This guide relates to HP (Hewlett Packard) printers such as the K550, K5400, L7580 and some earlier printers where the cartridges are static and completely separate from the printhead unit(s).

The first thing you should probably know about these particular printers is that they are already essentially a pre-built continuous ink supply printer. Their design is such that all the tubing, and printhead feeding system is already included in the printer itself (unlike in Canon and Epson printers where the CIS design has to include tubing and cartridges that feed ink to cartridges in a moving printhead). As a result these HP printers are ideal for the complete amateur who is scared of moving parts and of making a real botch job with the more complex CIS kits available for other printers.

So, with a K550, for example, when you're looking for a CIS unit for this printer you're really just looking to increase the capacity of the existing reservoir (ie: the cartridges).

Option 1 (High Capacity cartridges)

The obvious point to make here is that you can simple buy the high capacity cartridges for your printer and get between 17ml (cyan, magenta, yellow) and 58ml (black) of ink without changing anything.

Option 2 (Remanufactured cartridges)

As always you can purchase 3rd party (non HP) cartridges but it's important to go with cartridges that use the same design as HP originals (see below)

How the HP cartridges/printer work

Normally you'd expect an inkjet printer to work on the basis of gravity with the cartridge providing all the pressure required to get ink to flow into the printhead and then be jetted onto the page. With these particular printers however you will notice a small rubber bladder/bulb on the bottom of each of your HP cartridges. Without getting too technical, ink flows into this bulb from the cartridge and then a piston in the printer pushes against this to pump the ink into the printhead. This active "pumping" mechanism is vital for a number of reasons:

1. It's used by the printer to determine if the ink has run out (it can tell if there's not enough resistance to it's piston pump action)
2. It was designed this way to provide ink at the pressure required for the printer to work
3. It stops air being drawn into the printhead if there's insufficient ink supply

CIS kits (or refillable cartridges)

Many of the CIS kits for these HP printers (both on Ebay and generally) use cartridges that remove the rubber bladder (described above) with a hard plastic mould that provides no pumping function. Instead the kits rely on a constant pressure from the reservoir design to provide the necessary pressure. The hard plastic mould is there simply to fool the printer into thinking there is ink in the cartridge and allowing it to function. Whilst this can work it's less than ideal because it deliberately side-steps a system HP designed to make the printer work in the first place.

Potentially you could end up with one of these problems:

• Printhead printing with bands of ink missing
• Ink running out, forcing air into the printhead (because the printer thinks it can feel pressure in the cartridge and assumes ink is present)
• A burned out printhead

All of these things can happen because air has been pulled or pushed into the printhead through insufficient pressure or poor ink management in the reservoirs.

Tips

I can't comment on individual CIS kits on Ebay as each has it's own quirks but here's a few tips to reduce your risk of messing up your printhead(s) and subsequently your printer

1. Make sure you fill your kits properly and remove as much air from the cartridges as possible
2. If you can apply greater pressure by raising the level of the reservoir bottles, you should do so.
3. Make sure your ink levels never drop below the minimum (keep them topped up!) to ensure they don't run out
   
4. Black is more susceptible to the problems mentioned above than the other ink colours because it's a pigment (rather than dye) ink.

Because HP printheads are closed units that contain approximately 4ml of ink themselves, once they get air in them it's next to impossible to get it out. Once your printing starts to show extreme banding that won't go away for more than 5 minutes or even half an hour, it's time to consider purchasing a new printhead.

Final comment

A CIS kit may seem like the perfect way to improve the cost effectiveness of your printing using your K550 or K5400, etc... but if you experience any of the problems described above it's worth

• asking your kit provider if you made any mistakes in installation
• considering high capacity originals or remanufactured cartridges instead
• doing some searching for other kits (eg: inkbags, pouches) which utilise the existing HP cartridge technology instead of bypassing it.