Always stir ink well before putting into the screen, from top to bottom of the container. This may take a few minutes of forceful mixing, which is normal. This will modulate the inks molecules and increase the “flow” of the ink, making it easier to print with.
"Cure" occurs when the entire plastisol ink film fuses and becomes a single solid entity. For an ink to fuse fully, the entire film thickness must reach its correct cure temperature. (Surface to Garment) For Plastisol, that is 320 degrees Fahrenheit. Keep in mind that thicker ink deposits (e.g., High Density) take more time to reach their specified cure/fusion temperatures.*As a general rule this means that the surface temperature of your ink will need to register hotter than 320 degrees but should not exceed 350 degrees F.
Doing a 'wash test' on your first print is the best way to make sure you are curing your shirts properly. Take a sample print, cut it in half, and wash it 3 to 5 times in a conventional washing machine with 3 pairs of jeans or towels. Set the washer for 'Hot Wash/Cold Rinse'. Set the Dryer for 'Cotton/High' and dry for 30 minutes. Complete 3 to 5 wash cycles and compare the "washed" sample to the "unwashed" sample. If you see cracking of the ink film or ink loss, your inks are likely under-cured.
Another good way to ensure a proper cure is called a 'stretch test.' Since your white image will be a little thicker, this test is easy to read the results. After your shirt is cured, simply take a small section of your design and stretch it slightly between your fingers. If the design sticks together like plastic and stretches well, then it is cured and you can continue with those curing parameters.