About Ready to Quit and Give Up ......

Scuba_Vixen

Working on this HUD project

(crass solicitation for encouragement here)

The housing part of the project is driving me mad.

For those that haven't followed the project, I've had some issues with water ingress into the Delrin housing during pressure pot tests. It's not much, and sometimes it doesn't happen, but it usually does. The housing is milled out of a solid block of Delrin. (appearantly, solid is a relative term)

Initially, I figured it to be the lid seal. (what else could it be, right?) I've tried several flat gasket and O ring methods. Generally, with similar results. Then I figured maybe it was that I needed to use better tools, so I went over to a buddy's machine shop and did my machining there. Pretty much the same results...... This really cheezed me off and put me in a real quandry.

Well, to the rescue comes my housemate, she works at the refinery here. There's this spiffy paste that comes in a tube called Kolor Kut. It's normally tannish gold color, but turns bright red when contacted with water. They use it to tell how much water is at the bottom of the tankers tanks when they come to off load crude at the refinery. (they smear it on the bottom portion of a glorified dip stick)

Well, I smeared on a coat of KK on the inside of my housing, put the lid on as usual, and put it in the pressure pot. The nice thing about my pot is the clear top, and I can actually see into the housing (it has a clear lid) when it's in the pot, if I position my eyes at the edge of the pot top and hold the light in the right position.

So that's what I did.
Actually, I did it several times to see if the results were consistant ..... They were


Anyway, the ****er leaks thru the porosity line.

See this thread; post 8 for a mention of porosity line:

http://thedecostop.com/forums/showthread.php?t=18220


I took a few pics of a recent trial so you can see what I'm talking about.

The first pic is the housing with the KK smeared inside on all the surfaces, and ready to go into the pot.

The second pic is the housing in the pot

The third pic is the housing after coming out of the pot
Notice the RED splotch at the lower right area of the housing's end
Notice the RED trail running down the housing's bottom
this is from a similar seepage point at the other end of the housing

The fourth pic is the housing turned other end up, so you can see where the red trail was running down from

The fifth pic shows the left side of the box (relative to the 3rd pic)
Notice a small RED splotch forming at the center right area of the side.


Notice that there is no red anywhere near the o ring lid seal, it doesn't leak.

Notice that the red spots are not where there's a screw

I watched as the red splotches just spontaineously appeared and grew in size.

It's all really stinkin' weird to me, but it may explain some mystery (partial) floods of handsets and can lights.

Maybe I'll have another go at a housing, but you can bet it won't be Delrin again.

Polycarbonate is my first choice, but I can't find it in a small piece in 2" or 2.5" thickness

CellCast Acrylic is available in those thicknesses, and in suitable sized pieces, but PC would be a lot better.

Also thought about 6061 aluminum, it's available in suitable size and thickness.

Anyhoooooo,
Just thought I'd keep you all posted on how it's going.

Feedback is always welcomed.



Darlene

Attached Images

P4150303smlr.jpg (490.9 KB, 651 views)

RonMicjan

try acetal copolymer, same family as delrin, but less perosity problems

__________________
Ron
"Life is pain princess, anyone who tells you different, is selling something", The Dread Pirate Wesley.


www.tmishop.com Diving bits etc.

rdmmdr

darlene

try tap plastics on the web they will cut and ship to anywhere, and they have large acrylic. if you use alum you probably will not be able to get it anodized there so stop by the airport and get it alodined at the very least.
rick m.

Rebreatherlab

Hi Darlene,

I feel sorry for you! Try Acetal Copolymer. I have had many problems with Delrin and am now only using POM C. It is a tiny bit softer than Delrin, but less prone to cracking.

Maybe bad batch of Delrin or recycled from chips where other stuff got mixed in?

Andy

Genesis

Where 'ya getting the Delrin?

Wild stuff - I'll be doing these tests shortly on mine... let's hope I don't see similar problems!

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
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AD_ward9

Hope the following is useful:

1. It is not the Delrin causing the leak, unless you are using a very strange source of material. We have tested Delrin parts to very high pressures and no problem. The water absorption of Delrin is in line with most other plastics used for handsets. Delrin is actually stronger than most.

2. Your seal will likely be one problem, if not now then later. You might have a great seal, but we have never had any joy with face seals. We only get internal seals (piston seals, using an O Ring), to work really reliably. I know, it makes the unit about 2mm thicker. Sometimes face seals fail and give a spray into the unit, so it looks like it is leaking from somewhere else.

3. Deformation is likely to be a problem. We always fill with silicone oil. Without that, you need much thicker walls. You can put copper sulphate in with the silicone oil, and it still works - see below.

Thanks for posting the tip on Kolor Kut. It looks much better than the anydrous copper sulphate powder we use because it stays put. I have already emailed a company to order some in. However, copper sulphate can be packed in and when you unpack it, it is clear where the leak is (it goes from almost white to blue), and avoids the spray problem, or condensation running.

Cheers

Alex

Scuba_Vixen

Delrin came from McMaster Carr, it's half of a 2" thick, 4" X 12" block. All the leak points are right along the half thickness line.

It would be wayyy cool to have the whole thing "see thru" and make the housing from polycarbonate, but that kind of thickness is major expensive compared to copolymer acetal, which is a smidge cheaper than Delrin.

A 12" X 12" X 2.5" acetal block is ~$175

A 5" X 8" X 2.5" polycarbonate block is a tad over $200

Here is the only place I could find thick PC;

http://www.polymerplastics.com/mechanical_poly.shtml

I'm looking at 2.5" thickness cuz if I build another housing, I want a convex bottom shape like Karl's handsets, and I want to be able to have space in it to mill out a place for a battery pack of NiMH AA's for the HUD leds and controller.

I'm liking the idea of a charging port instead of having to take the cover off to replace batteries.


Darlene


Some body nudge me a little, the "see thru" idea seems unique.

RonMicjan

Actually it sounds like it is the delrin, ISC got a shipment of Delrin mismarked as acetal a while ago, (it looks identical) made a bunch of ADV's and they ALL leaked, caught during testing, the leak was right through the face of the unit, not a fitting, you could see the bubbles coming out right through the plastic.

__________________
Ron
"Life is pain princess, anyone who tells you different, is selling something", The Dread Pirate Wesley.


www.tmishop.com Diving bits etc.

AD_ward9

Very interesting behaviour. If you were in the UK we could send you material to try out. I definitely have not seen this problem with our designs and testing recently has been to much higher pressures. This suggests it is either a design issue with your case, such as the seal or a micro crack opening under pressure, or there is some defective material around.

The points alone the thin line are very suspicious. Is there a hairline crack?

Edit: In terms of material, one of our mech guys starting early and working this weekend, has just pointed out to me there are different types of copolymer and some do leak. So if it is Delrin, then I still have the view it is very unlikely to leak, but acetyl copolymer does look and feel identical as Rob points out, and if used could be the problem.

If you try paint the inside of the casing, to see if that affects the time before you see the problem? Using paint, you put in a moisture barrier behind the KK, and can see if the water is really coming from the back and not from a spray from somewhere else.

Cheers,

Alex

Genesis

Interesting on the face seals....

There's a lot of underwater video housings around with face sealed O-rings, including one that I own. Never had a problem with it and the unsupported surface area is MUCH larger than it is on a handset.....

The line that Darlene is getting with the Kolor Kut looks suspiciously like it is indeed coming through the material - a spray wouldn't tend to produce that kind of pattern....

It'll be interesting to see if my handsets exhibit the same sort of problem. Let's hope not! Of course the silicone oil fix is always available, but I'd love to avoid that one if I can.....

__________________
"A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American."
http://www.denninger.net
http://www.diversunion.org/liability.htm - Fix the Diving Cert racket