Analysis and suggestion of shrinkage mark on surface of PVC profile

December 20, 2017


The surface shrinkage marks on the surface of PVC profiles affect the appearance of the profile, and even affect the neat angle surface. It is a pity that the high quality level of doors and windows. The following analysis of the phenomenon of surface shrinkage marks is made.



1, shrinkage mark and inner rib thickness: in actual production, it is found that the inner rib is thicker than the outer wall, which is easy to cause shrinkage marks, and the inner rib is thicker than the outer wall, so the shrinkage mark is more obvious. On the contrary, the thinner the inner rib is, the more difficult it is to produce shrinkage marks. To be exact, when the thickness of the inner reinforcement is less than the 2/3 of the thickness of the outer wall, the shrinkage marks are generally not produced. It is considered that this phenomenon is due to the uneven cooling and contraction. When the cooling is done, the heat dissipation at the intersection of the inner and outer walls is larger than that of the other walls, and the cooling capacity is high, so there is a cooling shrinkage.  The thicker the inner tendons, the greater the heat storage and the more severe the back contraction.   



2, shrinkage mark and screw barrel temperature: the same mold set, under different process conditions, the degree of shrinkage mark on T section of profile section is obviously different. In production, many times have been found that increasing melt temperature will reduce the degree of shrinkage mark. For example, the location of the root of the push-pull window is prone to shrink marks, especially when the temperature of the screw barrel is low and the melt temperature is low (190 degrees), the shrinkage mark is very obvious. When the temperature of the cylinder is raised and the melt temperature reaches 198, the shrinkage mark is not obvious. This phenomenon is also found on the outer surface of the 75 profile grooves. This shows that the melt temperature is low, the PVC resin particle plasticizing particle structure as the number of damaged area, large void volume in the melt, the melt is not dense enough, thermal expansion, and thus the cooling shrinkage rate is high; as the increase of melt temperature, the quantity of PVC resin particles into particle structure damage plastic melt more, void volume small, dense melt, thermal expansion and cooling shrinkage rate is small, small. Therefore, the degree of T surface shrinkage is related to the melt temperature. Its essence is the performance of resin particles being plasticized into regional particles. In turn, it is also verified that the microstructure of PVC particles is the aggregation structure, the two structure particles and the regional particles.  


3, shrinkage mark and vacuum degree: the same set of mold, vacuum or vacuum mould in small hole blocked, it is easy to cause shrinkage mark.  After careful observation, vacuum or small dry mold vacuum hole blocked, the outer surface is not only compact profile mold, the cooling effect is reduced, the dry mould cooling is not complete, resulting in cooling shrinkage mark.   


4, shrink mark and mold runner: inner rib alone is not easy to generate channel shrinkage mark, T type channel binding shrinkage mark, the degree and type T two channel width handover ratio, ratio of type T cross channel width than vertical channel width, the ratio is higher, less shrinkage mark.


To sum up, the section design of profiles should pay attention to the ratio of inner and outer wall thickness to the wall thickness ratio of T type structure. A separate channel should be adopted to improve the stability of the extrusion and avoid the thickness fluctuation of the inner reinforcement. Attention should be paid to the regular cleaning of the dry mold to avoid the plugging of the vacuum hole.

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