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Strength between layers

Posted by charlieRC 
Strength between layers
June 06, 2013 06:23PM
I built a prusa mendal and am fairly happy with the print quality I am getting (could be better but getting there). Even so...

Is there anything I can do to improve the strength of the bond between layers. I have designed to take advantage of the strength of the XY plan, but sometimes, I need a bit more strength in Z as well. Is there anything I should be considering to improve the strength across the Z plain? Smaller extruder nozzle?

I am using .5 nozzle and 3 mm ABS.
Re: Strength between layers
June 06, 2013 06:38PM
Look at your layer height and try to tweak the "warm time". In other words, if you are using a fan for cooling the abs, adjust it to cool the extruded abs just a bit slower. It's a delicate balancing act but I believe it makes all the difference.
Re: Strength between layers
June 06, 2013 07:32PM
Actually, I'm not using a fan. I printed an x-carriage that would support a fan, but never installed it because I was getting feedback that it probably wouldn't offer much unless I was going to switched to PLA. Do you think I need to install it?

I wonder if reducing the layer height might help.
Re: Strength between layers
June 06, 2013 11:11PM
What is your layer height?
I wouldn't use a fan on ABS except for tiny parts that spend a lot of time near the hot nozzle.
Your nozzle size shouldn't be a problem.
Re: Strength between layers
June 07, 2013 08:08AM
waitaki: I am currently using .3 mm layer. Never changed from the default because I was not sure of the implications. If I were to change this to .2 or .25, are there other changes that would have to be made to accommodate this? I know it would slow down printing.

I did set the external perimeter speed to 40%. The regular perimeter speed is 30mm/s and 60mm/s for infill, which I think are both default. I have figured that slowing any of this down more would exacerbate the weakness between layer and speeding up would hurt quality.
Re: Strength between layers
June 07, 2013 12:23PM
There are a lot of factors that determine the bond strength of the traces. The nozzle temperature and layer width over height ratio are the two most important though. If you're using Slic3r, it is set by default to automatically determine the object's WOH ratio based on your nozzle diameter. A WOH ratio of about 2.2 is good for strong layer bonding but may be overkill in some cases. Think of the worst case scenerio (WOH ratio = 1), in that case the width of each trace is equal to the height which makes it cylindrical giving it the absolute least amount of contact with the trace below. A part printed like that would easily delaminate. As you increase the WOH ratio, you improve bonding at the expense of detail. If you set it too high (so each trace is wider than the nozzle's outer diameter) then the nozzle won't be able to squish it properly and you'll end up with a mess.

Also, I've found (as others have as well) that using a fan with ABS does not help at all. My first prints with ABS would delaminate like crazy since I used a fan to avoid melting my PLA x carriage. I've since upgraded to an ABS x carriage and shut the fan off and I no longer have issues with delamination.
Re: Strength between layers
June 07, 2013 03:30PM
Thank you very much sheck626!

That is an excellent description of the process. If I understand correctly the width is pretty much determined by the nozzle diameter, and the height by the layer height, is that correct? I am currently using a .5 nozzle and a layer height .3. That should yield a WOH of .5/.3 or 1.667, correct? If I want to try a 2.2 WOH, then I would want to try a layer height of .227.

Am I on the right track?

Thanks again.

Re: Strength between layers
June 07, 2013 05:02PM
The height of each trace is determined by layer height (the Z axis is raised by that amount before each layer, including the first). The width of the trace is a bit different, it's determined by the flow rate (how much plastic is being pushed into the extruder) and the feed rate (how fast the nozzle is moving with respect to the printed object) and the layer height. For a given layer height, a lower feed rate (slower speed) or a higher flow rate (moar plastic!) will cause the trace to be wider which makes sense if you think about it.. Remember, regardless of the nozzle diameter the same volume of plastic going into the machine is coming out (unless you're using a black hole extruder or something). The nozzle diameter defines the lower limit on the trace width since it has to be at least as wide as the nozzle inner diameter (and the trace can't be wider than the nozzle's outer diameter). That means that with low layer heights you have to use rather large WOH ratios which costs detail in the print. At 0.1mm layer height the minimum WOH ratio with an 0.5mm nozzle is 5!

The feed rate and flow rate necessary to produce the trace width you've set is calculated automatically by the slicing software. Slic3r takes it a bit further and automatically calculates (by default) an optimal WOH based on the nozzle diameter. In Slic3r, you can find the trace width by reading the top of the gcode (it's in the comments). If you divide that by your layer height you'll get the WOH ratio. All bets are off if your machine isn't properly calibrated however.

Sorry for the long response and anyone can feel free to correct me if I'm wrong.
Re: Strength between layers
June 09, 2013 01:48PM
I get my strongest prints by printing at low layer heights (0.1-0.15mm), very slowly (30 mm/s perimeters, 50 mm/s infill), and with a high infill (>50%). This gives the best bonding between layers because it leaves a lot of time for the hot end to heat the previous layer of plastic, while being very close to the previous layer. These settings may be annoying because of the extra time they add, but if you need the extra strength, they're a must.
Re: Strength between layers
June 09, 2013 04:07PM
I currently have my layer height set to 0.254, and have manually set my WOH to 220% giving me an width of 0.558mm. Prior to the manual setting of the ratio, I let slic3r calculate the value. I don't know what it was using, but I had issues with layers separating. Since the manual override, my results have been good.

BTW - my layer height is what its is because I have an Imperial (1/4-16) acme screw, and this value allows for an integer number of steps for the Z axis when changing layers.
Re: Strength between layers
June 11, 2013 12:26PM
Anyone considered using the post-process acetone vapor smoothing to increase bond layer adhesion for ABS prints? It might take some dry time afterwards, but from waht I saw on a friend's vapor smoothed prints, it seems to me that might help considerably.
Re: Strength between layers
June 12, 2013 01:49AM
I have done a bit of testing on layer bonding.

I printed the same piece at 210 and 230 degrees, at 230 degrees the parts was of course strongest.
When the parts was dipped in acetone, the strength of the parts printed at 210 degrees came to par with the parts printed at 230 degrees, but I didn't really saw any increase in strength on the parts printed at 230 degrees.

I might add that I have had a great deal of problems with layer bonding on my second RepaRap build. My problem was the hobbed bolt, that didn't had a consistent diameter, which lead to inconsistent extrusion.

Edited 1 time(s). Last edit at 06/12/2013 02:57AM by Ralf.
Re: Strength between layers
June 17, 2013 01:38PM
When I print big parts (more than 100mm tall) for my prototyping I will usually paint it over quickly with Acetone or spray some acetone onto the part. I then apply some pressure on the top of the part while it dries.

Most of my thin walled parts will not stick together unless I do this.
It will ruin the surface finish though.

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