Analysis and Countermeasures of Typical Causes of Foam Breaking of PVC Crust Foam BoardAnalysis and Countermeasures of Typical Causes of Foam Breaking of PVC Crust Foam Board
There are two main factors in the foaming or foaming of the foamed plastic plate section in theory: one is that the local strength of the melt itself is too low, and the foaming is formed from outside to inside; second, the pressure around the melt is biased Small, swollen local cells, weakened in strength, the bubble is formed from the inside out. In the production practice, there is no obvious difference between the two functions, which may exist at the same time. Most of the broken holes are caused by the uneven expansion of the local cells and the decrease in melt strength. The strength of the melt itself is too low and the pressure around the melt is too small. In summary, there are mainly the following aspects.
The molecular weight or polymerization degree of PVC resin is generally low according to its polymerization method can be divided into emulsion method PVC and suspension method PVC and bulk PVC. When producing rigid PVC foam products, if the emulsion method PVC resin is used, products with uniform cells and smooth surfaces can be obtained, but the dimensional stability of the product is difficult to control; and the production cost of the emulsion method PVC resin is higher; if the suspension method is used PVC resin, product appearance quality and cell uniformity are slightly worse.
In terms of comprehensive technology, price and performance, it is advisable to mix the two at 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.
Production practice has proved that SC-7 resin should be used for the production of skinned foamed plastic sheet, and SC-8 resin should not be used as much as possible, or SC-8 resin should be mixed with SC-5 or SC-6 resin.
Melt has poor thermal stability, improper extrusion temperature setting or control. Good melt plasticization is a prerequisite for foamed plastic sheet 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 foamed plastic sheets, 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.
During the extrusion operation, in addition to ensuring that the melt is well plasticized, the melt temperature in the extruder must be lower than the decomposition temperature of the blowing agent to prevent the premature decomposition of the blowing agent in the machine; the melt temperature of the outlet die must be To reach the decomposition temperature range of the blowing agent, in order 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 .
Insufficient addition of foaming agent The production of foamed plastic sheet generally adopts three different foaming agents, 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. The decomposition temperature should be lowered when using.
The exothermic foaming agent has a high foaming rate, about 190-260ml/g, with a fast decomposition rate 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.
Under the premise of not affecting the foaming rate, more endothermic foaming agents can be added as appropriate to replace part of the exothermic foaming agents, so as to suppress the foaming caused by the addition of the exothermic foaming agents. 1232 or BLA-616 foaming agent is an exothermic and endothermic balanced foaming agent. The decomposition has no induction period and the decomposition rate is fast. The maximum gas output can be reached in about 10 minutes. The gas release is slow, without sudden, and the maximum gas output Up to 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.
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