The film expand when heated and contract when cooled. Thus it wrinkles.
The film itself is easily to be scratched. Poor-quality film can only protect the screen. The film wrinkles means that it's not hard enough. Generally, film with goog quality won't wrinkle.
There are many reasons why it wrinkles. One is because the rerolling during production is not done well. When tension is not enough, it wrinkles.
Second reason is about the weather.
And the third is about the way of use. The film is not well coated. Thus wrinkles appear.
