Seam with Minimum Layer Time

[skc5741 0]

Hello,

 

I'm running off a RepRap printer, using Cura as my slicer. I've been having an issue with the Minimum Layer Time feature, specifically in accordance with the Lift Nozzle feature. I was starting to get some goopy prints, so I turned on the minimum layer time and lift nozzle features, thinking that would fix it. I no longer am making goopy prints, but when the minimum layer time feature first activates and the nozzle is lifted for the first time, a seam is left behind on the print that has been irritating me.

This is a Bishop I tried to print with 3 second min layer time, 5 mm/s min speed, with lift nozzle activated. Near the top, there is a distinct seam in the layer that it first begins to lift the nozzle on. I've had this problem with other prints as well, and I know it's not specific to the model.

 

What can I do to solve this problem?

Thanks,

Sam

[geert_2 558]

I have also noticed that when the nozzle is just waiting outside of the model, this may cause little defects due to the nozzle leaking, or due to its "take off" from the model, and due to the no-flow in the nozzle, which causes higher temps and thus more liquid material. Also sudden changes in layer area show up due to huge differences in cooling time.

 

That is why I usually print a dummy model next to the real model, when printing time is too little for good layer cooling. Instead of using the minimum layer time feature. Ideally, the dummy model would have the inverse layer area of the real model, so cooling time per layer is identical. This doesn't matter for big objects, but it does for very small ones. So what you could do is subtract this model from a cylinder, and use that inverse cylinder as dummy? Be sure to design a good bottom plate to get good bonding to the print bed (for example: something similar to a brim, but manually designed into the model).

 

For best results on small objects, layer printing time and material flow through the nozzle should be steady through the whole printing process.

 

Another option might be to print enough models at the same time.

 

[gr5 2,265]

I agree with geert_2 - that's what I always do.  Just a small 1cm by 1cm cuboid of the exact same height as the bishop will do.  And place the other object in a position such that fans are cooling your bishop while the other object is being printed.  Even just 3 seconds away from this part should give it plenty of time to cool.

 

I have a 30cm_cube.stl file I keep handy.  Cura lets me adjust the x,y,z scaling in mm (you can do % or mm scaling).  I make it the same height as the model and adjust the x/y to be about one fifth the height so that it's a stable part and won't fall over.  In other words if the part is 50mm high my base is 10X10.  If the part is 10mm high I could go as low as 2x2 (probably would do 5x5).

[geert_2 558]

This question comes up often enough to draw a quick demo in DesignSpark Mechanical, just to show the concept. Much easier to explain, and takes less time than typing.

 

So, here it is:

 

 

This is how I do it. But of course, depending on the models, dimensions, and materials, all sorts of derivations and approximations might work equally well.

 

The dummy cooling towers are waste, or could be re-used as "hotels" in Monopoly or other games.

 

 

Edited January 24, 2018 by geert_2

[skc5741 0]

4 hours ago, geert_2 said:

The dummy cooling towers are waste, or could be re-used as "hotels" in Monopoly or other games.

 

6 hours ago, gr5 said:

Just a small 1cm by 1cm cuboid of the exact same height as the bishop will do.  And place the other object in a position such that fans are cooling your bishop while the other object is being printed.  Even just 3 seconds away from this part should give it plenty of time to cool.

 

The dummy tower does feel like quite a waste, especially if the dummy towers end up using more material than the actual part. I like the idea of the 1x1 tower in addition though, I'll give it a try!

 

Thanks so much for your help!

Sam

Edited January 24, 2018 by skc5741

[peggyb 171]

and how about using the draft shield, will that have the same effect?

[geert_2 558]

@skc5741: The above "tiny real model" is a 10mm x 10mm x 10mm block, plus extension of 3mm x 3mm x 5mm on top. I will add these dimensions to the drawing for clarity. You are totally right that the dummy is waste, but otherwise the model can't be printed well (unless I would print 4 at once, which is not always desirable). So it wastes less than discarding too many real but malformed models. That is the idea.

 

Dummies are only required for tiny models. Once the model gets a bit larger, lets say 20mm x 20mm, this is no longer necessary usually, because each layer has enough cooling time anyway.

 

The dummy can have any shape you wish, but subtracting is sometimes easiest to hollow it out in areas where no extra cooling is needed, to reduce waste. Here I used a simplified dummy (model height = ca. 10mm) to provide enough cooling for the top part. It works good enough. This is part of a real design, not just a demo.

 

 

When printing solids or highly filled models (70...100% filled), like in my case, cooling is more critical than when printing hollow models, since solids require to dissipate much more heat through less surface area.

 

Concerning your bisshop: I don't know how big it is, but if its diameter would be 10mm, then subtracting it from a rod would be the easiest way to create a dummy. If its diameter would be 30mm, it could probably be printed without any dummy, by printing as cool as possible (ca. 185...190°C for PLA), and slow, and with a gentle desktop fan in front of the printer. But I think it looks quite good the way it is now, and I am not sure it would get much better with a dummy next to it. Even with a dummy, you might still have little defects where the nozzle takes off and lands.

 

@peggyb: if printing a draft shield takes enough time to let the rest cool, it could work, of course. Any method that provides enough cooling, enough equalisation of layer printing time (if required for quality), and that keeps the material flow constant enough (again, if required for quality), is a valid solution.

 

You could also add a desktop fan in front of the model, or carefully blow cold compressed air onto tiny areas for extra cooling (I have used these occasionally), although these methods might have other side effects.

 

Just don't let the printer print "spaghetti" somewhere else in mid-air while cooling your model, because the spaghetti would get dragged around and into your model, and it would accumulate on the nozzle (I tried it, out of curiosity). This is another reason why I add a baseplate under hollow areas in the dummy, to prevent spaghetti from the overhangs getting dragged around.

 

I would suggest: add this concept to your inventory of tricks, but don't throw out any other proven methods.