The reasons and solutions of the foam breaking of PVC foam board
The reasons and solutions of the foam breaking of PVC foam boardThe application of PVC foam board is relatively important in the current use, which is mainly because the product's use effect is very good. However, there are good and bad use of the product. For example, the foaming of the PVC foam board is a condition that affects the quality of the product. If you don’t pay attention, it will directly affect the use of the product afterwards.
If the local strength of the PVC foam board is too low due to its own melt, the resulting foam will be formed from the outside to the inside; due to the low pressure around the melt, the local cells will expand, the strength will weaken, and the foam will break It is formed from the inside out. Moreover, in production practice, the two effects may exist at the same time, most of the broken holes are caused by the uneven expansion of the local cells, and the melt strength is reduced. The strength of the melt itself is too low and the pressure around the melt is too small, and the PVC foam board currently used is one of the largest plastic products in the world today, and has a wide range of applications, only in the production of PVC foam board Holes often appear in the process, which has a certain impact on people's use. The following are the main reasons that affect the foaming of PVC foam board:
1. Poor thermal stability of the melt, improper extrusion temperature setting or control: good melt plasticization is the prerequisite for PVC foam board products. If there is a problem with the heat stabilizer, the extrusion temperature is too high, local melt degradation is easy to occur and will be torn due to reduced melt strength, too large bubbles; the extrusion temperature is too low, the melt is not plasticized, and the strength is very low , The same will appear bubble breaking. During the production of PVC foam board, the stability performance of the stabilizer must be regularly inspected at 180-200 degrees. Raw materials that do not meet the stability performance requirements should not be used or the dosage should be adjusted.
In addition to ensuring that the melt does not degrade during extrusion, the stabilizer also has an important function, which is to adjust the decomposition temperature of the blowing agent. If too much or too little stabilizer is added to cause the blowing agent decomposition temperature to be too low or too high, it is not conducive to foaming. The stabilizer should be adjusted according to the grade of resin used. For example, type 8 resin is used, the plasticization temperature is low, and the stabilizer should be appropriately increased so that the plasticization temperature of the material coincides with the decomposition temperature of the foaming agent. However, if too much stabilizer is added or the melt temperature is too high, it will cause the foaming agent to decompose in advance in the extruder, causing foaming gas to escape from the feed hole and the vacuum hole. If the stabilizer is added too little or the operating temperature If it is too low, the foaming will be incomplete.
The solution is to ensure that the melt is well plasticized during the extrusion operation, and also note that the melt temperature in the extruder must be lower than the decomposition temperature of the blowing agent to prevent the blowing agent from passing through the machine. Early decomposition; the melt temperature of the outlet die must reach the decomposition temperature region of the blowing agent to facilitate full foaming. The setting and control of the extrusion temperature should also be adjusted in time according to the material of the vacuum hole and the shape of the melt molding at startup. Ensure that the material is basically in orange peel when passing through the vent hole, and there should be no powder flow at the bottom of the screw; the surface of the melt when extruded from the die should be smooth and have certain elasticity, and it must not sag or rough crystal cross section .
2. The molecular weight or degree of polymerization of the product is too low
The PVC resin used can be divided into three kinds of emulsion method PVC, suspension method PVC and body PVC according to its polymerization method. In this way, when producing rigid PVC foam products, if the emulsion method PVC resin is used, the cells can be uniform and the surface is smooth. Products, but the dimensional stability of the products is difficult to control; and the production cost of the emulsion method PVC resin is higher; if the suspension method PVC resin is used, the appearance quality and cell uniformity of the product are slightly worse.
In terms of comprehensive technology, price and performance, the two should be mixed in a certain ratio, the ratio can be between 80/20-20/80. The forming cross-sectional area of the sheet is wide, and it should be distributed from the extruder head into the die to a width equivalent to about 1.3 meters. To obtain fully foamed low-density plastic products, the resin viscosity should not be too high. The dispersion and expansion of bubbles, the pressure of the entire cross section of the plate is consistent, and the melt fluidity is required to be high. If the viscosity of the PVC resin is too high, the melt fluidity is poor, the flatness of the board is difficult to ensure, and the cells are not easy to expand, resulting in low foaming ratio; conversely, if the viscosity of the PVC resin is too low, it will cause the melt strength to be low, which is likely to cause breakage. bubble. The solution is to use SC-7 resin for the production of skinned PVC foam board. Try not to use SC-8 resin, or mix SC-8 resin with SC-5 or SC-6 resin.
3. The quality of PVC foam board is poor or the dosage is insufficient
Materials used in PVC foam board during the foaming process, the gas decomposed by the foaming agent used to form bubbles in the melt. In this way, among these bubbles, there is a tendency for cells with small volume to expand to cells with large volume. The size and amount of bubbles are not only related to the amount of blowing agent added, but also to the strength of the polymer melt. If the intensity is too low, the gas will easily escape after diffusing to the surface of the melt, and the small bubbles merge with each other to form large bubbles. The long molecular chain of the foaming regulator is wound and adhered to the molecular chain of PVC to form a certain network structure. On the one hand, it promotes the plasticization of the material, on the other hand, it improves the strength of the PVC melt, so that the cell wall can withstand the pressure of the gas in the cell during the foaming process, so as not to be broken due to insufficient strength. The foaming regulator can make the product cells small and many, the cell structure is more uniform and reasonable, and the density of the foam is greatly reduced. Poor quality or insufficient addition of the foaming regulator will cause the foam to have low strength and break or foam. It should also be pointed out in use that the molecular weight and viscosity of foaming regulators produced by different manufacturers are very different. When foaming products break or burst, and other methods of treatment are not effective, the foaming regulator will be replaced or increased appropriately. The dosage will often produce obvious effects. However, increasing or replacing the foaming regulator with a larger molecular weight will cause the bubbles in the melt to expand due to the excessive viscosity, which will increase the density of the product. And because the melt viscosity is too large, the fluidity becomes worse, resulting in uneven die output, which affects the smoothness of the board surface, and even the production time is not long, and the mouth blur material failure occurs, especially when producing plates less than 10mm thick. The solution of the editor is to produce different thickness plates according to the production, the dosage of the processing regulator should also be different according to the situation. Even if it is the same formula, the thin plate does not have cells, and when the thick plate shows cells, you can add 0.3 more -0.5 parts of processing regulator, only based on the elimination of foam breaking, added in small amounts, avoid ignoring the consequences, and blindly add more, so as not to increase the density of the product and affect the effective production time.
4. Improper addition of foaming agent
Because the production of PVC foam board generally adopts three different foaming agents of heat-generating type, endothermic type or endothermic and exothermic compound balance type. Ammonium azodicarboxylate, also known as AC, is an azo-based activator. The decomposition temperature of AC foaming agent is high, reaching 232, far exceeding the processing temperature of PVC, and the decomposition temperature needs to be lowered when using. The exothermic foaming agent has a high foaming rate, about 190-260ml/g, with a fast decomposition speed and a great heat release, but the foaming time is short and the suddenness is also strong. Therefore, when the amount of AC blowing agent is too large, the amount of gas generated will be too large, which will cause the pressure in the bubble to increase quickly, the size of the cell will grow too large, and the gas will be released rapidly, which will destroy the cell structure, unevenly distribute the cell size, and even form an open The pore structure will produce large bubbles and voids locally. When producing foamed plastic products, the exothermic foaming agent AC should not be used alone, and should be used in conjunction with the endothermic foaming agent or a compound chemical foaming agent that balances heat and heat. 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 complement and balance. The exothermic foaming agent improves the gas generating ability of the endothermic foaming agent. The endothermic foaming agent cools the former, stabilizes its decomposition and releases gas in a balanced manner, suppresses the overheating degradation inside the thick plate, and reduces the precipitation of residues. Has whitening effect.
In this way, without affecting the foaming rate, it is possible to add more endothermic foaming agent, instead of part of the exothermic foaming agent, to suppress the addition of the exothermic foaming agent. For example, 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. The maximum gas output can be reached in about 10 minutes. The amount of gas generated reaches 156mL. Its decomposition temperature is within the range of PVC processing temperature, which can be used in the dynamic molding process of thicker and complex shaped products, so as to eliminate foam breaking and ensure the stability of foaming performance.
5. Improper use of molds during production
In the production of skinned PVC foam board, the length of the straight section of the die and the compression ratio are also different according to the thickness of the product. The thick plate die has a thick foam layer and has elastic deformation space. The generally designed straight die has a longer straight section and a larger compression in order to improve the melt pressure and foaming ratio; the thin plate die has a thin foam layer. The elastic space is small and the rigidity is large. The straight section of the generally designed die is shorter and the compression is relatively small to prevent excessive melting pressure.
If the thick plate die is misused due to improper production of thin plates, the flow resistance of the outlet mold will increase and the discharge will be uneven, resulting in uneven surfaces, reduced melt strength, or even the plate is broken, and the production cycle is shortened; production Thick plate, if the thin plate die is misused, it will cause bubble breakage due to the small melt pressure of the outlet die, which will increase the cell size. In this case, we must do a good job of differentiation. Before the production of skinned PVC foam boards with different thicknesses, we must correctly select the die.
6. Too much calcium carbonate, too large particle size, poor activity
When too much CaCO3 is added, although more bubble cores can be formed, if too much calcium carbonate is added or the particles are too large, the activity is not good, forming agglomeration, dispersing in the resin or not combining well with the resin cross section, resulting in melt strength When it is lowered, bubbles will easily break when expanding in the melt.
Solution: PVC foam board production must strictly control calcium carbonate dosage, particle size and activity. When the amount of calcium carbonate is too large, the foam regulator should be increased accordingly.
7. PVC foamed board has a cross-section foaming or uneven discharge, and partial material shortage
The cross-section of the sheet is foamed or the material is uneven, and there are many factors affecting the local material shortage, which runs through the entire process of mixing and extrusion. For example, the composition of the formula is unreasonable, the outer slip is too little, the temperature of the 5th zone of the extruder is easy to overheat, which leads to high temperature of the confluence core, large bubbles, string bubbles, yellowing, rough surface of the plate and other defects in the middle of the plate; single pot mixing Too much material, too low mixing temperature, too short mixing time, too little internal slip addition, it is easy to cause uneven dispersion of the components of the mixture. Poor fluidity and improper adjustment of die temperature or bolts during the extrusion process will cause the melt to be extruded from the die, the discharge is uneven, and the material is partially lacking, causing the foamed melt to expand from the weak link and break the bubble. Therefore, in the process of compounding and extrusion production, it is necessary to carry out recipes and process operating procedures, and conduct corresponding analysis on bubble breaking, and use different methods to deal with them. If the bubble breaking is always fixed in the same position, it means that the melt pressure in this part is too low, and the corresponding adjustment can be solved by using die bolts or temperature.
8. Adjusting the gap difference of the stereotyped template of each section of the PVC foam board also has a certain effect on eliminating foam cells. If the gap between the first styling plate and the second styling plate is too large, the melt will be pressed more tightly under the pressure of the styling template without sufficient cooling, resulting in an increase in density and foaming; if the third The gap between the template and the fourth template is too large. Due to the sufficient cooling of the melt, there is no room for compression and deformation between the templates, which makes it difficult for the fourth template to be in place and the thickness of the plate increases. The appropriate increase in the gap between the second and third blocks of the template can effectively prevent the foam melt from breaking before cooling, on the other hand, the third template can also be pressed into place at a certain temperature to prevent the plate Increased thickness. Secondly, when producing thick plates, appropriately reducing the screw temperature, die oil temperature, cooling water temperature of the first setting device, etc., also have a corresponding effect on eliminating foam breaking.
The foam breaking of PVC foam board is affected by various uses, not only to pay more attention when using it, but also to know more about the product when selecting the product, so that it can be used in the future. Better use effect.
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