Thermoforming against Injection for Volumes below Fifty Thousand Units
Thermoforming delivers lower total landed cost below fifteen thousand units by saving upfront tooling capital, beyond which injection piece price efficiency dominates.
Stress Cracking
Meaning
Premature mechanical failure occurs when a polymer is subjected to both tensile stress and a chemical agent simultaneously. This mechanism, known as stress cracking, results in brittle fracture at stress levels well below the yield strength of the material in air. It governs the service life of molded components like fluid reservoirs and pressurized caps.
The failure stops being a risk once the chemical agent or the tensile stress is removed.
Chemical Exposure
Certain fluids like surfactants, oils, and solvents accelerate the failure by localizing plastic deformation at the molecular level. In the presence of these agents, stress cracking occurs as the chemical molecules penetrate the amorphous regions and disentangle the polymer chains. This chemical action does not dissolve or chemically degrade the polymer itself.
Residual Stress
Molding parameters like high injection speed and low melt temperature lock in high levels of residual stress. This internal tension acts as the stress driver that initiates stress cracking even before any external load is applied to the part. Molders can reduce this risk by optimizing the cooling rate and using annealing processes.
Sourcing Remedy
High molecular weight resins with a high density or co-monomer content offer the best resistance to this failure mode. Specifying a resin with a low melt flow rate helps to prevent the crack propagation. Proper material selection is the most effective way to eliminate this defect.