Hauscook container cap

I have no choice since Haucook is my only supplier HOWEVER...

I really don't have a choice to purchase these caps through Hauscook. However, I question the fit of these caps. It goes on loose and does not seal even when I was told by the sales rep that the fit is intentionally loose and the heat will seal it. I'm no thermodynamics expert but I've been telling my customers the same thing. More often than not, I get suspectful looks. I tested it and it simply does not seal securely. Maybe I'm doing something wrong? I had to use a plastic bag and tie it close to the bowl to ease their and my concerns.