This document is intended to introduce you to the basic operation of how inkjet printers, and in particular Epson desktop inkjet printers work! Before you begin, please be aware that much of what we say here is just our understanding of how things work - this information has not been provided by Epson or any representative of Epson. We recommend you read it for information only, and in the light of our views on ‘understanding the way things work’ covered here. We’ll be adding some pretty pictures soon to make this text a bit more interesting.

Let’s begin:

Basic Systems

The way we see it, there are three key systems operating in any inkjet printer:

1. The ink system that starts with the ink in a cartridge, and ends up getting it onto the paper;
2. The system that moves the print-head across the page and seals it at the ‘home’ position (horizontal positioning);
3. The system that feeds the paper through the printer (vertical positioning).

They are each interrelated. But let’s take a look at each in turn:

The Ink System

To understand how ink flows through the system, we first need to understand a little bit of how the nozzles in the print-head work. Firstly, a quick definition that each colour ink has a printhead for that colour. Each printhead has many nozzles that are individually used to ‘fire’ a droplet of ink at the paper. The most basic concept, therefore, of inkjet printers as a whole is ‘fire lots of drops of ink at paper in such a way that they make the picture I want them to’.

Each nozzle is a tiny cylinder in the print head. This is the ‘barrel’ of the inkjet gun. Just down the cylinder, there is a little chamber with some ink in it, and in the walls of the chamber is a ‘mechanism’ for firing the ink. In Epson printers, its tiny piezo crystals, that vibrate when an electric current is applied - the vibration pushes an ink droplet out of the ‘barrel’. There are other mechanisms in use too, like the bubble-jet that heats the ink very rapidly - it literally boils a tiny amount of ink creating a bubbles that expands and pushes a droplet out-the-front. There are other mechanisms too. Anyway, the action of pushing the ink droplet out of the front creates a vacuum behind it... which in turn sucks another bit of ink into the chamber. This ink is taken from a tiny tunnel that leads to a nozzle onto which the ink cartridge is attached. In this way, ink is literally sucked out of the cartridge, as the nozzles are fired.

So what happens in the cartridge? It’s just ink, right? No, the ink in the cartridge tends to be held in some kind of sponge material which we believe must be there as a buffer zone between the ink channels in the print head and the weight of the ink. Instead of having potentially several grams of ink pushing down on those ink channels, and pushing their way through by gravity, the sponge support some of the ‘weight’ of the ink preventing just that. The cartridge also has an air vent - visible on Epson cartridges as the thin grooves - to allow air in as the ink is sucked out, but with a complex path that helps prevent evaporation of the ink. That also explains why you have to tear off those little coloured strips of plastic before installing a new cartridge - in shipping and storage, you want no evaporation at all. In use, you have to let some air in, but limit the supply to limit evaporation.

Before we move on, it is worth noting that all the desktop printers we know of are a type called ‘droplet on demand’ or similar. This means that the ink is only fired from the nozzle when you need a dot on the page. There is another kind, that always shoots a droplet of ink for every drop on the page. Those printers then use an electrostatic charge or similar to deflect the ink that is not required into a collection hopper. These printers tend to be high-end and expensive!

Horizontal Positioning - Head Movement and Timing

There’s nothing too unusual about how the head is moved across the page. The head unit is mounted on a guide rail, and is pushed and pulled across the width of the printer by a motor powering a loop of (in Epson’s we’ve seen) brown rubber-like material - we’ll call this the drive chain.

Now, how does it know where it is across the page? Just above the drive-chain is an apparently clear plastic strip - look closely and you’ll see it’s actually clear with tiny black bars in. We believe the print unit can read and count these in some way (possibly magnetically or optically) and therefore know how far across the page it has been moved. There will also be a switch of some kind to tell the printhead when it is at carriage left and carriage-right, but we haven’t found these yet! Mix in some clever electronics, and you have just given the printer the capability to accurately place dots anywhere across the printable area of the page.

You may also know that Epson printers use something called ‘bidirectional printing’ in some print modes. When this mode is on, the printer puts ‘dots on the page’ as it moves left across the page, and then also as it moves right. This could create alignment issues, as we will see soon.

There’s one final, clever, piece of information to know about Epson’s desktop printers (that we’ve seen). Whenever a print job ends correctly, the print head is returned to the carriage-home position, typically at the far right of the printer. As it passes a certain point, it pushes a widget that in turn mechanically raises a small sponge unit that fits snugly around the print head. The print head is now sealed, and after a few moments, it is locked in place which can be seen as a small plastic tag is lifted from below the print heads and locks it in place. You can not now ‘accidentally’ pull the head out of its home position without unlocking the head. We’ll cover this later!

Vertical Positioning - Paper Feeding and Movement

This is one area where printers tend to differ quite a lot in implementation, but not too much in generalisation. We won’t cover the why’s and wherefore’s of actually moving the paper through the printer, other than to define the platen, which is the ‘roller’ that really pushes the paper through, and typically works in conjunction with other small rollers pressed against it - the paper is fed between them, and as the platen is turned, it pushes or pulls the paper through the printer. There is also the printhead gap adjustment at the right of the printer, that moves the head up and down in relation to the platen.

We just want to generalise a lot on this topic and say - the quality of platen and paper feeding capability certainly does seem to vary between printer models, and even sometimes printers of the same model. As the platen is turned (by motor) the movement of the platen roller is measured using a ‘clear disk’ at the left-hand side of the printer. Again, look at the circumference of that disk and you’ll see tiny black bars that are almost certainly used for that measurement. Unfortunately, whilst the horizontal movement of the printer carriage tends to be rather accurate due to its nature (unless you put a brick in its path - not recommended), moving the platen accurately is possible - but less important (in a way) than the platen roller itself being able to translate that exact motion into moving the paper the exact same amount forward or back. It is this area that seems to be the challenge for some printers with some papers - we can only assume that the actual measuring system noted is as accurate as it needs to be. Please note at this point that we are in no way suggesting that the drive units of Epson printers are in any way substandard. What we are suggesting, though, is that if you are pushing a printer to the limits by using slightly heavier paper than it was designed for, or an ink system that requires very accurate feeding, then this could be an issue for you.

We should also note at this point the sets of tiny little wheels on the exit-path of the paper. These do differ between models - but are often referred to as ‘pizza wheels’ as they look like miniature, spiky, pizza cutters. These help push the paper through and away from the print carriage when a sheet is finished... but interest us as they can sometimes mark the paper!

Tying it all Together

Well, other than lots of rather important ‘ancillary’ subsystems like power supply(!) - we think we’ve covered the basics here. But there is one more point to note. After every print, we commented that the carriage is moved to the home position and locked in place. If you look carefully at the printer as it does this - you will see it is done in synchronisation with a short ‘reverse feed’ of the platen roller (on many Epson printers, at least). Some experimentation later - we find out that, if the carriage is at the home position, a reverse feed of the platen lifts the lock into place, and after that, a short forward feed lowers the lock.... this could become very useful to know indeed

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