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【Yuanbo Plastics Tips】Why skinning occurs during plastic injection molding?

【Yuanbo Plastics Tips】Why skinning occurs during plastic injection molding?

【Yuanbo Plastics Tips】Why skinning occurs during plastic injection molding?

  • Categories:Industry knowledge
  • Author:Xiao Cheng
  • Origin:
  • Time of issue:2021-04-14 14:34
  • Views:

(Summary description)When the plastic extrusion speed increases again, the surface of the extrudate will show a large number of irregular nodules, distortions or slub patterns, and even split and break into fragments or column segments, which we call "melt fracture" .

【Yuanbo Plastics Tips】Why skinning occurs during plastic injection molding?

(Summary description)When the plastic extrusion speed increases again, the surface of the extrudate will show a large number of irregular nodules, distortions or slub patterns, and even split and break into fragments or column segments, which we call "melt fracture" .

  • Categories:Industry knowledge
  • Author:Xiao Cheng
  • Origin:
  • Time of issue:2021-04-14 14:34
  • Views:
Information

In the process of plastic extrusion or injection molding, the extrudate through the die or gate has a smooth contour and an average shape when the fluid shear rate is low.

 

When the shear rate or shear stress increases to a certain value, the extrudate will be bright and exquisite on the outside;

 

When the plastic extrusion speed increases again, the surface of the extrudate will show a large number of irregular nodules, distortions or slub patterns, and even split and break into fragments or column segments, which we call "melt fracture" .

 

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These phenomena explain:

Under low shear stress or speed, small disturbances caused by various components are threatened by the melt;

However, under high shear stress or velocity, the disturbance in the fluid is difficult to suppress, and it will move smoothly;

When a critical shear force is reached, it will cause the fluid to break.

When a critical shear force is reached, it will cause the fluid to break.

 

How to deal with it?

1. Materials

The rupture of the fluid under high shear causes the skinning phenomenon of the manufactured product. Taking PC/ABS raw materials as an example, the two-phase relationship between PC and ABS is more prone to fluid discriminating and two-phase separation under high shear. Then stratified and peeling weather occurred.

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For PC/ABS raw materials, there is a small part of compatibility between the two modified plastics of PC and ABS. Therefore, a certain amount of compatibilizer can be added during the modification process to improve its compatibility. Of course, the bad peeling caused by the mixture is the first step we need to eliminate.

 

2. Mold design

The principle of mold design should follow the purpose of reducing shear. Products with dense skin textures are more prone to skinning (due to the high shear force of the melt in the cavity and the inner wall of the cavity during high-speed filling); at the same time, in the gate design, if the gate size is too small, it will cause As a result, high shear forces are generated when the melt passes through the gate, which in turn causes the surface of the product to peel. The mold design must follow the direction of minimizing shear.

 

Three, injection molding process

High speed and high pressure will cause the melt to produce excessive shear force at the gate position, and the shear between the melt and the inner wall of the cavity and the shear between the melt core layer and the skin layer will also increase sharply;

In addition, the injection temperature/mold temperature is too low, which reduces the fluidity of the melt and increases the flow resistance during the filling process, resulting in high shear forces. Therefore, reducing the injection speed, injection pressure, and increasing the injection temperature/mold temperature can effectively reduce the shear force and reduce or eliminate the occurrence of skinning. Try to avoid the high shearing force during the filling process, which is the best entry point to improve the peeling phenomenon of injection molding.

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