Blistering is a phenomenon that occurs in metal cans, especially in the weakened areas of the pannelling, where bubbles form due to acid attack. These bubbles can compromise the integrity of the can and affect the quality and safety of the packaged food.

Blistering can have a variety of causes, which include:

1. Presence of moisture in the varnish during the thermal process: If the varnish used in the can contains moisture, it can generate vapor during the thermal process, creating pressure and forming bubbles in the weakened areas of the pannelling.
2. Poorly cured varnish: If the varnish is not properly cured during the drying process, a softer and less resistant surface layer may remain, which facilitates the formation of bubbles.
3. Too high oven temperature: If the oven temperature is too high, it can generate higher internal pressure in the can, which increases the risk of blistering.
4. Excessive time in the oven: If cans remain too long in the oven, especially at high temperatures, the likelihood of bubble formation increases.
5. Excessive humidity or inadequate density of the paper: If the paper used in the manufacture of the cans has a high humidity or inadequate density, it can generate adhesion problems with the varnish, which increases the risk of blistering.

To prevent or solve blistering, the following measures can be considered:

1. Control the humidity in the varnish: It is important to make sure that the varnish used is free of humidity before its application. In addition, the manufacturer’s recommendations for temperature and drying time should be followed to ensure proper curing.
2. Adjust oven temperature: It is essential to maintain an adequate oven temperature during the curing process. It is recommended that tests and adjustments be made to find the optimum temperature to avoid the formation of bubbles.
3. Control the time spent in the oven: It is important to prevent cans from remaining too long in the oven, especially at high temperatures. Appropriate curing times must be established for each type of can and varnish used.
4. Verify the quality of the paper used: It is necessary to make sure that the paper used in the manufacture of the cans meets the required specifications in terms of moisture and density. This will ensure proper adhesion to the coating and reduce the risk of blistering.
5. Perform quality tests: It is advisable to perform quality tests on the cans, both during the manufacturing process and at the varnish approval stage. These tests will allow to detect possible blistering problems and to take the necessary corrective measures.

It is important to keep in mind that the measurements mentioned are general and may vary depending on the specifications and particular conditions of each manufacturing process. It is recommended to consult experts in the area and perform specific tests to find the most appropriate solutions in each case.