Picture of Clogged MakerBot Nozzle
If your MakerBot Replicator doesn't produce a good plastic stream in the "load filament" mode, the nozzle may be clogged. A "drooling, thin stream that wraps on the nozzle" is a sign of a partial clog.

What causes a nozzle to clog?  In my case, anytime "something happens" and plastic doesn't get pushed through a heated nozzle for an hour or more, I have clog issues.  An unlevel platform, dirty drive gear, shrinkage and "upcurl" of a big part can cause the nozzle to quit extruding.  If I "walk away" from a 14 hour build, Murphy's Law increases the chances of "no extrusion but the heat's on."

First, open a ticket with MakerBot support.  They are excellent and will help you troubleshoot the problem.  If the result is "clean the abs plastic out of your nozzle," then you might want to follow these steps.

Materials needed:

Acetone (get this in the "fingernail polish removal" area of a drug store or grocery store)
7 mm wrench
3/16 inch diameter rod
hex wrench supplied with the Replicator
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Step 1:

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Safety first. Take your time.  There are multiple "heat up" and "cool down" steps involved. 

This is no more hazardous than pulling cookies out of an oven--but you wouldn't grab a cookie tray with your bare hands.  Don't touch hot parts until they have cooled to room temperature--be patient.

First, unload the filament from the extruders.

Step 3:

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Turn the Replicator on and use the menu on the front panel to "preheat."

Step 4:

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Choose the nozzle (right in this case) and "Start Preheat."

Step 5:

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You should see the nozzle heating.  When it reaches about 220, you are ready to proceed.

Step 6:

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Using the 7mm wrench, remove the nozzle (turn counter clockwise).  The nozzle is hot; don't touch the nozzle or the "business end" of the wrench.  It should not take much force to free the nozzle.
PatrickW31 month ago

Why is it important to remove the nozzle while it is hot?

MikeTheMaker (author)  PatrickW31 month ago
This is based on some instructions that I originally received from MakerBot for exchanging a nozzle--the threads may be damaged if one exerts too much force since the nozzle may be "tight" when cool (or stuck in place with hardened plastic).
You could try to remove it cool, just be gentle . . .
robboz41 month ago

Thanks for a great Instructable! I'm now able to print again!

MikeTheMaker (author)  robboz41 month ago

I'm glad it helped. Judging from the number of views on this instructable, we're not alone with clogged nozzle problems. Hey, I just had a book published, "A Beginner's Guide to 3D Printing: 14 Simple Toy Designs to Get You Started." If you know any kids (or adults) who need a very basic guide to "designing with 3d software," ask them to take a look.

seanobrien1 month ago

Thank you for putting this together. Replicator 2x was clogged with ABS - these instructions worked great!

luclalande1 year ago
Was it PLA or ABS filament that first caused the problem?
MikeTheMaker (author)  luclalande1 year ago
ABS--I haven't had this type of problem with PLA using the Replicator 2; also, acetone won't work with PLA, so a clogged PLA nozzle will be another animal to deal with
cpalmer71 year ago
Wow, this worked like a charm.