FRP is simple to form, excellent in performance, and rich in raw materials. It has been increasingly used in all aspects of the national economy. The hand lay-up FRP technology (hereinafter referred to as hand lay-up) has less investment, short production cycle, low energy consumption, can produce products with more complex shapes, and occupies a certain market in China. However, at present, the surface quality of hand lay FRP products in my country is poor, which limits the promotion of hand lay products to a certain extent. People in the industry have done a lot of work to improve the surface quality of products. The surface quality of hand lay-up products in foreign countries is close to or reaches Grade A, and can be used as interior and exterior decorative parts of high-end automobiles. In this regard, we have absorbed advanced technology and experience from abroad, made a large number of targeted tests and improvements, and achieved certain results.
First of all, theoretical analysis is carried out on the characteristics of hand lay-up process operation and raw materials. The author believes that the main factors affecting the surface quality of the product are the following aspects: ① resin processability; ② gel coat resin processability; ③ mold surface quality.
Resin accounts for about 55 to 80% by weight in hand lay-up products. The performance of the resin directly determines the performance of the product. The physical properties of the resin in the production process determine the production efficiency and product quality. Therefore, the following aspects should be considered when selecting the resin:
The viscosity of hand lay resin is generally between 170 and 117 cps. Resin viscosity range is wide, which is convenient for selection. However, due to the difference between the upper and lower limits of the viscosity of the same brand of resin between 100cps ~ 300cps, the viscosity in winter and summer will also change significantly. Therefore, experiments are needed to screen and determine the resin with suitable viscosity.
This paper tested five resins with different viscosities. During the test, the speed of the resin impregnated glass fiber, the defoaming property of the resin, the compactness and the thickness of the paste layer were mainly compared. Through experiments, it was found that the lower the resin viscosity, the faster the impregnation of glass fiber, the higher the production efficiency, the smaller the porosity of the product, and the better the thickness uniformity of the product. However, when the temperature is high or the resin dosage is large, the glue flow is prone to occur. (Or called glue control); conversely, the speed of impregnating glass fiber is slow, the production efficiency is low, the void ratio of the product is high, the uniformity of the product thickness is poor, but the phenomenon of glue control and glue flow is reduced.
After many tests, it was found that the resin viscosity at 25 ℃ is 200 ~ 320cps, which is the best combination of product surface quality and intrinsic quality and production efficiency. In actual production, the phenomenon of high resin viscosity is often encountered. At this time, the resin viscosity needs to be adjusted to reduce it to a viscosity range suitable for operation. There are usually two ways to achieve: ① add styrene to the resin to dilute to reduce the viscosity; ② increase the temperature of the resin and the temperature of the environment to reduce the viscosity of the resin. When the temperature is low, raising the ambient temperature and resin temperature is a very effective way. In general, in order to ensure that the resin does not gel too quickly, two methods are usually used.
The gel time of unsaturated polyester resin is mostly 6～21min (25℃, 1% MEKP, 0.5% cobalt naphthalate). The gel is too fast, the operation time is insufficient, the product shrinks greatly, the heat is concentrated, and it is easy to damage the mold and the product. The gel is too slow, easy to flow, slow curing, the resin is easy to damage the gel coat layer, reducing production efficiency.
The gel time is related to the temperature and the amount of initiator and accelerator added. When the temperature is high, the gel time will be shortened, which can reduce the amount of initiator and accelerator added. If too much initiator or accelerator is added to the resin, the color of the resin will become darker after curing, or the resin will release heat quickly and be too concentrated (especially thick-walled products) due to the reaction too fast, especially the products and molds. Therefore, the hand lay-up operation is generally carried out in an environment above 15°C. At this time, the initiators and accelerators need not be added in a large amount, and the resin reaction (gel, curing) is relatively stable, which is suitable for hand lay-up operations.
The gel time of the resin is of great significance to the actual production. The experiment found that the gel time of the resin is 10~18min under the conditions of 25℃, 1% MEKP and 0.5% cobalt naphthalate, even if the operating environment conditions are slightly Changes, by adjusting the amount of initiators and accelerators, can also ensure production requirements.
Other properties of resin
(1) Defoaming property of resin
The defoaming property of the resin is related to the viscosity of the resin and the content of the defoaming agent. When the viscosity of the resin is constant, the amount of defoaming agent determines the degree of porosity of the product to a large extent. In the actual production, when adding accelerators and initiators to the resin, it is bound to mix more air. If the defoaming property of the resin is not good, the air in the resin cannot be discharged in time before the gel, and there must be more bubbles in the product. , The porosity is high, therefore, it is necessary to use a resin with good defoaming property, which can effectively reduce bubbles in the product and reduce the porosity.
(2) Color of resin
At present, when FRP products are used as high-quality exterior decorative parts, high-grade paints are generally needed on the surface to make the product surface colorful. In order to ensure the consistency of the paint color applied on the surface of the FRP product, the surface of the FRP product is required to be white or light-colored. In order to meet this requirement, when choosing a resin, a light-colored resin must be selected. Screening tests on a large number of resins show that the resin color value (APHA) Φ84 can better solve the color problem after the product is cured. At the same time, using light-colored resin, it is also easy to find bubbles in the paste layer and discharge bubbles in time during the paste process; and reduce the occurrence of inconsistencies in the color of the inner surface of the product due to uneven thickness of the product due to operational errors during the paste process .
(3) Air drying
In the case of high humidity or low temperature, the inner surface of the product is often sticky after curing. This is because the resin on the surface of the paste layer is in contact with the oxygen, water vapor and other polymerization inhibitors in the air, so that there is a layer of Fully cured resin. This seriously affects the post-processing of the product. On the other hand, dust adheres to the inner surface and affects the quality of the inner surface. Therefore, when choosing a resin, care should be taken to choose an air-drying resin. For resins that do not have air-drying, it is generally possible to add 5% paraffin (melting point 46-48°C)-styrene solution to the resin at 18-35℃ to solve the air-drying of the resin. 6 to 8%.
In order to improve the surface quality of FRP products, a layer of colored resin-rich layer is generally needed on the surface of the product. Gelcoat resin is such a material. Gel coat resin improves the aging resistance of FRP products, and at the same time provides a homogeneous surface for FRP products, improving the surface quality of the products. 6mm。 Generally in order to ensure that the product has a better surface quality, the thickness of the gel coat layer is required to be 0.4 ~ 0.6mm. In addition, the color of the gel coat is mainly white or light color, and there can be no color difference between batches. Secondly, it is necessary to pay attention to the performance of the gel coat: the viscosity of the gel coat and the leveling of the gel coat. The most suitable viscosity for gel coat spraying is 6000cps. The most intuitive way to measure the level of gel coat is to spray a layer of gel coat on the surface of the mold that has been demolded. If the gel coat layer has fisheye-like shrink marks , Indicating that the leveling of the gel coat is not good.
Different maintenance methods for different molds are as follows:
(1) New molds or molds that have not been used for a long time
The gel coat must be fully stirred before use. After adding the trigger system, it must be stirred quickly and evenly to achieve the best use effect. When spraying, if it is found that the viscosity is too large, it can be diluted by adding an appropriate amount of styrene; when it is small, it should be thinly sprayed and sprayed several times. In addition, the spraying process requires the spray gun to be about 2 cm away from the mold surface, the compressed air pressure is appropriate, the spray gun fan face is perpendicular to the direction of the gun, and the spray gun fan faces overlap each other by 1/3. This will not only solve the technical defects of the gel coat itself but also ensure the consistency of the quality of the gel coat layer of the product.
Influence of mold on product surface quality
The mold is the main equipment for forming FRP products. According to the material, the mold can be divided into steel, aluminum, cement, rubber, paraffin, FRP and other types. Glass fiber reinforced plastic molds have become the most commonly used molds for hand lay-up glass fiber reinforced plastics due to their characteristics of easy molding, easy availability of raw materials, low cost, short manufacturing cycle and easy maintenance.
The surface requirements of FRP molds and other plastic molds are the same, usually the surface of the mold is one grade higher than the surface finish of the product. The better the mold surface, the shorter the molding time and post-processing time of the product, and the better the surface quality of the product, the longer the service life of the mold. After the mold is delivered for use, in order to maintain the surface quality of the mold, the mold must be well maintained. The maintenance of the mold includes: cleaning the surface of the mold, cleaning the mold, repairing the damaged parts, and polishing the mold. The timely and effective maintenance of the mold is the final starting point of the mold maintenance. In addition, the correct maintenance method of the mold is the key.
First, clean and inspect the surface of the mold, and perform necessary repairs on the damaged mold and unreasonable structure. Secondly, use a solvent to clean the surface of the mold. After drying, polish the surface of the mold one or two times with a polishing machine and polishing paste. Finish waxing and polishing three times in a row, then wax again, and polish again before use.
Mold in use
First of all, ensure that the mold is waxed and polished every three times, and the vulnerable and difficult to demold parts should be waxed and polished before each use. Secondly, a layer of foreign objects (probably polyphenylene or wax) that is prone to appear on the surface of the mold for a long time must be cleaned in time. The cleaning method is: use cotton cloth items dipped in acetone or special mold cleaner to scrub (the thick part can be used for a strong tool) Scrape gently), and the part after scrubbing will be demoulded according to the new mold.
In broken molds For molds that cannot be repaired in time, wax blocks can be used first.
Materials that are easily deformed and do not affect the solidification of the gel coat will fill and protect the damaged parts of the mold and continue to be used. For those that can be repaired in time, the damaged area must be repaired first. The repaired area should be cured by no less than 4 people (at 25℃). The repaired area must be polished and demolded before being put into use. The normal and correct maintenance of the mold surface determines the service life of the mold, the stability of the product surface quality, and the stability of production. Therefore, there must be a good habit of mold maintenance. In short, by improving materials and processes and improving the surface quality of the mold, the surface quality of hand lay-up products will be significantly improved.