Meaning
Internal deformation of the polymer network occurs when molecular chains are locked into stretched configurations during rapid solidification. This localized stress is microstructural strain, which develops during the high shear conditions of the filling stage. It creates regions of high energy within the part that are susceptible to chemical attack and cracking.
Moulding Dynamics
High melt temperatures and slow injection speeds allow polymer chains to relax before they freeze. When relaxation does not occur, high microstructural strain is locked into the part, particularly near the gates where shear rates are highest. Annealing can be utilized to relieve these internal stresses after moulding.
Failure Mechanism
Environmental stress cracking occurs when a moulded part is exposed to chemicals that penetrate the strained polymer matrix. Regions with high microstructural strain have weaker intermolecular forces, making it easier for solvents to initiate microcracks. Microcracks propagate under low loads, leading to premature part failure in the field.
Tooling Impact
Sharp corners and sudden transitions in wall thickness increase shear rates and concentrate stresses during filling. Modifying the tool design to include generous radii reduces the local shear and minimizes microstructural strain in critical areas. The design change ensures more uniform physical properties throughout the moulded component.
Stress distribution analysis prevents the concentration of forces that leads to crack initiation during assembly or high-load operations.
