Skip to content

July 17, 2017

3d Hot End Temperature Problem

Ever have one of those “problems” where you spend endless amounts of hours trying to solve it, doing everything thing you can, “By The Book”, only to be stumped at every turn?

Yeah, me neither.

Until 3 days ago.

In the Beginning

Last year we had a new fence put in, and over the winter and the heat this summer, the not so stellar post caps decided to split, curl and look more like pagoda roof lines than post caps.

IMG 1201

Which is not a big deal until you consider than the rain will end up on the post instead of being diverted by the cap.

I spent some time with Tinkercad designing a new post cap, hollow up inside, over hangs the edges of the post so there is no way the rain can land on the post.

IMG 1202

The Design

The design is pretty simple. I mean, it’s a post cap. How complex can it be…

IMG 1228

It prints upside down, top is about 86mm square, base is about 146mm square (when mounted on the post). The sides are 45 degrees so no support. But it is fairly big so it does take some time to print out.

The Ugly

When I started the first print, I was running the perimeters at 30mm/s, the infill (what little I used) was 50mm/s. After all nothing but a bunch of long straight runs.

And then I looked at the temp stability. Or more specifically the lack thereof. Crazy, 10C higher than set point, 12C lower, and oscillate like that for the entire print job. if it settled down it was only for two or three minutes. Then back to high and low sweeps.

I managed to get the print done, decided I’d use the other printer I have. Exact same symptoms. Large temp swings during the print.

Next I loaded up a different print as a test and printed it out. Temp was rock solid. Back to the fence cap. Wicked temp swings again. Do this enough and the result is male baldness.

The Solution Search

Off to the internet I go to see if anyone has the same problems. Only about 500,000 people do. Safety in numbers you know.

So I started reading about possible “solutions”…keyword, “Possible”…

Arduino AREF isn’t stable. Turns out if you have flaky 5V supply line this can happen. Mine is rock steady.

Shield thermistor leads. According to the theory, the PWM signal from the heater element can create “noise” on the thermistor leads. So I stuck my scope on it. My cables are all wrapped in a bundle so you’d think there’d be lots of “noise”. My conclusion after testing was the “noise” was caused by someone not knowing what the heck they’re talking about. In the one case it did fix, turned out it was a bad connection not noise. Moving on.

Heater damaged. As I happen to have spare parts for my printers, I replaced the heating element. No change.

Thermistor. Now we’re getting into the grey area. Damaged thermistor, loose wire, bad Dupont connector, RAMPS problem (should be using RAMBO or some other 32 bit control board), fan blowing on it, wrong setting(s) in firmware, not PID tuned, and broken leads. Except this does not explain why a different design prints with perfect temp and this stupid fence cap doesn’t. None the less, I tested three different thermistors. And got the same huge temp swings.

None the less I made a lot of adjustments to the firmware in the section that controls the PID, temp curve and heating variables. All to no better outcome.

Small design printed better. Fence cap, huge temps swings. No cure.

Then I thought, ah ha! Slic3r. Yeah, no. Made no difference. Sigh…

PLA! Yeah, something wrong with it. Change it! Yeah, did that, no change…ugh.

Nozzle! Yeah, no. Not that either.

Arduino! Swapped out an Arduino MEGA with a 16U2 for the el sleaze bucket ones with a CH340G. Nah. No change.

MUST BE THE RAMPS!!! Yeah, tried two different RAMPS boards. Same temp swings.

I pretty much gave up.

Two Heads, Better than None

I was ready to throw in the towel (along with the hot end and the rest of the printer) when the wife came in to see what I’d been mumbling to myself about.

I showed her the problem with the temp swings, told her about all the changes I’d made. She looked at the test prints, perfect, looked at the fence cap print, not so good.

Told me to start the fence cap print again and she watched the temps and called them out to me…

We let it run for about 3 layers and she said, “Cancel the print job”. I cancelled it, she said, “Okay, re-slice it and slow it down.”. I quickly said, “That won’t do anything, I can print far faster than I have it going anyway and the test prints were printing out fine at 50-60mm/s.

At this point I’d have peed on an electrical outlet if I’d thought it would help. So I set the speeds, infill, perimeter to 20mm/s. Or roughly half the speed of the last ice age.

Started the print, swing started the same and then…yeah. Stable. I was amazed.

So we did three quick tests. Speeds at 20, 25 and 30mm/s. You can see them in the graph below as test 1, 2, 3.

Fence Cap Temp

Time for a Guiness…

The Problem

When you look at a typical hot end, there’s a big chunk of aluminum with a heating element and thermistor in it. If and when you do some PID autotune, the head is stationary. All is right with the world.

When you print, the head moves, that big chunk of metal becomes a mobile heating element. Swing your arm, feel the wind rushing around your hand. Now pretend you’re the FLASH (oh go ahead you know you want to) and make your hand only vibrate. Very little air movement (if time starts to go backwards I suggest you stop…:-)

So the problem is simple. When the head is doing those LONG FAST lines, that heater block is dissipating heat like crazy, so much so the firmware can not maintain a control on it. Like tossing a stove top element around and trying to maintain a stable temp. Not. Going. To. Happen.

The Solution

The obvious solution is that if you’re going to print big stuff where the head is moving fast and long, plan on slowing it down. A lot. If you find big swings and you have a print job running, slow it down.

There are folks who say that a silicon boot on the heater block or wrapping the block in cotton/kapton will help, but I haven’t tried either of those to see if there is any merit there.

I suspect this is why a number of printers are enclosed in boxes so air in the box is warmer, thus easier heat control.

Insulating the head would be easier on a delta. Hence my thinking is it should be able to maintain a stable temp when the head is moving long distances fairly quickly.

Then again, if you read back through the initial problem “solving” list I had in the first place, you might just want to smile, and knowingly nod your head a little bit…

Later that same day…

Good thing I’m not a cat. My nine lives would have been used up years ago with my curiosity…

I decided to do a test by making a sock for the hot end. I used the cotton strips and some Kapton tape to make a boot..The print cooling fans were not used, the only fan on was for the heatsink tower.

IMG 1230

The hot end is wrapped all around except for the back where the wires for the heating element and thermistor are. Kapton tape is holding it all in place.

I loaded up the same fence post cap, set the speed for the perimeter to 30mm/s, infill to 50mm/s. And did three layers.

Insulated Hot End

if you look directly above the 7 min mark, you’ll see the temp drop and thats where the second layer starts. At the very right edge it looks as if the temp is coming under control. Except at that point I’d set the printer to 50% speed.

The cotton insulation does help, a bit. But certainly no where near what I imagined it might. The back of the heating element where the thermistor is no doubt very sensitive to air flowing past it. Thus if the whole end was wrapped up like a mummy, maybe it would be better. Or not.

For now, print at a slower speed, it works.

Read more from 3D, Musings

Comments are closed.