Understanding Filament Extrusion

Hi All,

Now, that I am capable of depositing plastic into my surface, there are a few observations I have made which I want to corroborate with the experts on this arena. The following observations were made printing the whistle with white PLA at 220C, 20 mm/s feed rate, sprinter firmware, ReplicatorG software.

The extruder is always moving at the same speed, regardless of whether I feel the filament going in or not. At some times, I feel compelled to push on it with a pair of pliers, and I do know for a fact that in some cases there is plastic coming out only because I am pushing the filament. This is easier to see when the skirt is being placed. On some other cases, however, I am not pushing and plastic is being deposited so I am tempted to think the way this works is that the extruder is moving so that when the hot end reservoir is empty, more thread goes in and get molten.

Why is this important? Well, my impression was that only if thread goes in, plastic is deposited out. In other words, I need perfect friction and zero slipage at the shaft/filament interface if I want to provide a continuous plastic flow. But appatently this is not the case. Apparently, slipage is part of the game! Am I correct, or am I missing something?

If I am correct and slipage is part of the game, then it makes little sense to improve on the friction. For example, right now I have a ball bearing pushing against a serrated shaft. I was thinking about adding some laserations into the ball bearing and enhance the frictions. Seems to me I do not need this.

Opinions?

BTW, second question. As I am printing I see the nozzle kind of diving into the plastic. Almost as if it was carving the deposited plastic as more plastic is being deposited. Is this as it is supposed to be or is there something wrong with this? I must add the print looks awful but considering I haven't fully tuned the darned thing this is expected. But I need to know which is precisely what needs to be tuned so I want to make sure I do not waste time tuning that which is expected.

JIQ

Edited 1 time(s). Last edit at 10/27/2012 06:31PM by avayan.

Slippage is definitely not part of the game, and filament shouldn't slip. Maybe extruder is driven too fast, either because of incorrectly set STEPS_PER_MM, or using absolute units in slicer and relative in firmware. Or there's to much friction in hotend, temperature too low or problem with filament drive (hobbed bolt, springs, etc).

Edited 1 time(s). Last edit at 10/28/2012 04:59AM by miso.

Hello,

I have encountered the same problem with my printer, and I'm pretty sure that the steps/mm are ok.
Is there anything more we can control in the firmware?

If your plastic feed is slipping while printing at a feed rate 20 mm/s (x and y speed) and you're sure that you have set your extruder steps/mm correctly, then something is wrong with your extruder or hot end. How fast can you extrude in air? Note that in Pronterface, the speed is in terms of mm feed/s instead of mm of extrudate/s.

Hi avayan,

First thing first. I am not going to put on the "Expert Hat", but I have done a lot of research and built a few bits myself and this is my advice:

As mentionend above, NO slippage is allowed by the extruder. The extruder has no intelligence so to speak so it will never know when or where to extrude any plastic. This is solely controlled by the electronics (with firmware and software). If the electronics are telling the extruder motor to push plastic into the extruder then plastic should also be extruded from the extruder/nozzle. Never should plastic be extruded when NO plastic is pushed in by the extruder motor. If that happens it is normally called ozing. And to avoid that you would normally configure your printer to retract filament back up to reduce pressure inside the extruder (nozzle). This is done in the slicing program you use.
To be able to obtain full control of the plastic going in and out of the extruder the filament must not have any slippage at all against the hobbed bolt or you will get imperfections or total failure of your print.

If your nozzle dives into your print, you are maybe extruding way to much plastic (more than your layer hight in thickness) or is your nozzle coming loose and being pushed towards the bed as a result of the pressure in the nozzle?
Raise your nozzle up a bit and extrude in thin air. Carefully watch the nozzle and see if there is any vertical movement of the nozzle. Stop and start several times to check for any movement. No movement is allowed. Only z axiz is to control the up and down movement.

It is vital that you have an extruder and nozzle that works flawlessly or you will be chasing your tail working out why your printer is not printing properly. Make sure it is calibrated 100%. Make sure it can extrude plastic at the speed you want it to print at without failing.
Make sure your x,y and z are all perfectly calibrated.
It can at times be frustrating......

Regards
Jan

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