Definition: polyurethane is made of molded products with dense epidermis foam core, called whole skin self molded plastic foam.
The whole skin foam produces dense skin itself when it is manufactured. The whole skin foam is divided into soft foam hard foam. The latter is used as structural material, such as imitation wood. Most of the self formed foams are semi rigid foam plastics, which rely on foaming materials to form skins foam core materials in foaming molding. There is an obvious density gradient in the self foaming foam, the outer skin thickness is about 0.3-2.3m, the density is 1000-1200kg/m3., the core density is usually tens to hundreds of kg/m3. The products have elasticity wear resistance similar to that of polyurethane elastomers, but they are light in weight only a fraction of the density is elastic. Its mechanical strength is higher than that of ordinary foam plastics of the same density.
The typical density of the molded foam is gradually decreasing the epidermis to the inner core, rather than abrupt change, at the same time has a lower weight. Under ideal conditions, the density distribution of foam structure is reverse parabolic. The density of the skin layer is similar to that of the polymer parent material. The density of the product gradually decreases to the center of the profile, the density of the center is the smallest.
The basic elements of forming whole skin (self skinning) of products are: mold temperature, molding pressure foaming agent.
Production principle of self foam is: foam material in foam forming process, because of the reaction heat, foaming, foaming agent containing inert low boiling point physical foaming agent to produce bubble, but in excess of filling conditions, the mold internal high internal pressure mold low temperature make the material close to the mold foaming agent condensation, therefore, after curing, produce high density on the whole skin surface. Dense epidermis. By adjusting the mold temperature, the amount of foaming agent the balance of filling amount, the products with different thickness of skin can be prepared. When the mold temperature decreases, the skin thickness increases.
The results show that the formation of epidermis is mainly affected by the interaction of mold temperature compression. The pressure temperature can lead to dynamic equilibrium between liquid gaseous foaming agents in the reaction polyurethane mixture, which is close to the cold surface of the mold, which is beneficial to keep the foaming agent liquid ( vaporized), the material temperature close to the inner core is more than 100 degrees, the foaming agent vaporizes to form the foam inner core.
In general, the mold temperature should be at least 60 ℃ lower than the maximum reaction temperature. In fact, when CFC-11 is used as foaming agent, the mold temperature should be kept between 50 ℃ 70 ℃. The products made completely cooled mold are abruptly changed to the low density inner core the high density of the epidermis, lack of transition zone. Therefore, the mechanical strength is low, the epidermis is often stripped the inner core of the foam.
According to the different product material systems, the overfilling coefficient is generally between 50% 100%, so as to maintain a high molding pressure, which is conducive to the liquefaction of the skin layer foaming agent.
Carbon dioxide usually does liquefy at the temperature pressure of molding conditions. Therefore, it is suitable for foaming agents for self foaming skin. Commonly used low boiling point physical foaming agents can be used for the production of self molded foam. The skin thickness can be reduced by adding a small amount of water into the reaction mixture.