I've started on fixing plastic parts. So many of them are broken and I need to "weld" them.
This is first time I'm doing plastic welding so this should be fun.
I've started with test part from my wife's scooter. First I used hand-held soldering iron. To penetrate two pieces and mix melted plastic together I am making deep scratches first (not so deep to make a hole).
After that I'm trying to make rough surface a little bit more flat. I'm moving the tool constantly and trying not tu burn the plastic. It needs to be melting but not burning (no smoke!).
Once whole area is less rough and it is quite how it's easier to melt it slightly with fast moves. I'm using my finger to make surface even flatter - it's very hot so I'm suggesting using something else and melted plastic can stick to fingers

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Always both sides need to be fixed that way. For best results it is best to add more plastic to broken area with same parameters, ex. from another part. I didn't do that on test piece to check it strength after it's cooled.
I've put fixed part in vice and measured strength used to brake the piece. It was aroung 9 KG when it broke and the piece was bent so much that I'm curious if it would brake anyways if new.
One important thing to note is that I didn't try to break the piece by stretching it. I actually used worst case scenario which is pushing end of the piece like one would do to break branch on tree.
I've checked where it is broken, and it seems pieces where I was welding plastic got separated - probably because of too little/too much heat. Or maybe I should have cleaned it with acetone instead of welding very dirty piece

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Now test run is over and I need to fix my left (or right?) case. I have a donor for that, another broken case from same side (came with spyder).
Part which holds pin securing case from opening is broken. Hot knife, sanding paper, and hole for new piece is done, and replacement is prepared.
This time I've cut small 2mm wide straps from donor part to add more "meat". Also, I've used small hobby torch instead of soldering iron. That way it is easier to see when color changes and plastic starts to melt and not burn. I stull used my fingers to cool down and flatten some areas, fortunately without burning myself.
In general I was surprised how easy it is to weld plastic parts.
But, it is essential to use same type of plastic as a donor. There can be small difference but exact comparrision chart one should find by google.
Cases are made from PP-GF30 (Glass fiber-reinforced polypropylene with 30% fibers). I think all PP-GF** could be mixed but it's best to double check before welding or even do a test weld in place not visible.
[EDIT] I've continued to fix more damaged parts and I've bought additional sticks used for PP-GF welding. My observation:
- Soldering iron is best to make deep "scratches" and therefore bond plastics from two parts in deep groves. with torch it is not so easy.
- Torch helps to keep high temperature of bigger area and you can change shape of this area with fingers or flatten it to make it look better.
- When introducing more plastic (straps) from donor part into welded area it is best to use soldering iron because you can use one hand to heat up parts and use force to combine them together.
- You need to watch out to not overheat bigger area because it could collapse when you use force from soldering iron. If bigger area starts to get too hot on both sides, you can always wait for part to cool down, or use water to cool it, or support welded area by holding ex. metal plate under it.
- Even if you buy proper sticks for welding you part it doesn't mean they will melt in exactly same temperature and same way. In my case sticks started to melt sooner than parts from Spyder. When sticks were already too hot and started burning, Spyder parts were not melting yet. Because of that it was harder to bond these two and quality of bond was poor. If I used donor part with exactly same type of plastic, cut strips from it and use that for additional "meat", that was easier. Conclusion: Don't throw away damaged parts because you never know when you'll need them.