The sound of a successful Direct-to-Film (DTF) transfer is a smooth, consistent zip. But when that zip turns into a jagged tear—or worse, a total refusal of the film to release—you are witnessing a failure of film thermodynamics. This phenomenon, known in the industry as "locked peeling," is not just a nuisance; it is a catastrophic chemical fusion that occurs when thermal loads exceed the structural integrity of the film’s release coating.
Locked peeling occurs because temperatures exceeding 350°F (177°C) cause the release coating on the PET film to reach its melting point or undergo chemical degradation, transforming it from a non-stick barrier into a bonding agent. At this extreme thermal threshold, the Thermoplastic Polyurethane (TPU) adhesive powder becomes excessively fluid, migrating through the ink layer and chemically fusing with the compromised release coating. This creates a permanent molecular bond between the film and the garment, making a clean separation physically impossible.
To understand why 350°F is the "kill zone" for most DTF films, we must look at the architecture of the transfer media. A high-quality DTF film is a multi-layered PET (Polyethylene Terephthalate) substrate. It features a matte or glossy release layer, typically composed of specialized waxes or silicone-modified polymers, topped with an ink-receptive coating.
The release agent is designed to maintain a low surface energy environment, ensuring that the cured ink and TPU adhesive sit *on* the film rather than bonding *to* it. However, most wax-based release agents have a melting point between 320°F and 345°F. When your heat press hits 350°F, you aren't just melting the adhesive; you are liquefying the very barrier meant to provide the release. Once this layer liquifies, the boundary between the PET film and the ink disappears, allowing for interfacial adhesion.
TPU adhesive powder is the "glue" of the DTF process. Under standard conditions (280°F–320°F), TPU reaches a viscous, tacky state that anchors into fabric fibers. When pushed to 350°F+, the TPU’s viscosity drops significantly. It becomes a thin liquid that can permeate the pigment layers of the ink. If the release coating has simultaneously softened or degraded, the TPU makes direct contact with the PET substrate. The result is a "chemical weld" where the polymer chains of the adhesive and the film entangle at a molecular level.
While mechanical locking (where the ink "bites" into the texture of the film) can cause minor peeling resistance, chemical fusion is far more destructive. In a chemical fusion event, the heat triggers a cross-linking reaction. The water-based resins in the DTF ink, which are meant to remain flexible, can undergo secondary polymerization at extreme temperatures. This hardens the entire stack—film, ink, and adhesive—into a single, monolithic plastic sheet. Attempting to peel this "locked" film usually results in the ink being ripped off the fabric or the PET film delaminating, leaving a silver, plastic-like residue on the garment.
Thermodynamics also plays a role in the PET substrate itself. While PET has a high melting point (~500°F), its glass transition temperature (Tg) is much lower, around 158°F to 176°F. As the film stays under a 350°F load for 15+ seconds, the polymer chains within the PET become increasingly mobile. This "softening" of the base film makes it even more receptive to the migrating TPU and ink resins. By the time the press opens, the two surfaces have effectively become one.
In the 2026 DTF landscape, precision is the difference between a high-margin product and a pile of ruined blanks. Understanding the thermodynamics of your film ensures that your "locked peeling" issues remain a thing of the past, replaced by the consistent, high-quality results your customers expect.
