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Brief introduction of rigid polyurethane foaming (coarse MDI)

2020-07-16 12:04:47
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preface

The basic production method of rigid polyurethane foam is room temperature foaming, the molding process is relatively simple. According to the degree of construction mechanization, it can be divided into manual foaming mechanical foaming. According to the foaming pressure, it can be divided into high-pressure foaming low-pressure foaming. According to the molding method, it can be divided into pouring foaming spraying foaming. According to the specific application field product shape, the pouring foaming can be divided into block foaming, molding foaming insulating shell pouring. According to the foaming system, it can be divided into HCFC foaming system, pentane foaming system water foaming system. According to continuous production, it can be divided into batch process continuous process. Batch process is suitable for small batch production. The continuous process is suitable for large-scale production, the production line method has high efficiency. It can be divided into one-step method prepolymerization method ( semi prepolymerization method) according to whether prepolymerization is needed in operation steps.

Brief introduction of rigid polyurethane foaming (coarse MDI)

1. manual foaming mechanical foaming can be molded by manual casting without a small number of foaming machines molds small amount of foam products. The labor productivity of manual foaming is low, the utilization rate of raw materials is low. The yield is also low. In order to develop new formula, routine testing formula debugging of raw material system should be carried out in laboratory before production, i.e. manual foaming test. In production, this method is only suitable for small-scale temporary construction, producing a small amount of unshaped products making some foam plastic samples.

Manual foaming can be divided into several steps:

(1) The formula is determined, the volume of the product is calculated, the material consumption is calculated according to the density. According to the total material consumption of the product, the excess of 5% - 15% is generally required.

(2) Clean the mold, apply release agent preheat the mold.

(3) Weighing, mixing, pouring, curing demoulding. The mixing steps of manual pouring are as follows: after accurately weighing various raw materials, polyols auxiliaries are pre mixed, polyols premix polyisocyanates are put in different containers, then the raw materials are mixed evenly, immediately injected into moulds empty spaces filled with foam plastics, then foam plastics are obtained after chemical reaction foaming. In China, manual foaming still plays an important role in some small medium-sized factories. Manual pouring is also the basis of mechanical pouring. However, in the case of large batch many molds, manual pouring is appropriate. Mass production, large-scale construction, the general use of foaming machine mechanized operation, high efficiency.

2. one-step process pre polymerization method, at present, rigid polyurethane foam is produced by one-step process, that is, all kinds of raw materials are mixed foaming. In order to facilitate the production, many manufacturers mix polyether polyols other polyols, catalysts, foam stabilizers, foaming agents other raw materials together, call them "white materials". When used together with coarse MDI (commonly known as "black material"), it is still a "one-step method" because it does produce chemical reactions before mixing foaming. The early rigid polyurethane foam was produced by prepolymerization. This is because the polyisocyanate raw material used at that time was tdi-80. Because the viscosity of TDI is small, it does match the viscosity of polyols; TDI is volatile at high temperature; the reaction heat of TDI with polyols water is large; if one-step production is difficult, prepolymerization method is often used. If all TDI is reacted with polyols, the isocyanate terminated prepolymer has high viscosity is inconvenient to use. The prepolymerization in the production of hard foam is actually "semi prepolymerization". In other words, TDI reacted with some polyols, the NCO content of prepolymer was generally 20% - 25%. Due to the large excess of TDI, the viscosity of the prepolymer is low. The prepolymer is then mixed with polyester polyether polyols, foaming agents, surfactants, catalysts, etc., rigid foamed plastics are prepared through foaming reaction. The advantages of prepolymerization are: foaming relaxation, low foam center temperature, suitable for moulded products. The disadvantages are: complex steps poor material fluidity, which are suitable for thin-walled products complex products. Since the development of MDI, TDI has basically been no longer used as raw material for rigid foams, the prepolymerization method has been replaced by one-step method. Casting foaming is a common molding method of rigid polyurethane foam, that is to mix all kinds of raw materials evenly, then inject them into the cavity of the mold parts. Manual foaming mechanical foaming can be used for pouring of rigid polyurethane foam, intermittent continuous foaming methods can be used for mechanical foaming. The principle of mechanical casting foaming is similar to that of manual foaming. The difference is that various raw materials are successively weighed into the container for mixing, while the mechanical pouring foaming is that the raw materials are continuously input into the mixing chamber of the foaming machine by the metering pump according to the formula proportion for rapid mixing. The rigid foam casting method is suitable for producing block hard foam, rigid foam molding products, filling foam in the cavity of the workpiece, other on-site pouring foam. Block hard foam molded foamed block rigid foam refers to the large size rigid foam block, which can be produced by gap casting continuous foaming machine. After cutting, the blocky hard foam is made into a certain shape product. Molded rigid foam generally refers to the rigid foam products directly poured into the mold. The production method of block hard foam is similar to that of continuous method box type foam. A certain amount of solid powder paste can be added to the raw material



The foaming process is roughly as follows: polyols, foaming agents, catalysts other raw materials are accurately measured placed in a container for premixing. After the isocyanate is added, the mixture is fully mixed immediately. The reactive materials with fluidity are injected into the mold, which is chemically reacted foamed. The advantages of box type block foaming process are less investment greater flexibility. One mold can produce two hard bubbles per hour. The disadvantages are high loss of raw materials low labor productivity.

Molding foaming is foaming in a closed mold with a certain strength (such as a closed box). The density is determined by the formula dosage the set mold volume. It is generally used to produce some small hard foam products, such as whole skin hard foam, structure hard foam, etc. It is required that the foam mold can bear a certain pressure in the mold. The overfilling amount of raw materials depends on the required density skin quality. Large bulk foam generally needs to be mixed poured with foam machine. Both high low pressure foaming machines are available. In mechanical foaming, the Milky time of foaming material is much shorter than that of stirring mixing. Therefore, it is better to use large output foam machine to produce large foam plastics. The process of continuous production of hard block foam is similar to that of soft foam. The principle appearance of foaming machine used are similar to those of soft foam production. For example, planibloc flat top foaming device is also suitable for the production of block hard foam. The catalyst for casting foaming is mainly amine catalyst. Delayed amine catalyst can be used to extend the Milky time to meet the filling requirements of mold. This kind of catalyst can improve the fluidity of raw material system, but does affect its solidity. The isocyanate index is slightly higher than 100, such as 105. In the process of pouring foaming, the raw material temperature ambient temperature directly affect the quality of foam plastic products. The suitable ambient temperature is 20-30 ℃, the raw material temperature can be controlled at 20-30 ℃ a little higher. It is easy to get high quality products when the temperature is too high too low. If it is difficult to control the ambient temperature for large products such as ships vehicles, the raw material temperature can be properly controlled the amount of catalyst can be adjusted. The requirements of the die are reasonable structure, easy disassembly, light weight, pressure resistance, good surface finish. At the same time, according to the size of the mold different shapes, drill multiple vent holes in the appropriate position.

The material of the mould is usually aluminum alloy, sometimes steel mould is also used. The mold temperature directly affects the speed of reaction heat removal. Low mold temperature, low foaming rate, high product density thick skin; high mold temperature is the opposite. In order to produce high-quality foam plastic products, the mold temperature is generally controlled at 40~50 C. When the material is mild the mold temperature is low, the chemical reaction is slow, the foam curing time is long, the curing time is short when the temperature is high. In the injection mold foaming, the mold the product should be put together in a high temperature environment for curing before demoulding, so that the chemical reaction can be completed. If the mold is released too early, the ripening will be enough the foam will deform. The aging time temperature are different with the variety of raw materials the shape size of parts. The moulded foam must be solidified by 10min before the mold can be released. Because of the short mixing time, the mixing efficiency should be paid attention to. The agitator should have enough power rotation speed for manual pouring foaming. Mixing evenly, the pores of the foam are thin uniform, the quality is good, the mixing is good, the foams are coarse uneven, even the chemical composition within the local range does meet the requirements of the formulation, which greatly affects the quality of the products. The spray forming of rigid polyurethane foam is that the two groups of composite materials are quickly mixed directly sprayed on the surface of the object to form foam. Spraying is an important construction method of polyurethane rigid foam, which can be used in the construction of thermal insulation layer in the fields of cold storage, grain depot, residence workshop roof, wall, storage tank so on.

The advantages of spray foaming molding are: no mould is needed; whether it is on the horizontal vertical surface the top surface, whether it is on the surface of simple object complex surface, the rigid polyurethane foam plastic insulation layer can be formed by spraying method; the labor productivity is high; the rigid polyurethane foam plastic obtained by spray foaming has no seams has good heat insulation effect. It has a certain waterproof function. Low pressure high pressure spraying are generally divided into low-pressure spraying high-pressure spraying according to the pressure of spraying equipment, high-pressure spraying foaming can be divided into pneumatic hydraulic high-pressure spraying processes according to the type of medium providing pressure. Low pressure spray foaming is made up of two kinds of raw materials, namely "white material" (i.e. polyether) black material (i.e. aggregate MDI), which are extracted the raw material barrel transported to the nozzle of the spray gun by the plunger pump. Then the mixture of black white two raw materials is blown out the nozzle of the spray gun by compressed air.

The disadvantages of low-pressure spray foaming are: high loss of raw materials environmental pollution; black white raw materials are easy to cause blockage of gun nozzle pipeline, the nozzle should be cleaned manually every time the machine is shut down; in addition, the pressure of compressed air is unstable, the mixing effect is sometimes good bad, which affects the foaming quality the spraying surface is smooth. But the price of low pressure spray foaming equipment is higher the press is lower. Generally speaking, the low pressure spray foaming construction is to start the air compressor first, adjust the air pressure flow to the required value, then start the metering pump to start spraying construction. The distance between the muzzle the sprayed surface is 300-500mm, the flow rate is 1-2kg / min, the moving speed of the spray gun is 0.5-0.8


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