Warranty Void if Removed

Fortunately the search for a new fuser unit was short. I asked around campus for old laser printers, and in discussion with the ECS helpdesk people it transpired that they had a brand new fuser unit on the shelf that they were never going to use.

The fuser is for a much bigger, more industrial printer - the HP 4500, and is of much higher quality construction. It features two built in levels of thermal protection - a resettable "thermoswitch" that kicks in at 195°C, and a thermal fuse that permenantly blows at a higher temperature. I dismantled the broken fuser to investigate the cause of failure, and to ascertain the best way of fooling the printer into working without it.

It has a flat, ceramic heating element with a very small surface mount temperature sensor on the back. It had shattered, but it was unclear whether this is because of the mechanical munging of PCB through it, or simply overheating.

I measured the thermister as about 350k at room temperature. I breathed on it and it decreased to 290k.

I wired a pot up to the board in place of the fuser, and tried to fool the printer into working. I had no luck.

On friday I procured another printer from ECS - the HP 2200dn which is very similar to the 2200, but has a built in duplex unit. I hoped to learn how to fool the printer using the fuser from the dn.

Still no luck, the new fuser didn't appear to fool the printer. I looked up the error message in the service manual, and it was actually complaining of a missing fan! Replacing the fan made the printer happy again. I was too excited to try the pot instead, and jumped straight to printing on the new copper clad board Rob bought.

Instead of a test page, Rob connected up his laptop over ethernet and printed an actual PCB design. The image quality was reasonable, and definitely proved the concept, but was not good enough to yield a fully functioning board.  

I dismantled the new fuser unit, removing the extraneous rollers and paper sensors. I also removed the very strong springs that push the rollers together to allow space for a 1.6mm PCB. I wired it up to a mains plug via some chocolate block, proving that I hadn't learnt from the previous experiment!

The new fuser is of a much better design. Instead of a crappy ceramic board it has two high power halogen strip lamps inside metal rollers. It is far more robust, and I am not at all scared of mashing PCBs through it. So we connected it up to the mains and used Rob's remote IR thermometer to watch it heat up. It heated up nice and slowly, reaching 180°C over about 45 seconds. This made me much more confident about using it without any temperature control. 

We unplugged it at about 180°C and rolled the freshly printed PCB through slowly. I fed the 5" of board through in about 40 seconds. By the end, the temperature had dropped to about 160°C.

The toner was fused on incredibly well. Vigorous rubbing with a finger didn't remove any. So we etched it.

The finished result.

We used petrol to strip the toner, as it is basically plastic. The resultant copper corresponded exactly to the toner, proving that it fused well, and resists etch well.

There is much more work to be done to get a good image, however. The mechanics needs a revamp, as the board was free to slip slightly in the rollers. I think the imaging problems are deeper, and need more thought. On the plus side, the toner cartridge is running out, so a replacement has been ordered.