Meaning
Internal mechanical tension remains within a plastic part when the cooling process during injection molding is uneven or too rapid. This residual mold stress can lead to the warping, cracking, bowing or dimensional instability of the component over time. It is a latent defect that may not be visible immediately after production but appears when the part is exposed to heat or mechanical load.
Structural Integrity
Engineers measure the level of internal tension using polarized light or chemical stress testing to ensure the part meets safety standards. Excessive residual mold stress reduces the impact resistance of the plastic and makes it prone to sudden failure under pressure.
Failure Mode
Components used in high precision assemblies (such as electronic housings or medical valves) are particularly sensitive to these internal forces. If the residual mold stress is too high, the part may change shape slightly during storage, making it impossible to assemble or causing the final device to malfunction.
Acceptance Standard
Procurement agreements for molded parts include specific limits on allowable deformation and internal tension. The presence of residual mold stress above the agreed limit is grounds for the rejection of a batch because it compromises the long term reliability of the product. Manufacturers can reduce these stresses by optimizing the mold temperature and the duration of the cooling cycle.
This process adjustment increases the cycle time and the unit cost but prevents the much higher expense of field failures. A well controlled molding process ensures that the molecular structure of the polymer reaches a stable state before the part is ejected from the tool. Stability is required.
