I'm still working on developing a game plan for repair and customization of some action figures, and I'm wondering about making the broken parts a single piece again without the use of epoxy.
The issue with the epoxies is that the they need more surface to stick on, and some of these pieces have very thin surface areas where the break occurred.
If I could melt or reform the separate pieces into one, problem solved!
Back in my University days I worked at a place that used to modify vending machines to allow 12oz cans to be vended in 16oz machines. They did this by adding ABS blocks to the racks using a very liquid substance that lightly melted the ABS and allowed the bond to form. It was a very tight bonding.
The closest I've found to that, and probably your best bet if you don't mind buying it, is Plastruct Plastic Weld. This may even be the hobby version of the stuff I used back in the day, the effect is the same based on the description.
I also understand that acetone (otherwise known as nail polish remover) can have a similar effect on plastics of various sorts. If you have some junk plastic to test with, this is worth trying. You're not using a lot, you just need to dip the parts to be bonded in or brush it on with a non-plastic brush or cotton swab.
In the U.K. we often join household plumbing pipes together with a solvent glue something akin to the plastic model aircraft glue. It bonds the joint in seconds and causes a melting and fusing of the plastic local onto the break.
I know from personal experience (and the wording on the back of the pot I have) that it's excellent at joining ABS plastics. The pot I have is actually from a "pipes built into the walls" vacuum cleaner system, popular in other countries more so than in the U.K. so some variation on the solvent plumbing theme in your country is likely to be available from a builders or plumbing merchant that tradesmen use, if not from the general home diy store that the "weekend warrior" DIY crowd head to..
In similar fashion, for other kind of plastics such as car headlight lenses I've been able to find generic "hard plastics glue" on eBay. The last one was delivered from Poland, had a long list of unpronounceable and possibly highly toxic ingredients, but it did a good fast job of bonding a couple of ABS parts I had. The headlight lens took considerably longer but formed an acceptable bond eventually. A good glue to have a small amount of kicking around - the name of it was Technicqll R327
Many plastics, including ABS can be chemically welded. The result will be as strong as the original plastic. Thin parts may not have much surface area to join, and you may see a visible "melt" line at the repair.
Joining broken parts is essentially the same process as building things with a material like Lucite. You put the pieces together with a tight-fitting joint, then use a micro-pipette or syringe to apply solvent to the joint and nowhere else. Capillary action draws the solvent into the joint (which is why it needs to be tight-fitting, and why not much solvent is required). It isn't necessary to dissolve a huge amount of the material into a blob of plastic that runs together. The welding happens right at the mating surfaces.
For repairing broken parts that fit together tightly, a common technique is to fit the parts together, then use tape on one side to hold them in place. Apply the solvent on the other side. Once the joint is dry, remove the tape and check that side. On a really thick piece, the solvent may not wick all the way through the crack, in which case you can add a little solvent on that side.
If you're repairing broken parts that no longer mate tightly, you need a different method. You will need a filler material for the gaps. That's typically done by dissolving some of the same type of plastic in the solvent (most plastic cements are a mix of solvents and some dissolved resin; for welding, the cement needs to contain appropriate solvents and the same type of resin). Apply a thin layer to both broken edges, then push the pieces together.
This link, Intro To Solvent Welding Plastic, gives a list of suitable welding solvents for all the major types of plastics. I would test some of those first, though. For example, it lists acetone as a welding solvent for polyethylene and polypropylene, but my recollection is that dissolving would be too slow to be useful for welding. Similarly, methylene chloride is listed as not a solvent for ABS, but it's commonly the main ingredient of commercial ABS welding solvents. As for ABS, acetone, or a mix of acetone and other solvents, is commonly used for welding.
Many of the common solvents are available in the paint department in big hardware stores. There are also commercial solvent mixes sold in small bottles for this purpose. A little goes a long way, and the mix is optimized for specific types of plastics. But you'll spend almost as much for a few ounces of the proprietary magic mix as buying a big can of a specific solvent at a hardware store. That said, I've used this solvent and applicator from Micro-Mark pretty extensively, and both are great.
