First layer AFTER raft is printing too fast!

[dkightley]

Perhaps I might make a suggestion to all who have posted on this thread convinced that there is a "bug", and also to those who have posted on the thread I linked in my post above....also firmly convinced there is a "bug".

Everyone is looking at either the gcode or the gcode simulation and concluding the first layer of the model after printing a raft is wrong because it's different to the layers above it. Nobody seems to be thinking about WHY the layer is different.....or even WHY SHOULD it be different! Let's go back to first principles.....

The raft has been printed and now we want to print the model on top of it.....but we don't want the first layer of the model to adhere to the top of the raft in the same way as we want layer 2 to adhere to layer 1, etc. So...how do we extrude the first layer so it doesn't amalgamate with the layer below it but does stick very slightly?

The way in which the S3D boffins have done this is to firstly lift the extruder up so the extruded plastic is not "squeezed" into the layer below it. In fact it is extruded somewhat into free space and gravity is used to drop it down onto the top of the raft...enough to make it very slightly adhere, but not that much so it sticks so it can't be separated. To aid this delicate "sticking but not grabbing" process, the layer is extruded at the default printing speed...and the fine control over the process is achieved using the Separation Distance....ie the height at which the filament is "dropped".

This method must have worked well when it was developed....otherwise it wouldn't have been done that way!! Perhaps nowadays with changed and more exotic materials, this process now needs the capability of having more adjustments to cater for the varying properties of the plastics used.

I don't see this as a "bug"...either as the layer being "printed too fast", or as the second layer being "printed below the first layer" (as reported in the other thread). What I do see is that the way in which the first few layers of the part are printed so to be able to control the adhesion to the raft does need revisiting to see if it can be enhanced to make getting the right conditions for adhesion and separation somewhat easier and more consistent. It could be that printers in general are printing much faster now that when this bit of functionality was developed...and some slowing of the print speed is now needed. Again....it needs revisiting, not fixing!!