RTM (resin transfer molding) is a process in which resin is injected into a closed mold to infiltrate the reinforcing material and solidify it. It is suitable for the molding of multiple varieties, medium batches, and high-quality advanced composite materials. This advanced technology has many advantages. It can use a variety of fiber reinforced materials and resin systems and has an excellent surface. It is suitable for manufacturing high-quality products with complex shapes, with high fiber content, less volatile components during the molding process, less environmental pollution, strong production automation adaptability, low investment, and high production efficiency. Therefore, the RTM process has been widely used in the automotive industry, aerospace, defense industry, mechanical equipment, and electronic products. The primary factor that determines RTM products is the mold. Because RTM molds generally use the male and female mold alignment method, finding ways to improve the surface quality and dimensional accuracy of the male and female molds has become a key factor in determining product quality.
Material selection is a key to the quality of the mold. The material selection is based on the characteristics of the RTM molding process.
(1) Gel coat layer: RTM molding has a higher heat release, and the heat release of 4mm thick products can generally reach above 120°C. This requires gel coat resins with thermal shock resistance, gloss and heat resistance. Vinyl mold gel coat is used in this process, its heat distortion temperature is 160℃-172℃, and it has good mechanical properties.
(2) Surface layer: Mainly considering heat resistance and crack resistance, 30g/㎡ surface mat and 300g/㎡ alkali-free chopped strand mat are used as reinforcing materials, and the resin is bisphenol A epoxy vinyl ester resin. The resin has good continuous high temperature resistance and low shrinkage.
(3) Reinforcement layer: Pay attention to strength and shrinkage. The selected reinforcement material is 0.4 alkali-free cloth and 300g/㎡ alkali-free chopped strand mat as the reinforcement material, and zero shrinkage resin is used as the matrix material.
(4) Reinforcement layer: increase the overall rigidity of the mold, facilitate the operation of opening and closing the mold, and adopt a steel frame reinforcement method.
Making the original model
Many of the original molds of FRP molds have used plaster, wood, cement, paraffin, etc. as the base material, and they are made by hand. The surface of the master mold made with these materials and production processes is difficult to reach the A-level surface, and the dimensional accuracy cannot be controlled. , The production process is complicated, the cycle is long, defects are easy to occur, and the flatness is poor. It is only suitable for the production of FRP molds that require low precision and low surface quality. If the above method is adopted, the production requirements of the RTM model cannot be met at all.
In order to meet the requirements of the RTM model, block machinable resin can be used as the raw material to be manufactured by CNC machining. Block processable resin is commonly known as Yogi. Use epoxy resin, ABS plastic, glass microbeads, aluminum oxide, carboxymethyl fiber to mix and stir evenly. After mixing dibutyl resin in a certain proportion, heat to dissolve the mixture into a paste, and enter after vacuum defoaming The mold is molded, and the molded solid is heated to form a cured product. Substitute wood has the advantages of good thermoplasticity, strong stability, and good mechanical processing performance.
When making a model with substitute wood as the material, firstly, according to the design requirements of the product, use 3D drawing software such as Pro/e, UG, CATIA to draw the 3D model of the product. In order to ensure the accuracy of the product when designing the three-dimensional digital model of the product, a certain margin should be reserved according to the shrinkage rate of the selected resin. Then it is processed by numerical control, and the original model is generally processed into the actual product, and the product size and thickness must meet the design requirements. In this way, the step of making the mold cavity is omitted during production, and the product size and thickness accuracy are greatly improved.
Material selection and original mold making provide a prerequisite for the successful production of an RTM mold. However, the control over the production process cannot be ignored. The original mold is processed by CNC, and the dimensional accuracy meets the requirements, but the processed tool marks still have a great impact on the mold surface. It needs to be polished and finely polished to make the model meet the A-level surface requirements.
First, start the production of the first mold gel coat layer. For the gel coat layer to obtain a good surface effect, the gel coat layer needs to be made by spraying. When spraying gel coat, the general gel coat dosage is 1000g/㎡, sprayed in 3 times, the first layer of gel coat dosage is 200g/㎡, the color is different from the last two, the last two gel coats dosage is 400g/㎡. The airflow of the gel coat spray gun should be uniform, and the glue should be sprayed evenly on the surface of the model. After the glue is sprayed for the last time, the surface layer will be laid. The surface layer is made of one layer of 30g/㎡ surface mat and 4 layers of 300g/㎡ chopped strand mat. The surface felt should be spread smoothly without laps and wrinkles. Be especially careful about the edges and corners. Use the brush corners to fully press to drive away air bubbles. After the surface layer is cured, it is necessary to carry out the work of picking up the bubbles. The bubbles are pierced and then polished with sandpaper to remove the burrs and wiped with acetone. Then, the following paste can be carried out.
After demolding, remove the excess flash, and blow off the small particles in the mold with compressed air.
Sanding: After soaking in water with 400# water sandpaper, wrap it on a smooth sanding board and sand it back and forth repeatedly. Until the first layer of gel coat layer is sanded off; after washing the mold with clean water; sanding with 600# water sandpaper according to the above method, until the surface of the abrasive tool does not see obvious scratches of uneven thickness Wash the mold, use 800# water sand, and polish according to the above method, until there are no obvious scratches on the surface of the mold, and the hand feels smooth and delicate; rinse with water and finally use 1200# water sandpaper to repeat the above method.
Polishing: Apply 1000# polishing paste evenly on the surface of the mold. After 2-3 minutes, use a polishing machine with cloth wheels to perform polishing step by step until the mold surface is mirror-reflected and there is no residual sanding surface. (It usually needs to be polished twice); After washing with water, apply 2000# polishing paste evenly on the surface of the mold. After 2-3 minutes, use a polishing machine with a wool wheel to polish step by step until the mold surface is highly clear. Until the light is reflected. After polishing, you can wash off the polishing paste with washing powder and water and then rinse with water to dry.
After that, the sealing agent is applied and the release agent is applied.