Discussion on the core breaking of the hottest pol

2022-08-09
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Compared with ordinary rubber materials, polyurethane elastomer has the advantages of high strength, excellent wear resistance, adjustable hardness and so on. When used to replace rubber products, it can greatly prolong the service life of products and reduce the use cost. The service life of rice roller made of polyurethane material can be extended by 3 ~ 5 times than that of rubber roller. Therefore, in recent years, rice processing plants are gradually paying attention to it and winning a certain market. At present, the total annual output of several domestic polyurethane rice roller manufacturers is only tens of thousands, which is not commensurate with the annual demand of nearly one million rice rubber rollers in our large rice production and processing country. China also exports a considerable number of rice milling cots and rice processing machinery every year. Polyurethane cots have been widely used abroad

polyurethane cots have a bright future. Polyurethane rubber roller is composed of iron roller core and cast polyurethane elastomer coating. In the manufacturing process of polyurethane cots, if the process formula is not handled properly, problems such as coring may occur during the use of the cots. Combined with years of research and production accumulation, the author discusses the main factors causing the core separation of polyurethane rice roller

1 choice of adhesive

according to the theory of bonding valence bond, valence bond and van der Waals force can be formed between polymer materials with high molecular polarity and the surface of metal materials, so as to form a close bond between materials and metals

pour polyurethane liquid raw materials around the metal roll core to form a certain bonding force between the polyurethane elastomer and the roll core, but under the dynamic working state of the rice milling roll, a large shear force will be generated between the polyurethane elastomer and the roll core, which has a high requirement for the bonding strength of polyurethane and metal. Through practical tests, the primer (adhesive) containing multiple isocyanate groups is coated on the roll core, which has a good effect on enhancing the adhesive force. This is because polyisocyanate can react with polyurethane materials and form a rigid adhesive layer, which SABIC believes is integrated with the iron roll core. The isocyanate group, benzene ring and carbamate group in the bonding layer interact with the iron atoms and their oxides on the surface of the metal roll to form chemical bonds or secondary valence bonds (such as hydrogen bonds), resulting in high bonding strength between polyurethane and metal

2 treatment of iron core

treatment of cast iron roll core is also one of the keys to prevent rubber roll from coring. To ensure a good combination between polyurethane elastomer and iron core, the iron core usually needs to be cleaned, such as acid pickling for rust removal, organic matter for oil stain removal, etc., to ensure that its surface is clean and free of dirt. Mechanical sand blasting is usually one of the effective methods to improve the bonding strength. Through sand blasting, the surface roughness of the roll core is increased, and the surface area of the bonding interface can be increased several times or even dozens of times, which correspondingly improves the adhesion between polyurethane and metal iron core. The uneven surface produced by sand blasting will produce more bite connections between polyurethane and iron core and improve the bonding strength

the iron core should be kept dry after treatment to prevent rusting again. Especially in summer, the water in the air will form a layer of water film on the surface of the treated iron core, causing the iron core to rust. The water adsorbed on the surface of the iron core will also react with isocyanate, forming carbon dioxide bubbles in the bonding interface layer, which will greatly reduce its bonding strength

3 influence of mold and iron core temperature

in the pouring process of polyurethane material liquid, the temperature of iron core and mold is required to be close to that of polyurethane material liquid. When the temperature difference between the two is large, the curing speed of the material will be uneven, resulting in large internal stress after curing of polyurethane material. Due to the low strength of the just cured polyurethane, the internal stress in the elastomer will be concentrated on the joint surface of the iron core after demoulding, forming more cracks and local coring, and obvious cracks will appear in serious cases

in actual production, the temperature difference between polyurethane castable and iron core and mold can be controlled within 10 ℃

4 requirements for mold design and demoulding

when designing and manufacturing the mold, it is required to have good sealing, easy to close, open and remove the film. When designing the mold, we should also consider the heat balance of polyurethane, which is an important factor that is easy to be ignored by manufacturers. If the rubber roll is unevenly heated during high-temperature vulcanization, incomplete vulcanization of polyurethane, low strength of elastomer, or cracks usually occur at the junction of iron core and polyurethane. Improper demoulding of vulcanized cots will also cause cots to lose core. Because the strength of polyurethane just vulcanized at high temperature is not high. In case of excessive force or strong impact when moving during demoulding, cracks and invisible coring will occur

5 influence of synthetic process

synthetic process is one of the key links in production. Here is some experience about the process of mdibd polyurethane system

(1) viscosity of prepolymer the viscosity of prepolymer is very important for the production of polyurethane rice roller. In order to obtain prepolymer with low viscosity, the isocyanate index of prepolymer prepared by MDI and polyester polyol is usually greater than 2.5. Otherwise, it is difficult to defoaming in vacuum due to the increase of viscosity of prepolymer, and when the prepolymer with high viscosity is mixed with chain extender 1,4 butanediol (BD), it is easy to mix more bubbles and cannot be eliminated

(2) reaction time and temperature the preparation reaction time of prepolymer should be controlled within 2 ~ 3H, and it is not suitable to react for a long time. The reaction temperature is strictly controlled below 100 ℃. After MDI is added, the reaction temperature should be controlled to avoid excessive heat accumulation caused by exothermic reaction and rapid rise of system temperature. Vacuum defoaming should be completed within 1.5h

(3) when mixing temperature and time, the temperature of prepolymer and chain extender BD is usually between 60 ~ 90 ℃. The temperature difference between the two should not be too large. If the temperature of mixed materials is low, the viscosity is high, and bubbles that cannot be eliminated are easy to appear in the adhesive layer, which may cause coring or cracking; If the temperature is high, the viscosity of the mixed liquid rises too fast, and it is difficult to pour, so there is no time to eliminate bubbles in the later stage. When mixing prepolymer and BD, avoid mixing bubbles as much as possible, and the mixing time should not exceed 90s

6 influence of pouring process

improper method in the pouring process of rubber roller will form bubbles on the surface of iron core and reduce the bonding strength. Due to the limited pouring time, it is required to shorten the pouring time as much as possible, and ensure the uniform injection to prevent excessive injection. Otherwise, bubbles in the material and improper pouring will produce bubbles on the surface of the iron core, greatly reducing the bonding strength. During pouring, ensure the fluidity of polyurethane casting material for a period of time after entering the mold, so that the residual bubbles float up. Some manufacturers use rubber rollers to pressurize when the elastomer is just formed, so that the combination of the two is closer. Of course, this will improve the bonding strength between the iron core and polyurethane, but the pressurization time should be selected, otherwise the opposite effect will be achieved

in theory, pressurization is carried out when polyurethane is still plastic, and the screw holding force, that is, the three-dimensional cross-linking between macromolecules has not yet formed. The operation of plastic granulator touches a wide range of areas of the national economy. If the pressure is advanced, a large amount of materials will overflow, and grooves will appear at the upper end of the rubber roll after forming, which will become defective products; If the pressurization lags behind, cracks will appear on the surface of the rubber roller in the light, and large cracks will be formed in the heavy, which will be discarded due to the displacement between the polyurethane and the iron core when it is under pressure. These are all operational skills that need to be mastered through experience accumulation. As long as you observe the "gel point", it is not difficult to master

6 conclusion

application of fixture to the production process of high-efficiency urethane rice roller is complex, there are many factors affecting product quality, and the operation skill is strong. However, as long as we strictly follow the operating procedures and pay attention to the above aspects in production, it is not difficult to produce rice milling Cots with high qualification rate. Production workers should have a good foundation of chemical knowledge and good production and operation skills and experience, which is necessary to ensure the quality of cots

source: electronic dehydration Forum

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