1. In the four extrusion foaming process, EVA first to ensure that the polymer and the formation of homogeneous solution foaming agent, and to establish a high enough pressure of extruder head to suppress foaming system near the extrusion die mold foaming ahead of time.
2. During the solidification stage, a reasonable cooling rate has an important impact on the density and performance of the final product.Therefore, processing equipment if using physical foaming agent, the injection and its measurement and process parameters on the final foaming material of bubble structure, density, mechanical properties and thermal properties will have a significant impact.
3. The foaming system through molding the nose for rapid nucleation and growth, the process to ensure that induce faster and greater pressure of supersaturated supersaturated pressure drop, to nucleus to form the bubbles at the same time, as far as possible in the process of growth, the right head temperature to increase the stability of the bubble and the final products foaming ratio control is the key.
4. EVA foam foaming process generally can be divided into two stages: the first is in the plastic melt or liquid form in large nuclear, uniform, fine bubble continues to swell as required by the second plastic bubble structure, the final curing finalize the design fixed will bubble structure, get the foam.Common molding methods of EVA foam plastics include extrusion, injection molding and molding.Among them, extrusion molding is one of the main processing methods of EVA foam plastics, and the foaming products such as general shaped materials, plates, pipes, diaphragm and cable insulation layer are all made by this method.Physical or chemical foaming methods can be adopted, in which physical foaming mainly USES some fluorocarbon compounds and their mixtures, as well as their low-boiling liquid.
5. According to the source of foaming power in the foaming process of polypropylene foam plastics, it can be generally divided into three types: mechanical foaming, physical foaming and chemical foaming.Mechanical foaming is the mechanical strong stirring, so that the gas is evenly mixed into the resin to form bubbles;Physical foaming is by means of the change of the physical state of the foaming agent in the resin, forming a large number of bubbles;Chemical foaming refers to the foaming of a resin by means of the gas produced by the chemical change of the foaming agent.Most of EVA foaming USES chemical foaming, and the foaming agents used are azodicarbonamide, diisopropyl azodicarboxylic acid, etc., of course, nitrogen can also be used as physical foaming agent for physical foaming.
6. In addition, the physical foaming method of direct injection of gas is also applied in the extrusion and molding process of EVA foam plastics.The main process parameters influencing extrusion foaming include: extrusion pressure, extrusion temperature, material retention time in the extruder, die axial pressure, etc.Generally speaking, the higher the extrusion pressure, the smaller the size of the bubble hole, and the more bubbles there are, the higher the quality of the foaming body is only possible to be obtained in a narrow temperature range.The higher the melt temperature and the lower the strength of the polymer melt, the more likely the bubble pressure in the bubble will exceed the surface tension of the foam and cause the bubble to burst.Therefore, it is necessary to optimize the specific polymer system in production to determine a suitable foaming temperature, so as to prolong the retention time of materials in the extruder and increase the number of bubbles.
7. In EVA extrusion foaming, the foam structure will be affected by melt crystallization and solidify and finalize at the extruder head.Therefore, the extrusion of EVA crystallization temperature is very important, if the crystallization occurred in the early days of foam, so early curing will make enough gas to promote the growth of bubbles, lead to bubble growth is not enough, it will affect the foaming ratio and the density of the final products.If the crystallization rate is too fast, it will also cause the above situation.And if they remain high head temperature, crystallization time is longer, the outward diffusion of a gas is increases the chance of escaping, as a result, the appropriate crystallization temperature and crystallization rate has important influence for EVA extrusion foaming.