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How to seal a gap between hard PVC pipe and copper coil as visible on the picture below? It needs to stand small vacuum (45 mmHG = 1.8inHg = 6kpa = 60mbar = 0.87psi).

PVC-u and copper coil pipe

What is the best way to do it? Is there any special PVC mass / epoxy suitable for that?

At the moment I used tick layer of white silicone and it stills cures (after 15h) and not sure if it was a good idea in the first place:

gap repaired using silicone

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    Separate mechanical strength and air-tightness. Cut a "washer" out of a copper sheet. Solder it onto your pipe so that it is right inside of your PVC bellhousing. Drill 3 or 4 holes going through both the washer and the PVC. Apply a glob of silicone to those holes before inserting the fasteners next. Rivet (or bolt) your washer ad PVC together. Repeat for the gash by the lip. Mechanical strength: done. Apply silicone to all the joints from the inside and outside for air-tightness. – Zach Mierzejewski Apr 19 '16 at 19:48
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I too have struggled with gluing copper and PVC for certain experiments. Silicone is an ok choice, depending on the pressure, but white silicone just happens to be less effective for sticking to stuff than clear silicone... I don't really know why, I just know that it is.

It's kind of too late to use anything else now that you put silicone on it (actually PVC cement seems to stick to silicone, but it's a poor glue overall); but chances are the white silicone will separate or peel off after a while. If (or, after) that happens, you will have another chance to use another glue. The main thing to be concerned with is gluing to copper. I think that Just for Copper epoxy would be a better choice. I have not used this before, but based on round-about experience, I think it would be good to try. In general, copper oxidation (patina) is a problem for glue on copper (in the long run). Gluing to PVC is probably not going to be as much of a problem... at least for most kinds of glue, silicone, and/or epoxy. Maybe the the answer is to use something like a copper putty to adhere to the copper, then use silcone or another epoxy to join the PVC to the copper epoxy.

Something else that might help would be to use a piece/sheet of plastic or rubber over the area (like a shield). Use something, in which you can cut tight, accurate holes to help seal the deal. Some plastics don't stick to some epoxies. So if you decide to try this, test the plastic for compatability first.

Edit- See where I think a weak spot could be; unless maybe another pipe is inserted which helps support this area.

enter image description here

  • I updated information about pressure. Will my white silicone stand that vacuum? – Lukasz Apr 19 '16 at 13:36
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    Unfortunately, that is a complicated answer. Variables include thickness of the silicone, bonded strength, and area/span of the hole. But generally, I would say that 0.87PSI is a maybe. I added a picture to my answer, pointing out the area that I would most likely expect to see it fail from pressure (but I really don't know). If your "fitting" is attached to another pipe that obviates or reinforces the spot that I pointed out, then I think that it might hold until the copper oxidizes and the bond at the copper surface is lost. – Ben Welborn Apr 19 '16 at 14:09
  • I dealt with the weak point by adding PVC fill / fitting into it (perfectly cut shape). I also discovered that the silicone used was overdue (never dried up) therefore washed it out easily and replaced with a new silicone recommended by shopkeeper at repair shop. New silicone has with extremely strong bond (also white BTW), claimed to be 150 tons strength. We will see how this one works. – Lukasz Apr 19 '16 at 19:37
  • By PVC fill, do you mean the epoxy, like for filling in holes on house siding? If you washed out the silicone, with water, then it was not silicone. Perhaps it was caulk? There certainly are things that can be done with or added to silicone to affect the properties... like adding rubber, caulk, or even tin and aluminum. Can you share with us what you are using, and let us know how it goes? – Ben Welborn Apr 19 '16 at 20:28
  • By PVC fill i mean piece of PVC cut into the shape to fit the hole. Then I glued that piece in. I will let you know the effect and silicon details once the result is clear. – Lukasz Apr 19 '16 at 20:32
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I see that you have a copper tube coil inside that enclosure. My guess is that the experiment you are trying to accomplish requires that to be copper? If so, is there a reason that you couldn't transition away from copper to PVC inside the enclosure?

If that is an option, then you are left with sealing PVC to PVC which is much easier.

Of course, you would still have to address the weak point that Ben Welborn mentions in his answer.

  • It must be copper (or aluminium in the worst case) - it is a cooling coil and it's performance is one crucial point here so I do not have an option unfortunately. I dealt with the weak point by shaping PVC fitting into the biggest gap. – Lukasz Apr 19 '16 at 19:33

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