Patent ID: 7419249

Claim:
An inkjet printhead comprising: a plurality of nozzles; a bubble forming chamber corresponding to each of the nozzles respectively, the bubble forming chambers adapted to contain ejectable liquid; and, a generally planar heater element disposed in each of the bubble forming chambers respectively, the heater element being bonded on one side to the chamber so that the other side faces into the chamber, the heater element being configured for receiving an energizing pulse to heat some of the ejectable liquid above its boiling point to form a gas bubble on the side facing into the chamber, whereby the gas bubble causes the ejection of a drop of the ejectable liquid from the nozzle; and, the chamber having a dielectric layer proximate the side of the heater element bonded to the chamber; a print engine controller to control the ejection of drops from each of the nozzles such that it actuates any one of the heaters to eject a keep-wet drop if the interval between successive actuations of that heater reaches a predetermined maximum; wherein, the dielectric layer has a thermal product less than 1495 Jm −2 K −1 s −1/2 , the thermal product being (ρCk) 1/2 , where ρ is the density of the layer, C is specific heat of the layer and k is thermal conductivity of the layer, and during use, the density of dots on the media substrate from the keep-wet drops, is less than 1:250 and not clustered so as to produce any artifacts visible to the eye.