Fixing an oozing or leaking heat block
If you have done 3D printing, you may have experienced the oozing or leaking heat block problem. This is where you have filament oozing from the heat block on either the top side by the throat tube, the bottom side by the nozzle or both. This plastic will start to melt and drip down on your prints, and over time will burn on to your heat block leaving a big mess. In this section I will discuss why this happens and how you can fix it.
Why does this happen?
You are probably wondering how this happened in the first place. The main reason is that the heat block assembly became loose at some point. This may be from movement and vibration when printing, or maybe you had to work on the assembly at some point to clean it or something else. In the illustration on the right you see that there is a gap between the throat tube and nozzle inside the heat block. This gap is what allows the heated filament to ooze out. The heated plastic will be forced through the threads of the throat tube and possibly the nozzle and start pooling up at the top and bottom of the block. If left long enough, the plastic will run down the side of the heat block and drip onto your nice print.
Now you are wondering, how can there be a gap there? You say to yourself, I tightened the nozzle in when I re-assembled it, there can't be a gap. There is something you need to understand about metal. When it is heated, it expands. When a metal ring is heated, that expansion will be outwards. If the throat tube and nozzle are not tightened together enough, the expansion of the block can cause this gap.
How can I fix this?
So now you are wondering, is this something I can fix, or do I need to buy a new heat block assembly. There is a good chance you can fix this. The main thing is to tighten things when the block is heated and the metal is expanded during reassembly. This will reduce the chance of that gap forming the next time it is heated.
Since your heat block was leaking, you should first disassemble it and clean it. Clean parts reduce the chance of leaking in the future. Remove your heat block assembly from the extruder carriage and set your printer to preheat. preheating for ABS is probably the best as it will get your heat block to around 240 C (464 F) making things easier to clean. With pliers holding the heat block, use a wrench or another set of pliers to remove the nozzle. Once that is removed, your throat tube should come out easily. Be VERY careful as the parts are extremely hot. Use a wire brush to clean away any plastic from the throat tube, heat block and nozzle. You may need to re-heat the parts to clean them good. Once cleaned, you can begin to put it all back together.
These first steps are easier to do when the parts are cool and you can use your hands to fit them together. The proper way to assemble the heat block is to first thread the nozzle into the heat block, and once it is in all the way, back it off 2 turns. Next, thread the throat tube in until you feel it hit the end of the nozzle threads. DO NOT use pliers on the throat tube. With the throat tube in all the way, grab the heat block with a pair of pliers and use a wrench to tighten the nozzle in place. Once that is all together, heat up the heat block to temperature while holding it with the pliers. This can be done with the heater on the printer, or the burner on your stove. Don't use a torch or it may get too hot. Once heated, tighten the nozzle a bit more. This will ensure that the gap between the throat tube and the nozzle will be closed not letting the filament ooze out. In the illustration above, you will notice that after assembly, the nozzle is still not tight against the heat block. This will allow you to tighten it again if you need to. One more thing to note. Do not put a nut on the throat tube and tighten it to the heat block. This can cause more issues.
Now you can re-assemble the extruder assembly. Use the nut on the throat tube to tighten the extruder assembly to the carriage to prevent the heat block assembly from spinning when printing. Be careful not to over-tighten the nut as you can easily break the throat tube.