Hotend Safety.

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AndrewRutter
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Hotend Safety.

Postby AndrewRutter » Sat Apr 19, 2014 4:36 pm

As some of you are probably already aware, the style of hotend that we have been using from the beginning here at Type A Machines has some limitations.

One of these is down to the use of a plastic thermal insulator. This can melt if the hotend gets too hot.
There are several ways in which a hotend can fail, but some of those failure modes included uncontrolled heating.
See here:
viewtopic.php?f=5&t=889
We have had this happen in a small number of machines, including machines that are part of the 2014 Beta (but not in any production 2014s).

What does that mean?

While the Series 1 is safe, any device which deals with high levels of heat is a risk. We recommend any 3D printer using this style of hotend not be used unaccompanied.

Should you keep using your printer?

Assuming you have not encountered overheating, you can continue using your printer. The number of machines affected is very small and no damage was done beyond that to the part itself.

Should I be worried?

Remember, a 3D printer is a CNC machine that has a tool that spends many hours at high temperatures. This is failure mode that is known throughout the industry and is a rare occurrence to the best of our knowledge. If you do encounter overheating, unplug the printer and contact support@typeamachines.com

What is Type A Machines going to do about this?
One solution is to switch to an all metal hotend. We have one in development, but this still does not meet the strict requirements we have decided to set ourselves for a new safer hotend. So we are going to develop something that does.

A new hotend must be of a 'fail safe' design (something we are not aware of existing in any current hotend design).
And whatever design we come up with, we will be making public and open source. Because this is a common problem across the industry and one we call all help solve.

And I am posting here in the Forum not just to remind everyone that hotends get hot! But to ask people to offer their suggestions in this thread, because you (our users) are our most powerful asset.
We will also be reaching out to other manufactures and those in the wider RepRap community so that we can all collaborate and share a solution.

edited for phrasing and an errant apostrophe.

AndrewRutter
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Re: Hotend Safety.

Postby AndrewRutter » Sat Apr 19, 2014 4:40 pm

Our requirements so far are:

The fail-safe must be integral (not removable without disabling the hotend).

It must be wired in series with the heater.

It must use standard industrial solutions.

It must not interfere with normal operation of the hotend.

It must be robust.

AndrewRutter
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Re: Hotend Safety.

Postby AndrewRutter » Sat Apr 19, 2014 4:43 pm

Our current best idea, and currently in development, is using thermal fuses.

These would be in series, integral to the heater leads, and spec'd to fuse when the hotend reached a high enough temperature to experience a failure of the thermal insulator.

I will post more data and images when we have them.

latentspark
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Joined: Tue Jun 18, 2013 7:41 pm
Location: Seattle, WA

Re: Hotend Safety.

Postby latentspark » Mon Apr 21, 2014 7:09 pm

I have just upgraded to the E3D Extruder (actively cooled, all metal) and that coupled with the new glass build platform and upgraded extruder wheel has changed the way I look at my printer. Before, I was having trouble with reliable extrusion and with an elongated melt zone like the existing extruders from TAM, I would be lucky if I got 4 prints out of a head before needing an overhaul.

If you haven't read the RepRap magazine's hot-end review article, I highly recommend it.
http://reprapmagazine.com/issues/3/index.html

LaserGnomes
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Re: Hotend Safety.

Postby LaserGnomes » Tue Apr 22, 2014 1:45 pm

It's our understanding that the e3d extruder would not resolve this issue of safety.
The melting plastic portion is the only failsafe... if it where all metal it would be worse frankly. Nothing would fail until the metal was red hot and eventually smoked the wires themselves.


Open ears and wallets to be proven otherwise.

-LG

Miloh
Posts: 74
Joined: Wed Oct 10, 2012 12:02 pm

Re: Hotend Safety.

Postby Miloh » Wed Apr 23, 2014 12:03 am

Correct about the e3d, and other hot ends.

This is a problem that most extruders in the reprap world have, and we believe that fixing it with the steps outlined is the way to proceed with hot ends. It's the next step to take to guarentee safety and I think you'll see this approach adopted by more and more reprap hot end manufacturers in a short time.

Going back to what Andrew wrote earlier in the thread, the design must have an integral cut off that is designed to work in the case that a functioning thermistor or thermocouple circuit is not colocated with the heat source.

What do we mean by integral? The cutoff must be electrically in series, and be physically & mechanically colocated to a region of the hot end heat envelope that gives useful results before catastrophic failure. With an integral thermal cut off, the heater circuit would fail open and break permanently if a temperature above a certain point is reached. The single use cutoff sensor should be colocated at the heat envelope and cannot be removed without destryoing the hot end.

Preliminary testing with hot ends at 230C shows heater leads reaching 160C. Since the heater leads are less than inches away from heater blocks, this looks like the first place to install the first tests using thermal cutoff devices with a breakdown region around 160C. This will only add a few dollars to the bom in parts cost, but a great deal of ease mind.

latentspark
Posts: 32
Joined: Tue Jun 18, 2013 7:41 pm
Location: Seattle, WA

Re: Hotend Safety.

Postby latentspark » Fri May 09, 2014 3:27 pm

I'm a little unclear as to what is causing this problem in the first place, so hopefully I'm not sounding naive.

Have you guys considered throwing a sensing line in between the fuse and the heater element? You'd have to add a subroutine to the Marlin firmware or a simple but custom impedance-checking circuit to watch for deviations there, but something like this could be used to trigger an emergency shutdown without destroying the fuse permanently. (IE, use a resettable fuse)

If these failures are firmware or heater driver related, then this won't help too much. If it's a sensing solution problem, then it would allow you to have a software driven emergency shutdown. Another option is to have a latching relay that gets thrown by this line going open. This relay could be tied to the power supply, cutting all power to the printer in the case of a failure. I'd consider this to be a more elegant solution than just permanently disabling the hotend and having the printer continue unknowingly to try and extrude filament until it finishes the print.

ccrome
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Re: Hotend Safety.

Postby ccrome » Wed May 14, 2014 10:16 pm

In order to pass UL on a different type of product, I've had to go through the following (for a Li-Ion Battery powered product):

* 2 separate over voltage protections
* 3 separate over current protections
* 2 separate over temperature protections
* 2 separate under-voltage lockouts.

Of course, Li-Ion batteries tend to explode when mistreated, so there was more protection in place than normal.

For the hot end (and for all the high power circuitry in general), I would consider multiple, redundant, safety features. Here's where I would start:

* Thermal fuse connected in series with the heater, thermally connected to the cold-end of the hot-end. (temp safety 1)
* Temperature PID loop (temp safety 2)
* PTC fuse in series with the heater block (over current protection 1)
* current monitor in series with the heater (over current protection 2, software)
* Maybe even a non-resettable fuse too, though I'm not sure about that -- the beauty of the PTC is that it can withstand a short duration short like something a hacker may do.
* Monitor the voltage going to the heater block, so you can compute POWER to the heater, and set a software limit.

That would do a pretty good job: 2 over current, and 2 over temp protections (or more). Definitely should talk to UL to see what they'd require.

-Caleb

nonispohn
Posts: 1
Joined: Tue Feb 10, 2015 4:26 pm

Re: Hotend Safety.

Postby nonispohn » Tue Feb 10, 2015 4:28 pm

I love you guys!

mrSEADEMON
Posts: 140
Joined: Fri Apr 18, 2014 9:35 am
Location: BY Minsk

Re: Hotend Safety.

Postby mrSEADEMON » Sat Feb 21, 2015 12:45 pm

Use E3D and don't have problems with hot end. Before this hot end was died


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