Under normal circumstances, it is normal to have bubbles in the plastic mouth of thin-walled milk tea cups. This is because the air between the inner wall and the outer wall of the milk tea cup is sealed in the cup, forming a tiny air gap. When drinking hot drinks, the temperature of the internal liquid rises, which leads to the expansion of the temperature difference between inside and outside, thus generating bubbles at the rubber mouth.
These bubbles will not have any influence on the use of the cup, nor will they affect the heat preservation effect of the cup. However, if it is found that there are too many bubbles or obvious defects on the surface of the cup, it may be necessary to pay attention to the quality of the cup.
To better illustrate this problem, here is a related picture:
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