A bonded thermoformed assembly can fail adhesively at the plastic surface, cohesively within the adhesive, or through the plastic itself. Identifying the failure plane is essential.
Oil, release agent, dust, moisture, low-surface-energy materials, incompatible solvents, residual forming stress, poor joint fit, and incorrect cure conditions are common causes. A cleaner that improves wetting may simultaneously attack a stressed polymer.
Corrective action
The plastic grade, adhesive, primer, and cleaning process must be treated as a qualified system. Surface preparation should be controlled from forming through assembly so contamination is not reintroduced. Bond-line thickness and clamping should match the adhesive design.
Failed samples should be examined for the actual failure mode. Increasing adhesive quantity will not correct chemical incompatibility or a joint that continuously peels under thermal expansion.