Monday, 15 August 2011

Preventing Jams: Contradictory Constraints

Recently (okay, a couple weeks ago) after leaving my reprap printing while out buying food, I came home to find this calamity:
Extruder body print ended with some wispy filament and then a big gap of nothing.

A quick look in the side of my Adrian's extruder showed the brass M4 insert driving the filament to be very clogged up with plastic, to the point that I'd have to dis-assemble the extruder to be able to clean it properly.
Those teeth would not get a grip on the filament.
While taking the gear from the body I also found that I couldn't retract the filament that was already in the hot end, and so had to dis-assemble that too. Upon releasing the PTFE insulation, I found that after the extruder stalled, the heat had travelled up the hot-end assembly far enough to cause a small bulb of melted ABS to form in the slight gap where the filament left the bottom of the extruder body and entered the PTFE insulation.

The blob on the right that jammed the filament in place while I tried to remove it was a bugger to get out even after heating the whole thing back up.

After re-assembling everything I supervised the reprap trying to print another extruder part. It seemed that sometimes after making a few moves that were quite short, just as it was about to start another line of extrusion after retracting the filament for the last move, the extruder would make a short buzzing sound as the pinch-wheel lost its grip on the filament.

It took many days and tests to figure out what was going on here, and I narrowed it down to these somewhat contradictory issues:
  1. As the filament is retracted further, the pinch-wheel has to pull it harder and eventually loses grip, however if it is not retracted far enough, some plastic will ooze from the nozzle and leave strings across gaps in an object.
  2. As the nozzle temperature drops, the extruder has to push the filament harder to get it through the nozzle, however as the temperature rises, more heat can travel up to the extruder body and soften the filament coming in; this is especially pronounced with PLA.
  3. The standard brass M4 insert used as a pinch-wheel in Adrian's old extruder design does not give much grip on the filament, reportedly compared some of the hobbed bolts used in Wade's extruder, and especially wears out faster, however to use that I first need to build the extruder parts, and it was whilst attempting such a build that my extruder failed...
Anyway, to solve the problem I have reduced the retraction distance from 2mm down to 1.4mm (this seems to have made the most difference), and I no longer go by nophead's practice of dropping the temperature after the first layer unless I'm making a very small one-off object that would stay too molten otherwise. It should also be noted that the speed of printing places a limit here, as if the extruder is trying to work too fast then the power going to heat the nozzle won't keep it up to the target temperature during extrusion.
The best way that I know of to solve the problem of waste heat softening the approaching filament is to attach a small PC cooling fan to the body of the extruder, however that will require me to design and make a bracket to hold the fan in place, as it is not standard to fit one unlike Adrian's mini extruder.
Nevertheless, I am back on track now and should be able to make some plastic component sets for sale.

On a big tangent, I was also reminded recently of one reason that some people are keeping a 3D printer around, in short, to counteract the shoddy workmanship and occasional planned obsolescence characteristic of products made by for-profit companies.
For example, normally I would steer clear of any kitchen utensil with a bi-material handle, but had no choice on one occasion...
This can-opener that had a thick bit of steel travelling into the handle seemed sturdy enough, until this happened.
At first, I thought the plastic had given way from the steel, until I saw that beneath the top of the handle was a hole drilled or punched into the steel.
The plastic doesn't fill the hole, which serves no purpose but to cause the material to fail by creating a stress concentration alongside it. One definite case of planned obsolescence.
I'll probably make a better handle for it if I have time. Not all is bad however, as my nice friend peter has lent me some old calipers built in an era when such shoddy production was generally unthinkable. From the looks of them, they will likely outlive me, and just as much could be said for some other things we have, such as solid stainless steel cutlery.
If we lived in a sane society, we could make just about all our kitchenware out of this world's abundant iron and chromium, and share a kitchen equipped with it between several flats, with dishwashers more like today's hospital autoclaves than the rubbish that fills the white goods market. There would be no more wasted glass and clay from my own clumsy breakages, but sadly we don't live in such a society yet.

No comments:

Post a Comment