Welcome! Log In Create A New Profile

Advanced

Heating barrel length

Posted by cptwinder 
Heating barrel length
December 17, 2008 09:29AM
The other night I was cooking my own dinner (not something I do every often) and messed up horribly. Apparently 450 doesn't mean the oven cooks faster then at 350.

While cleaning up the burnt mess and waiting for the second round to finish cooking a thought about the heating barrel stuck me. Right now we have a relatively short heating barrel that needs to liquefy the plastic rod. Has anyone tried using a longer barrel to heat the plastic over a greater distance? I'm wondering if this will lower the pressures needed to extrude the plastic.
Re: Heating barrel length
December 17, 2008 09:45AM
actually the reverse is true. the heating barrel may be any length, as long as the heated portion (at least the portion heated beyond the "yeilding" temp) only needs to be localized to where the plastic deforms, i.e. the nozzle. the longer heating barrel (note, not necessarily the element portion) may make it easier to isolate the heated part of the system.

I've been thinking a bunch about the way our heating system is designed and there may be some improvements that are possible down the road. I see many people with PTFE insulators becoming deformed due to heat and pressure, when realistically all at that heat doing the deforming is unnecessary and would be considered waste heat.

I've only just started my personal reprap project, but i intend to build an extruder first and work on some of these theories. i understand that the design must remain simple and accessible to as many people as possible. but i envision temporarily expanding the current setup (my own) to possibly multiple heating elements and heating zones incorporating heat sinks and reservoirs. and naturally multiple sensors. This would simply create a good foundation for experimentation and material profile building. it is definitely a very complex system with variable feed rates and various external environments but hopefully we can create a clear instruction to produce a system which would be easier for the software to stabilize. ultimately i'd prefer to have a final design which, while complex, is more proven and preferably do not tax the system with any additional elements and sensors that what exists already.

anyway. just some thoughts. my background is mechanical engineering, and i am currently working as a corporate quality engineer (and therefore have plenty of access to material test equipment)
Re: Heating barrel length
December 17, 2008 10:19AM
What I was calling the heating barrel I was referring to the element portion. What I saw in my burnt dinner that started me thinking about the heating element portion of the barrel was that the outsides were way over cooked, yet the insides where still raw. With this in mind, it seemed that if you heated the filament slower over a longer period of time it would melt more evenly.

However, I don't have an extruder to play with, and mechanical engineering is not my specialty. I'm a network admin by trade. So I was just bouncing ideas...
Re: Heating barrel length
December 17, 2008 10:20AM
cptwinder Wrote:
-------------------------------------------------------
>
> Has anyone tried using a
> longer barrel to heat the plastic over a greater
> distance? I'm wondering if this will lower the
> pressures needed to extrude the plastic.
>
I use a longer (75 mm), thin-walled barrel made of copper. I apply heat only to the orifice end of it, however. The longer, uninsulated barrel allows heat to radiate and convect away before it gets to the PTFE thermal break. It also confines the melting process to the copper barrel and doesn't get the PTFE thermal break involved as is often the case with the classic Mk II barrel.

Further, I make a non-threaded, compression connection between the barrel and the PTFE thermal break rather than the threaded barrel the Mk II uses. This allows me to use 5/32ths inch hard copper tube for the barrel and an 0.005 inch piece of copper braised onto the end which I drill for the extruder orifice. That minimizes the distance that the polymer is constricted before it leaves the barrel and cuts way down on the torque needed to extrude molten plastic.


-------------------------------------------------------

Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison
Re: Heating barrel length
December 17, 2008 10:48AM
Jeremy,

part of what i want to look into is "preheat", but it will really vary between materials. any given material to be used in the extruder would have a specific melting profile. basically at X temperature the material is at Y viscosity/deformable. at the nozzle, where most of your deformation occurs, you want the plastic as liquid as possible, in order to limit your pressure build up. of course all of the preheat will be needed to be balanced with friction it would cause in the barrel.

if we had a good experimental setup, we could determine the best heater profile for different materials.

Forrest,

your solution is very much in line with what i envision. the heat conducted from the heating zone, acts as this preheater, and his concentrated heating zone keeps only the necessary plastic at the correct heat. the problem doesn't lie with the execution, just thats it is a balancing act that may be hard for another amateur repraper to recreate elsewhere. the barrel and nozzle materials and cross-sections chosen will have a great impact (heat conductivity), the air flow and ambient temperature will effect dissipation and "waste" energy. feed rate and material as well.

i really think it may be a good idea to look into some sort of heat reservoir to help with varying feed rates. (and i don't just mean slow vs fast, but also the necessary pauses during a build). it would take much of the work off of the heating element. less wasted energy is of course also a very good thing.

anyway, like i said, i don't have a reprap yet, i've gotten together with some others from chicago and we intend to help each other through our builds and hopefully soon i can test out a few things and document them on the wiki, best case we will have incremental improvements which are worthy of this awesome community effort.

andres
Re: Heating barrel length
December 17, 2008 11:01AM
Andres Wrote:
-------------------------------------------------------

> Forrest,
>
> the problem doesn't lie with the
> execution, just thats it is a balancing act that
> may be hard for another amateur repraper to
> recreate elsewhere.
>
It shouldn't be. I documented mine quite well.

[3dreplicators.com]

[3dreplicators.com]

[3dreplicators.com]


-------------------------------------------------------

Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison
Re: Heating barrel length
December 17, 2008 11:22AM
Forrest,

I definitely agree that much of your documentation is absolutely great. i've certainly spent plenty of time going over any information i could find on your Tommelise. and it's all been wonderfully helpful. unfortunately some of the other instructions i've found skim over the heating element specifically. naturally aiming for something that works rather than works well. i just think that it may be at a point now here that refinement can take place. i certainly hope that this doesn't come across as an attack towards anyone or anyone's designs.

furthermore it's the difficult balancing act i'm referring to. there may be ways that we could improve the thermal stability of the extruder, so that someone at the equator and someone in siberia could get similar control over nozzle temperatures. i think a resevoir of some kind can help. and it may be as simple as added thermal mass in the form of some washers stacked along the barrel. to borrow from the method that the reprap itself creates parts, it is very easy and possible to form complex cross-sections by stacking washers of varying diameters. a little thermal paste and now you have a thermal mass that could store heat in some areas and dispense heat from others. (just need to worry about weight now, hrm)

(don't worry i also realize that temperature comes into play again when the part is cooling and possibly warping. i think the heated substrate some people are working on is a good move towards a possible solution there)
Re: Heating barrel length
December 17, 2008 11:48AM
It's usually a good idea to have a baseline system working before you try to do kaizen refinements.


-------------------------------------------------------

Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison
Re: Heating barrel length
December 17, 2008 01:15PM
wow, i wasn't expecting to get such a dismissive response.

feel free to ignore me if i bring this idea up again. and yes, i fully intend to look into if it will actually improve anything though isolated experimentation as well as normal reprap use once i have a baseline system. I'll present the data and people can discuss if the design adjustment has any merit. if it turns out to have no benefit, then at least we looked down that road.

I may not have a working system, but is there anything amiss in my understanding of the nozzle/heater system requirements?

Andres
Re: Heating barrel length
December 17, 2008 02:14PM
Andres Wrote:
-------------------------------------------------------
> wow, i wasn't expecting to get such a dismissive
> response.
>
Try not to confuse a dismissive response with an honest sharing of personal experience which is what I was giving you.

I begged my way onto the Reprap team back in late 2005 when there were just a very few people on it and no system of any sort actually working, just the Mk II extruder design. I built a complete positioning system (Godzilla) and got it completely wrong. From there I build another one (Tommelise 1.0) which used shaft-encoded gearmotors which didn't work all that well, but let me do a bit of printing in HDPE with a redesigned Mk II. I've got maybe 500-600 hours of extruder operation that informs the design changes I'll be making in my next generation extruder.

The point of all this, is that I've discovered that if you have something that is already working running on your bench you get an incredibly clearer idea of what improvements are needed, or even a complete redesign because you know what works and what doesn't. If you borrow somebody elses design for a first round of development you'll make progress a lot more quickly than if you start from scratch.

Edited 1 time(s). Last edit at 12/17/2008 02:15PM by Forrest Higgs.


-------------------------------------------------------

Hell, there are no rules here - we're trying to accomplish something.

Opportunity is missed by most people because it is dressed in overalls and looks like work.

Thomas A. Edison
Re: Heating barrel length
December 17, 2008 05:06PM
I think it is well worth experimenting to find the best heater length. If too short then it will limit the speed you can extrude at because the plastic will not have time to melt. If too long then the larger volume of molten plastic will cause the extruder to overrun longer when the motor stops because molten plastic compresses under pressure and expands again. Also a heater that is longer than it needs to be it will waste some energy.

Other things that have a big effect are: whether there is a temperature gradient along it, how slippery it is, and how good a thermal conductor it is.

E.g. I made a stainless steel barrel with a gradient along it so the the start was below the meting point of the plastic. I found it impossible to push HDPE through, even with an open end. Forrest has a copper barrel with a gradient down it and can push filament through it, presumably because copper is a much better conductor. Maybe a shorter SS barrel would work.

With the current brass heater barrel it is all well above the melt point of the plastic, so the plastic actually melts in the PTFE, which is very slippery. What happens when extruding I don't know. Does the melt point move down into the brass barrel or does it stay in the PTFE?

There is a lot of experimenting to be done to find the best solution. At the moment I am playing with a hollow soldering iron heater that 'sid found. That appears to be stainless steel and has a big gradient along it. I have no idea whether it will work yet. I have also made an aluminium barrel and nozzle to see how that compares to brass. The list goes on an on. The more people actually trying things and reporting results the better.


[www.hydraraptor.blogspot.com]
Sorry, only registered users may post in this forum.

Click here to login