Wednesday, February 24, 2010

Extruders Pt. 2: Add oil

In China, you can encourage someone to work harder by saying "加油!" (jia you), which means "add oil". Apparently that's encouraged our extruder to work harder as well, because adding oil did just the trick. A drop or two of machine oil, applied sparingly to the inner surface of our stainless steel tube on a scrap piece of PLA filament, has got everything finally working smoothly.

I should add that our plastic extruder nozzle tip diameter is unusually large; this is done "just to make sure it works". As the plastic printing is a secondary aspect of our project, we're not too concerned with plastic print resolution and precision for now. On the other hand, an oversized nozzle gives us something like twelve times the print speed, allowing for the printing of bigger objects. Which leads to one of my other observations; perhaps in a multiple-print-head system, a fine nozzle could be used for printing the part perimeter, while a fat one handles the infill. The infiller could even be something as simple as a glue gun running on hot glue, which would actually be quite ideal (cheap, widely available, tough, sticky).

While backflow was undoubtedly an issue in our old extruder, some problems persisted in our new, tight tolerance one. The plug was gone, but extrusion would still fail after cooldown. To see if backflow was causing the problems again, we tried this procedure:

1. Feed fresh PLA
2. Heat up
3. Cool down
4. Heat up
5. Attempt to extrude

As predicted, it was unable to extrude upon reheating. No attempt was made to extrude on the first warm-up this time, so there would be no pressure to cause backflow. If backflow was still occurring it would be due only to the capillary effect. To see if this was causing the problem we removed the filament for postmortem analysis; however, no plug was visible. The diameter was very slightly wider near the tip, where it had warmed up.

We concluded that the softened PLA either had too much friction with the stainless steel surface, or was able form some kind of chemical bond to it, and this was strong enough to prevent extrusion.

Adding a drop of machine oil to the extruder resolved the problem nicely, either by lubrication or by physically preventing the chemical bond. You might ask if this would foul up the extruded PLA, but the oil doesn't appear to get consumed; it apparently stays inside, clinging as a thin film to the steel surface.

In summary: If you find that you can successfully extrude for a little while using a fresh piece of plastic, but it jams the second time, "加油!"

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