EVA high foaming (also known as secondary foaming) will complete the cross-linking of fully mixed materials under heating and pressure in the mold, and part of the foaming agent will decompose to produce partial foaming. The mold is cooled to a certain temperature and taken out. Perform secondary heating and foaming under pressure. The expansion ratio can be 30 to 45 times.
The surface of the EVA shaped foot pad is soft; the tensile toughness is stronger than that of LDPE, and the hand feels sticky (but no glue on the surface); white and transparent, high transparency, the look and feel are very similar to the PVC film, and attention should be paid to distinguish it. EVA foam is used on the bottom of the interlayer to make the soft bag easy to support or protect the contents in the box. Foamed EVA is expensive and has good elasticity and is not easy to deform. Determine its softness and hardness by the degree, increase the hardness and strengthen the commonly used ones:
(1) High foaming EVA
(2) 35-40 degrees EVA
(3) 50-55 degrees EVA, etc. For example, it is used for cushions, which has strong impact resistance; it is comfortable to wear and has a strong protective effect. The thickness of the protective pad is 17 cm, and the thickness after hot pressing (finished product) is 14-16MM.
How to choose the lubricant for eva foam?
EVA foam should be added with a smoothing agent? Yes. The EVA foaming process is inseparable from the smoothing agent. The purpose of using the smoothing agent is to make the EVA foam extrusion process easy.
EVA foam smoothing agent has two types: internal smoothing agent and external smoothing agent.
The difference between the inner smoothing agent and the outer smoothing agent in the use effect is: the outer smoothing agent is used to reduce the melt and metal parts. The internal smoothing agent changes the rheological behavior and viscosity of the melt by modifying the intermolecular conflict, and has the function of improving the fluidity and uniformity of the melt.
The difference between the effects of the two EVA foam smoothing agents is the compatibility between the polymer and the smoothing agent. The inner smoothing agent has greater compatibility, while the outer smoothing agent has less compatibility. However, compatibility tends to improve with the rise of temperature and pressure. At this moment, what was originally an external smoothing agent can become an internal smoothing agent, so there is no strict boundary between the two. In many cases, a kind of smoothing agent often has the effect of smoothing the surface and the inside together, but a certain kind of smoothing effect is the main one.
An element that should be paid attention to in the EVA low foaming extrusion molding process is that the melt should flow smoothly through the cavity to form a smooth and not easy to break its appearance. The smoothing agent has an effect on the surface finish of the cavity, the mixing and distribution of the foaming gas, and thus the cells of the EVA foam.
In the extrusion formula of EVA foam, these three smoothing agents are often required to exist together. The author has done a lot of tests on this. Smoothness has a great influence on the quality of the extruded product, the energy consumption of the extrusion process, and the length of the mold cleaning cycle. Extruding products of the same quality has a difference in energy consumption of 30～, and a difference of 1～5 times in cleaning cycle. It can be seen that the smoothing agent is very effective in the extrusion process of EVA low-foaming cotton products.
EVA foam must be dried before thermoforming. Hook adhesive thermoforming requires heating the EVA material. The heating process can be carried out in an oven, heating plate or infrared heater. The duration of heating depends on the thickness of the sheet (approximately 1mm/min).