as high cost, long production cycle, and difficulty in molding complex and large components.

[0004] While wet molding is a relatively low-cost alternative suitable for rapid prototyping, it faces significant challenges in fabricating such complex laminated structures. First, there is the issue of thickness adaptability. The introduction of damping intercalation changes the total thickness of the preform, and different products have different layup schemes, resulting in varying thicknesses. Traditional fixed-thickness molds cannot adapt to this change, leading to either insufficient mold closing pressure and non-dense products, or excessive mold closing, damaging the mold or fibers.

[0005] Secondly, there is the challenge of resin wetting and interface control. Viscoelastic damping layers are typically impermeable to resin, hindering resin flow in the thickness direction and easily leading to defects such as dry spots and bubbles at the fiber / damping layer interface. Simultaneously, aramid fibers are hygroscopic and orientation-sensitive, easily disrupted under resin pressure, affecting the final mechanical properties. Existing molds lack effective methods for guiding and controlling the resin flow path.

[0006] Secondly, there is the issue of process synergy. There is a lack of integrated molds that can simultaneously meet the requirements of "variable thickness", "vacuum-assisted defoaming" and "controllable directional overflow" to adapt to the complex molding physics of high-damping hybrid composite materials.

[0007] Due to the significant differences in wetting characteristics between carbon fiber and aramid fiber in the wet molding process, the resin is unevenly distributed between different fiber layers, making it difficult to achieve sufficient wetting within a limited process time. Even increasing the amount of resin cannot completely eliminate the wetting blind spots between layers. These insufficiently wetting areas will form interface defects and pores, which seriously affect the damping performance and mechanical integrity of the product.

[0008] Therefore, there is an urgent need in this field for a complete solution that integrates specialized molds and optimized processes to achieve high-quality, high-efficiency, and low-cost manufacturing of high-damping hybrid composite materials. Summary of the Invention

[0009] The technical problem to be solved by the present invention is to provide a wet molding process for high-damping hybrid composite materials and its molding die, so as to solve the technical problems faced by traditional processes in preparing such materials, such as unsuitable thickness, insufficient wetting, easy fiber disturbance and poor interface bonding of damping layer.

[0010] To address the above problems, this invention provides a wet molding process for high-damping hybrid composite materials, comprising the following steps: Step 1): Prepare a hybrid laminated preform; the hybrid laminated preform is a layup structure, which includes a reinforcing fiber layer and a damping intercalation layer, wherein the reinforcing fiber layer includes a carbon fiber layer and an aramid fiber layer, and the damping intercalation layer is located in the middle layer or functional layer of the hybrid laminated preform. Step 2): Transfer the hybrid laminated preform into a mold, cover the surface of the hybrid laminated preform with resin containing damping filler, and perform wet molding to cure the resin.

[0011] Preferably, the damping intercalation layer is nylon mesh, the resin is vinyl resin or epoxy resin-based, and the intermediate layer is the central layer of the layup structure.

[0012] More preferably, the reinforcing fiber layer adopts a hybrid layup structure of T700 grade carbon fiber plain weave fabric and TH5314DX para-aramid fiber plain weave fabric, with an areal density of 200 g / m². 2 and 255g / m 2 The (viscoelastic) damping intercalation layer adopts a nylon mesh structure with a thickness of 0.1-0.5mm; the vinyl resin is a flame-retardant vinyl ester resin system.

[0013] Preferably, the method for preparing the hybrid laminated preform is as follows: the reinforcing fiber layer and the damping intercalation layer are cut into shapes that conform to the mold, and stacked into a plane in a predetermined order to form the hybrid laminated preform; or, the reinforcing fiber layer and the damping intercalation layer are stacked into a plane in a predetermined order to form the hybrid laminated preform, and then the hybrid laminated preform is cut into shapes that conform to the mold.

[0014] More preferably, the preparation method of the hybrid laminated preform is as follows: the carbon fiber layer, aramid fiber layer and damping intercalation structure are respectively cut into shapes that conform to the mold surface, and stacked into a plane in a symmetrical order of "carbon fiber / aramid fiber / carbon fiber / aramid fiber / nylon mesh film / aramid fiber / carbon fiber / aramid fiber / carbon fiber" to form a hybrid laminated preform with a gradient transition structure; or, the carbon fiber layer, aramid fiber layer and damping intercalation structure are first stacked into a planar composite material in a prescribed symmetrical order, and then the composite material is cut into a shape that conforms to the mold surface.

[0015] Preferably, the wet molding process includes the following steps: Step 2.1): Heat the mold to bring the temperature inside the mold cavity up to the temperature required for resin curing and molding; Step 2.2): Vacuum the mold to increase the fluidity of the resin; Step 2.3): Close the mold, and then perform a multi-stage pressurization and pressure holding process; Step 2.4): When the pressure inside the mold cavity reaches the pressure required for resin curing, stop applying pressure and maintain the pressure until the product is cured and formed; Step 2.5): Open the mold and remove the product.

[0016] Preferably, the wet molding process includes the following steps: preheating the lower mold to 40-50°C to achieve the optimal resin impregnation temperature inside the mold cavity; evacuating to maintain the absolute pressure at 5-10 kPa; applying a pressure of 3-5 MPa and maintaining it for 1 hour when the resin temperature reaches 55°C to complete the initial impregnation and pre-gelling of the resin; raising the temperature to 65°C and maintaining a pressure of 3-5 MPa for 2 hours to achieve sufficient resin flow and intermediate curing; continuing to raise the temperature to 90°C, increasing the pressure to 7-8 MPa and maintaining it for 1 hour to promote the later curing of the resin; and finally raising the temperature to 120°C and maintaining a pressure of 7-8 MPa for 1 hour to complete the complete curing of the resin.

[0017] The present invention also provides a molding die for a wet molding process of high-damping hybrid composite materials, comprising an upper die and a lower die; the outer edge of the inner surface of the lower die is provided with a plurality of pad receiving grooves, in which pads are placed; the working surface of the lower die is provided with an annular guide groove and a sealing groove, the sealing groove is located on the outer edge of the annular guide groove, and a sealing strip is provided in the sealing groove; the lower die is provided with four ejection devices for ejecting products.

[0018] Preferably, the depth of the annular guide groove is 2-5 mm and the width is 5-10 mm; the depth of the sealing groove is 3-6 mm and the width is 8-12 mm.

[0019] Preferably, the ejection device includes an ejection block, the bottom of which is connected to an ejection rod, and the ejection block is located in the mounting groove of the lower mold. When the mold is closed, the ejection block does not protrude from the working surface of the lower mold, and the ejection rod passes through the lower mold and protrudes from its bottom. When the mold is opened, the ejection rod moves upward, driving the ejection block to lift the product.

[0020] Preferably, each of the four corners of the lower mold is provided with a guide post, which cooperates with the guide groove of the upper mold.

[0021] More preferably, the guide post also has the function of final precision positioning. Its guide section that cooperates with the upper mold is a tapered positioning structure, which is respectively set at the four corners of the mold, and the positioning accuracy can reach ±0.05mm.

[0022] Preferably, a wear-resistant plate is provided on the contact surface of the lower mold and the upper mold where there is relative sliding friction.

[0023] Compared with the prior art, the present invention has the following beneficial effects: Through an innovative "rigid-flexible-rigid" symmetrical layered structure design, a mesh nylon damping film is precisely inserted into the carbon fiber / aramid fiber hybrid reinforcement layer, achieving the best match between material stiffness and damping performance. By employing optimized multi-stage thermo-pressing process parameters (55℃ / 1h / 3-5MPa→65℃ / 2h / 3-5MPa→90℃ / 1h / 7-8MPa→120℃ / 1h / 7-8MPa), gradient curing of the resin and interface optimization were achieved, reducing the internal defect rate of the product to below 1%. Through the dual-function design of the flow guide-sealing groove system, the sealed environment required for vacuum-assisted molding is achieved, and a controllable resin overflow path is provided, which increases the resin wettability of aramid fibers by more than 40%. The use of a replaceable pad system enables precise adjustment of the mold cavity thickness within the range of 4-15mm, adapting to the needs of different layup schemes, improving mold versatility by more than 80%, and significantly reducing mold investment costs; The integrated heating system and four ejection devices in the lower mold ensure the stability of the process and the convenience of demolding. The product dimensional tolerance can be controlled within ±0.1mm, and the pass rate reaches over 98%. The entire process and mold system work together to enable the economical wet molding process to stably produce high-end damping composite material components with performance close to that of autoclave processes, reducing production costs by more than 60%, and demonstrating significant economic benefits and industrialization prospects. Attached Figure Description

[0024] Figure 1 This is a three-dimensional view of the upper mold in the forming mold; Figure 2 This is a three-dimensional view of the lower mold in the forming mold; Figure 3 This is the front view of the molding die; Figure 4 for Figure 3 Side view; Figure 5 This is a cross-sectional view of the multi-layer component in Example 1; Figure 6 This is a perspective view of another working condition of the molding die in Example 3. Detailed Implementation

[0025] To make the present invention more apparent and understandable, preferred embodiments are described in detail below with reference to the accompanying drawings.

[0026] like Figure 1-4 As shown, this invention provides a molding die, which includes as follows: Figure 1 The upper mold 1 shown and as Figure 2 The lower mold 2 is shown. Multiple pad receiving grooves 23 are distributed along the outer edge of the inner surface (i.e., the working surface) of the lower mold 2. Replaceable standard thickness pads 22 are placed within the pad receiving grooves 23. The thickness specifications of the pads 22 include 1mm, 2mm, 2.5mm, and 4mm. The working surface of the lower mold 2 is provided with an annular guide groove 21 with a depth of 3mm and a width of 6mm, and a sealing groove 24 with a depth of 4mm and a width of 10mm. The sealing groove 24 is located on the outer edge of the annular guide groove 21 and is also annular. A sealing strip 9 is provided within the sealing groove 24. The sealing strip within the sealing groove 24 forms a vacuum chamber, and / or a controllable overflow channel is formed by selectively setting notches in the sealing strip at specific locations.

[0027] Each of the four corners of the lower mold 2 is equipped with a guide post 3, which mates with the guide groove of the upper mold 1. The guide post 3 uses a tapered locating pin 25 with a tolerance fit of H7 / g6. The lower mold 2 has four ejection devices inside for ejecting the product, with an ejection stroke of 20-30mm. Each ejection device includes an ejector block 4, the bottom of which is connected to an ejector rod 6. The ejector block 4 is located within the mounting groove 5 of the lower mold 2. The ejector rod 6 has a diameter of 12mm, is made of GCr15 bearing steel, and has a hard chrome plated surface. When the mold is closed, the ejector block 4 does not protrude from the working surface of the lower mold 2; the ejector rod 6 passes through the lower mold 2 and protrudes from its bottom. When the mold is opened, the ejector rod 6 moves upward, driving the ejector block 4 to lift the product.

[0028] On the lower mold 2, a wear-resistant plate 8 is provided on the contact surface with relative sliding friction between it and the upper mold 1. Its function is to protect the more expensive and critical components on the mold body from wear by sacrificing replaceable wear-resistant blocks.

[0029] 12.9 grade carbon steel screws 7 are installed on the sides of both the lower mold 2 and the upper mold 1 for safe hoisting, handling and installation of the molds.

[0030] Example 1 A wet molding process includes the following steps: S1: The carbon fiber layer 11, aramid fiber layer 12, and nylon mesh 13 are cut into the same shape and stacked in a predetermined order to form a multilayer component 1, wherein the nylon mesh 13 is located in the middle layer of the multilayer component 1; or, the carbon fiber layer 11, aramid fiber layer 12, and nylon mesh 13 are stacked in a predetermined order to form a multilayer component 1, wherein the nylon mesh 13 is located in the middle layer of the multilayer component 1, and then the multilayer component 1 is cut (e.g. Figure 1 (as shown) S2: Transfer the multi-layer part 1 into the lower mold 2; S3: Spray, coat or pour resin containing damping filler onto multilayer component 1 so that the resin covers the surface of multilayer component 1. S4: Sequentially execute the processes of heating the lower mold 2, closing the mold, vacuuming, pressurizing, and holding pressure to allow the resin to cure and solidify. S5: Open the mold and remove the product.

[0031] When designing the laminated structure, fiber content, mechanical performance requirements, and damping performance requirements must be considered simultaneously. Nylon mesh 13 not only does not affect the overall thickness of the product, but also forms a stable damping structure between layers, significantly improving the vibration damping characteristics of the product.

[0032] Taking a 2.5mm thick damping structural component as an example, four layers of T700 carbon fiber twill fabric and two layers of TH5314DX aramid plain weave fabric are used. An 80-mesh nylon mesh 13 is interlaced between the third and fourth layers. The regular grid structure of the nylon mesh 13 can establish an effective resin delivery channel, while its flexible characteristics can form a micro-damping structure between the layers. Through a stepped thermo-pressing process of 55℃ / 4MPa / 1h→65℃ / 4MPa / 2h→90℃ / 7.5MPa / 1h→120℃ / 7.5MPa / 1h, the vibration damping performance of the product is significantly improved.

[0033] By creating a vacuum environment by completely laying a sealing strip in the lower mold sealing groove, and using the guide groove and preset overflow port to achieve controllable resin flow, the problem of impregnation of carbon fiber / aramid hybrid layup is effectively solved.

[0034] In this embodiment, the resin paste adopts a vinyl resin-based system and adds 20-30wt% of nano-silica damping filler with a particle size of 50-100nm.

[0035] The carbon fiber layer 11 can be made of carbon fiber fabrics with different moduli, and the dynamic performance of the part can be further optimized by hybrid layup of high and low modulus carbon fibers.

[0036] The aramid fiber layer 12 can be made of aramid fiber fabrics with different fineness, or it can be mixed with other high-damping fibers.

[0037] The multi-layer fiber arrangement adopts a symmetrical lay-up design. Through the precise positioning of the nylon mesh intercalation, the damping characteristics of the product are maximized while ensuring mechanical properties.

[0038] Nylon mesh 13 can be replaced by other polymer materials with regular pore structures, as long as they can form a stable mesh structure, play a dual role in guiding resin flow and constructing interlayer damping structure, and do not affect the test results of the overall thickness and fiber content of the product.

[0039] Example 2 A wet molding process includes the following steps: S41, Select a 4.0mm thickened pad according to the 4.8mm product thickness and install it in the lower mold receiving groove; S42, four layers of carbon fiber fabric and four layers of aramid fabric are laid on the working surface of the lower mold, and two layers of nylon mesh damping layer are inserted in the middle of the layup. S43, No sealing strip is placed in the sealing groove, so that it can be used directly as a large-capacity annular overflow groove; S44, executes an enhanced temperature and pressure process of 55℃ / 3MPa / 1h→65℃ / 5MPa / 2h→90℃ / 8MPa / 1h→120℃ / 8MPa / 1h; S45 achieves smooth demolding of the product through four ejector rods.

[0040] By eliminating the sealing strip and transforming the sealing groove into an overflow groove, a large amount of excess resin generated during the molding process of thick-section products can be accommodated, effectively preventing excessive pressure in the mold cavity. A 4.0mm thickened pad is used to accommodate the increased layup thickness, and tapered locating pins and wear-resistant plates are used to ensure mold closing accuracy during the thick section forming process; The optimized temperature and pressure process parameters ensured thorough wetting while improving the density and interfacial bonding strength of the product.

[0041] The upper mold 1 and lower mold 2 are made of P20 mold steel, with an overall size of 410×400mm; the lower mold 2 is 63mm thick, and the upper mold 1 is 47mm thick; the guide post 3 is made of 20Cr material, with a diameter of 25mm and a length of 52mm; the ejector block 4 is made of 20Cr material, with a size of Φ27×20mm; the ejector rod 6 is a standard part with a diameter of 12mm; the wear-resistant plate 8 is made of 20Cr material, with a size of 50×20mm and thicknesses of 10mm, 11mm, 11.5mm, and 13mm respectively.

[0042] The above structures are all common structures in molding dies, and their specific shapes and fit relationships can be adjusted according to actual needs.

[0043] The spacer block 23 can be made of H13 mold steel, and its hardness reaches HRC48-52 after heat treatment; The ejection device may include an ejector rod and an ejector plate, and the surface of the ejector rod may be hard chrome plated. The bottom of the guide channel 21 can be set with a drainage slope of 0.5°, and the transition radius of the channel wall is R=1mm; The cross-section of the sealing groove 24 can be trapezoidal, with a bottom width of 8mm and an opening width of 10mm.

[0044] Example 3 A wet molding process includes the following steps: S51, select a 5.5mm extra-thick pad according to the 6.5mm product thickness and install it in the lower mold receiving groove; S52, five layers of high-modulus carbon fiber fabric and five layers of aramid fabric are laid on the working surface of the lower mold, and three layers of nylon mesh are evenly set at the second, fourth and sixth layers to construct a gradient damping structure. S53, a sealing strip is placed in a section of the sealing groove (2 / 3 of the total length) to achieve partial sealing, and a gap is reserved in the remaining 1 / 3 section to form a controllable overflow path; S54, executes a high-pressure forming process of 55℃ / 5MPa / 1h→65℃ / 5MPa / 2h→90℃ / 8MPa / 1h→120℃ / 8MPa / 1h; S55 optimizes the interaction between the ejector block and the ejector rod, and adopts a segmented ejection method to achieve complete demolding of complex laminated structures.

[0045] By using a gradient-distributed damping intercalation design, a continuously varying damping characteristic is formed in the thickness direction of the product, effectively expanding the product's vibration reduction frequency band.

[0046] By combining local sealing with controlled overflow, the necessary vacuum environment can be maintained, and the resin flow front can be precisely controlled to avoid fiber orientation disorder.

[0047] The optimized high-pressure molding process, combined with the use of extra-thick pads, ensures full impregnation and dense curing of thick-section products.

[0048] The segmented ejection mechanism effectively reduces the demolding risk of complex laminated structures and prevents interlayer separation.

[0049] In this embodiment, the carbon fiber fabric can be T700 high modulus carbon fiber, and the aramid fabric can be TH5314DX high performance aramid. Nylon mesh can be made in 120 mesh size, and the thickness of each layer can be selected differently according to the damping requirements; The arrangement of the sealing strips can be optimized based on the results of resin flow simulation. The ejection system can use hydraulic control to achieve a precise segmented ejection sequence; The heating system must ensure the uniformity of the temperature field for thick-section products, with the temperature difference controlled within ±2℃.

[0050] The above embodiments fully demonstrate that the mold system of the present invention can adapt to the molding requirements of various high-damping hybrid composite materials, from simple to complex, through different configuration schemes, reflecting the high flexibility and reliability of the invention in practical applications.

Claims

1. A wet molding process for high-damping hybrid composite materials, characterized in that, Includes the following steps: Step 1): Prepare a hybrid laminated preform; the hybrid laminated preform is a layup structure, which includes a reinforcing fiber layer and a damping intercalation layer, wherein the reinforcing fiber layer includes a carbon fiber layer and an aramid fiber layer, and the damping intercalation layer is located in the middle layer or functional layer of the hybrid laminated preform. Step 2): Transfer the hybrid laminated preform into a mold, cover the surface of the hybrid laminated preform with resin containing damping filler, and perform wet molding to cure the resin.

2. The wet molding process for high-damping hybrid composite materials as described in claim 1, characterized in that, The damping intercalation layer is nylon mesh, the resin is vinyl resin or epoxy resin-based, and the intermediate layer is the central layer of the layup structure.

3. The wet molding process for high-damping hybrid composite materials as described in claim 1, characterized in that, The preparation method of the hybrid laminated preform is as follows: the reinforcing fiber layer and the damping intercalation layer are cut into shapes that match the mold, and stacked into a plane in a specified order to form the hybrid laminated preform; Alternatively, the reinforcing fiber layer and the damping intercalation layer are stacked in a predetermined order to form the hybrid laminate preform, and then the hybrid laminate preform is cut into a shape that conforms to the mold.

4. The wet molding process for high-damping hybrid composite materials as described in claim 1, characterized in that, The wet molding process includes the following steps: Step 2.1): Heat the mold to bring the temperature inside the mold cavity up to the temperature required for resin curing and molding; Step 2.2): Vacuum the mold to increase the fluidity of the resin; Step 2.3): Close the mold, and then perform a multi-stage pressurization and pressure holding process; Step 2.4): When the pressure inside the mold cavity reaches the pressure required for resin curing, stop applying pressure and maintain the pressure until the product is cured and formed; Step 2.5): Open the mold and remove the product.

5. The wet molding process for high-damping hybrid composite materials as described in claim 1 or 4, characterized in that, The wet molding process includes the following steps: preheating the lower mold to 40-50℃ to achieve the optimal resin impregnation temperature inside the mold cavity; evacuating to maintain the absolute pressure at 5-10 kPa; applying a pressure of 3-5 MPa and maintaining it for 1 hour when the resin temperature reaches 55℃ to complete the initial impregnation and pre-gelling of the resin; raising the temperature to 65℃ and maintaining a pressure of 3-5 MPa for 2 hours to achieve full resin flow and intermediate curing; continuing to raise the temperature to 90℃, increasing the pressure to 7-8 MPa and maintaining it for 1 hour to promote the later curing of the resin; finally, raising the temperature to 120℃ and maintaining a pressure of 7-8 MPa for 1 hour to complete the complete curing of the resin.

6. A molding die for a wet molding process of a high-damping hybrid composite material, characterized in that, It includes an upper mold (1) and a lower mold (2); the outer edge of the inner surface of the lower mold (2) is provided with multiple pad receiving grooves (23), and a pad (22) is placed in the pad receiving groove (23). The working surface of the lower mold (2) is provided with an annular guide groove (21) and a sealing groove (24). The sealing groove (24) is located on the outer edge of the annular guide groove (21), and a sealing strip (9) is provided in the sealing groove (24). The lower mold (2) is provided with four ejection devices for ejecting products.

7. The molding die for the wet molding process of high-damping hybrid composite materials as described in claim 6, characterized in that, The ejection device includes an ejection block (4), the bottom of which is connected to an ejection rod (6). The ejection block (4) is located in the mounting groove (5) of the lower mold (2). When the mold is closed, the ejection block (4) does not protrude from the working surface of the lower mold (2), and the ejection rod (6) passes through the lower mold (2) and protrudes from its bottom. When the mold is opened, the ejection rod (6) moves upward, driving the ejection block (4) to lift the product.

8. The molding die for the wet molding process of high-damping hybrid composite materials as described in claim 6, characterized in that, The lower mold (2) has a guide post (3) at each of its four corners, and the guide post (3) cooperates with the guide groove of the upper mold (1).

9. The molding die for the wet molding process of high-damping hybrid composite materials as described in claim 8, characterized in that, The guide section of the guide post (3) that cooperates with the upper mold (1) is a tapered positioning structure.

10. The molding die for the wet molding process of high-damping hybrid composite materials as described in claim 6, characterized in that, Wear-resistant plates (8) are provided on the contact surfaces of the lower mold (2) and the upper mold (1) where there is relative sliding friction.

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