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When we PVC-weld an ending with a flexible pipe, the pipe is usually very snug in the ending. The weld makes the bond permanent and ensures that no fluid can pass through.

I'm trying to double-purpose a shop-vac to become an air pump. The ending pictured (white plastic) is not quite snug in the shop-vac air outlet. It wobbles ever so slightly—to the tune of perhaps 1 mm on each side (1/32" or thereabout).

I'm concerned that the welding product meant for this sort of joint will not quite cure while filling such a big gap, that it will either fail by leaking or by not curing outright.

Will a PVC weld cure to fill 1 mm? What other bonding agent could I use as a filler before using, or in addition to, the PVC weld?

repurpose shop-vac

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    I'm trying to double-purpose a shop-vac to become an air pump. - I'm not sure what you're hoping to pump with this setup, but a Shop-Vac will develop an ultimate pressure of about 50"wc, or 1.8psi under ideal circumstances. This will be a terrible pump. Maybe enough to fill an air mattress but that's about it. Shop vacs are optimized for airflow, not for pressure.
    – J...
    Nov 2, 2021 at 8:31

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Will a PVC weld cure to fill 1 mm? What other bonding agent could I use as a filler before using, or in addition to, the PVC weld?

As others have mentioned, PVC weld only works on PVC. The black part in your image reads >PP<, which means it is probably made of polypropylene.

My recommendation would be a flexible polyurethane or silicone based adhesive, e.g. construction glue. Even RTV silicone can work. As usual, clean the surfaces well and if possible, scratch them a bit with sandpaper to make the glue stick better.

Epoxy may work, but it does not bond very well to polypropylene and may start to come apart as the machine vibrates.

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  • Good eye catching the >PP< (and how nice of them to identify the material). This is apparently a distinct form of identification. I'm used to a single-digit number inside a triangle (or an oval?), with some values indicating suitability as, say, a water bottle, and others not, but not to a pair of letters.
    – Sam7919
    Nov 2, 2021 at 14:40
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    Yeah, the "resin identification code" triangle is more common. But also beware, the marking is part of the mold and it does happen that the material gets changed but the mold is not updated.
    – jpa
    Nov 2, 2021 at 14:46
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It might work or it might not work.

PVC pipe and fittings are not really 'glued' together but are solvent welded. The inside of the fittings are designed with a tapered hub (the inside diameter of the hub gets smaller further in, slightly smaller than the pipe's outside diameter). The solvent cement, 'melts' the PVC and when pushed together the PVC of the pipe and the fitting mix together. That is why the fittings are so tight.

Because this is a chemical weld, you need to be sure that you are using the correct chemical with the correct material. Are you sure that the outlet of your shop vac is PVC and not ABS or some other kind of plastic?

You might have better results using a two-part epoxy product designed for plastics.

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    Ah... good point. It's indeed likely ABS, not PVC. But epoxy is rather problematic. It's quite harmful if inhaled, and is best left for the professionals who know what they're doing? The pump is not that strong. It's only 5 HP—good for a shop vac, minuscule for a pump. Might a multi-purpose construction adhesive (that handles wood, drywall, plaster, ceramic, brick, concrete) not be a suitable substitute?
    – Sam7919
    Nov 2, 2021 at 0:38
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    Epoxy is not that harmful in the amounts we are talking. Get one of the two part epoxies or JB weld from the hardware store.
    – redlude97
    Nov 2, 2021 at 4:36
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    Epoxies are great for specific applications. They function as both fillers and adhesives, which is what this 1mm gap need.
    – Nelson
    Nov 2, 2021 at 8:29
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    @Sam I hope you're not breathing PVC cement vapors either! And yes, construction adhesive should work well. In any case, always make sure your work area is well ventilated.
    – Self Evident
    Nov 2, 2021 at 10:06
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These things are not designed to go together. They are not made of the same kind of plastic.

And PVC cement is not made to bridge gaps. That would never hold against pressure.

Since leakage is not mission critical here, all you need is a gasket of the right thickness shaped like a cylinder. Nothing easier; wrap the white part with a few laps of masking tape until it is the right thickness. “How many” will be found by experimentation, it's easy to peel off and re-do if wrong.

Keep in mind, average shop vacs are NOT rated to run at low airflow (unless they are). When you hear the vac motor "whine up" because the inlet is blocked, that is what they cannot do continuously. Certain vacs are made for this duty, they are built with a separate air stream to cool the motor, so "revving up" cools them even more.

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    I like the gasket idea; it's cheap and doesn't require messing around with sticky compounds that might or might not stick. Also, it is a good warning about keeping the motor cool. In the case of blocking the outlet side of the vac, it might not actually "whine up" because you are not creating a "partial vacuum" in the impeller housing (or whatever the main fan is called). I've seen vacs used for inflating air mattresses and such-like, but would emphasize the warnings that these machines aren't really designed for low-flow, high pressure applications.
    – Conrado
    Nov 2, 2021 at 19:44
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The PVC welding cements rely on the PVC fitting fitting together in a very tight fashion. I doubt that you'd have satisfactory results based on my experiences. Your best solution would be to get a two part epoxy and roughen up both surfaces with sand paper to the maximum depth and apply the epoxy and let cure for the required time.

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Another option might be to model the two parts combined, and 3d-print a replacement. However threads are incredibly faffy to mock up.

The advantage is that if it breaks you can print another one reasonably easily. Same if the part needs a minor tweak, and you can have a smooth(ish) bore through the middle for improved airflow.

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  • I intend to do this on my dust extractor system, which has up to three adapters cobbled together.
    – Criggie
    Nov 3, 2021 at 0:53

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