Solvent welding/casting plastics

[Kram]

Decided to fix up some bike plastics and solvent welding seems to be the best way. Ive done a few bits very happy with how easy it is. I looked up the solvents, acetone would do it, but I chose MEK as it also works on POM, and seems to be the common one in cements. Good reference here for plastics and solvents.

http://nerfhaven.com/forums/topic/18527-intro-to-solvent-welding-plastic/

I disolved some ABS in a 30ml glass jar to make a thick paste. After use it takes a good 24hours is a minimum to cure, but has a touch dry skin in half hour. Parts must be clamped to get a good join.

I think this disolved paste could be used to cast plastic parts as an alternative to 3d printing. I need a battery adaptor so might try casting one with this paste. I could either mask off a tool and paint onto that, or take a mould from a battery and pour the paste in around solid filler to minimise solvent use. Tools, batteries etc all seem to be nylon which is resistant to the solvent, but thick plaste wont go past masking tape.

I am unsure if it is best painted in thin layers to let it evaporate or poured thick.

Anyone have experience?

 

[graffian]

Not of ABS but I recently made some Q-Dope to paint some copper windings and the form they were wound on.
It's said to be acetone and expanded polystyrene, 3:7 ratio by weight gives a treacley liquid that turns to
polystyrene when all the smelly stuff evaporates.
For some reason my acetone n polystyrene didn't work, it turned to a blob of gloop in acetone; which has hardened
to a layer of polystyrene under acetone!.
I tried lechler epoxy thinners and polystyrene, it's the consistancy of honey and turns into plastic.

I might give ABS a try.

I am going to have to go and make some honey sarnies.

 

[Screwdriver]

Plastic can generally only be recycled or melted so many times before it becomes degraded. After two or three cycles my understanding is the long chain polymers unravel and/or become chopped up and shorter. The plastic becomes brittle.

Dissolving ABS in MEK will hasten the process and you will make a brittle join. There are plenty of powder products out there which claim to fix ABS but in my experience, they're all weak. Using a solvent weld on unbroken clean plastic pipe for example is a different matter, there's a huge surface area, the bond is strong and thin. Using it to fix a lug on a broken fairing, forgetabout it.

For a super strong secure fix for a broken part, use hot staples and plastic weld with a heat gun. Something like the Leister hot air welder is ideal.

But don't take my word for it, do a simple test. Mould a piece to match an existing scrap of virgin material and test them. A small bit of round or a flat would do. The virgin material will resist bending, then remain springy until you hit the elastic limit and it will probably (lit.) plastic deform. Do the same with your solvent casting and it will snap immediately with surprisingly little effort. Same goes for welded joins or repairs.

The one thing I have learned over the years I have been playing with plastic is that the temperature you take it to to make a weld is absolutely critical. It is very, very easy to burn if your heat source is hotter than the melting temperature of the plastic. Obviously when you heat steel to weld it, the plasma is vastly higher temperature than the melting temp for the workpiece but metals are a great conductor and the part is heated gradually. Plastic is not a good conductor for heat , in fact it is a really good insulator so if you heat it with a source having a much higher temperature than the plastic melting temp, it will burn!

 

[Kram]

No the abs disolved with MEK seems to be strong and just as flexible as the original plastic. Ive repaired a fairing and very happy with the result. I put too much paste in places so that needs to be sanded off for the screen and light unit to fit.

Never had good results with hot welding nylon, but I am experimenting with extruding it for some parts I need to make. Recieved a temperature controller today should allow me to get good results, as I was overheating it before.

 

[Screwdriver]

No the abs disolved with MEK seems to be strong and just as flexible as the original plastic. Ive repaired a fairing and very happy with the result. I put too much paste in places so that needs to be sanded off for the screen and light unit to fit.

Never had good results with hot welding nylon, but I am experimenting with extruding it for some parts I need to make. Recieved a temperature controller today should allow me to get good results, as I was overheating it before.

OK, that's interesting thanks. I'll give it a try next time I have a smashed fairing.

 

[Kram]

I filled up all the holes as they are sloppy moulded holes with too much draft in one direction. I will drill them to be exact. The upper screen/mirror mounts on these are weak so I have added plenty of meat. The big patch is mostly fresh sheet plastic.

The grey and black repairs were disolved abs printing filament while red was from sheet. I think the sheet gives better results.

I will try to find some time to sand it down and prep it for paint next week.