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Need retraction only when crossing or starting or ending in an outer wall

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When printing gears or other prints with small distances between objects, there is a real need to have retraction only when crossing, ending in or starting in an outer wall.  It would be really nice if there were an option to set retractions to only occur at those times.  The minimum distance feature is nice, but does not completely cover the issue when there are outer walls close together.  As a result of having to set the minimum distance so that retractions can occur between the objects, there can be a HUGE number of unnecessary retractions, which causes extra wear and tear on the printer (especially the extruder) and a much longer print time than is necessary.  Combining the minimum distance with a forced retraction when crossing outer walls would result in a near optimum number of retractions.

P.S. Sorry if this has been covered before.  I couldn't find anything after reading a number of pages worth of forum topics.

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As far as wear and tear - I wouldn't worry about that.  I've seen prints that do a kilometer (a kilometer!) of retractions in just one print (voronai vase).

 

If you don't retract you will get stringing, right?  I'm confused - how can retractions be bad when going from one gear to another?

 

In cura it shows light blue and dark blue lines.  One is retraction moves and one is non-retraction (and non extruding moves).  I assume you know this.

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So the slicer, when doing one slice, does it by mathematically intersecting a plane with all the triangles (all of them!) to get line segments.  Then it stitches these randomly ordered (unfortunately there is no helpful ordering in an STL file - no reference of which triangle is supposed to be connected to another) lines into "islands" or "loops".  So let's think of what you are printing as "loops".  There are "inner loops" which surround a void inside your part such as a vertical cylindrical hole but most loops are meant to be printed with material on the *inside* of the loop.  An island is a loop with material on the *inside*.

 

Now that I have definied "island" and "loop" we can talk about retractions.

 

If the printer is moving within a single island and "combing" is enabled it will not retract.  It will move to the other position without leaving the island and without crossing an inner loop.  Combing is on by default so the printer SHOULD NOT RETRACT.  Maybe you disabled combing?

 

I often recommend to turn on combing except for skin layers.  Maybe you did that and you are talking about a skin (top or bottom most in that region of the part) layer?

 

When moving from one island to another it will always retract (well - except for some other rare rules that you seem to have noticed like minimum retraction distance).

 

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On 8/30/2018 at 9:48 AM, gr5 said:

So the slicer, when doing one slice, does it by mathematically intersecting a plane with all the triangles (all of them!) to get line segments.  Then it stitches these randomly ordered (unfortunately there is no helpful ordering in an STL file - no reference of which triangle is supposed to be connected to another) lines into "islands" or "loops".  So let's think of what you are printing as "loops".  There are "inner loops" which surround a void inside your part such as a vertical cylindrical hole but most loops are meant to be printed with material on the *inside* of the loop.  An island is a loop with material on the *inside*.

 

Now that I have definied "island" and "loop" we can talk about retractions.

 

If the printer is moving within a single island and "combing" is enabled it will not retract.  It will move to the other position without leaving the island and without crossing an inner loop.  Combing is on by default so the printer SHOULD NOT RETRACT.  Maybe you disabled combing?

 

I often recommend to turn on combing except for skin layers.  Maybe you did that and you are talking about a skin (top or bottom most in that region of the part) layer?

 

When moving from one island to another it will always retract (well - except for some other rare rules that you seem to have noticed like minimum retraction distance).

 

I usually have combing turned except for skin layers.  I still see a lot of retraction in the infill.

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That doesn't sound right.  Can you show a screen shot of one layer showing this?  Make sure to include the blue lines (I think the blue are disabled by default).  Also keep in mind that skin layers are any layer where any part of the layer is a "top" layer.  So if you print a pyramid then every layer is a skin layer (I think - I'm not 100% sure on the definition).

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For printing gears, which are usually low profile, maybe you could print them one by one, instead of all at the same time? Then the printer will complete the first gear without jumping anywhere else, before starting the next? Not ideal, but maybe it is a temporary work-around?

 

However, if the gears would be very small, insufficient cooling might become an issue. For diameters like 20mm, it should be okay.

 

Another disadvantage is of course that you don't get as much models on one plate: the head neads enough room so it doesn't crash into already printed items.

 

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