Research and Application of RTM Molding Die for Composite Materials Used in Oil Fields

 

Structure and Characteristics of Composite Insulation Parts

1.1 Appearance and Size of Composite Insulation Parts

 

The insulation parts are glass fiber reinforced composite materials. The appearance and size of the insulation parts are shown in Figure 1. The insulation parts are composed of a barrel and connection parts at both ends.

 

 

Figure 1 Appearance and Dimensions of Insulating Parts

 

1.2 Performance Requirements and Process Difficulties of Composite Insulation Parts

Composite insulation parts are components of nuclear magnetic resonance logging instruments. They are large in size and their function is to remove mud, so that the mud near the probe is relatively small, and to reduce the hydrogen atoms in the mud that affect the logging effect. In addition to meeting the basic mechanical performance requirements, they are also required to withstand temperatures of 200 °C and pressures of 170 MPa, and have corrosion resistance and high wear resistance. Due to the presence of thin-walled areas, composite insulation parts have certain difficulties in meeting the above-mentioned index requirements, so the selection of appropriate materials and process systems is the premise for ensuring the performance of molded products.

2. Insulation Process Design

2.1 Determination of the Molding Process of Insulating Parts

According to the use requirements of insulating parts, epoxy resin fiber reinforced materials are selected for manufacturing, and three process tests are carried out:

① Winding process molding, and then machining to the product size;

② Vacuum bag infusion molding, and then machining to the required size of the product;

③ Steel mold RTM (resin transfer molding, resin molding process) infusion molding. By comparison, the products molded by process

③ have better quality stability and better appearance. There are no defects such as pores and cracks inside and on the surface, and raw materials are saved compared with the previous two process schemes. The optimized steel mold RTM infusion process route is shown in Figure 2.

 

 

2.2 Design of Insulation Layer Structure

 

During RTM molding, the preform or continuous fiber fabric is placed in the molding mold, and the resin is injected into the cavity. The degree of resin and fiber infiltration is controlled by adjusting the pressure inside the mold. After curing, the composite product is demolded. In order to ensure the mechanical properties and other comprehensive properties of the molded product, the product should avoid the presence of high-resin areas during the RTM infusion molding process. Therefore, the layer of the fiber braid should be designed. The layer section of the fiber cloth is shown in Figure 3. Multiple layers of fiber cloth are stacked and laid, and the thickness of each layer is 0.2 mm. To ensure the regularity of the laid layer, the fabric is sewn or tightened with yarn during the laying process. When laying the braid, it needs to be symmetrical along the center line of the core axis. Use as little spray glue as possible during the laying process, and use point spray glue when necessary. During the laying process, ensure that the work site is carried out in a dust-free, pollution-free and dry environment.

 

 

3 Mould Structure Design and Manufacturing

3.1 Mould Structure Design

In the RTM molding process, the mold structure is related to the quality of the molded product, production efficiency, and service life of the mold. A reasonably designed mold structure can ensure that the molded product has good dimensional accuracy and appearance. Therefore, it is necessary to reasonably design and select the structure and material of the mold for the RTM process. The mold clamping accuracy between the mold modules is high, and an auxiliary demolding device should be provided to ensure that the molded product can be demolded smoothly; the mold has sufficient strength and rigidity under the injection pressure, which is the premise to ensure its sealing performance; to ensure the rationality of the resin flow in the cavity, the gate and vent need to be designed. The overall structure of the mold is shown in Figure 4. In addition, the selection of mold parts materials should also consider the influence of the temperature of the resin curing exothermic peak on the mold. The overall sealing of the mold is the key to RTM infusion. The sealing structure of the upper and lower molds and the end cover is shown in Figure 5.

 

 

1. Glue outlet cover
2. Mandrel
3. Molded product
4. Lower mold
5. Upper mold 6. Glue inlet cover

 

 

3.2 Mold Manufacturing

When using the RTM process for molding, the resin is injected into the cavity at a high flow rate and pressure. Therefore, the structural strength and rigidity of the mold must be large enough to not deform or break under the maximum injection pressure, and the sealing performance must be good. Common molding mold parts materials include fiberglass, aluminum or steel, and their material parameters are shown in Table 1. As can be seen from Table 1, fiberglass has the lowest density, but its mold parts are easily damaged and have a short service life. Aluminum has a large thermal expansion coefficient, and mold parts are prone to deformation after processing. Therefore, most RTM molds are made of steel, especially low-alloy steel, which has higher strength and rigidity. P20 hot working mold steel is often used as the molding mold part material.

 

 

Table 1 Common Mold Material Parameters

 

According to the shape of the product, the designed mold structure is slender. When the rigidity of the mold parts is insufficient, it is easy to cause twisting and flexural deformation during mold closing and hoisting, resulting in certain deformation of the product after molding. In order to avoid deformation of mold parts, the wall thickness of the upper and lower molds can be appropriately increased. Since the structure of the molded product requires the addition of a reinforcing frame and reinforcing ribs to the mold shape, its anti-deformation ability is improved.

 

The upper and lower molds of the designed mold are made of cast blanks. After rough machining and elimination of residual stress, the sealing of the mold parts needs to be tested. If there is air leakage in the mold cavity, the leaking part is locally polished, repaired with argon arc welding, and then fine-machined and polished. After production is completed, the surface roughness of the mold cavity is ≤Ra0.8 μm to ensure that the molded product can be demolded smoothly.

 

4.1 Finite Element Analysis of Mold Structure

• Finite Element Analysis of Mold Clamping Force

The Abaqus finite element analysis software is used to analyze the stress and deformation of the upper mold when the braided body is loaded into the mold and clamped before the insulating part is formed. According to the overall structure of the mold, the upper mold is prone to deformation. The density of the mold steel is 7.8×103 kg/m3, the elastic modulus is 207 GPa, and the Poisson’s ratio is 0.27. The allowable stress of the mold is 260 MPa, and the limit flexural deformation of the molding mold is taken as 0.4 mm.

 

The working condition mainly examines the reaction force of the fiber braided body on the mold when the preform is loaded into the sealed mold cavity after the fiber braided body is laid on the mandrel. During the analysis, the surfaces of the 12 groups of bolt holes connecting the upper and lower molds of the mold were fixed, and a total pressure of 20 kN was applied vertically on the inner surface of the upper mold. The maximum stress value was calculated to be 76 MPa, and the stress concentration was at the bolt hole mouth, as shown in Figure 6 (a), which was less than the allowable stress of 260 MPa. The maximum deformation occurred in the middle of the mold section, and the deformation gradually increased from both ends to the middle. The maximum deformation was 0.018 mm, as shown in Figure 6 (b), which was less than the limit flexural deformation of 0.4 mm. The structure met the requirements.

 

 

4.2 Simple Support Analysis of Mold

The simply supported working condition mainly examines the flexural deformation caused by the gravity of the mold itself due to frequent lifting and transportation during operation. The various modules of the mold are connected by bolts. During simulation, the various parts of the mold are bound and constrained. The lifting positions at both ends of the mold limit its circumferential displacement, apply gravity to the mold, and apply a total pressure of 5 kN (1.5 times the gravity of the corresponding mass of the mold) on the upper surface of the mold.

 

The maximum stress value of the mold is calculated to be 17 MPa, which is less than the allowable stress of 260 MPa. The maximum deformation occurs in the middle of the mold along the length direction. The deformation gradually increases from both ends to the middle. The maximum deformation is 0.023 7 mm, and it is less than the limit flexural deformation of 0.4 mm without installing the mandrel. As shown in Figure 7, the mold structural stiffness meets the requirements.

 

 

Figure 7  Simulation Results Of Stress And Deformation Of The Simply Supported Molding Die

 

4.3 Analysis of thermal Stress of Mold Curing

The RTM process needs to be cured at high temperature after the infusion is completed. At this time, the thermal effect of the temperature of the resin curing exothermic peak on the mold needs to be considered. The heat source input enters the mold cavity through heat conduction from the outer surface of the mold. The temperature is 170 ℃. The thermal conductivity of the mold material is 52 kW/(m·℃), and the thermal expansion coefficient is 1.5×10-5/℃. The thermal deformation of the mold due to temperature is calculated to be 0.29 mm, and the support effect of the core shaft and the product on the mold is not considered. The mold thermal deformation cloud diagram is shown in Figure 8.

 

 

5 Insulation Parts Production Process

After the insulating parts are laid according to the prescribed process, they are loaded into the sealed cavity of the mold, the auxiliary tooling at both ends is removed, the mold is closed, and the pouring system is connected for injection, as shown in Figure 9. After demoulding according to the overall molding process, the surface of the product is smooth, without wrinkles or defects. Finally, the two ends of the product are cut, punched, and deburred. The produced product is shown in Figure 10

 

 

Figure 9 Insulation Parts Production