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May 29, 2023
Explanation of Polyurethane PU Sole Forming Process
  Polyurethane (PU) microporous elastomer sole material achieved industrialization in Europe in the late 1960s, entered the US market in the 1970s and has been rapidly developing globally since the 1980s. The sole made of microporous polyurethane elastomer has the characteristics of high strength, good elasticity, comfort, wear resistance, folding resistance, oil resistance, corrosion resistance, lightweight, and anti-slip. It has a buffering effect on the ground impact and can be made into various colors. The main varieties of soles are leather shoes, off-road skiing shoes, sandals, slippers, sports shoes, travel shoes, and anti-static, oil-resistant safety shoes.
1、 Production process
(1). Preparation of PU sole raw materials:
PU sole raw materials can be divided into two types: polyester and polyether. In the early years, the system developed was a polypropylene glycol system, but now it is mostly a polyester-type system. Due to their different properties, the preparation methods are also different. The preparation of polyester-type raw materials is mostly done using the prepolymer method or semi-prepolymer method, and can generally be made into two or three components. Silicone for PU shoe sole XH-1193.
Component A is composed of some polyester, chain extender, foam stabilizer, and foaming agent, and is obtained by static degassing after mixing for 40e~70e. In a full water foaming system, the foaming agent is water, and the water content in component A must be determined, with a general content of around 0.4%.
Component B is a prepolymer of isocyanate-terminated polyester polyols obtained by reacting with isocyanates. The relative molecular weight of polyester should be between 1500 and 2000. The ratio of pure MDI to liquefied MDI in isocyanates is 19B1. During the reaction process, one-thousandth of an inhibitor must be added to prevent the occurrence of side reactions. Holding for 2 to 3 hours at a certain temperature is sufficient, with NCO% controlled at around 19%.
Component C is the catalyst (when added to component A, it becomes a dual-component stock solution). The three-component system is suitable for dual-color, low-hardness sports shoes, and low-density sandals.
(2). PU sole forming process:
PU soles are generally formed by low-pressure or high-pressure casting, with a few also using injection molding. The molding equipment is a sole pouring machine. The atmospheric pressure pouring equipment used for polyester PU molding mainly consists of pouring machines, circular or rotary drying channels, and other devices. Due to the intense mixing reaction of components A and B in the PU sole stock solution, the accurate measurement of equipment and the uniformity of component mixing are two important factors that directly affect product performance during molding. For dual-color soles, they are molded using a dual-color pouring machine, usually using a mold with an additional intermediate plate, and undergoing secondary pouring and heating curing.
2.  matters needing attention:
(1). Raw material storage and pretreatment:
The raw materials for shoe soles are usually prepared into three components: A, B, and C, which are mainly composed of polyols, isocyanates, and catalysts. Their storage periods under room temperature and sealed conditions are 6 months, 3 months, and 6 months, respectively. During storage, attention should be paid to moisture and light protection. Strictly prohibit exposure to sunlight and rain, and pay attention to ventilation. Before use beyond the shelf life, analysis, and sample foaming test must be conducted to check for failure. The raw materials should be heated and melted before use.
(2). Color matching:
The color paste should be added to component A and thoroughly stirred before use to disperse evenly. The catalyst component is usually added to component A in the production plant. But because organic catalysts are prone to hydrolysis, and sometimes due to different production conditions and environments in different regions. The catalyst can be treated as a separate component to facilitate regulation. Catalysts should be added to component A before use. After stirring for at least 10-20 minutes, add component A to the working tank of the foaming machine.
(3). Ratio:
The correctness of the raw material component ratio is the most critical condition to ensure product quality, therefore, daily production must test and check the accuracy of the foaming machine's metering ratio. When adding new raw materials or measuring pump errors with significant fluctuations, special attention should be paid to monitoring and checking the component ratio measurement. The measurement of raw materials by foaming machines is mostly based on volume, which is directly related to viscosity and even temperature. Therefore, in production, the pressure of each raw material must be strictly controlled, and the temperature must be within the predetermined range. Otherwise, it will seriously affect the accuracy of component measurement.
(4). Mold:
Shoe sole molds are usually made of aluminum alloy and steel materials, and in recent years, resin molds have been widely used. The pattern of the mold cavity must be clear, and special attention should be paid to the design of deep concave-convex patterns during mold design. It must be conducive to the discharge of bubbles, avoid sharp angle concave-convex patterns, and if necessary, change to a circular arc transition. The best angle for a direction parallel to the bending direction should be around 45 °C. Previously, some people believed that the finished sole should not be too thick, generally ranging from 6 to 8mm and usually not exceeding 12mm; But now we have fully mastered the casting of soles with a thickness greater than 12mm. When adding inserts such as wood cores, the wood cores must be dry. The mold temperature should be controlled between 45~55 ℃.
(5). Measurement:
According to the shape and density requirements of the sole, the injection amount of foaming material should be adjusted appropriately. When pouring high-heel soles, the heel part should be appropriately raised when installing the mold. When pouring, first pour a portion of the material in the front and then pour the heel (so that the foam center is not concentrated at a single point), and the injection amount should be between 40% and 70%; When pouring flat top shoes, it is advisable to lift the sole of the mold slightly upwards. Adjust the appropriate angle of the mold on the production line, and adjust the mold tilt angle accordingly according to the shape of the shoe sole to avoid defects such as cavities and bubbles on the surface of the shoe sole.
(6). Forming time:
The forming time of polyurethane sole is generally 5-7 minutes. At this time, the performance of polyurethane sole has not yet reached its optimal level, and excessive force should not be used during demolding. The semi-finished product after demolding should be left at room temperature for 1 day, and the demolding agent adhered to the surface of the product should be cleaned within 24 hours to facilitate processing after color spraying.
3. Facility conditions:
(1). Production site: 8m wide, 20m long, approximately 160 m2, with corresponding raw materials, semi-finished products, and finished product warehouses. Due to the lightweight equipment and large floor area, it can be placed upstairs (for easy installation and movement, the equipment does not have foundation screws). If there is a spray painting area for producing PU sole; If producing continuous upper shoes, additional equipment and places for sewing and shaping the upper should be added.
(2). Materials: The raw materials should be kept at around 0.5-1 tons, with A material of 18kg per barrel and B material of 20kg per barrel. Although the amount of C material is small, it is not easy to deteriorate and can be prepared at around 10kg. Additionally, 50kg of release agent, a large bucket of cleaning agent (dichloromethane), and several color pastes need to be prepared.
(3). The electricity consumption is about 50kw. If the production line oven does not require electric heating, the electricity consumption is about 20kw.
(4). One set of air compressor (with a gas transmission capacity of 0.6m3/min, a pressure of 0.6-0.8MPa, and a power of 5.5kw) is used as the main air source and for spraying release agent.

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