PC / ABS as the main material of automobile interior decoration and electronic and electrical shell has its irreplaceable advantages. However, in the process of injection molding, it is easy to lead to peeling phenomenon of product surface due to improper material itself, mold design and injection molding process.
Let's first look at the description of melt fracture: the polymer in plastic extrusion or injection molding has smooth surface and uniform shape when the fluid shear rate is low. When the shear rate or shear stress increases to a certain value, the surface of the extrudate will lose luster and the surface will be rough; when the extrusion rate of plastic increases again, there will be many irregular nodules, twist or slub lines on the surface of the extrudate, and even split and break into fragments or columns, which is called "melt fracture".
Picture: peeling plastic parts
These phenomena show that the small disturbance caused by various factors is restrained by melt at low shear stress or rate, but it is difficult to restrain the disturbance in the fluid at high shear stress or rate and develop into unstable flow; when a critical shear stress is reached, it will cause the fracture of flowing melt.
In general, when the shear rate of melt is more than 50000, PC / ABS materials will be prone to peeling. In addition, what other factors will affect the peeling of injection molded parts?
Material factor
Compared with other materials, the two-phase structure of PC and ABS is more prone to fluid fracture and separation under high shear, which leads to peeling phenomenon. For PC / ABS materials, PC and ABS are partially compatible, so it is necessary to add appropriate compatibilizer in the modification process to improve its compatibility. Of course, we need to eliminate the skin defect caused by mixing.
Mold factor
The principle of die design should follow the direction of minimizing shear. Generally speaking, the products with dense textured surface are more prone to peeling (caused by the friction and shear of 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 lead to excessive shear when the melt passes through the gate, and then lead to the peeling of the product surface.
Technological factors
The main direction is to avoid excessive shear. When it is difficult to fill the product, high speed and high pressure can be used to improve. However, high-speed and high-pressure will lead to excessive shear force of melt at gate I, and shear between melt and inner wall of mold and between core layer and skin layer will increase sharply. Therefore, in the actual injection process, we can also consider increasing injection temperature / mold temperature and improving material fluidity to reduce the flow resistance in the actual filling process Avoid excessive shear due to high speed and high pressure.
