In the foaming process of polyurethane foam machine, there are sometimes larger bubbles, which we call vacuoles. Cavitation is a common foaming defect. How to solve the problem of cavitation in polyurethane foaming machine is a problem to be considered in the construction process. Here, experts put forward the following suggestions, which are helpful to solve the cavitation defects of high-pressure foaming machine:
1、 Strictly control the proportion and injection quantity of raw materials
Control the ratio of black material, combined polyether and cyclopentane. After many experiments and researches, experts of Tianjiang chemical industry in Luoyang found that: under the condition of constant total injection volume, if the proportion of black material is too large, there will be vacuoles, if the proportion of white material is too large, there will be soft bubbles, if the proportion of cyclopentane is too large, there will be bubbles, if the proportion of cyclopentane is too small, there will be vacuoles. Misalignment of black and white materials will cause uneven mixing and foam shrinkage.
The injection quantity should be based on the process requirements. When the injection quantity is lower than the process requirement, the foam will have low density, low strength and even the phenomenon of non dense void. When the injection volume is higher than the process requirements, there will be bubbling and leakage, which will lead to the deformation of the box (door body).
2、 Temperature control of polyurethane foaming machine is a key to solve the problem of cavitation
When the temperature of polyurethane foaming machine is too high, the reaction is violent and difficult to control. It is easy to see that the foam liquid injected into the larger tank has uneven performance. The chemical reaction viscosity of the foam liquid injected at the beginning has increased rapidly, and then the foam liquid injected has not yet reacted. As a result, the later injected foam liquid can not push the first injected foam liquid to the front end of the foaming process of the box body, resulting in local cavity of the box body.
Black and white materials should be treated with constant temperature before foaming. After repeated tests, experts found that when the foaming temperature is controlled at 18-25 ℃, the preheating furnace temperature of foaming equipment is controlled at 30-50 ℃, and the foaming mold temperature is controlled at 35-45 ℃, the foaming effect is the best. When the temperature of the foam film is too low, the fluidity of the system is poor, the curing time is long, and the reaction is not complete, which leads to cavitation; when the temperature of the foam film is too high, the plastic liner is heated and deformed, which leads to violent reaction of the system. Therefore, it is necessary to strictly control the temperature of the foaming film and the ambient temperature of the foaming furnace. Especially in winter, when the line is opened in the morning, the foaming film, preheating furnace, foaming furnace, box body and door body must be preheated for more than 30 minutes. After a period of foaming in summer, the foaming system of polyurethane foaming machine must be cooled.
The relationship between polyurethane foaming machine and temperature is very large, because foaming is carried out by heat. If there is no heat, the foaming agent in the system can not be evaporated and can not form foam plastics. The heat comes from two sources, one is the heat generated by chemical reaction, the other is the heat provided by the environment. The heat of chemical reaction is a fixed value, which is not affected by external factors. The heat provided by single environment changes with the change of ambient temperature. When the temperature is high, the environment can provide heat for the reaction system, increase the reaction speed and shorten the reaction time. The foam is fully foaming, and the density of the foam surface and core is close. When the ambient temperature is low, part of the reaction heat is released to the environment. The loss of heat, on the one hand, prolongs the curing period of foam and increases the shrinkage rate of foam molding. The results show that the foaming volume of the same foaming material at 15 ℃ is 25% smaller than that at 25 ℃. On the other hand, it will form a layer of hard skins without foaming or even foaming on the surface of foam, which will increase the amount of foam perfusion and increase the cost of foam production.
3、 Pressure control of polyurethane foaming machine
The low pressure of foaming machine will make the mixture of black and white and cyclopentane unevenly. It shows that the density distribution of polyurethane foam is uneven, and there are large bubbles, bubbles and local bubbles in the area. The white, yellow or black stripes appear on the foam, and the foam shrinks. Therefore, the injection pressure of the foaming machine should be controlled at 13-16mpa.
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