Meaning
Structural separation of material layers on molded plastics results from high localized stress during mold filling. The shear induced skin delamination causes the outer layer of the molded part to peel away from the core. This defect occurs primarily in thin walled parts.
Defect Mechanism
High injection speeds generate extreme shear rates between the flowing polymer melt and the cold mold wall. When these shear rates exceed the melt strength of the polymer, shear induced skin delamination occurs as the frozen skin layer is torn from the hot, flowing core. This separation is aggravated by low melt temperatures which reduce interdiffusion between the layer interfaces.
Material Influence
Incompatible polymer blends or excess moisture can significantly increase the risk of surface peeling. When regrind or contaminated resin is used, the shear induced skin delamination is often triggered at lower shear rates due to the poor interfacial adhesion between the different plastic phases. Resin drying and material purity are essential to prevent this weak boundary formation.
Process Optimization
Adjustments to molding parameters are necessary to eliminate surface separation defects. Molders can reduce shear induced skin delamination by increasing the mold and melt temperatures, which lowers the viscosity of the flowing polymer and reduces the shear stress generated during filling. Alternatively, slowing the injection speed or widening the gate geometry decreases the flow velocity, allowing the polymer molecules to relax and form a cohesive, uniform structure across the part cross section.
This molecular relaxation prevents the formulation of distinct, unbonded boundary layers that are susceptible to mechanical peeling when the finished part experiences external stresses or friction during demolding and subsequent assembly operations.
