Typical cause analysis and Countermeasures of blis

Analysis and Countermeasures of typical reasons for blister breaking of PVC skinned foamed board

there are two main factors in theory: first, the local strength of the melt itself is too low, and the blister is formed from the outside to the inside; Second, because the pressure around the melt is too small, local bubbles expand and the strength is weakened, and the broken bubbles are formed from the inside to the outside. In production practice, there is no obvious difference between the two functions, which 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

molecular weight or polymerization degree is too low

according to relevant information, PVC resin can be divided into lotion PVC, suspension PVC and bulk PVC according to its polymerization method. When producing rigid PVC foamed products, if PVC resin by lotion method is used, products with uniform pores and smooth surface can be obtained, but the dimensional stability of products is difficult to control; And the production cost of PVC resin by lotion method is high; If suspension PVC resin is used, the appearance quality and cell uniformity of the product are slightly poor

in terms of comprehensive technology, price and performance, the two should be mixed in a certain proportion, which can be between 80//80. The forming cross-sectional area of the plate is relatively wide, and the width from the extruder head to the die should be distributed to a width equivalent to about 1.3 meters. 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 area of the plate is coordinated, and the melt fluidity is required to be high. If the viscosity of PVC resin is too high, the melt fluidity is poor, the flatness of the board is difficult to ensure, and the bubble is not easy to expand, resulting in the foaming times, the coke price in Xingtai, Hebei will be 20 yuan/ton lower; 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 plates, and sc-8 resin should not be selected as far as possible. It is expected that by 2017, sc-8 resin will be used, or mixed with sc-5 or sc-6 resin

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

good plasticization of melt is the prerequisite for foaming plastic sheet products. If there is a problem with the heat stabilizer, the extrusion temperature is too high, it is easy to cause local degradation of the melt, and it will be torn due to the reduction of melt strength and too large bubbles; If the extrusion temperature is too low, the melt is not plasticized, and the strength is very low, bubbles will also break. During the production of foamed plastic plates, the stability of the stabilizer must be regularly tested at 180~200 degrees. Raw materials that do not meet the stability requirements must not be used or reused after adjusting the dosage 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, to adjust the decomposition temperature of the foaming agent. If the stabilizer is added too much or too little, causing the decomposition temperature of foaming agent to be too low or too high, it is not conducive to foaming. The stabilizer should be adjusted according to the brand of the resin used. For example, if type 8 resin is used, the plasticizing temperature is low, and the stabilizer should be appropriately increased, 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, causing the foaming gas to escape 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 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 should be smooth and elastic to a certain extent, and it is not allowed to sag or have rough cross-section crystallization as soon as the die is exported

improper addition of foaming agent

three different foaming agents are generally used in the production of foamed plastic plates: 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 ℃, which is far beyond the processing temperature of PVC. It is necessary to reduce the decomposition temperature when using it

exothermic foaming agent has high foaming rate, about ml/the quality of printing material plays a leading role in the appearance quality of products. G, it has 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, the gas generation is too large, which will cause the pressure in the bubble to increase rapidly, the cell size to grow too large, and the gas will be released rapidly, which will destroy the cell structure, uneven distribution of cell size, and even form an open cell structure, which will produce large bubbles and cavities locally. When producing foamed plastic products, exothermic blowing agent AC shall not be used alone, but shall be used in combination with endothermic blowing agent or composite chemical blowing 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 gassing capacity of endothermic foaming agent, which cools and stabilizes the decomposition and balanced release of gas, inhibits the internal overheating degradation of thick plates, reduces the precipitation of residues, and has whitening effect

on the premise of not affecting the foaming rate, more endothermic blowing agents can be appropriately added to replace some exothermic blowing agents to inhibit the bubble breaking caused by adding more exothermic blowing agents. 1232 or bla-616 foaming agent is an exothermic and endothermic balanced foaming agent. There is no induction period for decomposition, and the decomposition rate is fast. In this way, the maximum gas generation can be achieved in about 10 minutes to form a virtuous cycle. The gas release is slow, there is no burst, and the maximum gas generation is 156ml. Its decomposition temperature is within the range of PVC processing temperature. It can be used in the dynamic molding process of products with thick size and complex shape, so as to eliminate foam breakage and ensure the stability of foaming performance

poor quality or insufficient dosage of processing regulator

during the foaming process of materials, 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 not only related to the amount of blowing agent, but also related 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 shape structure. On the one hand, it promotes the plasticization of materials, on the other hand, it improves 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 rupture due to insufficient strength. Foaming regulator can make the foam cell of the product smaller and more, the 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 foaming body, broken foam or bubble string

it should be pointed out that the molecular weight and viscosity of foaming regulators produced by different manufacturers are very different. When foaming products have broken bubbles or bubbles, and other methods are ineffective, replacing foaming regulators or increasing the dosage appropriately will often produce obvious results. However, increasing or replacing the foaming regulator with higher molecular weight will make the bubbles in the melt unable to expand due to excessive viscosity, which will increase the density of the product. 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, causing 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. With the same formula, there are no bubbles in the thin plate, and when bubbles appear in the thick plate, you can add 0 5 parts of processing regulator, only based on the elimination of broken bubbles, should be added in a small amount. Do not add more blindly regardless of the consequences, so as not to increase the product density and affect the effective production time

when calcium carbonate is added too much, the particles are too large, and the activity is not good

caco3 is added too much, although more bubble cores can be formed, if calcium carbonate is added too much or the particles are too large, the activity is not good, forming agglomeration, dispersing in the resin or poor combination with the resin cross-section, resulting in the reduction of melt strength, and bubbles in the melt are easy to break when expanding

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

in the production of skinned foamed plastic plates, the length and compression ratio of the straight section of the configured die are also different according to the thickness of the products. The thick plate die has elastic deformation space due to the thick foaming layer. Generally, the straight section of the 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 the thin plate die, the straight section of the die in general design is short and the compression is small, so as to prevent excessive melting pressure

if it is used improperly, the thin plate is produced, and the thick plate die is misused, the flow resistance of the outlet die increases, and the discharge is uneven, resulting in uneven surface, reduced melt strength, even the plate is broken, and the production cycle is shortened; In the production of thick plates, if the thin plate die is misused, the bubble will be broken because the melting pressure of the outlet die is too small, which will increase the bubble

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

uneven foaming or discharging of plate section, local material shortage

uneven foaming or discharging of plate section, and there are many influencing factors of local material shortage, which runs through the whole process of mixing and extrusion. For example, the composition of the formula is unreasonable, the external slip is too small, and the temperature of zone 5 of the extruder is easy to overheat, resulting in high temperature of the confluence core, bubbles, yellowing, rough 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, causing the foaming melt to expand from the weak link and break the bubble. Therefore, in the process of mixing and extrusion production, we must implement the formula and process operation procedures, and conduct corresponding analysis on the bubble breaking, and adopt different methods to deal with it respectively. 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 finalized templates in each section also has a certain effect on eliminating blisters. If the gap between the first sizing plate and the second sizing plate is too large, the melt will be squeezed closer under the pressure of the sizing plate because it has not been fully cooled, 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 and deformation between the templates after sufficient cooling of the melt, resulting in the fourth template is difficult to be in place and the thickness of the plate increases. On the one hand, properly increasing the gap between the second and third templates can effectively prevent foaming melt from breaking before cooling, on the other hand, the third