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Discussion Starter · #1 ·
Have you ever thought, wouldn't it be cool to make something like the RPG Diffusers? I have, especially since the number of them I need would be cost prohibitive. I've thought of how to do it, but I have one sticking point. Maybe someone out there can fill in the blank.


The idea is that you make a box about 2'x2' or however big you want the mold to be. You then take 1"x1" cut to various sizes and fix them to the bottom of the box. You then take the lid of the box and drill a few holes. Place the lid on the box and inject foam into the box. Once finished, remove the lid and remove the foam diffuser. Ok, sounds simple doesn't it? The problem is how to stop the foam from sticking. I know that there are many types of foam out there, but I can' think of any way to stop it from sticking. Any ideas? How would you prevent foam from sticking to would, or what type of foam to use. Preferably it comes in a can and can be sprayed through a hole.


Am I nuts or is this a workable idea?
 

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I don't know that any of these would work or are good ideas, but:

1) Wax...sometimes used to prevent sticking

2) Something like a cooking spray? PAM? (no butter flavored ;))

3) How about a wax paper?

4) Why not make the diffusers out of the 1"x1" wood?
 

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Discussion Starter · #3 ·
That's a start. The pam & wax are good ideas. Couldn't do the wax paper because of shaping it around the 1x1's would'nt work. Wouldn't want to make the diffusers out of 1x1 because they'd be too heavy and I just don't like the idea of it :) Wax definately sounds promising though. I wish there was a foam that was expanding but didn't have a tendancy to stick.
 

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Even a light coast of Crisco may keep it from sticking. Now that I think about it, the wood MAY absorb the PAM too much. I'd try something with a little weight to it...like the wax or a grease of some kind. Let us know how it works out. I too have thought about this, but always thought about doing it from the wood only.
 

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Wont this give you the inverse of the diffuser? The foam will give you a mold but you then need to fill the mold to get the original design of the wood diffuser.


What about skipping the box and building the diffuser out of wood. Then use fiberglass sheets and epoxy to coat the wood. When it dries seperate the fiberglass from the wood. You would have a rigid fiberglass mold that you could shoot foam into that could be re-used. They make releasing agents for molds like that. A big art store may carry it.
 

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My father many years ago used to make the green foam "Aquadome" covers that you used to see cover many pumps and wells. The foam was created by mixing two chemicals that reacted together to form foam. The molds were stainless steel and had about 20 clamps to hold the inner mold to the outer mold. From experience in making these, I have some observations:


Pressure...Expanding foam, if you are trying to make a dense product, will exert an enormous amount of pressure. Even with metal clamps and a 50 lb stainless mold, it would still pop if you put in too much foam. Better have good pressure relief ports.


Heat: With rising pressure comes rising heat. These forms would get to about 200 degrees F.


Silicone: We used silicone spray to coat the molds to prevent sticking. But this was on a smooth stainless surface. Wood is porous, and I don't care how much you coat it, it will absorb your coating, and your foam will still stick to it.
 

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Robert,

That is what I was thinking...the porosity of the wood would absord the spray.


But, what if you sealed the wood first? either with a good sealer, or even a thin coat of poly? Wouldn't that allow you to spray a lubricant on without absorption?


As for the pressure, as long as the foam is forced around the 1x1's, and there is enough pressure to form your mold on that end, would it matter what happened on the back side? Make the mold, put a back on it, and begin to fill. Once the pressure is great enough, remove the back. The foam should have found its way into the 1x1 mold, and then you can very easily cut the back flush with the mold after curing.


Would any of this work?
 

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Quote:
Originally posted by Jay Wilson
Robert,

That is what I was thinking...the porosity of the wood would absord the spray.


But, what if you sealed the wood first? either with a good sealer, or even a thin coat of poly? Wouldn't that allow you to spray a lubricant on without absorption?


As for the pressure, as long as the foam is forced around the 1x1's, and there is enough pressure to form your mold on that end, would it matter what happened on the back side? Make the mold, put a back on it, and begin to fill. Once the pressure is great enough, remove the back. The foam should have found its way into the 1x1 mold, and then you can very easily cut the back flush with the mold after curing.


Would any of this work?
Not ever having done it, it's hard to say. Depending on the sealer/poly, it might have an adverse reaction to silicone, especially under a bit of heat/pressure.


Forcing foam into all the nooks and crannies will still require a bit of pressure, IMHO. Otherwise, you'll end up with pock marking and air bubble cavities and a less than uniform product. I don't know if that matters in this application, but it's something to be prepared for.
 

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Keep in mind that the construction of the RPG diffusors is not just random pieces in random spaces. Most commercial diffusors are based on number theory - Quadratic Residue sequences and Primitive Root sequences. Any surface irregularity is good, but most commercially available diffusors are actually engineered to provide more efficient diffusion over a broader frequency range. Have you looked at pricing on the Auralex products for your needs? They are less expensive than RPG.
 

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Quote:
Originally posted by John G
Keep in mind that the construction of the RPG diffusors is not just random pieces in random spaces. Most commercial diffusors are based on number theory - Quadratic Residue sequences and Primitive Root sequences. Any surface irregularity is good, but most commercially available diffusors are actually engineered to provide more efficient diffusion over a broader frequency range. Have you looked at pricing on the Auralex products for your needs? They are less expensive than RPG.
Do you really think so? the fact that the product is universal in design makes me think there may be some engineering to it, but as a whole, wouldn't the room, equipment, etc., etc. need to be taken into consideration on these formulas?


I looked at the Auralex site, and some of their diffusors appear to be mounted to be aesthetically pleasing. Rather than a random pattern, or an engineered pattern, they were laid out just ao theylook good.


This is just my opinion though, I have no idea, just what I would have thought.
 

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The quantity and placement of a diffusor has to do with the design and purpose of the room, but the design of the actual diffusor itself is based on engineering. In other words when the sound strikes the surface of the diffusor it should be diffused evenly. The depth and quantity of the wells will effect the quality and frequency spectrum of the diffusion. If the diffusor is not designed properly then often a narrower band of frequencies gets diffused, and the diffusion pattern becomes less even. The patches of reflective areas (drywall) that you see between the mounted diffusor panels also helps to diffuse the sound as do patches of absorbents.
 

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Here are yet more ideas:


Line the mold with something less porous:

sheet metal for ducts

formica or plastic sheets

masonite (smooth side inward)


Or sand the wood very smooth, and try painting or sealing it.


Then cover the wood with a slippery substance that won't react with the foam, and will run. Silicone spray sounds like a good choice since it is probably won't react chemically with the foam.
 

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Discussion Starter · #14 ·
I was hoping someone would post - sure, just use this foam :) I'm going to give it more thought. The plastics/composit is a good idea, but the cuting thoughs would result in a rough surface that would require more treatment. I thinking about using a higher grade wood that is more finished that 1x1 and potentially sealling it then using a shalack of some sore to give it a non-porous coat. I may test this out with available products such as foam in a can for construction. I know this stuff won't stick to tin-foil, so maybe I can find a creative solution. There was also a dual-component mix foam I used years ago for hobby purposes. I think someone mentioned a similar product above. Does anyone know where I can find this type of stuff?
 
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