glas insert repair

[Idris]

Now if only we knew a chemical engineer...

To my mind, this falls under the remit of a chemist or polymer scientist. (I don't want to get involved in any demarcation disputes! )

[Tinman]

I had a clear window insert for the #66 Greyhound bus from which wanted to quickly remove paint. I put a dish of water in the microwave and brought it to a point below boiling.  As a rule of thumb, I always test the water to make sure it's not too hot. This time I made two errors, I failed to test the water's temp and I had added a secret additive to aid in the softening and release of the paint.  While my first and major error was not checking the water's temp, the additive increased the mineral content and would have allowed the temp to be higher before reaching the boiling point. 

I dropped the window insert into the bowl and right before my eyes it crinkled up into a shriveled useless mass. As soon as it began to deform, I made my third mistake - reaching into the water to rescue the deforming window insert (ouch).  I have pics of the deformed window insert but cant seem to locate them at the moment.  A sad day for my clear window bus restoration which has remained uncompleted to this day. I have not used hot water as a tool for window work since then. I now soak painted plastic parts in undiluted Simple Green cleaner and allow them to soak overnight.  Works like a charm.

My elevation is 112 feet above seal level (34 meters) so I'm guessing the water temp to be somewhere near yet below 100* Celsius  (200ish* Farenheit).  That is below the temp required to get the plastic to flow so I'm figuring Lesney window insert + heat = ruined shriveled piece of plastic.  My one off clinical test (LOL) confirmed my hypothesis and the result was a painful disaster. My pain was physical (first degree burns to my fingers), emotional (I had just ruined a sought after vintage part) and financial (even though the bus was play worn, it was still a clear window bus which always costs a premium in the U.S.).

[DrJeep]

I'm guessing the water temp to be somewhere near yet below 100* Celsius (200ish* Farenheit). That is below the temp required to get the plastic to flow so I'm figuring Lesney window insert + heat = ruined shriveled piece of plastic.

Apparently the melting point of acrylic is around 220C but its moulding temperature is 50 to 80C. So at 100C it would become plastic. In practice, as Joe says, that means boiling water ruins window inserts. I don’t think heat is the answer to the cracked window problem!

[GHOSTHUNTER]

A crack is visible because of the gap between the two surfaces and while this gap is pretty narrow to us, it allows light to be reflected, All we need to do is fill the gap with a substance that has the same mechanical properties of the moulding with the crack in it. There must be a liquid that will set to allow similar flexability to the original windows moulding (if it were to set hard it would just crack again when the moulding is manhandled back into the model's body casting) and by the use of capillary action we should be able to perform a servicable repair on the crack(s).

This is sort of what they do with real damaged windscreens on cars etc but of course on a bigger scale and they use optical quality resin.

Ghosthunter.

[Tinman]

This is sort of what they do with real damaged windscreens on cars etc but of course on a bigger scale and they use optical quality resin.

Ghosthunter.

I've seen my fair share of failures with that process too.

[DGinBC]

I wish I had a Matchbox window to test it on, but something done by restorers is dipping in Future floor polish. It is thin enough to fill the cracks and dries to a hard, shiny surface. It definitely does not harm (craze) the acrylic like CA glues can do.

[Tinman]

I wish I had a Matchbox window to test it on, but something done by restorers is dipping in Future floor polish. It is thin enough to fill the cracks and dries to a hard, shiny surface. It definitely does not harm (craze) the acrylic like CA glues can do.

That will give you a window insert with a very shiny crack.

[Idris]

A crack is visible because of the gap between the two surfaces and while this gap is pretty narrow to us, it allows light to be reflected, All we need to do is fill the gap with a substance that has the same mechanical properties of the moulding with the crack in it. There must be a liquid that will set to allow similar flexability to the original windows moulding (if it were to set hard it would just crack again when the moulding is manhandled back into the model's body casting) and by the use of capillary action we should be able to perform a servicable repair on the crack(s).

This is sort of what they do with real damaged windscreens on cars etc but of course on a bigger scale and they use optical quality resin.

Ghosthunter.

There is a difference between repairing the crack and repairing it invisibly. (We want the latter.)
Ghosty is correct when he says that the crack is visible because of light being reflected from the two surfaces. this is why (as previously stated), the material used to effect the repair must have the same refractive index as the parent material. If it doesn't, even if a mechanical repair has been made, the path of the crack will remain visible.

I've been looking online to see if there's any information which would enable me to identify the plastic used for the window inserts. Unfortunately, that does not seem to be the case. However, I have found reference to PET bottles having residual stresses after moulding which mean that when the plastic is heated to its softening point, the material attempts to relieve those stresses by deforming. (Try pouring boiling water into a PET bottle!) I suspect something similar may be the case here, which would mean that even if the heat could be applied with almost pinpoint accuracy, there would be a danger of the plastic deforming/pulling away.
The solution is therefore either a second, undamaged window insert, or filling the crack with a material having the same optical properties as the surrounding plastic. However, if the plastic cannot be identified, that second option is a tall order!

[GHOSTHUNTER]

I'll send a message to Paul Carr on facebook as he was a member of the R&D team through the 1980's and may know.

Ghosthunter.

[paelzermaen]

1-DSC00269.JPG (176.07 KiB) Viewed 5281 times

First try was with a hot nail to get the heat on a very small area of the plastic, but was not succesfull at all. Plastic just melted and sticked to the nail. OK, that was expectable.
Next try was heating up slowly in the oven, over a wooden form which follows the shape of the window insert. but between 75 and 90 degrees Celsius it just becomes soft and went banana. I think, when the softening point is reached, instead of filling the cracks inside, it just follows the internal tensions and shrink together.

The good thing is, now I HAVE to dismantle another Ford Group 6 to get a undamaged insert.