Idle extruder hitting printed object

I wonder if someone can help me. As I have started printing larger and taller parts I have the problem of the idle extruder knocking against the object being printed. I have checked the heights of the two extruders and they appear to be identical. This problem does not occur until the object is at least 40mm in height.

To illustrate the sort of problem I am experiencing, imagine I am printing a part that has two separate identical vertical sections that are spaced the same distance as the separation of the two extruders. Now assume that the slicer software has chosen to print the left hand section first. The active right hand extruder will lay down whatever the layer height is and then move to the right hand section with the left hand section over the layer that has just be laid. Now if the extruders were, as I stated earlier, at equal heights then it is not obvious why the left hand extruder should start hitting the part on the left that has just had a layer added.

All of this is new to me so I would appreciate others perspective on this particular problem and how they ensure that their twin extruders are at the same height.

Check if there’s not a small amount of filament bulging out of the unused nozzle.
Also check if the bed is perfectly flat and at the same distance from the 2 nozzles everywhere.

Thank you for your reply Raby. I do get what appears to be a high spot in the middle of about 0.05mm if measured by the height to the nozzle. However, I am printing on borax glass which I have checked on my milling machine bed and can detect no such imperfection in the glass. I therefore think that what appears to be a high spot is actually the print head deflecting under its own weight when it is in the centre.

I am redesigning my hotend mount as I am using e3d v6 heads and I am trying to minimize its length on the y axis and I will see if the problem exists with the new mount.

My guess is that others who have read my post haven’t responded as they probably consider that it is obvious that if the two nozzles are at the same height then what I am experiencing can’t occur. They may be right, but I did wonder if I was over extruding then when the plastic cools would it draw back in the centre and the resulting deposited height be greater than the gap between the active extruding nozzle and the object it was printing on and hence the following nozzle would be at risk of hitting the newly extruded material?

That could be it too. The default values are too high for the Vertex hot-end and it’s over-extruding. It could be the same for the E3D. 190° for PLA and 225 for ABS is enough. And I reduced the flow to 75%.

If the flow rate is too high, I’ve seen extrusions pile up on either side of their intended path and be hit by the extruder head during travel. I don’t know from dual extruders yet, but I would assume they are both heated during a print? Because if not, both extruders may actually be hitting the print but the printing extruder, being heated, would melt its way across the print, producing a less-noticeable impact than if it were cold. Conversely the idle extruder, if cold, would slam into the plastic with more impact. /theory

Like several others on here,I also have a high spot in the centre of the print bed. Thought it was the aluminium bed, tried hitting it with a hammer, didn’t improve it much! Tried padding the corners of the bed with tape. This was when I realised that I can slip a fine feeler gauge between the underside of the glass and the aluminium bed. I can do it even in the centre, therefore the raised centre is nothing to do with the aluminium bed underneath!

I’m very curious what you mean by the weight of the head deflecting in the centre - how does that give you a raised bed?

As to one of the heads hitting a print, this has only ever happened when I’ve over extruded over several layers. Once you get nice even surfaces layers where there are no ‘head’ marks as the head moves away, you shouldn’t really have either head hit a print.

But there is one possible issue with this. As your prints get taller, they may shrink unevenly, depending on their orientation to the fans. If that happens, it could raise one edge of the print high enough to catch one of the heads. You probably won’t notice it catching the hot head, but you would certainly notice the cold head catching it as there is no give, just crack!

I’ve not had this happen to me, but this is one of the reasons that Ultimaker gave up trying to do dual head configurations.

In answer to your question what I meant was that the cross bars supporting the print head are like two beams which are supported at their respective ends. As the load (print head) moves to the centre so the deflection, which all beams are subject to, will increase. (Remember I only measured .05mm difference or 1.2 thousandth of an inch.) Therefore the gap between the print head and bed will decrease which is exactly the same measurement as one would get if the bed had a high spot.

If you are correct that it is the fact that the idle head is cold that exacerbates the problem then when it next occurs I might repeat the print but with the idle head also hot.

Thanks for your comment. I am intrigued by your observation that you observed a gap between the glass and the aluminium bed both at the edges and the centre. I can’t understand what was raising the glass up if it was not in contact with the aluminium below, or have I misunderstood you.

[quote=“rrekwa”]Thanks for your comment. I am intrigued by your observation that you observed a gap between the glass and the aluminium bed both at the edges and the centre. I can’t understand what was raising the glass up if it was not in contact with the aluminium below, or have I misunderstood you.[/quote]That was because I had (mistakenly) padded the corners thinking the aluminium bed was at fault and it was deflecting the glass!

I see what you mean about the beams / rods. I think you are right, which means there is basically no cure to that issue except to use larger diameter rods (or stiffer / different profile? Or maybe stainless steel tubes?) which would require a major redesign - something for @Velleman to consider for the next iteration? Not something that we can fix here, unless someone knows of something with the same diameter that could be used which wouldn’t sag?

It’s an annoyance, particularly when you have large prints as there is a limit to how close you can let the print head get to the bed in the centre which means the gap at the four corners can be quite large, leading to poor adhesion and those corners lifting on long prints.

I know exactly what you mean as I have had the same problem although I am now using a0,4mm noxxle with a 2.025 layer height so the problems becomes less acute. I have also taken to increasing the first layer height which seems to be quite effective.