ABS layer adhesion problems

Dear All,
I have switched from using PLA to ABS and I am having some problems. First, I had difficulty in getting the ABS to stick to the heated bed. Running the bed continually with a foil insulator blanket gets it up to 120 degrees - but once it is uncovered the temperature rapidly drops to about 95 degrees. The room temperature is relatively warm (~26 degrees). I have tried improving the below bed insulation with expanded polystyrene, but I don't think it makes much difference - heat loss must be dominated by radiation from the top surface. Using an ABS slurry (in acetone), on the bed, produces excellent adhesion for the first few layers, but higher layers are very weak/brittle. I am running the hot end at 260 degrees (which was what I needed to get the ABS to stick to glass without the slurry). From what I have read, I think the ABS must be cooling too quickly once it leaves the nozzle. So, I think I need to look at the fan. I don't quite understand what the fan is doing. I had presumed that it was cooling the nozzle block so the temperature could be controlled (i.e. heater fighting a cooler), but I have also read that it cools the PLA to allow for better bridging. So, can I slow the fan in someway? I thought it just ran off the main power supply rails. Or can I get rid of it? Or, should I block some of the duct outlets that point at the nozzle? Or, should I just try and raise the hot end temperature some more - I assume that it can't really be 260....(I had to use 240 for PLA). Or, should I increase the extrusion rate since more hot ABS will cooler slower (but then I am concerned that I will be left with blobs that catch the print head). It is interesting that the parts I printed in PLA are very strong in Z. The parts I am printing really need better temperature resistance than PLA provides (and they need to be smooth) hence the venture into ABS. Any pointers would be greatfully received. The part that I am printing is here: [www.thingiverse.com]
Since a 'part breakage' is a major safety issue I am doing lots of destruction testing. I think PLA may actually be a better material since it tends to bed nicely before breakage, giving some warning of failure.
Greetings,
Christof

Christof, I suggest you take a look at [reprappro.com] if you haven't already. Regarding the fan, for ABS you need to cool the heatsink (which cools the upper part of the nozzle) but not the print. See [forums.reprap.org] for one way of doing it.

Edited 1 time(s). Last edit at 07/24/2014 10:21AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Excellent - that was just what I was looking for. I will print the modified fan housings and see whether it improves things. Many thanks, Christof

I posted a few notes in ABS printing a while ago, have a look at [forums.reprap.org]. I don't provide that custom heatsink duct any more, but there are better ones anyways (see dc42's reply).

[www.thingiverse.com]