Why sometimes transfer lines appears when dye sublimation printing?

Transfer lines are where you end up with a faint line in the fabric that corresponds to where the edges of the transfer sheet were located during pressing. They are caused by the shirt fibers melting along the edges of the transfer sheet. If you are an Embroiderer, transfer lines look kind of like hoop marks, except they are permanent!

The white line is heat transfer line
Your first option for preventing transfer lines on apparel is to experiment with your heat press settings. Consider dropping the temperature down to 385℉ and reducing the pressure to a very light setting. You might also cut back the pressing time from 60 seconds to about 35 seconds. Changes should be made
gradually, as too much variance from the standard settings could affect image quality.

One important thing to remember with apparel, is that different garments may need different settings. Regardless of which approach you use, be sure to document your settings for future reference.

These are some of the most common issues encountered by Sublimators, and each of them is relatively easy to resolve. If you are still having trouble, then make sure you contact your equipment supplier to discuss. Don’t let production challenges slow you down or intimidate you. Most issues can be resolved over the phone! But first you have to make the call.
More infos:
E-mail: sales@feiyuepaper.com
Tel: 86-025-83228884
Fax: 86-025-85288894
Whatsapp: +86 18252072197
Address: Central Road 323, Nanjing, Jiangsu, China