K2 -- filler content vs. resin content
（2） Calculation of resin dosage
The resin content of FRP is an important process parameter, which can be calculated by the following two methods.
(1) According to the principle of gap filling, the formula of glue content is calculated. Only when the unit area mass and equivalent thickness of glass cloth are known (a layer of glass cloth is equivalent to the thickness of products), the glue content of FRP can be calculated
(2) First calculate the quality of the products, determine the percentage of glass fiber mass, and then calculate.
① Product surface area × thickness × fiber reinforced plastic density = product quality:
Product quality × glass fiber mass percentage = glass fiber mass;
Product quality - glass fiber quality = resin quality.
② The surface area of the product × the number of glass fiber layers × the mass of glass fiber unit area = the mass of glass fiber;
Glass fiber mass △ glass fiber percentage content = product quality;
Product quality - glass fiber quality = resin quality.
The amount of resin needed in paste preparation can be estimated according to the quality of glass fiber. If short cut felt is used, its glue content is generally controlled between 65-75%. For example, when glass cloth is used as reinforcement material, the glue content is generally controlled between 45-55%, so as to ensure the quality of products.
（3） Glass cloth paste
For products with gel coat layer, impurities shall not be mixed into gel coat. Before pasting, contamination between gel coat layer and backing layer shall be prevented, so as to avoid poor adhesion between layers and affect product quality. The gel coat layer can be reinforced by surface felt. The impregnation of glass fiber by resin should be paid attention to during pasting. Firstly, the resin should infiltrate the whole surface of the fiber bundle, and then the air inside the fiber bundle will be completely replaced by resin. It is very important to ensure that the first layer of reinforcement is fully impregnated with resin and tightly bonded, especially for products to be used at higher temperatures. Due to poor impregnation and poor adhesion, air will be left around the gel coat layer, which will generate bubbles due to thermal expansion during the curing treatment and use of the products.
When pasting, brush, scraper or dipping roller are used to evenly brush a layer of prepared resin on the gel coat layer or mold forming surface, and then a layer of cut reinforcement material (such as diagonal strip, thin cloth or surface felt) is laid. Then, the molding tool is used to brush and compress it to make it closely fit. Besides, attention should be paid to eliminate air bubbles so that the glass cloth is fully impregnated Two or more layers of reinforcement are laid at the same time. Repeat the above operation until the required thickness is reached.
If the geometric dimensions of the products are complex, the reinforcement materials are not laid smoothly in some places, and the bubbles are not easy to be eliminated, the scissors can be used to cut the parts and make them paste flat. Attention should be paid to the staggered parts of each layer to avoid strength loss.
Glass fiber and resin can be used to fill the parts with a certain angle. If some parts of the product are relatively large, it can be thickened or reinforced to meet the use requirements.
Due to the different direction of the fabric fiber, its strength is also different. The laying direction and laying method of glass fiber fabric should be in accordance with the process requirements.
（4） Lap joint treatment
The same ply fiber should be continuous as far as possible, and it is not allowed to cut or splice at will. However, when the product size, complexity and other reasons are difficult to meet, butt laying can be adopted during pasting, and the lap joints of each layer must be staggered until the thickness required by the product is reached. During paste making, the resin is impregnated with brush, wool roller and pressure bubble roller, and bubbles are drained out.
If the strength requirement is high, in order to ensure the strength of the product, lap joint should be used between the two pieces of cloth, and the lap width is about 50 mm. At the same time, the lap position of each layer should be staggered as much as possible.
（5） Paste making of chopped felt
When using chopped felt as reinforcement material, it is better to use different specifications of impregnation roller for operation, because the impregnation roller is particularly effective in removing bubbles from the resin. If there is no such tool and it needs to be dipped with a brush, the resin should be applied by point brush method, otherwise, the fiber will be disordered and displaced, resulting in uneven distribution and different thickness. If it is difficult to make the reinforced material close to each other by brush or dipping roller, it can be smoothed and pressed by hand.
During paste making, glue liquid is applied on the mold surface with glue roller, and then the cut felt is laid on the mold by hand and smoothed. Then glue is applied on the rubber roller, and the resin glue liquid is immersed in the felt by repeated rolling, and then the glue liquid in the felt is extruded out of the surface with the rubber bubble roller, and then the second layer is pasted. If the corner is encountered, the felt can be torn off manually to facilitate the covering. The overlap between the two felt is about 50 mm.
Many products can also be laminated alternately with chopped mat and glass cloth. For example, the fishing boats made by Japanese companies adopt the method of alternate pasting. According to the introduction, the FRP products made by this method have good performance.
（6） Paste making of thick wall products
If the thickness of the product is less than 8 mm, it can be molded at one time. When the thickness of the product is more than 8 mm, it should be molded in several times. Otherwise, the product will be scorched and discolored due to poor heat dissipation during curing, which will affect the performance of the product. For the products that have been molded many times, the burr and bubble formed after the first paste curing should be removed before the next layer can be pasted. In general, it is recommended that the thickness of one-time forming should not exceed 5mm. Of course, there are also low exothermic and low shrinkage resin developed for forming thick wall products. The thickness of this resin in one molding is larger.
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