Flow Lines?

[The Rookie]

Hello all! It has been too long since I have been on to post any questions/responses.

Today I was taking pictures of PET tubes with extra back lighting for our quality department and they immediately started questioning me about "flow lines" on the part. The parts all pass inspection criteria and are acceptable, however, they want to know why.

I told them that I would get back to them after talking with my team (since I am still a newbie at work and didn't want to misspeak).

My team said that they are flow lines and we always have had them, so I wanted to get other opinions/explanations. I have also noticed these lines before that were slightly discolored and looked a little degraded, but my team said it comes and goes as well.

I am inserting the image to the text and also attaching so that you can see the lines. The lines are very hard to see, so I traced the lines as well as lightening the image in hopes of making it stand out.