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The RTM Process

Jun 11, 2020

The RTM process usually includes the following four steps: one, cutting the reinforcement material from the coil and laying them together; two, making the reinforcement material have a certain shape, trimming the preform according to the shape of the product; three, filling the mold (including pre Laying of molded bodies, resin injection and curing); 4. Demoulding and processing.

5. Winding

(1) Classification

According to the different physical and chemical states of the resin matrix during fiber winding molding, it is divided into dry winding, wet winding and semi-dry winding.

Dry winding Dry winding is the use of prepreg yarn or tape that has been treated with prepreg, which is heated and softened to a viscous flow state on the winding machine and wound onto the mandrel. Since the prepreg yarn (or tape) is professionally produced, it can strictly control the resin content (accurate to within 2%) and the quality of the prepreg yarn. Therefore, dry winding can accurately control product quality. The biggest feature of the dry winding process is the high production efficiency, the winding speed can reach 100 ~ 200m/min, the winding machine is clean, the labor and hygiene conditions are good, and the product quality is high. The disadvantage is that the winding equipment is expensive and requires additional prepreg manufacturing equipment, so the investment is large. In addition, the interlayer shear strength of dry winding products is low.

Wet winding Wet winding is the process of directly dipping the fiber bundle (yarn tape) into the mandrel under tension control. The advantages of wet winding are:

① Cost is 40% lower than dry winding;

②The product has good air tightness, because the winding tension makes the excess resin glue squeeze out the bubbles and fill the gaps;

③ The fibers are arranged in parallel with good parallelism;

④When wet winding, the resin glue on the fiber can reduce the fiber wear;

⑤High production efficiency (up to 200m/min). The disadvantages of wet winding are: ① resin waste is large, and the operating environment is poor; ② the amount of glue and the quality of the finished product are not easy to control; ③ there are fewer types of resin available for wet winding.

Semi-dry winding is the process of fiber dipping. After winding to the core mold, a set of drying equipment is added to remove the solvent from the immersion yarn. Compared with the dry method, the prepreg process is omitted. And equipment; compared with the wet method, the content of bubbles in the product can be reduced.

Among the three winding methods, wet winding is the most common application; dry winding is only used in the cutting-edge technology field with high performance and high precision.

(2) Raw materials

Reinforcement materials Reinforcement materials for winding molding are mainly various fiber yarns: such as non-alkali glass fiber yarn, medium alkali glass fiber yarn, carbon fiber yarn, high strength glass fiber yarn, aramid fiber yarn and surface felt, etc.

Resin matrix Resin matrix refers to a glue system composed of resin and curing agent. The heat resistance, chemical resistance and natural aging resistance of the winding products mainly depend on the resin properties, and also have a great influence on the processability and mechanical properties. The commonly used resins for winding molding are mainly unsaturated polyester resins, and sometimes epoxy resins and bismaleimide resins are used. For general civilian products such as tubes and cans, unsaturated polyester resins are mostly used. For winding products that require high mechanical strength, compressive strength and interlayer shear strength, epoxy resin can be used. Aerospace products mostly use bismaleimide resin with high fracture toughness and good moisture resistance.

Fillers There are many types of fillers, which can improve some functions of the resin matrix after being added, such as improving wear resistance, increasing flame resistance and reducing shrinkage. Adding hollow glass microbeads to the glue solution can increase the rigidity of the product, reduce the density and reduce the cost. When producing large-diameter buried pipelines, 30% quartz sand is often added to increase the rigidity of the product and reduce costs. In order to improve the bonding strength between the filler and the resin, the filler should ensure cleanliness and surface active treatment.

(3) Mandrel

The inner mold for forming hollow products is called a core mold. Under normal circumstances, after the winding product is cured, the mandrel must be released from the product. Basic requirements for mandrel design

① Must have sufficient strength and rigidity to withstand various loads applied to the mandrel during the forming process of the product, such as its own weight, product weight, winding tension, curing stress, cutting force during secondary processing, etc.;

②It can meet the requirements of product shape and dimensional accuracy, such as shape size, concentricity, ellipticity, taper (release), surface smoothness and flatness, etc.;

③Ensure that the product can be removed from the product smoothly after curing;

④ Simple to manufacture, cheap to manufacture, and convenient to obtain materials.

Mandrel material Winding mandrel material is divided into two categories: molten, soluble materials and assembled materials. Molten and soluble materials refer to paraffin, water-soluble polyvinyl alcohol sand, low melting point metals, etc. These materials can be made into hollow or solid core molds by casting. After the product is wound and formed, hot water or high-pressure steam is passed from the opening to make It melts and melts and flows out of the product. The flowing solution is cooled and reused. The commonly used materials for assembled mandrel are aluminum, steel, sandwich structure, wood and gypsum. In addition, there are lining materials. The lining material is an integral part of the product. It is not taken out of the product after curing. The function of the lining material is mainly anti-corrosion and sealing. Of course, it can also play the role of a core mold. Rubber, plastic, stainless steel, aluminum alloy, etc.

(4) Winding machine

The winding machine is the main equipment to realize the winding forming process. The requirements for the winding machine are:

① It can realize the winding law and accurate yarn arrangement of product design;

② Easy operation;

③High production efficiency;

④ Equipment cost is low.

The winding machine is mainly composed of two parts: core die drive and wire winding nozzle drive. In order to eliminate fiber loose thread during the reverse movement of the wire nozzle, keep the tension stable and accurately arrange the ribbon on the head or tapered winding product, and realize the winding with a small winding angle (0°~15°), the vertical core is designed on the winding machine Axial infeed (extending arm) mechanism. In order to prevent the yarn belt from twisting during the reverse movement of the wire winding nozzle, the extension arm is provided with a mechanism capable of turning the wire winding nozzle.

(5) Winding molding process:

The winding process generally includes the manufacturing of mandrels and linings, the preparation of resin glue, the heat treatment and drying of fibers, the dipping of glue, the drying of rubber yarns, the winding, curing, inspection, and processing into products under a certain tension. .

The main process parameters that affect the performance of winding products are fiber dipping, glue content and distribution, yarn drying, winding tension, yarn winding position, curing system, winding speed, ambient temperature, etc. These factors are mostly closely related to together. Reasonable selection of process parameters is an important part of giving full play to the characteristics of raw materials and manufacturing high-quality winding products.

Fiber dipping: The level of glue content is directly related to the weight and thickness of the product. If the rubber content is too high, the strength of the product is reduced, and the glue flow is severe during molding and curing; if the rubber content is too low, the porosity of the product is increased, and the air tightness, aging resistance, and shear strength are all reduced.

Winding tension: the size of the tension, the uniformity of the tension between the fiber bundles and the uniformity of the fiber tension between the winding layers have a great influence on the quality of the product. The appropriate winding tension can make the resin prestressed, thereby improving the resin The ability to resist cracking. If the tension between the fiber bundles is uneven, when the load is applied, the fibers will be broken by each, which will greatly affect the overall strength. In order to prevent the inner and outer looseness of the winding layers of the product under tension, the winding tension should be decreased regularly to ensure that each layer has the same initial stress. Winding tension will directly affect the density and void ratio of the product, and will have a great impact on the fiber impregnation quality and the product's gum content.

Winding speed: Winding speed means that the yarn speed should be controlled within a certain range. If the speed is too low, the production efficiency is low, and if the speed is too fast, the resin is easy to splash, the glue is impregnated or the impurities are easy to suck in.

Curing system: The curing system refers to an important condition to ensure the resin is fully cured, which directly affects the physical and mechanical properties of the product. The curing system includes heating temperature range, heating rate, constant temperature and time, cooling and so on. According to the different performance requirements of products, different curing systems are used, and different resin systems have different curing systems. Generally, they should be determined through experiments according to the resin formula, product performance requirements, and product shape, size, and structure. Reasonable curing system.

6. Extrusion

In the pultrusion process, continuous glass fiber bundles, belts, or cloths impregnated with resin glue are formed and solidified by an extrusion die under the action of traction force to continuously produce glass fiber reinforced plastic profiles with an unlimited length. This process is most suitable for producing FRP profiles of various cross-sectional shapes, such as rods, tubes, solid profiles (I-shaped, slotted, square profiles) and open profiles (doors and windows profiles, blades, etc.). Pultrusion is a special process in the composite material molding process. Its advantages are:

①The production process is fully automated and the production efficiency is high;

②The fiber content in the pultrusion products can be as high as 80%, the dipping is carried out under tension, which can fully play the role of reinforcing materials and the product has high strength;

③The vertical and horizontal strength of the product can be adjusted arbitrarily, which can meet the use requirements of products with different mechanical properties;

④There is no corner waste in the production process, and the product does not need post-processing, so it saves labor, raw materials and energy consumption compared with other processes;

⑤The product quality is stable, the repeatability is good, and the length can be cut at will. The disadvantage of the pultrusion process is that the product shape is monotonous, only linear profiles can be produced, and the lateral strength is not high.

(1) Raw materials for pultrusion process

In the pultrusion process of resin matrix, unsaturated polyester resin is the most widely used, accounting for more than 90% of the resin used in this process, in addition to epoxy resin, vinyl resin, thermosetting methacrylic resin, modified phenolic resin, Flame retardant resin, etc.

Reinforcement materials The reinforcement materials used in the pultrusion process are mainly glass fibers and their products, such as untwisted rovings and continuous fiber mats. In order to meet the special performance requirements of products, aramid fiber, carbon fiber and metal fiber can be selected. No matter what kind of fiber is used in the pultrusion process, its surface must be treated to make it bond well with the resin matrix.

Auxiliary materials The auxiliary materials in the pultrusion process mainly include release agent and filler.

(2) Pultrusion mould

The mold is an important tool for pultrusion technology, and generally consists of two parts: a preform mold and a forming mold. ① During the pultrusion molding process, after the reinforcement material is impregnated with resin (or while being impregnated), before entering the molding mold, it must pass through a preforming mold composed of a group of yarn guide elements. The role of the preforming mold is The reinforced material after dipping is gradually formed into a preform of approximate shape and shape according to the profile configuration of the profile, and then enters the forming mold, so as to ensure uniform yarn content in the cross section of the product. ②Molding mold The ratio of the cross-sectional area of the forming mold to the cross-sectional area of the product should generally be greater than or equal to 10 to ensure that the mold has sufficient strength and rigidity, and the heat distribution is uniform and stable after heating. The length of the pultrusion mold is determined according to the traction speed and the resin gel curing speed during the molding process to ensure that the product reaches the degree of mold release and curing when pulled out. Steel chrome plating is generally used, and the surface of the mold cavity needs to be smooth and wear-resistant, so as to reduce the friction resistance of pultrusion and increase the service life of the mold.

(3) Pultrusion molding process

The pultrusion process is composed of yarn feeding, dipping, preforming, curing and setting, traction, cutting and other processes. After the untwisted roving is drawn out from the creel, it enters the dipping tank through the yarn discharger and soaks the resin glue solution, then enters the pre-forming mold, discharges the excess resin and bubbles, and then enters the forming mold to gel and solidify. The cured product is continuously pulled out of the mold by the tractor, and finally cut by the cutting machine to a fixed length. In the forming process, each process can have different methods: such as the yarn feeding process, you can add continuous fiber felt, hoop winding yarn or use three-way fabric to improve the transverse strength of the product; the traction process can be a crawler traction machine, also It can use a mechanical hand; the curing method can be in-mold curing or heating furnace curing; the heating method can be high-frequency electric heating, or molten metal (low melting point metal), etc.

(4) Other pultrusion process

In addition to vertical and horizontal units, pultrusion technology also includes pultrusion technology for curved products, reaction injection pultrusion technology and pultrusion technology with fillers.