What Causes Layer Separation?

Lately I have begun printing larger objects in ABS, and I notice that there are weak bonds between some layers, but not others. I thought to raise the temperature to 240C to get better adhesion, but this seems to create greater stress in the parts, and I actually had one part rip itself apart as it began to cool.

My thoughts of possible causes are:
1. Layer height is a bit too high
2. Temp needs to come down to 225
3. All the advice I give about extruded current settings is wrong.
4. ABS Has absorbed too much water
5. Print head speed is too fast?

If only some layers are weak it sounds like the flow rate drops.

Generally larger width over thickness and higher temperature makes stronger bonds.

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[www.hydraraptor.blogspot.com]

Width over thickness. Hmmm...

Meaning that the flow rate of extruded plastic is increased relative to travel speed over the X and Y?

That actually makes a lot of sense. If my problem is that the filament is being laid down tensioned (in other words, stretched), then increasing the flow rate over speed effectively reduces the stretching, which in turn would probably reduce the amount of shrinking that the plastic "wants" to have.

Even without stretching, if what I have is variable flow rate to begin with, then I must be right at the edge of a good setting for my extruder tension, since most layers are fine. I suppose either adjusting the tension, OR increasing the width over thickness will take it into a better zone.

Thanks for the response. I will try your suggestions and post my results.

Now, I wonder what one of these parts would do if I put it in the oven for a while...

This happens to me also usually at the corners but occasionally even on round objects. I suspect that the nozzle temperature might be dropping too low occasionally causing poor adhesion. When the extruded layers shrinks as it cools down, the layers separate at the weak points. I was hoping to fix this by enclosing the Mendel in a box as Nophead has done to keep it at 50 C or above.

I'm also using the Adrian's push-on design with PTFE lined PEEK thermal insulation and I notice that when hot, the aluminum heater block is loose around the nozzle. I wonder if this might cause the temperature of the tip to vary from the block temperature which is being measured by the thermistor.

Quote

Width over thickness. Hmmm...

Meaning that the flow rate of extruded plastic is increased relative to travel speed over the X and Y?

Well if you keep the layer hight the same then yes you would need more plastic but I am assuming the amount of plastic is correct to give the correct width, if not it will be very weak.

Given the correct flow of plastic, a wider filament is squashed more and has more contact area with the layer below, so has a stronger bond. [hydraraptor.blogspot.com]

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[www.hydraraptor.blogspot.com]