Does a material with these characteristics exist? 4

[bcavender]

The problem is to perform a secondary operation on a hollow and irregularly shaped CPVC shape.

The secondary opp potentially can take the CPVC shape above the temperature where it can become soft and have its shape compromised. (Performing the operation earlier in the build cycle is out of our control.)

This ideal material, possibly a powder or granular at STP, could be poured into the irregular cavity to fill it completely (as a physical internal support device would be expensive if not impossible to fab or even insert if one could be made).

At a temperature of appx 180F, this ideal material would experience a change where it would begin to build a light support matrix to support the CPVC before it goes limp and loses its shape.

Once the secondary opp is complete and both are cooled back to STP, this support material would lose its internal matrix structure and return to a powder or granular state that could simply be poured out.

This is likely a bit of a shot in the dark question as this is far afield from my expertise, but I have been impressed so many times with good ideas and suggestions from Eng-Tips I just have to run this up the pole to see if there is some possible approach that will get the job done.

All comments, alt suggestions welcome!!!
B

 

[Compositepro]

Sand in a rubber bladder can be easily shaped in an external mold. Pulling vacuum on the bladder will make the sand particles lock together by friction into a pretty hard shape. Releasing the vacuum makes it go soft again. The bladder can be made from a thin film (Nylon vacuum bagging film used for composite molding), which is disposable, or the bladder can be pre-molded (usually from silicone) and cured to near net shape, and even fabric reinforced for long-term durability and reuse.

 

[MintJulep]

Is heat a necessary part of your secondary operation, or just an undesired side-effect?

Can you just manage the temperature instead?

If your thing is open, or you can otherwise manage the expansion, ordinary water/ice would provide support and cooling.

Maybe a slurry of some type of filler bead and a water-soluble binder like rice starch paste?

 

[bcavender]

CPro,

Using atmospheric pressure to lock the shape via friction is really intriguing. Cost and tech are straightforward. Release would be fast. That ticks all the boxes elegantly. I like it.

Have you ever experimented w this for sand types/vacuum level used?

I can see where variables such as grain size or variations in sand types could affect the amount of friction produced for a given pressure exerted.

I’m wondering if this technique has a process name?

Great response!!!
B

 

[bcavender]

Mint,

Yes heat is required in the secondary and the amount required risks kinking/sag etc.

Filler beads are interesting if there would be a way to create a locking effect like CPro suggested.

For example, if you poured the cavity full of a silicone rubber and let it cure you could perform the secondary w no sweat as the silicone handles the temp just fine, conforms perfectly and would absolutely prevent kinking … but the problem is the SiRubber can’t be removed and isn’t inexpensive.

So maybe some material with a high enough viscosity that could be reasonably recovered afterwards would be a candidate.


Hmmm …

 

[SnTMan]

Are you doing lots of these or a few?

Regards,

Mike

The problem with sloppy work is that the supply FAR EXCEEDS the demand

 

[bcavender]

Mike,

The company that asked me to look into this likely will not have a big volume to do, but like anything else, the efficiency of how well you can do the technical side, the more upside potential there is as it unfolds. I could see a couple hundred units a month to begin. It’s pretty much a niche market and non-necessity based.

Still a very interesting technical challenge.

 

[EdStainless]

You might have to play with different sand to get the locking effect.
You could try contacting a local foundry supplier and see if they would give you samples of various sands.
In addition to particle size there are rounded grain and angular grain sands.
My hunch is that you should start with fine grain angular sand.
And it is preferable to use non-silica sand for health reasons.
There are a few available such as Olivine (Mg oxide).
Molded Silicone bags would make life much easier for operators.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[bcavender]

Ed,

My first run at ‘angular sands’ turned up “lava sand” which is both angular as well as porous.

That would be a candidate as it appears to have 50-60% less weight compared to common sand. That would save some handling headaches … be interesting to see what grade or screen sizes are available. It is an aluminosilicate so probably it would need similar handling precautions for prevention of silicosis, etc.

Olivine def makes the grade as a candidate as its density makes it only a little heavier than lava sand.

I’m going to order in samples and see how they behave.

Thank you for your excellent suggestions!!!

Best regards,
B

 

[Compositepro]

I have played/experimented with diatomaceous earth (pool filter media), which is highly porous, low-density and cheap. This makes air withdrawal fast, handling is easy, and friction between particles is high. It is somewhat soft under vacuum because the particles can crush under pressure (pressing hard with your finger will leave a dent in the bladder). That is the issue with all angular particles as well, because the contact points between particles can crush relatively easily. Quartz sand would be harder and denser, but heavier.
Round particles, like 200 micron glass beads, can be so free flowing the they act like water (they become fluidized). But under vacuum compaction round particles will lock together into a closest-packed configuration. In all cases you need a fabric filter/breather to keep particles from flowing out with thé air to your vacuum pump. You want a good vacuum level of greater than 25 inches of mercury.

 

[bcavender]

Pro,

DiaEarth and 200u glass beads are two that were off my radar, but sound like potentially good fits. The glass beads might be a tad easier to keep out of the pump. Hmmm…

Appreciate the vacuum parameter, that was in the back of my mind. I need to get some materials on the bench and get cracking.

Appreciate your experienced help!!!

B

 

[EdStainless]

If you what some bladders to play with just buy some bicycle innertubes, either in latex or TPU (you can get them very thin) and use those as test pieces.
They won't take the heat but they will give you a feel.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

 

[bcavender]

Ed,

TPU is an interesting material (after a little reading). Pretty strong for a film.

Thanks for the recco!
B