Why are PVC profiles would appear surface shrinkage?

September 9, 2019

Why are PVC profiles would appear surface shrinkage?
 

Shrink marks
The surface shrinkage marks of PVC profiles affect the appearance of the profiles and even affect the neatness of the clear angles. I have regretted the high quality doors and windows. The following is an analysis of the surface shrinkage phenomenon.

 

1. Shrinkage and vacuum: The same set of molds, when the vacuum is small or the vacuum hole of the mold is blocked, it is easy to cause shrinkage marks. After careful observation, when the vacuum degree is small or the vacuum hole of the dry sizing die is blocked, the outer surface of the profile cannot be closely attached to the sizing die surface, so the cooling effect is lowered, and the cooling in the dry sizing die is incomplete, resulting in post-cooling shrinkage marks.
 

2, shrinkage marks and mold flow path: the inner ribs are not easy to produce shrink marks, and the T-type flow path is easy to produce shrink marks. The degree of occurrence is related to the width ratio of the T-shaped two-way flow path. The ratio is T-type. The width of the cross flow channel is larger than the width of the vertical flow channel, and the larger the ratio, the less likely the shrinkage mark is generated.
 

3. Shrinkage and internal rib thickness: In actual production, it is found that the inner rib is too thick compared with the outer wall, which is easy to cause shrinkage marks. If the inner rib exceeds the outer wall thickness, the shrinkage marks are more obvious.

 

On the contrary, the thinner the inner rib is, the more difficult it is to cause shrinkage. mark. Specifically, when the thickness of the inner rib is less than 2/3 of the thickness of the outer wall, no shrinkage marks are generally generated. The analysis considers that this phenomenon is caused by uneven cooling and shrinkage. In the case of shaping cooling, the intersection of the inner and outer walls is larger than the other wall thicknesses, and the heat is high, so there is a post-cooling shrinkage. The thicker the inner rib, the greater the heat stored, and the more severe the post-contraction.
 

4. Shrinkage and barrel temperature: The same set of molds, under different process conditions, the degree of shrinkage marks on the surface of the T-shaped structure of the profile is obviously different. It has been found many times in production that increasing the melt temperature will reduce the shrinkage marks. degree.
 

For example, the wind cover of the sliding window is prone to shrinkage at the root of the card foot, especially when the temperature of the barrel is low and the melt temperature is low (190 ° C), the shrinkage marks are obvious. When the temperature of the barrel is increased and the melt temperature reaches 198 ° C, the shrinkage marks are not noticeable. This phenomenon was also observed on the outer surface of the 75 profile top groove structure.
 

This indicates that when the melt temperature is low, the plastic resin particles of the PVC resin are destroyed into a small amount of regional particle structure, the void volume in the melt is large, the melt is not dense enough, the thermal expansion rate is large, and the cooling shrinkage rate is large; for example, the melt temperature is increased. The plasticization of PVC resin particles is broken into a large number of regional particle structures, the void volume in the melt is small, the melt is dense, and the thermal expansion rate is small, so the cooling shrinkage rate is small.
 

Therefore, the degree of T-type surface shrinkage is related to the temperature of the melt. The essence of the T-shaped surface is the representation of the degree of plasticization of the resin particles into regional particles. Conversely, it is also verified that the microscopic morphology of the PVC particles is agglomerated structure, secondary structure particles, and regional particles. Constituted.
 

In summary, the section design of PVC profiles should pay attention to the thickness ratio of the inner and outer walls, and the wall thickness ratio of the T-shaped structure. The mold design inner channel should adopt a separate flow channel to improve the extrusion stability and avoid the fluctuation of the inner wall thickness. It should be noted that the dry sizing die is regularly cleaned to avoid clogging of the vacuum holes.

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