If one uses the IS 1020 pigment ink in the aftermarket cartridges, the sponge/ink combination is not proper. I carry both 1020 and 1128 Black Text pigment ink. 1128 will interface better with the aftermarket sponges and also with newer OEM carts. Sponges used in the aftermarket cartridges are not ideal for pigment ink especially the 1020...and likely other pigment ink that are similar..possibly the one used.
Thus based on my experience on tests performed by myself Precision Colors, it appears that you might have used ink similar to the 1020 pigment whose lineage is older. In addition to this, the other differences between the 1020 and 1128 are in particle size. 1128 is better suited for printing on Color Lok papers which has a denser/finer surface structure.1128 smaller particles allow pigment to enter the small pores and resist rub off with high lighter pens. However, when 1128 is used on older type copy papers or lower grade copy papers, because the particles are smaller, it tends to dive in too deep and get absorbed too deeply and its blackness is compromised. The older 1020 will tend to stay closer to the surface and thus appear darker. However 1020 on newer Color Lok papers will tend to smudge when a high lighter pen is used on it because its larger particle size allows more pigment to remain exposed.
Since more and more papers are using the Color Lok process or it appears to be very common today, and the 1128 performs well where the 1020 was previously used, I will tend to give the nod to the 1128 for more universal use today on even older Thermal head text printers.
Thus based on my experience on tests performed by myself Precision Colors, it appears that you might have used ink similar to the 1020 pigment whose lineage is older. In addition to this, the other differences between the 1020 and 1128 are in particle size. 1128 is better suited for printing on Color Lok papers which has a denser/finer surface structure.1128 smaller particles allow pigment to enter the small pores and resist rub off with high lighter pens. However, when 1128 is used on older type copy papers or lower grade copy papers, because the particles are smaller, it tends to dive in too deep and get absorbed too deeply and its blackness is compromised. The older 1020 will tend to stay closer to the surface and thus appear darker. However 1020 on newer Color Lok papers will tend to smudge when a high lighter pen is used on it because its larger particle size allows more pigment to remain exposed.
Since more and more papers are using the Color Lok process or it appears to be very common today, and the 1128 performs well where the 1020 was previously used, I will tend to give the nod to the 1128 for more universal use today on even older Thermal head text printers.