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Technology of hand lay up FRP

Hand lay up is a simple, economical and effective FRP molding process, which can realize the return of capital in a short time without a lot of equipment and capital investment.

1. Spraying and brushing of gel coat

In order to improve and beautify the surface state of FRP products, increase the value of products, and ensure that the inner layer of FRP is not eroded and prolong the service life of products, the working surface of products is generally made into a layer of adhesive layer with pigment paste (color paste) and high resin content. It can be pure resin or reinforced by surface felt. This layer of adhesive layer is called gel coat layer (also known as surface layer or decorative layer). The production quality of gel coat directly affects the external quality, weather resistance, water resistance and chemical corrosion resistance of the products. Therefore, the following points should be paid attention to when spraying or brushing the gel coat layer:

(1) When collocating gel coat resin, it is necessary to mix fully, especially when using pigment paste. If the mixture is not uniform, spots and stripes will appear on the surface of the product, which will not only affect the appearance, but also reduce its physical properties. Therefore, mechanical mixing should be used as far as possible, and it is better to use a mixer without vortex to avoid air mixing.

(2) Gel coat can be sprayed with brush or special spray gun. 5-7% styrene should be added during spraying to adjust the viscosity of resin and replenish the volatile loss of styrene during spraying.

(3) The thickness of gel coat layer should be accurately controlled between 0.3-0.5mm, which is usually controlled by the quality of gel coat used per unit area, that is, the dosage of gel coat is 350-550g / m2, so as to achieve the required thickness.

The thickness of the gel coat layer should be appropriate, not too thin, but not too thick. If the gel coat is too thin, the curing may not be complete, and the glass fiber on the back of the gel coat is easy to be exposed, affecting the appearance quality, and can not play the role of beautifying and protecting the FRP products; if the gel coat is too thick, it is easy to crack and not resistant to impact, especially from the opposite direction of the product to attack. The uneven brushing of gel coat can easily cause cracks in the process of demoulding, which is due to the stress in the resin due to the different curing speed on the surface.

(4) The gel coat should be painted evenly to avoid local accumulation of gel coat.

(5) The curing degree of gel coat layer must be well controlled.

The best way to check whether the gel coat layer is cured properly is to touch the surface of the gel coat layer with clean fingers. If you feel that the gel coat layer is slightly sticky but not sticky, it means that the gel coat layer has been basically cured. At this time, the next paste operation can be carried out to ensure the integrity of the gel coat layer and the backing layer.

2. Determination of process route

Process route is related to product quality, product cost, production cycle (production efficiency) and other factors. Therefore, before organizing production, the technical conditions (environment, temperature, medium, load, etc.) of the product must be determined In general, the following aspects should be considered: 1

(1) According to the use requirements of the products, the raw and auxiliary materials, formula and laying method should be selected reasonably.

(2) According to the product geometry, production quantity, determine the mold structure and die material.

(3) According to the climate conditions, task priority, determine the curing mode.

3. Main contents of process design

(1) According to the technical requirements of the product to select the appropriate materials (reinforcement materials, structural materials and other auxiliary materials, etc.). In the selection of raw materials, the following aspects are mainly considered:

① Whether the product is in contact with acid and alkaline medium, the type, concentration, use temperature and contact time of medium.

② Whether there are performance requirements such as light transmission and flame retardant.

③ In terms of mechanical properties, it is dynamic load or static load.

④ Whether there are anti leakage and other special requirements.

(2) Determine the mold structure and material.

(3) Selection of release agent

(4) Determine the resin curing mix and curing system.

(5) According to the given product thickness and strength requirements, determine the type, specification, number of layers and laying mode of reinforcement materials.

(6) Draw up molding process specification.

4. Paste of FRP layer

Paste making is an important process of hand lay up process. It must be operated with precision, fast, accurate, uniform resin content, no obvious bubble, no poor impregnation, no damage to fibers and smooth surface of products, so as to ensure the quality of products. The quality of the product is closely related to the proficiency of the operator and the seriousness of the working attitude. Therefore, although the paste making work is simple, it is not easy to paste the product well, which should be taken seriously.

(1) Thickness control

The thickness control of FRP products is a technical problem encountered in the process of hand lay up process design and production. When we know the required thickness of a product, we need to calculate to determine the content of resin and filler and the specifications and layers of reinforcement materials used. Then calculate its approximate thickness according to the following formula.

T = (g1n1, 10 g2n2 +...) ×(0.394+0.909K1+0.4×K1K2)

Where: - calculated thickness of FRP (mm);

G1, G2 -- mass per unit area of cloth or felt of various specifications (kg / m2);

N1, N2 -- the number of layers of cloth or felt of various specifications;

394 -- thickness constant of fiber substrate;

0.909 -- thickness constant of polyester resin;

0.400 -- thickness constant of filler;

K1 -- ratio of resin content to glass fiber content;

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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