PVC foam board, also known as Chevron board and Andy board, is widely used in passenger cars, train car roofs, box core layers, etc. However, foam breaking sometimes occurs in the production process, so PVC foaming What are the reasons for the bubble burst? Let me take you to find out.
In production practice, most of the broken pores are caused by the reduction of melt strength after the uneven expansion of local cells. There are many factors for the low strength of the melt itself and the low pressure around the melt. To sum up, it is mainly manifested in the following aspects:
According to the polymerization method, PVC resin can be divided into three types: emulsion method PVC, suspension method PVC, and bulk PVC. When producing rigid PVC foam products, if the emulsion method PVC resin is used, a product with uniform cells and a smooth surface 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 high; if the suspension method is used PVC resin, the appearance quality and cell uniformity of the product are slightly worse. In terms of comprehensive technology, price, and performance, it is advisable to mix the two in a certain ratio, and the ratio can be between 80/20-20/80. The cross-sectional area of the plate forming is wide, and the width from the extruder head to the die should be distributed to a width equivalent to about 1.3 meters. To obtain fully foamed plastic products, the viscosity of the resin used should not be too high. In dispersion and expansion, the pressure of the entire cross-section of the sheet is coordinated, 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 surface is difficult to ensure, and the cells are not easy to expand, resulting in a low foaming ratio; on the contrary, if the viscosity of the PVC resin is too low, the melt strength will be low. Bubble.
If the mold is used improperly, the thin plate is produced, and the thick plate die is misused, the flow resistance of the exit mold material will be large, and the discharge will be uneven, resulting in an uneven surface, reduced melt strength and even the plate will be broken, and the production cycle will be shortened; In the production of thick plates, if the thin plate die is misused, the melting pressure of the exit die will be too small, which will cause the bubbles to be large and cause bubble breakage. During the foaming process of the material, the gas decomposed by the blowing agent forms bubbles in the melt. In these bubbles, there is a tendency for small cells to expand to larger cells. The size and number of bubbles are not only related to the amount of foaming agent added, but also to the polymer melt strength. If the strength is too low, the gas is easy to escape after diffusing to the surface of the melt, and the small bubbles merge to form large bubbles. The long molecular chain of the foaming regulator is entangled 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, and 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 rupture due to insufficient strength. The foaming regulator can make the product cells smaller and more, the cell structure is more uniform and reasonable, and the density of the foam is greatly reduced. Poor quality or insufficient addition of foaming regulators will result in low foam strength and bubble breakage or bubble stringing.
The above are some of the reasons for the broken foam of PVC foam board, I hope it can be helpful.