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I printed Peter Van Den Berg's optimized J-tube

 
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I printed Peter's 3d Model of his optimized cast rocket core.

I'm a visual thinker so I had to see it in person, even if at 25% size.



 
Rocket Scientist
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Way Cool Solomon!!!  Can you print it full size?
 
pollinator
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Hmmmm.  Could you print a mold for a castable core? Even if it had to be multiple prints and fitted together.  That would solve a BUNCH of issues.
 
Solomon Parker
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I could print out forms for a castable core, but my present printer can only print 250x250x300mm. So I would have to split it up in a bunch of pieces which is a little beyond my skills at this point.

But if I get a much larger printer, perhaps a belt printer or one with a 1 meter square print bed, I could pull it off.
 
R Scott
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It would be much easier to ship plastic pieces and bags of mortar than a delicate already cast core.

It would be easier if you had a big printer, but if you could find someone to help split the files (that only has to be done once) then anyone could print them locally.

Way beyond my abilities, but you have me thinking in a whole new direction!
 
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Looks cool, I can see the potential for casting intricate shapes but one would need a high frequency vibrating table to produce a good end product.
Are Dragon rocket stoves still being made?
 
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I do functional mechanical 3d modelling on a daily basis. If someone could send me the original model or drawing, I would be happy to split it up into pieces that could be printed on the typical 200x200x200 printer. Purple Moosage for email addy!
 
pollinator
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Oooh, pretty. That's a good teaching tool as well.

I did this the old-fashioned way: I made a reverse mold from wood and then poured castable refractory. I did top and bottom halves instead of L-R so that joining them wouldn't be critical. The RMH is a 4" system in my glasshouse and it performs well, but I will need to demolish and rebuild the core sometime soon (hopefully before cold weather returns) because it has cracked and spalled pretty badly with use. I have gotten three seasons out of it and lots of education, so it's been a good first step.

Here is what I learned:

* I really should have set up a vibrating table for the casting process. I suspect a large part of the deterioration of the refractory material is down to the voids and inconsistencies in the cast body.

* The next iteration, if it's a J-tube, will have some extra depth to allow ash buildup without throttling airflow. Several times I've gotten reverse draft because too much fuel lodged as coals in the burn tunnel.

* Those intricate castings are weak at corners and transitions. If I do another cast core, I will make slab forms instead of the hollow molded halves. Sort of like firebrick splits, but with the detail of Peter's design fully incorporated. Fit of the pieces will be done with simple rabbets or grooves to allow for differential expansion and contraction, as I think this has been the number one driver of cracks in my core. I want the parts to shift within limits and not stress the weak spots.

Or, I might go for the gusto and build a batch box. Traipsing out to the glasshouse every 30 minutes on cold winter nights does get old.
 
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I have been thinking of how one could do this using super wool inserts....    


What if instead of squares we made it circular so super wool could be put inside....

I was thinking taking aluminum sheeting and make the form to hold the superwool in place we could print this form and have the high heat protection with the wool in place.......   or...   how about chicken wire with spikes  that pierce the superwool to hold it in place....


I was thinking if we had superwool thread  one could sew the chamber together. to a chicken wire exterior frame...




 
Solomon Parker
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Scots John wrote:Looks cool, I can see the potential for casting intricate shapes but one would need a high frequency vibrating table to produce a good end product.
Are Dragon rocket stoves still being made?



Somewhere on Peter's site (or somewhere) is instructions on how to make a vibrating table. They are indispensable for casting cores.

Dragon stoves are still being made, but for the $1100 price tag, I can think of a lot of ways around buying one. That's just me though, I rarely buy something I can build. I can justify a lot of development costs and tooling to figure it out for myself for that price.
 
Solomon Parker
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R Scott wrote:It would be much easier to ship plastic pieces and bags of mortar than a delicate already cast core.

It would be easier if you had a big printer, but if you could find someone to help split the files (that only has to be done once) then anyone could print them locally.

Way beyond my abilities, but you have me thinking in a whole new direction!



I'm trying to see what's possible in a more technically advanced direction. RMHs have been around and popularized for over a decade now, and there are new things that can be applied, like 3d printing.

So many people have printers now, with the right models, anyone could put together a core mold in no time, even if you had to make it out of a bunch of smaller pieces.

And that goes for batch boxes too.

Peter has worked out a lot of this stuff in Sketchup, so the models already exist. It's simple to convert them to be printed like I did here. But the real fun would be to create negatives to make molds with.
 
Solomon Parker
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Joshua Rimmer wrote:I do functional mechanical 3d modelling on a daily basis. If someone could send me the original model or drawing, I would be happy to split it up into pieces that could be printed on the typical 200x200x200 printer. Purple Moosage for email addy!



Sent you a moosage.

In the spirit of the makerverse, it would be so awesome if we could work out an open sourced project with this. The models are out there, I got this one from Peter and the batch box models are on his website.  It's the splitting of the models and making negatives that I need to learn how to do.

Because this seems to me to be the road to printing out our own rocket cores (sort of). Being able to quickly modify molds and cast new cores, this can only lead to newer and quicker innovation in the RMH world.
 
Scots John
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Solomon Parker wrote:

Scots John wrote:Looks cool, I can see the potential for casting intricate shapes but one would need a high frequency vibrating table to produce a good end product.
Are Dragon rocket stoves still being made?



Somewhere on Peter's site (or somewhere) is instructions on how to make a vibrating table. They are indispensable for casting cores.

Dragon stoves are still being made, but for the $1100 price tag, I can think of a lot of ways around buying one. That's just me though, I rarely buy something I can build. I can justify a lot of development costs and tooling to figure it out for myself for that price.



Hmm that is quite expensive, especially as the average rocket stove builder seems to be on a tight budget!
I am only guessing but, perhaps there would of been around $1500 2000 investment for a proper vibrating table, $5000 for the mold construction and $100 of refactory for each unit?
I don’t have much experience with casting ‘refractory’ cement but I work as a formwork engineer on large building sites and I did train with the use of vibrating tables, (35 years ago) I think the version of table you mentioned is based around an offset shaft electric motor that tend to shake rather than vibrate the cement, they would probably work ok but would not settle a dry mix very well.
Great work in any case....
 
Solomon Parker
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Scots John wrote:I am only guessing but, perhaps there would of been around $1500 2000 investment for a proper vibrating table, $5000 for the mold construction and $100 of refactory for each unit?



What? No!

If you want to just go buy everything, sure. But nobody was talking about that.

IIRC, one can build a vibrating table suitable for casting cores for less than $20.

The molds we are talking about PRINTING would cost $20-$30 in PLA filament, plus a few bucks for wood backing boards, and whatever for the refractory.

If we can't do this for HALF of the cost of a Dragon Heater core, then we're doing something wrong. Plus we'd get to keep the tools and forms. Or perhaps if the forms didn't survive, we could just print new ones.

AH! I found a video on making a vibrating table.  Just a motor, a tire, a piece of plywood, some bolts, and a dimmer switch.
 
Scots John
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I think you miss read my reply to your answer... I was suggesting the the presently available Dragon core was expensive and I was also suggesting some figures that they might of experienced while manufacturing the product!
I would of through their  mold is a multi use structural construction and would of cost quite a lot of money?
Yes those type of vibrators do not work particularly well  with dry mixes and as far as I know, refectory is mixed very dry?
I did not say it would not work as I don’t actually know that but casting course dry mixes and expecting good results with fine detail would normally require a high frequency vibrating table?
I was not implying anything negative about your model!
 
Solomon Parker
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Scots John wrote:I did not say it would not work as I don’t actually know that but casting course dry mixes and expecting good results with fine detail would normally require a high frequency vibrating table?



The vibrating table is in the video I just posted. I believe this is the sort of thing that Peter uses.

I have no interest in the dragon heater. I'm figuring out how to do this for people who want to make their own stuff. Money ruins everything.

 
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I made a styrofoam cavity model to cast my 8" refractory J-tube core, with top and bottom sections instead of left-right to minimize differential expansion and stresses between areas. It has served well for four and a half heating seasons so far. Both sections have cracked in several places, but are still locked in place and not in danger of failing. Notably, the burn tunnel roof with trip wire molded in is still unbroken and almost perfect. I do see a difference between the bottom which I cast first with cement that had started to set and was honeycombed in spite of vibrating, and the top which cast perfectly. The top is largely still smooth from the plastic bag mold release material, while the bottom (patched solidly where honeycombed) has spalled to a rough surface in a number of places.

https://permies.com/t/60784/a/45819/IMG_0860-w600.jpg
styrofoam core mold on form box
 
Solomon Parker
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That looks great. We're figuring out that exact sort of thing only 3d printed.

One of the good things about Peter's designs is that they're already in sections. Smaller sections means fewer problems with cement batching. And parts can be replaced if they fail. The J tube core is two pieces, the best extent batch box is three pieces.

Another thing we can do is create true circular and exponentially curved edges, rather than squares and octagons.
 
Glenn Herbert
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If designed with proper draft and good mold release surfaces, a core mold can release easily and be perfectly reusable. With my experience of the behavior of my core, I might do another with different section divisions, like floor, complete sidewalls left and right, and burn tunnel roof. Or maybe just bottom and top as the original but feed sidewalls full height and burn tunnel roof and riser base as one part, since those don't seem to have any cracking issues. Running the feed tube sidewalls all the way up would leave fewer narrow sections of casting. My core mold is good for many more uses, but I think I will be making batch boxes for future projects unless I get a job for a simple installation.
 
Mart Hale
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I believe this would be the next step for making a mold.....




 
Solomon Parker
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I don't think that's quite the next step. What I'm suggesting is not people building their own printers or CNC machines, I'm just talking about printing molds on standard off the shelf printers that are already all over the market.

This is certainly an option, especially if someone is using EPS or XPS foam. But I think building a CNC machine is going to be a bit daunting for most people interested in casting their own core.
 
Mart Hale
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Solomon Parker wrote:I don't think that's quite the next step. What I'm suggesting is not people building their own printers or CNC machines, I'm just talking about printing molds on standard off the shelf printers that are already all over the market.

This is certainly an option, especially if someone is using EPS or XPS foam. But I think building a CNC machine is going to be a bit daunting for most people interested in casting their own core.



All options are on the table,  if it works use it.

Yes, not everyone has these machines or the desire to build them.
 
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Very cool!  3d printing is *very* cool technology.

It's awesome to see the "guts" of this thing like this.

Very educational!
 
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Glenn Herbert wrote:I made a styrofoam cavity model to cast my 8" refractory J-tube core, with top and bottom sections instead of left-right to minimize differential expansion and stresses between areas. It has served well for four and a half heating seasons so far. Both sections have cracked in several places, but are still locked in place and not in danger of failing. Notably, the burn tunnel roof with trip wire molded in is still unbroken and almost perfect. I do see a difference between the bottom which I cast first with cement that had started to set and was honeycombed in spite of vibrating, and the top which cast perfectly. The top is largely still smooth from the plastic bag mold release material, while the bottom (patched solidly where honeycombed) has spalled to a rough surface in a number of places.


Just curious where the cracks actually end up?

 
Glenn Herbert
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A few random vertical cracks in feed tube sidewalls. The whole assembly is locked in place with surrounding insulating firebricks, so most of the cracks can't even be wiggled.
 
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I have considered the potential for using the water soluble material used for biodegradable packing peanuts to cast pieces for a "lost wax" "Lost styrofoam" type of casting.  Except in this case you would let the casting set and then dissolve the mold with water.  Though admittedly, I have not yet done any research on the composition of said biodegradable packing peanuts.
 
Glenn Herbert
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At least some of those are based on cornstarch.
 
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Solomon Parker wrote:I printed Peter's 3d Model of his optimized cast rocket core.

I'm a visual thinker so I had to see it in person, even if at 25% size.



So great to see another 3D Printer! Thanks for sharing this :)
 
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Thomas Tipton wrote:I have considered the potential for using the water soluble material used for biodegradable packing peanuts to cast pieces for a "lost wax" "Lost styrofoam" type of casting.  Except in this case you would let the casting set and then dissolve the mold with water.  Though admittedly, I have not yet done any research on the composition of said biodegradable packing peanuts.



The casting itself is wet. Wouldn't the moisture in the casting material dissolve the fill in that case?

Polystyrene should work nicely, as once it is all properly cured you can burn/melt the poly. Maybe a hot air gun to get it to melt and clear the tubes, then light a small fire to drag through the rocket tubes? Do it outdoors and it will be burned clean pretty quickly.

 
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Wouldn't doing a "lost wax" casting with polystyrene produce dioxins, especially if you do a low-temperature degradation in oxygen?

I like the idea of using that technique with something resilient but breathable, such that it would be possible to cast and install the burn tunnel, and then fire it up once everything is working properly.

My candidate for this would be corrugated cardboard. If it's cut into strips that can be rolled to the appropriate diameter, it would be no problem at all to make straight cylindrical sections. Likewise, with a 3D-printed form for the bends, one could easily wet a cardboard roll and press it into a form, perhaps closing it with carriage bolts and wingnuts to form the damp cardboard to shape. If necessary, a starch could be added to the water to help the formed cardboard keep its shape for assembling and casting of the refractory.

It should be noted that, while it would still maintain a certain angularity, rolls of cardboard could be angle-cut in sections to provide turns, and the fluidity of the turns is limited only by the width of the sections; you could make a smooth curve working with cut wedges whose thin edges narrowed sufficiently.

The best part of this approach, to my mind, would be that the corrugations form perfect parallel channels for airflow through the form. Once assembled, once the refractory has set, and once the core has been installed, all one needs to do is fire it up, and the cardboard would incinerate. As it would happen largely at burn-tunnel temperatures, there would be nothing left but carbon dioxide, water vapour, and a little heat, even if some plastic coatings somehow made their way in.

The other best part might actually be the use of cardboard, which is pretty ubiquitous in much of the human-inhabited world. Even dried-out food-contaminated cardboard might be used, provided the channel structure is uncompromised.

Great ideas here. I do like the idea that the physics of RMHs could be refined by more organic flowpaths. This seems a good way to make, test, and disseminate changes or refinements.

-CK
 
Glenn Herbert
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There was experimentation with more organic flowpaths a number of years ago by Peter van den Berg and others, and it was found that the square/angular shape was as good or better than anything else they tried. A few tweaks like the sloped "tripwire" molded into the ceiling, and a sweep to the back of the burn tunnel floor, were all that proved useful. The angular path, especially from horizontal burn tunnel to vertical riser, promotes beneficial turbulence and better fuel-air mixing.
 
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I am very interested in the progress,
its great to watch thank you fellas.
 
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In order to be complete: here's the link to the complete design. You'll need a free version of SketchUp and strip the parts around the core in order to view it.
https://pberg0.home.xs4all.nl/Bestanden/J-tube/6inch-Jtube.skp
 
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Chris Kott wrote:Wouldn't doing a "lost wax" casting with polystyrene produce dioxins, especially if you do a low-temperature degradation in oxygen?

...
-CK



It is now common in the sand casting  and investment casting industry to 3D print patterns in wax, which is burned off in the casting process (we're talking cast iron, bronze, etc.) As long as you provide a path out for the melted liquid, you don't have to burn it, you could just melt it out.


The most common 3d printing material is Poly Lactic Acid (PLA) which has a relatively low melting point (around 185 Celsius)  and is cornstarch-based. The water-soluble stuff is generally Poly Vinyl Alcohol (PVA) which has a similar melting point as PLA, It takes quite a bit of water to actually dissolve it. my understanding is that the refractory mix is fairly dry, so it may or may not dissolve before the refractory mix is self-supporting.

Printers that can print in metal are now starting to drop down into the $5k price range.
 
Thomas Tipton
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Improvised Vibrating Table.

Take the blade off a sawzall, plug it into the electrical outlet, press the head onto what ever you want to vibrate, pull the trigger.
 
Glenn Herbert
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Or an oscillating handheld sander... That is what I used.
 
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Would a jigsaw without the blade also work?
 
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For vibrating cement products inside a mold, the higher frequency vibrator methods will work the best.

Starting at the most basic level would be tapping  the mold with a hammer,  working up to a purpose designed vibrating table.

There is a post higher up the page that shows a DIY vibrating table that uses an off set vibrating motor, I have tried that exact method using the same motor, the set up can  produce pretty good results but the motor is not the same as a high frequency commercial vibrating table and very often air voids will still be present in the finished product!

For really fine detail and a completely air free end product a purpose vibrating table is far more efficient that any diy system that I have come across.
However not many  folk are going to invest the sort of money required to purchase a full on commercial vibrating table just to produce a single item.

Vibrating pokers can be hired very cheaply from most town centers, these devices are used to settle concrete mainly in the building industry and are designed to be sunk into the concrete on large projects.
They can be used on small molds but are very powerful and can easily blow apart a weak mold or damage or deform the mold!
Having said that, pokers can work extremely well if used with care on a strong mold, I have used then for hundreds of cement castings.

Next step down would be an orbital sander placed on the outside of the mold, this can work OK and is the best ‘cheap an easy’ option….  but don't expect perfect results as the vibrating frequency and power will not even come close to a poker or or a proper table.
From my own experience the other suggestions will have limited results and probably not much better than a stick and hammer!

One big issue with refractory cement is it must be mixed very dry with minimal water to get the best long lasting finished casement
From my experience, only  a high frequency device will completely settle a dry mix.


 
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