Flatten.

Merging every layer, blend mode and transparency effect into a single image, so that what the printer receives is what the screen showed. Wall and PDF routes ask for it by name: SAIC wants files flattened with no alpha channels, and Gelato recommends flattening all layers and all transparencies in a print-ready PDF. Printful warns that a PDF can carry hidden layers, blend modes and transparency effects that render differently during production, and asks for it flattened if a PDF is sent at all. Apparel is the exception. Flatten a DTG or DTF file onto a background and the alpha channel the RIP reads for white is gone, so the background prints as a box round the design.