Problems with a new build

I finally got my prusa to a somewhat working state. After the initial build, I had several problems with the makergear hot end, so I decided to go with a jhead and I am having MUCH more luck. However, I am still having some issues.

I am printing PLA, so according to the hotends.com web site, I need to blow a small 70mm fan across the insulator of the hot end. I did this, but because of the mounting technique of the extruder and hot end, I had to angle the fan back so that it is more pointing towards the heated build platform (MK2). Two long-running prints in a row had lifting of the print prior to completion. In both cases, it was in the rear of the printer - near where I had the fan. I tried again with the fan off, and it was somewhat better, but I'm worried about not using the fan with the jhead hot end. What happens with printing PLA if I don't use the fan? I observed no ill effects but don't want to cause long-term issues..

This lack of adhesion also makes me wonder about the heatbed. For PLA, I have it set to 60 degrees, and it stays pretty much at that level. I tried lowering it for the second and subsequent layers, but that had no effect. I AM suspicious of the accuracy of the thernistor. I have an IR thermometer, and it reads 50- 53 degrees while the thermistor is reporting 60, but I'm not sure how accurate the thermometer is because they disclaim its use on reflective surfaces - like glass!!

So I'm not sure where to go next. I think I'm going to bump the bed temperature up a bit assuming the thermistor is inaccurate based on the thermometer readings.

Any other ideas?

Thanks

You can use a smaller fan than that, you just need to disturb the air around the Hotend.
What commonly happens with PLA is as the heat creeps up the Hotend, the PLA starts to soften inside the PTFE tube in the Hotend, the extrusion pressure cause the softened PLA to mushroom and create a jam in the PTFE tubing.
Some people seem to run fine without a fan, my first printer wouldn't run for more than 20 minute without one, I've never run PLA without a fan since because I hate cleaning out the blockages.

I actually mistyped my first message. The fan is 40 - not 70 - millimeters. It.s actually small enough to fix between the x axis smooth rods, but I had to mount it in the rear until I can print a different x carriage. I might try to rig up something temporary to mount the fan on the side. I can then raise it a little higher and perhaps even pitch it up.

Try setting the heated bed temp higher. I sometimes have trouble getting certain PLA to stick to plain glass. If you get some PVA glue (basic white glue, such as Elmer's) and dilute it 1:4 or so with water, and spread that on the glass, it'll make the PLA stick really nicely.

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I can run PLA through a J-head without a fan. It might be because the end is sunk into the Wade's body by 10mm. I think that keeps the entrance cool.

The outside of the J-head, where it meets the extruder base is definitely too hot due to the convection from the heater block but the inside, 10mm higher, must be cooler.

To measure the temperature with an IR thermometer you can stick a square of black tape to the glass.

Edited 1 time(s). Last edit at 07/27/2012 03:06AM by nophead.

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I haven't tried the PVA glue yet - a definite thing to do. But one thing I noticed after some experimentation is a lack of temperature uniformity on the heat bed. Within pronterface I'll set the temp to 60 for PLA and I'll measure 65 on one side of the bed and 50 on the other. I suspect that no matter what I do I'm going to have lifting issues until I get the heat uniform. The vendor I purchased from actually reached out to me before I realized I had a problem, and a replacement PCB is on its way to my house as I write this.

Aside from this lifting issue, everything is working beautifully. The only other thing I noticed is that things tend to print just a bit too big. I have double checked all of my steps settings and triple checked the e steps setting. But when I print things that I would expect to snap together - like the planetary gears from thingiverse - they don't. The holes in the gears need to be reamed out and the planet gear assembly does not fit into its ring gear. It's as if I'm extruding too much plastic making everything just a bit bigger. But like I've said, I have repeatedly calibrated my e steps setting. I was thinking of setting my flow rate less than my feed rate (skeinforge). Based on my rudimentary understanding on the slicing math, it seems to me that this would thin the plastic a bit; maybe it would give me the desired result.

I feel it's very much dependent on the quality of the PLA. Mine runs fine with no fan but I use a 50mm fan with a cone anyway. you can always change up the fan speed if it is too fast and try your hand at designing a directional cone thing, tinkercad is really simple otherwise search thingiverse.