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The common causes and solutions of breaking bubbles in PVC foam board

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The common causes and solutions of breaking bubbles in PVC foam board

Date:2020-03-25 Author: Click:

 Poor thermal stability of melt, improper setting or control of extrusion temperature


Good plasticization of the melt is the precondition of PVC foam board products. If there is a problem with the heat stabilizer and the extrusion temperature is too high, the partial degradation of the melt is easy to occur and it will be torn due to the decrease of the melt strength and the excessive bubble; if the extrusion temperature is too low, the melt is under plasticized and the strength is very low, the bubble will also be broken. During the production of PVC foam board, it is necessary to regularly check the stability performance of the stabilizer at 180-200 degrees. The raw materials that do not meet the requirements of stability performance shall not be used or the dosage Bathroom plateshall be adjusted, and they shall be reused after passing the inspection.


In addition to ensuring that the melt does not degrade in the extrusion process, the stabilizer also has an important function of regulating the decomposition temperature of foaming agent. If the amount of stabilizer is too much or too little, the decomposition temperature of foaming agent is too low or too high, it is not conducive to foaming. The stabilizer shall be adjusted according to the brand of resin. For example, if type 8 resin is used, the plasticizing temperature is low, and the stabilizer shall be increased appropriately, so that the plasticizing temperature of the material coincides with the decomposition temperature of the foaming agent. However, if the stabilizer is added too much or the melt temperature is too high, the foaming agent will decompose in advance in the extruder, and the foaming gas will escape from the charging hole and the vacuum hole. If the stabilizer is added too little or the operating temperature is too low, the foaming will be incomplete.


Solution: during extrusion operation, in addition to ensuring the good plasticization of the melt, the melt temperature in the extruder must be lower than the decomposition temperature of the foaming agent to prevent the premature decomposition of the foaming agent in the extruder; the melt temperature of the outlet mold must reach the decomposition temperature area of the foaming agent to facilitate the full foaming. The setting and control of extrusion temperature should also be adjusted in time according to the vacuum hole material and the melt forming shape during startup. Ensure that the material is basically orange peel when passing through the vent hole, and there shall be no powder flow at the bottom of the screw; when the melt is extruded from the die, the surface shall be smooth and elastic to a certain extent, and it is not allowed to sag or crystallize rough on the cross section once the die is exported.


2、 Low molecular weight or polymerization degree


PVC resins can be divided into three types: emulsion method PVC, suspension method PVC and bulk PVC according to their polymerization methods. In the production of rigid PVC foam products, if emulsion PVC resin is used, the products with smooth pore and smooth surface can be obtained, but the dimensional stability of the products is difficult to control. Moreover, the cost of producing PVC resin by emulsion method is relatively high. If the suspension PVC resin is used, the appearance quality and uniformity of the foams will be slightly worse.


In terms of comprehensive process, price and performance, it is advisable to mix them in a certain proportion, which can be between 80 / 20-20 / 80. The cross-sectional area of the plate forming is relatively wide, and the die from the extruder head to the mouth should be distributed to a width equivalent to about 1.3m. In order to obtain fully foamed low-density plastic products, the resin viscosity used should not be too high. In order to facilitate the dispersion and expansion of bubbles, the pressure of the whole cross-sectional area of the plate should be consistent, and the requirements for melt fluidity are high. If the viscosity of PVC resin is too high, the melt fluidity is poor, the flatness of the board surface is difficult to guarantee, and the bubble hole is not easy to expand, resulting in low foaming ratio; otherwise, if the viscosity of PVC resin is too low, it will cause low melt strength and easy to cause foam breaking.


Solution: sc-7 resin should be selected for the production of skinned PVC foam board, and sc-8 resin should not be selected as far as possible, or sc-8 resin should be mixed with sc-5 or sc-6 resin.


3、 Improper addition of foaming agent


The production of PVC foam board generally uses three kinds of different foaming agents: heating type, endothermic type or endothermic and exothermic compound equilibrium type. Ammonium azodicarboxylate, also known as AC, belongs to the azo group activator. The decomposition temperature of AC foaming agent is as high as 232 ℃, which is far beyond the processing temperature of PVC, so the decomposition temperature should be reduced when using.


Exothermic foaming agent has high foaming rate, about 190-260ml / g, fast decomposition speed and great exothermic, but it has short foaming time and strong suddenness. Therefore, when the amount of AC foaming agent is too much, the air volume is too large, which will make the pressure in the bubble increase rapidly, the bubble size growth is too large, the gas will be released rapidly, the bubble structure will be destroyed, the distribution of the bubble size is uneven, and even the pore structure will be formed, which will produce large bubbles and holes in the local area. In the production of foamed plastic products, exothermic foaming agent AC shall not be used alone. It shall be used in combination with endothermic foaming agent or compound chemical foaming agent with heat and exothermic balance. Sodium bicarbonate (NaHCO3), an inorganic foaming agent, is an endothermic foaming agent. Although the foaming rate is low, the foaming time is long. It can play a complementary and balanced role when mixed with AC foaming agent. The exothermic foaming agent can improve the gas generating ability of the endothermic foaming agent. The endothermic foaming agent can make the former cool, stabilize its decomposition and release gas in a balanced way, inhibit the internal overheating degradation of the thick plate, reduce the precipitation of residues, and have a whitening effect.


Solution: under the premise of not affecting the foaming rate, the endothermic foaming agent can be added appropriately to replace some exothermic foaming agent, so as to inhibit the foam breaking caused by the addition of exothermic foaming agent. 1232 or bla-616 foaming agent is an exothermic and endothermic equilibrium foaming agent, which has no induction period and fast decomposition rate. It can reach a large gas volume in about 10 minutes. The gas release is slow and no sudden, and the large gas volume reaches 156ml. Its decomposition temperature is in the range of PVC processing temperature, which can be used in the dynamic forming process of products with thick size and complex shape, so as to eliminate the foam breaking and ensure the stability of foaming performance.


4、 Improper use of mould


According to the thickness of the products, the length and compression ratio of the straight section of the die are different. Thick plate mouth


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