Polyurethane (PU) microporous elastic sole material has been industrialized in Europe since the late 1960s, entered the US market in the 1970s, and has developed rapidly worldwide since the 1980s. This sole made of microporous PU elastomer is famous for its high strength, excellent elasticity, comfort, wear resistance, folding resistance, oil resistance, corrosion resistance, light weight and skid resistance, which can effectively cushion the ground impact, and can be made in a variety of colors and different styles. Including but not limited to dress shoes, cross-country ski shoes, beach shoes, indoor shoes, sports shoes, travel shoes and anti-static, oil-resistant safety shoes.

Production technology

Preparation of raw materials for PU sole

Raw materials can be divided into polyester type and polyether type two categories. In the early days, the polypropylene glycol system was mainly used, and now the polyester system is mostly used, and the preparation methods are also different due to the differences in performance.

Polyester PU sole raw materials are usually prepared by prepolymer method or semi-prepolymer method, which can be made into a two-component or three-component system.

Component A is composed of some polyester, chain extender, foam stabilizer and foaming agent, etc., which is prepared by mixing evenly 40e~70e and then standing degassing. In the all-water foaming system, the foaming agent is water, and the moisture in component A must be accurately measured, and the content is usually about 0.4%.

The B component is a terminal isocyanate prepolymer obtained by the reaction of some polyester polyols with isocyanate. The relative molecular weight of polyester is 1500~2000. In the selection of isocyanate, the ratio of pure MDI to liquefied MDI was 19:1. In the reaction process, it is necessary to add 1/10,000 of the inhibitor to prevent side reactions, maintain a certain temperature for 2 to 3 hours, and control the NCO% at about 19%.

Group C is divided into catalysts (added to component A is a two-component raw material), and the three-component system is suitable for two-color, low-hardness sports shoes and low-density beach shoes.

PU sole forming technology

PU soles are generally molded by low pressure casting or high pressure casting, and a few use injection molding. Molding equipment for sole casting machine. The normal pressure pouring equipment for polyester PU molding is mainly composed of pouring machine, ring or turntable drying channel, etc. Since the A and B components of PU sole raw materials are liquid, and the mixing reaction is violent, the accurate measurement of the equipment and the mixing uniformity of the components are two important factors that directly affect the performance of the product during the molding process. For two-color soles, they are usually molded using a two-color casting machine, using a mold with an external intermediate plate, and are poured twice and cured by heat.

Matters needing attention

Raw material storage and pretreatment

Sole raw materials are usually supplied in the form of composite materials, with polyols, isocyanates and catalysts as the main components of A, B, C three components. They can be stored at room temperature and sealed for 6 months, 3 months and 6 months respectively. During the storage period, should be moisture-proof, avoid light, strictly prohibit exposure to sunlight, rain, and pay attention to ventilation. After expiration date, analysis and small foam test must be carried out before use to check for failure. Raw materials should be heated and melted before use.

Color matching

The color paste should be added to component A and fully stirred before use to ensure uniform dispersion; The catalyst component is usually added to the A component in the production plant, but because the organic catalyst is easy to hydrolyze, and sometimes because of the production conditions and environmental differences in different places, the catalyst can be used as A component alone, easy to adjust, the catalyst should be added to the A component before use, stirring at least 10 to 20 minutes, and then the A component is added to the working tank of the foaming machine.

matching

The accuracy of the ratio of raw material components is the key to ensure product quality, therefore, the daily production must be tested to check the accuracy of the measurement ratio of the foaming machine. When the new raw material is added or the error fluctuation of the metering pump is large, it is especially necessary to pay attention to the monitoring and inspection of the component ratio measurement. The measurement of raw materials is mostly based on volume, and the volume is directly related to viscosity and temperature, so the pressure and temperature of each raw material must be strictly controlled in the production, otherwise the accuracy of component measurement will be seriously affected.

mould

The sole mold is usually made of aluminum alloy and steel materials, and in recent years, the use of resin molds has become very common. The mold cavity pattern must be clear, and special attention should be paid to the deep concave and convex pattern design in the mold design, which must be conducive to the discharge of bubbles, avoid the sharp Angle concave and convex pattern, and should be changed to a circular arc transition if necessary. Parallel to the direction of bending, the best Angle should be about 45°. In the past, some people think that the finished sole should not be too thick, generally 6 to 8 mm is appropriate, usually not more than 12 mm; However, the technology of casting soles with a thickness of more than 12 mm has been fully mastered. When adding inserts such as wood cores, the wood cores must be dry. Mold temperature should be controlled at 45 to 55℃.

measure

According to the shape and density requirements of the sole, the injection amount of foaming material should be appropriately adjusted. When pouring the bottom of the high heel, the heel part should be appropriately raised when installing the mold. When pouring, the first part of the material in the front can be poured after the heel (so that the foaming center is not concentrated in one point), the injection amount should be between 40% and 70%; When pouring flat top shoes, the sole part of the mold should be slightly lifted upward. Adjust the proper Angle of the mold on the production line, and adjust the inclination Angle of the mold according to the shape of the sole to avoid defects such as holes and bubbles on the surface of the sole.

Forming time

The molding time of PU soles is generally 5 to 7 minutes, at this time, the performance of PU soles has not reached the best state, and the force should not be too large when demoulding. The semi-finished product after release should be placed at room temperature for 1 day, and the release agent attached to the surface of the product should be cleaned within 24 hours to facilitate subsequent spray processing.