The stuff that you get at the store to put together pieces of PVC is labeled "Cement." This is a misnomer. The "cement" class of adhesives, such as contact cement, rubber cement, etc, all are used to bond two, usually dissimilar, materials together. PVC cement is actually a solvent. Welding involves melding two pieces of the same material into one. (Hmm, I wonder if there is a connection).
When you solvent weld PVC, you are actually turning the two sides that you are joining into PVC mush, the molecules all blend together, and what you are left with is essentially one single piece of PVC.
Step 1: Materials
-PVC Primer (it's usually purple, some places sell a really nice clear variety, if you want it to look nice you can get this stuff. Some spudders like the industrial look of the purple stains)
*Often you can get dual-packs of PVC cement and primer
PVC primers and solvents contain mostly ketones. Using commutative logic here, ketones are strong solvents of PVC. Aside from bonding applications, PVC should NOT come in contact with ketones, as it stands a chance of weakening the pipe. Ketones, such as Acetone and Methyl Ethyl Ketone, should NEVER be used as fuel for combustion spudguns.
Step 2: Get ready to get messy
Put down some sort of cover on whatever surface you're working on because a lot of times when you squish the parts together, cement will gooze everywhere. That's also what the paper towels are for.
Have some paper towels ripped off and at the ready, mostly so you can quickly clean off excess cement, because you don't have long before the cement starts eating away at PVC you don't want solvent welded. The can instructions say to wear gloves and goggles. You probably won't get cancer from it being on your hands for five seconds, but it will de-fat your skin. I did this all back when I was 16 and thought I was indestructible, as all teenagers do, and did a lot of solvent welding without gloves. Nowadays, I wear gloves.
Step 3: Deep Purple
Primer for PVC serves almost the same purpose as priming in painting applications. Paint primer prepares the surface and gives the paint molecules a nice material to adhere to. PVC primer cleans debris, then "digests" the surface molecules of the PVC part, so that when the solvent/cement is applied, it can easily dissolve the outer layers.
This stuff contains 13-17% Methyl Ethyl Ketone, 70-80% Acetone (dimethyl ketone), 5-10% cyclohexanone, and negligible amounts of other nasty chemicals.
Open the can of (purple) primer. If it's the first time it's been opened, you might need a really big guy or a pipe wrench to break the seal. Most of the time, the swab is built right into the top.
Pull out the applicator, which is attached to the lid, and dab it against the side of the can to let the excess run out. This stuff runs like water and stains EVERYTHING. It will probably saturate the top of the can.
Cover each surface to be welded on both parts thoroughly. Just be careful about runs. If you are doing severalf fittings, you can go ahead and prime everything, it's perfectly fine to prime parts ahead of time, so long as they don't get dirty before cementing.
Priming is a chemical process. The solvent (acetone and MEK) molecules diffuse into and are absorbed by the surface of the PVC plastic. This roughs up the shiny surface from the molding process, and swells the plastic, loosening the long polymer chains. Both of these processes contribute to a strong weld. The primer is extremely fast at absorbing and evaporating, so the plastic won't be "wet to the touch" for very long. However, it is important for the plastic to be chemically "wet" with absorbed solvent, so I would not let the primed parts sit for more than 5 minutes before cementing. It can't hurt to re-prime.
Some people claim it's not necessary to prime. I'd personally rather not find out the hard way.
See external link:
Step 4: Cement the fittings
Open the can and dab off excess cement. Apply cement to the inside of the joints.
This is where a judgment call comes in.
If you put too much cement on, it will gooze everywhere, and possibly ruin threaded fittings. If you don't use enough, especially on large parts, the parts might seize before you can push them together all the way.
With small fittings, one application should be more than enough.
With large fittings, you might need to dunk the applicator and add more to make sure it's thoroughly wet.
Step 5: PUSH!
With small parts, this is easy.
With large fittings, it's often more like, "EEERGGGAAARRRRDAMNIT#@$~!STUPIDPIECEOF$%&~!!!!."
If you're working with 4" fittings or larger, I'd suggest having a friend help.
Once you push and twist the parts together, hold them like that for about 10-30 seconds (longer for larger parts). You don't have to hulk-crush them the whole time, you are just making sure that the pieces don't back out. PVC slip connections are tapered, which means that sometimes, if you were to push it together and just let go, the inner piece would just slight right out, which is extremely frustrating.
After about 10-30 seconds the bond should be strong enough to keep the part from jumping out.
Wipe off any excess runny PVC cement. If it sits too long (more than about 3 minutes) on parts of the PVC you aren't welding, it can weaken the structural integrity.
Step 6: Wash your hands.
I don't think it's terrible for you, compared to things out there like benzene, lead, uranium, arsenic, asbestos etc., but I wouldn't want to keep it on my skin for too long. So go scrub down.
If the cement has dried on your skin, it will give you a lovely plastic coating that will peel off like a massive blister and, along with the purple discoloration, will make people think you have a skin disease. Abrasive soaps work really well for getting it off, like Gojo, but any dish soap will work too.
Other than that, you're done. It will be about an hour before the PVC is really workable, but I personally wouldn't put pressure in it for 24 hours. The instructions say 1-4 hours (depending on diameter) at 60 degrees F, but if you're using it for a pneumatic, you're pretty much doing something that shouldn't be done in the first place with it, so it's good to be safe.
For more info on cure times, see this link:
If you wanna see this parts in action, check out the Colossal Cannon