Fabric
100% polyester
Polyester is the fabric sublimation is for, and the fabric most likely to shine, scorch or bleed dye under ordinary HTV.
Sublimation
This is the job polyester is good at, if the shirt is light. Sawgrass list polyester fabric at 205°C (400°F) for 45 seconds, medium pressure. Xpres press polyester T-shirts at 190°C for 60–70 seconds, medium, and peel hot. Dark polyester cannot go lighter, because sublimation has no white ink. The full method is on the polyester sublimation page.
Heat transfer vinyl
Siser EasyWeed lists 100% polyester at the same 150°C, 10–15 seconds, medium, hot or cold peel as cotton. The heat is not the hard part. The colour already in the shirt is.
If you are pressing over a sublimated or strongly dyed polyester, put down Sub Block first: 130°C (265°F), medium, 10–15 seconds, hot or cold peel, then layer the colour you want. Siser say a black film can hide migration until you put another colour on top.
For a cooler vinyl, Siser HI-5 is 120°C for 5 seconds on textiles up to 150 g/m², and 150°C if the fabric is heavier, at high pressure. EcoStretch is 120°C for 10–15 seconds, medium, hot peel. Do not silently drop a 160°C glitter or holographic film to 120°C. Those films do not publish that number.
DTF
DTF is published for polyester as well as cotton. The Xpres examples sit at 150–160°C. Cover the shirt. A shiny halo around a DTF design is scorch, not a pale transfer. See scorch marks.
Common questions
Why did the white vinyl turn pink?
That is dye migration. Disperse dye in polyester can move into the film when you heat it. Siser say high temperature and high pressure make it worse, and they publish EasyWeed Sub Block at 130°C (265°F) as a blocker layer.
The shirt has a shiny rectangle. Did the vinyl fail?
Usually the fabric scorched or glazed. Siser’s advice for heat-sensitive textiles is a cover sheet, a test on an inside seam, and a lower temperature with a longer time where the film allows it. Their HI-5 film is 120°C for 5 seconds on lighter polyester.