An oven using bellows

I've been thinking about one thing for some time now. One of the original
assumptions of this project was that in order to keep the cost down for the
Reprap equipment, one would not want to preheat materials, because that would
mean all its components had to be made to withstand that temperature.

As I understood it, this is how commercial rapid prototypers operate, by
setting a bias temperature higher than ambient air and operating in a closed
space (?)

When extruding, one only has to go over that temperature with a few degrees to
pass the melting point. The object is then allowed to cool after it is
finished and less or at least more even strain occurs (?)

I have played with the idea of using bellows as a complement to the standard
Reprap construction (see my crappy picture attached).

The inside of the bellows would act as an oven keeping the object at a higher
temperature, thus reducing the stress put on when printing HDPE and other
materials with higher melting points.

In the picture I have tried to describe how the bellows would stretch when the
Reprap plate would move downwards as the construction is being made. When the
plate reaches the bottom, the bellows would be stretched into an egyptian
pyramide.

Apart from being an obvious risk of interfering with the object being built
and a general nuisance the bellows would also mean you cant see a damn thing
what is happening.
Possibly this could be augmented either by finding a material that is both
transparent, flexible, heat-tolerant and thermically insulating or by setting
glass inspection windows on the side(s) as in a normal kitchen oven.

Another possible addition would be fans blowing on the upper side to keep the
plastic on the outside from melting, possible this would only be needed while
using CAPA parts.

The extruder parts themselves would already be tolerant to the temperatures
involved anyway. Of course the MDF board on the underside also had to be
insulated.

What do you think?

Hmm... You'd have to keep a positive pressure inside and there would be the issue of what the membrane would be made of. You could have a Teflon gasket around the extruder mouth so that the membrane could be of something less heat tolerant. You are looking at temperatures inside of 75-190C.

Nice concept. Bears more thinking about. About the only downside I can see is the heat loss through the membrane. You have, however, neatly taken the positioning system out of the hot zone.

What is the theory behind this? Is there an advantage to keeping the whole object being printed at the higher-than-ambient (but lower than melting) temperature?

Would a gentle flow of warm air around the extruder and onto the object be sufficient? That would let you dispense with the bellows. (But might not be energy efficient.)

As far as I understand there is an advantage since the plastic shrinks when cooling.

Flat objects thus has a tendency to curl (see the builder blog btw).

If the whole object is finished before cooling off less curling will occur.

Hmm maybe a hot air gun could provide both the heat and the preassure to the bellows, otherwise I though I'd use a large iron spring to keep the whole thing risen.

I think Forrest knows more about this!

Personally, I'd rather build a repstrap in an oven than a reprap with a tent, but that's aesthetics.

Have you considered a couple inflated tubes to support your bellows? Imagine a pyramid or tetrahedron of those balloons they twist into animal shapes, with the fabric of your bellows suspended between.

Of course, you could just use sufficiently springy rods, such as found in dome tents, and such as found a use in the current model, to support your bellows without resorting to positive pressure.

Roach_S Wrote:
-------------------------------------------------------
> Of course, you could just use sufficiently springy
> rods, such as found in dome tents, and such as
> found a use in the current model, to support your
> bellows without resorting to positive pressure.

What's wrong with positive pressure? That seems like the most straightforward approach to me. I mean, the bellows would have to be mostly airtight anyway to seal the heat in, so why not take advantage of that and use it to hold a bit of pressure as well?

Kyle

The problem would only be to get parts that withstand the heat... CAPA would make an interesting site in a 175 centigrade oven...

The reason I thought the commercial machines where so expensive was that all parts had to be fabricated to withstand the temperature.

Also self-replication is out of the question with that approach, parts in thermoplastic would be rather doughy ;-)

Hence why I said repSTRAP in an oven over repRAP with bellows.

One potential problem with positive pressure will be inflating it when making the initial laydowns. Actually, that'd be true of support sticks too.

Essentially, in my mind at least, on the first few layers, the fabric will try to "inflate" into the mechanical elements of the XY system, as there will be surplus fabric to need.

Think of it this way. You have a tent. It wants to erect itself, although fortunately, it has a hole in the top.

When you're stacking something on top of a large stack, the fact the tent stays erect isn't a problem.
When you're stacking something on the floor of the tent, if you, for some reason, have to keep your elbow level, you're going to be banging into the tent repeatedly.

I can see about four ways around this, but each presents its own problems.

It's best to think of the "tent" as a balloon with the extruder extending into the balloon via a gasket. When the extruder is moved the balloon deforms to accommodate the movement of the gasket and the extruder.

It should work. It ain't rocket science.

As for myself, I haven't given up on printing at room temperature just yet.

Sorry I just reread your post and saw the "strap" where I previously read the "rap", so I guess I deserved a rap from the strap ;-)


My latest thought was to use a cylindric housing with a large steel spring, something like you see in "space corridors" or "infectios disease"-movies. Then only have a little tent to accommodate for sideways moving of the extruder. As the plate moves downwards, the cylinder would expand and keeping the tent part of the bellows independent of the altitude (would that be the z-axis?)

It doesnt have to be a cylinder, just thought a helix spring would be the easiest to implement or even find something ready made.

I happened to stumble across this page while wandering the internet: [jcflowers1.iweb.bsu.edu]

It appears to be a university lesson on how to use their Stratasys. The interesting part is where they mention the temperature settings for the machine - the filament is set to 270C for extrusion, but the modeling area is set to only 70C. This tells me that it only needs to be kept "really warm", rather than near-melting-temp, so this might make things a little bit easier. Forrest, have you by any chance tried pointing a hair dryer or similar air heater at the extruded object to keep it warm while extruding? It might be an interesting experiment.

Eric Morgan Wrote:
-------------------------------------------------------
> Forrest, have you by any chance tried pointing a
> hair dryer or similar air heater at the extruded
> object to keep it warm while extruding? It might
> be an interesting experiment.

Nope, haven't done that. Energetically it is fairly suicidal given that you'd be running that hair drier for most of a day to make something like the polymer pump for the extruder. My marginal rate for electricity is about $0.23/kwhr.