According to HP Media Test Expert, Thom Brown, the cost of printing has come down 30% in the last three years. The average user will get through 3.8 ink cartridges each year which works out as a cost of $6 per month. It's a good way of rationalising it but 30% or no 30% it still feels like bill shock every time we have to we have to play the replacement game.  The real trouble is that few of us ever thinks of the purchase of a printer as a subscription or a model with significant running costs. So, for those who can't take the pain at the checkout, Pocket-lint went over to HP's ink factory in Dublin to find out exactly where all that money goes.

The process of printing is like dropping grapes into a bucket from the top of a 30 story building at speeds of 50km/h and at intervals of 30,000 times a second. We're asking those cartridges to make single dots on a page from 32 separate super-heated vapour explosions but yet be cool enough by the time they arrive to be dried and in good condition. And all of that through a nozzle just one third the breadth of a human hair. If the droplets are not the exact right shape, the lines on the documents on the photographs will become blurred and raggedy, the contrast poor and the images far from sharp. And if the results aren't good, then the consumer will blame the printer manufacturer, so it's small wonder that they put so much into the production of the cartridges.

There's a holy trinity of printing between the machine, the paper and the ink cartridge but out of them all it's arguably the cartridge that's the hardest working of them all. The ink comes into contact with both the machine side of things as well as the paper too. The paper itself is specialised to some degree with a choice between porous and swellable technologies - you can ask us about that one another time - but the cartridge has become such a piece of work that more often than not these days, it's even the place where you'll find the actually printing head which was previously always found in the machine.

The engineering and assembly that goes into each cartridge is certainly more than you'd expect especially when standing there with your bill shock face. As well as the tiny nozzle, each one contains a sponge and a filter as well as the ink. The sponge is there to ensure that every last drop of ink is soaked up from the cartridge container but is not so greedy that it starves the nozzle. The filter is 7 microns in size and is designed to block whatever impurities that may have got in there that could clog up the nozzle and ruin the drop flow.

The cartridges are filled in a vacuum to make sure they're up to the brim and, once done, the lids are placed on top and ultra-sonically vibrated against the main body until the two heat up and fuse together. The heads themselves are made from an incredibly thin film and held down by the inert and rather pricey metal Palladium to make sure that they're perfectly in place. Each cartridge also has its own unique ID chip to make sure it's gone through all the quality control and, if there is a problem, it can be traced back to a particular batch.

But of course, it's not just the plastics where all the money goes. A huge amount of the expense is concerned with the ink itself. Each new ink takes around 3-5 years to develop. The fluid dynamics are selecting for viscosity, anti-smudge factors, quick drying and colour permanence to last years of exposure to light. The droplets are examined under electron microscopy for size, shape and speed to make sure they'll give that perfect image reproduction when required.

It's small wonder that manufacturers like HP recommend using their ink cartridges and no one else's when it's time to change them over. Sure, a significant part of their business is the ink cartridge money spinner but it's easy to see how a refill of their specially engineered hardware could produce some very patchy results. The new ink is not going to have the same viscosity which will give a dramatically different performance but, more than anything else, the cartridges simply aren't designed to last for a second life cycle. Many of the parts such as the delicate filters and nozzles might well have worn out. HP describes refills like second hand cars. There's just no guarantee that they'll run properly.

Thankfully, you can buy into a printer type and system where new cartridges cost less. You can make the choice whether you'd rather spend more on the outlay or pay higher running costs but, for now at least, shelling out for new cartridges is a part of life. From an eco point of view, companies like HP are crying out for us to send back the empties so that they can recycle them. 70% of the new ones are made from the plastic of the old. But, until they run a decent incentive schemes to help with further ink purchases, it seems unlikely that many people will bother.

So, until then, you'll just have to remember these facts the next time your printer runs dry and it's time to stump up the cash again. You won't like it and it's still not cheap but at least you'll have a better idea of where the money is going.