Start of print not extruding for several cm

[BaronWilliams]

It would seem to me that it would only take a few lines of code. All of the prompts and controls are in the GUI already. The software is already generating the skirt geometry. It just needs to apply it to both extruders with an offset. This is how it should work (similar to Repetier):

Edit Process Settings > Layer/Right Extruder > [check] Include Skirt/Brim > Set Distance > Select Left Extruder > [check] Include Skirt/Brim > Set some different distance (S3D should know this from the right extruder settings)

You can also select how many layers and perimeters as done now. Then there needs to be a good way to save these settings that don't require an export and import (entirely separate problem).

If we're going the skirt route, then it also needs to have an option to build the skirt to the last printed layer for each print head that prints a skirt. You won't know how many layers your print object will need ahead of time unless you manually calculate it from the layer height (keep in mind that the first layer height can be different). And if you have multiple processes where layer height changes here and there, then it becomes a mess to figure out how high to make your skirt based on the number of layers to use. It's not practical to have to enter the layer height when you wan't to use it this way.

Is there a patent on the "purge wall" concept?

[jimc]

i cant say anything for a skirt for both nozzles BUT to do a skirt the same height as your model set the layers to 9999.

[BaronWilliams]

Thanks for the tip jimc! I just tested that out. If I specify a large number for the skirt, like 99999, it automatically clips the skirt height to the highest layer of the printed model. So automatic height matching is already there for the skirt. I never knew that. Very nice to know

So all we need is the ability to have each head make it's own skirt at a different offset and we're all set.

[BaronWilliams]

After learning about being able to make the skirt auto-adjust to the layer height of the printed model (I didn't know this was possible) I found a way to get 2 purge walls. I haven't tested it with a print job yet but I plan to later. It looks correct in the G-code viewer.

Here's what I did.

Make 2 processes, one for each extruder. I made the right and left extruder processes use a skirt of height 99999 so they auto-adjust to the height of the printed object. I set the skirt offset to 4 for the right and 8 for the left so they have their own devoted purge wall. I made the right process print the object without support by disabling support. I made the left process print the support only by enabling support and setting "Top Layers", "Bottom Layers" and "Shells" to zero.

It seems like this should work fine. It looks perfect in the G-code viewer. I will test this later. If this works, I'm very happy

[jimc]

Baron, give it a go and let us know how that works out. There are all kinds of things that s3d can do. Problem is many are hidden like the 9999. More people would know of its capabilities if there was a stinkin manual.

[dennisjm]

Another thing you might try is in the tool change script you could do something like

retract [old_tool] (some bigger amount than normal retract)
advance [new_tool]

I think you'd do this in just the 1st process.
That may help with ooze like normal purge walls do. I haven't tried it yet though. I've just been using Makerware for dual extruder prints instead.

I can see a problem with this in getting the sync right with the normal retract and change code so that it doesn't retract a tool that's already retracted or something. Maybe it can work though.

[BaronWilliams]

Using my dual head FlashForge Dreamer printer I tested a print using the method I mentioned above and it worked great. I am quite happy with the results

So two independent "purge walls", one for each head, are easily created by using separate skirts each set at auto height (a skirt layer height value of 99999), and each using a different skirt offset. Of course, each head is using it's own independent process.

This did the job.

I didn't have any extra retraction settings. Since this print wasn't all that large, but large enough to always show a dual head print start problem (hence the main need for a "purge wall"), I wasn't concerned about retraction. On a larger print, I will need to be concerned about retraction.

Had I printed this object without the dual skirts rising to the top of the print, each print head would have failed to print for about 10-20 mm for each layer. The "purge wall" created using the skirt prevented this problem.

I do have some concern about wasting plastic. If this is a very large print, the skirt is going to waste too much plastic. It's useful for about 30 mm, and beyond that it's just wasting plastic.

So while this trick to get a simulated "purge wall" is working, it still needs a bit more options to make it work well.

If we had the option to specify the maximum skirt layer length in mm, we could prevent the skirt from wasting plastic on very large prints.

[BaronWilliams]

I can see a problem with this in getting the sync right with the normal retract and change code so that it doesn't retract a tool that's already retracted or something. Maybe it can work though.

As long as the "purge wall" is long enough to take care of such sync problems, it should be fine.

The problem in doing all this stuff manually is that different sized prints are going to require different amounts of retraction and different sized "purge walls". With a very tiny print, a "purge wall" and retraction isn't really needed. As the print gets larger, the needed "purge wall" size and retraction distance gets larger. The length of the retraction that's needed also varies by filament type.

The "purge wall" only needs to be so long. Once it goes beyond a certain size, it's just wasting plastic. That's the problem in using a skirt to make a "purge wall". Once your print is larger than 10x10, the skirt becomes too large and begins wasting lots of plastic.

I am still quite happy though. The skirt "purge wall" trick is working great depiste a few shortcomings.

A LOT OF PEOPLE WILL NEED TO KNOW THIS DUAL EXTRUDER PURGE WALL TRICK
Where is the best place to post this? This thread is not the best place for this. Ideally this trick should be posted on the Simplify3D web site, not on the forum. How can we get this information posted on the Simplify3D web site? It really belongs there. Should I contact customer support?

[dennisjm]

If you're really missing several cm of plastic each extruder switch, your "purge wall" is going to have a hard time sticking when the 1st layer misses a portion of the circle.

Part of the magic of the makerware purge walls is that each wall is started with a bucket purge so that the wall itself is always actually extruded. If you use the wall as the purge process it'll have issues.

Anyhow, you can simulate the makerware purge walls with enough separate processes and tweaking settings. It's too much work to do more than on a tinkering basis though. I'm printing all the time for various different priorities/needs so I just load up Makerware if I need a dual extrusion print.

Maybe S3D 2.2.. guess we'll see whenever that day arrives.

[BaronWilliams]

If you're really missing several cm of plastic each extruder switch, your "purge wall" is going to have a hard time sticking when the 1st layer misses a portion of the circle.

My prints always miss a portion of the skirt. The skirt still sticks and still builds up several layers with different missing portions on each layer. Skirt failure from missing sections a few centimeters long has not been a problem for my prints.

UPDATE:
I am now doing a very large dual head print. It is 56% complete. On every single layer the two skirts are missing 1-4 centimeters at the start position of the skirts. The left head is missing more than the right head (because the filament is different). This isn't causing any problems at all because as the heads go around their own skirts they eventually start extruding. If there's nothing below to support the extrusion, because the previous skirt has missing extrusion at a certain portion of the start of the skirt, the new extruded filament hangs from the head until it gets to a portion of the skirt that's not missing anything. then it all comes off in one lump. This makes the skirt look bad, but who cares.

The 2 skirts are now about 64 layers high. With the missing pieces on every layer, and some lumps, the skirt is sticking just fine to the Kapton tape and it's building just fine. It's wasting a bit of plastic because I only need about 5 centimeters for each head's purge. But at least it is working fine.

Thanks to the two auto-height skirts the model is coming out PERFECT!

We just need some minor tweaks to get this perfected.

It would be nice to have a real "purge wall" in the next version, but until then, I am happy with this double skirt trick