Unstable flow: Macromolecular chains will be completely straightened under the action of extremely high shear rate (≥106s-1), and will show great elasticity if they continue to deform, which will lead to unstable laminar flow, and the melt will fall into elastic disorder state, and the flow rates at all points will interfere with each other. This phenomenon is usually called unstable flow of polymer melt. Melt fracture: after the polymer melt passes through the runner in the die in an unstable state, it will become uneven in thickness and dull, and rough shark skin will appear on the surface. In this case, if the shear stress or shear rate continues to increase, the melt will appear wavy, slubby or periodic spiral, and more seriously it will break into irregular fragments or small cylindrical blocks, which is called melt fracture. Solution: ① Adjust the linear velocity of melt in the barrel of injection molding machine; (2) When the melt causes flow instability, increasing the temperature can increase the ultimate stress and shear rate; (3) In the transition section from large cross-section to small cross-section, reducing the convergence angle of the flow passage and streamlining the surface of the transition section can improve the ultimate shear rate of the unstable flow.