Introduction of polyurethane rigid foam machinery with excellent performance (1)

　　Urethane foaming equipment is a kind of building insulation material. Many people don’t understand it very well. Discussion on the causes of foam shrinkage and solutions: China’s solar water heaters account for about 33 million m2, accounting for 70% of the world’s possession, with an annual output of 780 10,000 m2, the annual sales volume of products is 10 times that of Europe, becoming the first user and producer of solar water heaters. Among them, the vacuum tube solar water heater with the largest market share is an authentic Chinese product. From design and development to production, there is no foreign experience to learn from, and there are many problems that need to be gradually improved in practice. As we all know, for solar water heaters, especially the quality of the polyurethane thermal insulation layer of vacuum tube solar water heaters is very important, it determines the thermal insulation effect of the water heater. Polyurethane products have always been "three-point product, seven-point foaming", that is to say, not only good polyurethane foaming materials, but also a scientific foaming process and strict operating procedures are needed. Foam shrinkage is the most common problem of solar water heaters. It can cause deformation, bulging, and even cracking of the water tank, which is what solar manufacturers often call "peeling", "shelling", "extracting a backbone" and other phenomena. .

　　1. Causes of foam shrinkage: Poor foam formation. Poor foam is one of the main reasons for foam shrinkage: At present, polyurethane foams for solar water heaters are made by free foaming by one or more castings, mainly by multiple castings. During the foaming process, it is not affected by any external pressure except atmospheric pressure. The polyurethane foam filling space must be completely filled before the gelation process arrives. After gelation, if the volume of the foam continues to expand, the resulting foam will have significantly elongated cells. In severe cases, it may be fibrous. Its physical and mechanical properties Very bad, this article calls it a "bad bubble". After cooling, it shrinks severely due to the condensation of the gas in the cells, and expands with the expansion of the gas in the cells after being heated. The foaming method of multiple pouring increases the proportion of this "bad foam". In addition, the foam at the junction formed by two adjacent pourings is likely to cause foam when cold due to different shrinkage rates.

　　2. Foam density: In the harsh working environment of solar water heaters, the temperature difference between inside and outside during working (may reach 100℃), and the requirements of working life, we recommend that the minimum foam density should not be less than 35kg/m3. If the blowing agent is HCFC-141b, the foam density should be increased to more than 38kg/m3. Some foaming material suppliers, in order to improve their market competitiveness, reduce the foam density to less than 30kg/m3. Although this reduces the cost of foam production on the surface, the strength of the foam is low and is not enough to carry the weight of the water tank filled with water, so that the foam is slowly squashed. Part of the weight of the water tank is supported by the top of the vacuum tube. The vacuum tube is stored in the crushed Danger.

　　3. Material ratio deviation: The material ratio of most foaming machines is fixed at 1:1, which is inconvenient to adjust, but the actual material ratio often deviates from 1:1 due to the influence of foaming material viscosity, temperature and other factors. This can be roughly seen from the difference in density between machine free foam and manual free foam. Taking the manual free foam density as a benchmark, when the machine free foam density is lower than the manual free foam, it indicates that material A (combined polyether) is excessive.

　　4. The influence of the ring Polyurethane foaming has a great relationship with temperature. Because foaming relies on heat to proceed. If there is no heat, the blowing agent in the system cannot evaporate, and foam cannot be formed. The heat comes from two aspects, one is the heat produced by the chemical reaction; the other is the heat provided by the environment. The heat of chemical reaction is a fixed value and does not follow the influence of external factors, but the heat provided by the environment changes with the change of the ambient temperature. When the ambient temperature is high, the environment can provide heat to the reaction system, which can increase the reaction speed and shorten the reaction time. It shows that the foam is fully foamed, and the density of the foam surface layer and the core part are close. When the ambient temperature is low (such as below 18°C), part of the reaction heat is dissipated into the environment. The loss of heat, on the one hand, causes the foam maturation period to be prolonged and increases the foam molding shrinkage rate.

　　5. Shell material and thickness factors The thickness and strength of the shell also have a certain effect on the shrinkage of the foam. Generally, under the same conditions, the thicker the thickness, the higher the strength, and the smaller the shrinkage rate of the foam. Common shell materials include stainless steel, color steel plate and embossed aluminum. Embossed aluminum has a large foam shrinkage rate due to its large thermal conductivity, and it is gradually eliminated.