A look inside the design of Genuine Epson T0691, T0692, T0693, T0694, T0781, T0782, T0783, T0784, T0785, T0786, T0791, T0792, T0793, T0794, T0795 & T0796 ink jet cartridges. 
INSIDE THE EPSON T0781 INK JET CARTRIDGE.
If you have not yet had the opportunity to look inside your Epson 78 ink jet cartridge then you'll see the design and thought Epson has put into this cartridge. Every design detail clearly states: "don't refill me"*
The design of this cartridges is the same for Epson T0691, T0692, T0693, T0694, T0781, T0782, T0783, T0784, T0785, T0786, T0791, T0792, T0793, T0794, T0795 & T0796. So this document also is applicable to those cartridges as well.
The Epson T0781 ink jet cartridge is a multiple chamber cartridge that holds about 9ml of ink.
First thing you will notice when taking the side of the cartridge off is the multiple chambers as shown on the right. What you may not realize when viewing the cartridge like this is that ink in the cartridge dances from one side of the cartridge to the other.

A Peek Inside The Epson T078, T077 Series Cartridges
 
  The side of the cartridge where the label appears displays indentations which are part of the path ink travels through the cartridge. Ink flows from the chambers (shown on image above ) through the wall of the cartridge and into the paths shown on the right and then back into another chamber.

One may think that you could easily overcome this type of cartridge design simply by pressure filling the cartridge in a similar manner to refillable style cartridges (see bottom of page for brief description). This is not the case though. Pressure filling with a luer slip injector does not work with this design. It did not work with the couple of cartridges that were tested, nor did it work when a couple of clients tried the same thing.

You may be tempted to take the side cover off of the cartridge (top image) and inject ink into one of the chambers hoping to fill all of the chambers in the process. This also does not work as once the chamber you are filling has reached capacity ink will flow up and out through the fill hole you have created. Ink does not flow through the ink path of the cartridge filling other chambers. This is probably because the ink will follow the path of least resistance and in this case that is the fill hole. Even keeping constant pressure on the fill hole using a sealant so that only the injector needle pierces the plastic (in other words no ink can get out of the cartridge around the fill hole) crates a problem as air needs to get out of the cartridge during the refill process.
 
Even if you do get past the design of the cartridge you will have to contend with Epson's newest intellidge chip design. This is a two part design as you can see from the image on the right. With the OEM chip removed from the cartridge you will notice two contacts attached to a piece of clear silicon and a spring.

 

If you were to pull the silicon piece out of the cartridge you would notice another small chip on the cartridge. Look carefully into the black portion and you will see what appears to be another very small chip.
The silicon portion of this piece used to create a seal on inside of the cartridge.

 

The seal is an interesting part of the cartridge.
When the cartridge is viewed closely you can see a small opening exists leading from #1 to the #2 section of the cartridge. This is a very tiny and somewhat difficult opening to locate. What I can not determine at this point is why this is here in the first place. Part of my confusion may be that all cartridges I have opened thus far have been installed into a printer.
I may find that the seal is not created on an uninstalled cartridge or better stated is created once a cartridge is installed. If this is the case then the creation of the seal when installed may create required pressure in the cartridge.
 

We should probably go back to the chip one more time. I have had conversations with two individuals who have removed these chips from cartridges and attempted to reset the chips without the connections in the back of the chip. I'll state (possibly prematurely) that I do not recommend doing this. In the two cases that I am aware of two individuals removed the Epson chips from their cartridges and installed the chips onto aftermarket cartridges for refilling purposes. The chips were reset using a hand held resetter . Neither chip was recognized by the printer they were installed into. This would make it appear that the two part connection is required for chip to function. This may or may not be completely accurate because the basis of this is on reset chips alone.
 
  I should point out also that one individual who attempted to do this with a cyan and photo magenta cartridges using an Epson Stylus Photo R280 has never been able to use another Genuine Epson cyan or photo magenta cartridge in his Epson Stylus photo R280 and have the cartridge recognized by the printer. Two new cyan and two new photo magenta cartridges was the basis for his determining that the printer would no longer accept Epson cartridges. This person was able to use aftermarket refillable cartridges though.

I initially took apart these cartridges to figure out a successful way to refill them. By successful I mean a complete refill, not one that only injects 3cc to 5cc of ink into a cartridge. This is very important as the chip resetters for use with these cartridges resets the chip back to full. If using some type of refill method that only fills to 1/2 capacity you are forever running the risk of damaging print heads as the ink level indicator of the printer will always show that there is more in in the cartridge then is really there.

To make the refill process as a whole even more complex is that every chip resetter up through the writing of this article must have the chip reset before it falls below the 25% mark. If chip is allowed to fall below this mark then the chip reset will be unsuccessful.

All in all I would consider these cartridges to be very difficult to refill. Very successful design on Epson's part if I do say so myself.

*Disclaimer: This article does not state cartridges can not be refilled. It expresses an opinion that the cartridges appear to be designed in every little detail to prevent you from refilling these cartridges.

Pressure Filling in Theory: Many cartridges can be pressure filled by filling an injector to 1/2 the injector capacity, inserting into cartridge, pulling back on plunger so that air is remove from cartridge. When plunger of injector is released is pulls ink down into cartridge.
to see how this is done CLICK HERE