Viscosity

Plastisol straight out of the pail sits thick, closer to peanut butter than paint, typically holding a high viscosity that a shop measures in centipoise on a Brookfield or similar viscometer when they need to spec it precisely. Water-based and discharge inks run thinner by design so they can soak into cotton fiber rather than sit on top of it. Viscosity isn't a fixed number for a given ink either — temperature, agitation, and how long a batch has sat all shift it during a production run.

Printers adjust viscosity constantly on press: a curable reducer thins plastisol that's printing too heavy or clogging a fine mesh, while an extender or thickening agent bulks up ink that's running too thin to hold opacity. Get it wrong in either direction and it shows on the shirt — ink too thin bleeds past clean edges and loses halftone dot definition; ink too thick won't level out after the squeegee pass, leaving visible mesh texture or scattered pinholes in solid fills.

Pros treat viscosity as a press-floor variable tied to the specific mesh count, squeegee durometer, and ambient shop temperature that day, not a fixed setting written once into a job ticket. A print that ran clean at 8am on a cool morning can behave differently by 2pm once the shop warms up and the ink loosens.

Buyers sometimes ask for “thinner ink” hoping it'll produce a softer-feeling print, not realizing viscosity is adjusted at the press, invisible in the artwork file, and that a soft hand actually comes from a lighter ink deposit and finer mesh, not simply a runnier ink.