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Analysis and Countermeasures of typical causes of foam breaking of PVC skinned foam board

Visualizações:3 Horário de publicação: 2022-01-12 : Louis Lu

Analysis and Countermeasures of typical causes of foam breaking of PVC skinned foam board

Theoretically speaking, there are two main factors for foam breaking or penetrating in the section of foamed plastic plate: one is that the foam breaking is formed from the outside to the inside because the local strength of the melt itself is too low; Second, because the pressure around the melt is too small, local bubbles expand, the strength decreases, and the broken bubbles are formed from the inside to the outside. In production practice, there is no obvious difference between the two effects, and they may exist at the same time.  Most of the holes are caused by the reduction of melt strength after the uneven expansion of local bubbles. There are many factors that the strength of the melt itself is too low and the pressure around the melt is too small.  To sum up, there are mainly the following aspects.


Low molecular weight or degree of polymerization


According to the relevant information, PVC resin 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 the emulsion PVC resin is used, the products with uniform pore surface and smooth surface can be obtained, but the dimensional stability of the products is difficult to control. Moreover, the cost of emulsion PVC resin production is high. If suspension PVC resin is used, the appearance quality and cell uniformity of the product are slightly poor.


In terms of comprehensive process, price and performance, the two should be mixed in a certain proportion, which can be between 80 / 20-20 / 80. The forming cross-sectional area of the plate is wide.  The width from the extruder head into the die should be distributed to a width equivalent to about 1. 3m.  In order to obtain fully foamed low-density plastic products, the resin viscosity should not be too high.  In order to facilitate the dispersion and expansion of bubbles, the pressure of the whole cross-sectional surface of the plate is coordinated and has high requirements for melt fluidity. If the viscosity of PVC resin is too high, the melt fluidity is poor, the flatness of plate surface is difficult to ensure, and the cell is not easy to expand, resulting in low foaming ratio; On the contrary, if the viscosity of PVC resin is too low, it will lead to low melt strength and easy to cause bubble breaking.


Production practice has proved that sc-7 resin should be selected for the production of skinned foamed plastic sheet, 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.


High performance plastic foam board has excellent performance and has a wide range of applications.


Poor melt thermal stability, improper extrusion temperature setting or control


Good plasticization of melt is the precondition of foamed plastic sheet products. If there is a problem with the heat stabilizer, the extrusion temperature is too high, the melt is prone to local degradation, and will be torn due to the decrease of melt strength and too large bubbles; Bubble breaking also occurs when the extrusion temperature is too low, the melt is under plasticized and the strength is very low. During the production of foamed plastic plates, the stability of the stabilizer must be inspected at 180 ~ 200 degrees regularly.  Raw materials that do not meet the stability requirements shall not be used or used after adjusting the dose and passing the inspection.


In addition to ensuring that the melt does not degrade during extrusion, the stabilizer also has an important function, that is, adjusting the decomposition temperature of foaming agent. If too much or too little stabilizer is added, resulting in too low or too high decomposition temperature of foaming agent, it is not conducive to foaming. The stabilizer shall be adjusted according to the brand of the resin used.  For example, 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 is consistent 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, resulting in the escape of foaming gas from the feeding hole and vacuum hole.  If the stabilizer is added too little or the operating temperature is too low, the foaming will be incomplete.


During extrusion operation, in addition to ensuring good plasticization of the melt, the melt temperature in the extruder must be lower than the decomposition temperature of the foaming agent to prevent 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, so as to facilitate 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 at startup. Ensure that the material is basically orange peel when passing through the exhaust hole, and there should be no powder flow at the bottom of the screw; When the melt is extruded from the die, the surface shall be smooth and have a certain elasticity.  It is not allowed to sag or crystallize rough in the section as soon as the die is exported.


Improper addition of foaming agent


The production of foamed plastic sheet generally adopts three different foaming agents: heating type, endothermic type or endothermic and exothermic composite equilibrium type. Ammonium azodicarboxylate, also known as AC, is an azo activator. The decomposition temperature of AC foaming agent is high, up to 232 ℃, far exceeding the PVC processing temperature.  It is necessary to reduce the decomposition temperature during use.


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 burst. Therefore, when the amount of AC foaming agent is too much and the gas generation is too large, the pressure in the bubble will increase rapidly, the cell size will grow too large, the gas will be released rapidly, the cell structure will be damaged, the cell size distribution will be uneven, and even the open cell structure will be formed, which will produce large bubbles and cavities locally. When producing foamed plastic products, exothermic foaming agent AC shall not be used alone, but shall be used in combination with endothermic foaming agent or composite chemical foaming agent with heat and exothermic balance. Inorganic foaming agent sodium bicarbonate (NaHCO3) is an endothermic foaming agent.  Although the foaming rate is low, the foaming time is long.  When mixed with AC foaming agent, it can play a complementary and balanced role. Exothermic foaming agent improves the gas generating capacity of endothermic foaming agent.  Endothermic foaming agent cools and stabilizes the decomposition and balanced release of gas, inhibits the internal overheating degradation of thick plate, reduces the precipitation of residues, and has whitening effect.


On the premise of not affecting the foaming rate, more endothermic foaming agents can be added to replace some exothermic foaming agents, so as to inhibit the foam breaking caused by more exothermic foaming agents. 1232 or bla-616 foaming agent is an exothermic and endothermic equilibrium foaming agent.  There is no induction period for decomposition, and the decomposition rate is fast.  The maximum gas generation can be reached in about 10 minutes, with slow deflation and no burst.  The maximum gas generation is 156ml. Its decomposition temperature is within the range of PVC processing temperature.  It can be used in the dynamic forming process of products with thick size and complex shape, so as to eliminate foam breaking and ensure the stability of foaming performance.


Poor quality or insufficient dosage of processing regulator


During the foaming process of the material, the gas decomposed by the foaming agent forms bubbles in the melt. In these bubbles, there is a trend that small bubbles expand to large ones. The size and number of bubbles are related not only to the amount of foaming agent, but also to the strength of polymer melt. If the strength is too low, the gas is easy to escape after diffusing to the melt surface, and small bubbles merge with each other to form large bubbles. The long molecular chain of foaming regulator is wound and adhered to the molecular chain of PVC to form a certain network structure. On the one hand, it can promote the plasticization of materials, on the other hand, it can improve the strength of PVC melt, so that the cell wall can withstand the pressure of gas in the cell during the foaming process and will not break due to insufficient strength. The foaming regulator can make the product have small and more foam cells, make the foam cell structure more uniform and reasonable, and greatly reduce the density of the foam. Poor quality or insufficient dosage of foaming regulator will lead to low strength of foam, broken foam or string foam.


It should be pointed out that the molecular weight and viscosity of foaming regulator produced by different manufacturers are very different.  When foam products break or string bubbles and other methods are ineffective, it will often produce obvious results to replace the foaming regulator or increase the dose appropriately. However, increasing or replacing the foaming regulator with higher molecular weight will make the bubbles in the melt unable to expand and increase the product density due to too large viscosity. And because the melt viscosity is too large, the fluidity becomes poor, resulting in uneven die discharge, affecting the flatness of the plate surface, and even the production time is not long, resulting in die paste failure, especially when producing plates less than 10mm thick.


When producing plates with different thickness, the dosage of processing regulator should also be different according to the situation.  In the same formula, when there are no bubbles in the thin plate and bubbles in the thick plate, 0. 3-0. 5 parts of processing regulator can be added appropriately.  It is only based on the elimination of bubble breaking.  It is forbidden to add more blindly regardless of the consequences, so as not to increase the product density and affect the effective production time.


Too much calcium carbonate is added, the particle size is too large, and the activity is not good


When CaCO3 is added too much, more bubble cores can be formed, but if calcium carbonate is added too much or the particles are too large, the activity is not good, agglomeration is formed, dispersion in the resin or poor combination with the resin cross section, resulting in reduced melt strength and bubble breaking when bubbles expand in the melt.


The dosage, particle size and activity of calcium carbonate must be strictly controlled in the production of foamed plastic plates. When the dosage of calcium carbonate is too large, the foaming regulator should also be increased accordingly.


Improper use of mold


The length and compression ratio of the straight section of the die are also different according to the thickness of the products. Due to the thick foaming layer and elastic deformation space of thick plate die, the straight section of die is long and the compression ratio is large, so as to improve the melting pressure and foaming ratio; Due to the thin foaming layer, small elastic space and large rigidity of thin plate die, the straight section of die in general design is short and the compression is small, so as to prevent excessive melting pressure.


In case of improper use, production of thin plate and misuse of thick plate die, due to increased flow resistance of outlet die and uneven discharge, the surface is uneven, melt strength is reduced, and even the plate is broken, and the production cycle is shortened; If the thin plate die is misused in the production of thick plate, the bubble will be broken due to the small melting pressure of the outlet die, which will increase the bubble.


Before the production of skinned foamed plastic plates with different thickness, the die must be selected correctly.


Foaming or uneven discharge of plate section and local shortage of material


There are many factors affecting the foaming or uneven discharge of plate section and local shortage, which runs through the whole process of mixing and extrusion. For example, the formula components are unreasonable, the external sliding is too small, and the temperature in zone 5 of the extruder is easy to exceed the temperature, resulting in high confluence core temperature, large bubbles, string bubbles, yellowing, rough plate surface and other defects in the middle of the plate; Too much mixing in a single pot, too low mixing temperature, too short mixing time and too little internal sliding addition are easy to cause uneven dispersion of mixture components. Poor fluidity and improper adjustment of die temperature or bolts during extrusion production will lead to uneven discharge and local lack of material when the melt is extruded from the die, resulting in expansion and bubble breaking of foamed melt from weak links. Therefore, in the process of mixing and extrusion production, the formula and process operation procedures must be implemented, and the bubble breaking shall be analyzed and treated by different methods. If the broken bubble is always fixed at the same position, it indicates that the melt pressure at this position is too low.  It can be solved by adjusting the die bolt or temperature accordingly.


In addition to the above corresponding measures, adjusting the gap difference between the shaped formwork of each section also plays a certain role in eliminating bubbles. If the gap difference between the first sizing plate and the second sizing plate is too large, the melt will be extruded more tightly under the pressure of the sizing template due to insufficient cooling, resulting in increased density and bubble breaking; If the gap between the third template and the fourth template is too large, there is no room for compression deformation between the templates due to sufficient cooling of the melt, resulting in the difficulty of the fourth template in place and the increase of plate thickness. On the one hand, properly increasing the gap between the second template and the third template can effectively prevent the foam melt from breaking before cooling.  On the other hand, the third template can also be pressed in place at a certain temperature to prevent the thickness of the plate from increasing. Secondly, in the production of thick plate, appropriately reducing the screw temperature, die oil temperature and the cooling water temperature of the first shaping device also has a corresponding effect on eliminating bubble breaking.