All plastisol inks will eventually start to thicken up and harden in their containers. There are several different reasons for this: the main reasons are fluctuation in shop temperature (cool temperatures naturally harden inks), and time sitting on a shelf. White ink is especially susceptible to thickening in the bucket. This is because white ink typically contains high pigment concentrations. White ink pigments settle over time, which leads to the white plastisol ink becoming thicker than when it was initially manufactured. Opening up a bucket of screen printing plastisol ink and finding it stiffer than usual does not mean the ink has gone bad, or that you have to throw it away. Here are two ways for how to fix thick plastisol ink.
1. Stir the Plastisol Ink
You can fix a thick ink by stirring it. The more you mix an ink prior to pouring it onto your screen printing frame that is on the press, the more it will loosen up. The reason behind this is that by stirring your ink causes the ink to heat up. Generating heat loosens the ink's molecules, which creates a creamier and smoother ink. You can manually mix the ink with a wood spatula or a stainless steel spatula. If you do not want to stir your plastisol ink by hand, you can use an ink mixer instead. Use a mixer such as the Multi-Mix Ink Mixer.
2. Use an Ink Thinner or Reducer on Thick Plastisol Ink
Add a small amount of curable reducer. Using a reducer, for example the Lawson Thinner DT/Reducer #4 is especially helpful for very old ink. Just a cap-full works for an entire quart to help achieve the desired ink viscosity.