Retraction causing Major Major Major underextrudion

[miburns]

Hi All,

Normally I just bang my head against a wall for months, but I have a child on the way and really want to print some baby toys in the next month. Has anybody seen this problem???


I just finished setting up my printer with major upgrades. It is a large delta printer with a flying extruder. The extruder is a BMG clone. There are about 8 inches of Bowden tube between the flying extruder and the hotend.

Retraction is KILLING my prints. The prints look fine without any retraction (as long as no jumps that would make strings are present). But once I add retraction, I get MAJOR underextrusion. I had this problem on my previous DIY extruder (which I just switched out for the BMG clone, so I know it isn't particular to the physical extruder). I printed the main 4 tower retraction test at 100% scale and got the following results (shown in image: from left to right, retraction = 0, .1, .2, .3, .4, 1). My retraction is fast (3000 mm/min). From the looks of this small scale retraction test, .4mm is the winner, but then when I printed benchy I still got stringing. My assumption is that The skinny towers don't require a lot of plastic so not a lot of plastic pressure builds up and less retraction is needed. I did the same retraction test with the object scaled to 200% (simplify 3D). This image is also attached. The item on the left is a solid print with no retraction (there are a lot of strings and blobs, but the item is solid). The item on the right is .5mm of retraction and if I poke it, the towers fall over. That is how brittle and underextruded they are. Has anyone seen this. Can someone point me in the right direction?

[S3D-Jake]

In my experience bowden extruders need far more than 0.4 or 0.5mm retraction. I'd start with 2mm retraction and 0.4mm of coasting. This may be extreme for the small pillar tests but I'd bet that'd solve the issues with your benchy tests.

[miburns]

Hi Jake, thanks for the reply. I know those are typical for large Bowden tube setups. I have a flying extruder, so my Bowden is short. Either way. Above .5 mm retraction, it was like a step function. Boom, the print would become frail and brakeable. I found the problem was with stealthchop. As soon as I turned off stealthchop for my extruder, I started getting great retraction ability and was able to increased it quite a bit to make my prints good.

For configuration tracking for anyone else who has this problem....

I am running:

Marlin 2.0
BTT SKR V1.3
TMC2130
Stealthchop (which I turned off for extruder)
BMG clone extruder

Best of luck everyone!