\"Headlight\" polishing...

[Don Y]

On 2/27/2023 6:46 AM, Martin Brown wrote:

But, I see lots of cameras with clear \"domes\" housing the
mechanism (allows the camera to be aimed without modifying the
mount).  These *feel* like plastic.  But, don\'t exhibit the
fogginess of headlights.

I asked my tame polymer chemist and their best suggestion was to try
thermoforming sunbed grade acrylic sheet which is a lot more tolerant. It
limits your choice of shape a bit though. eg

Yeah, and that limitation then becomes something that you have to codify
before you can settle on an actual design. I suspect it\'s a semi-complex
relationship between material thickness, extent of curvature, size, etc.

Note that the \"domes\" on security cameras are pretty \"regular\" surfaces.

https://plasticsdirect.co.uk/products/sunbeds-acrylic-screens-and-parts-sunbed-perspex

Of course, I have no idea how much exposure they\'ve seen.

But, I am starting to reexamine the idea that abrasion (and
car exhaust/pollutants, suggested here) might have more of
an impact than *just* UV exposure.  Again, I cite the fact that
tail lights don\'t seem to suffer the same fate!

I suspect that is because they are on the trailing edge of the car and don\'t
get anything like as many energetic impacts from stuff flicked up off the road
by other traffic. I think abrasion takes most of the UV protective surface
coating off and strong sunlight does the rest.

That was my initial thought:

\"Does the \"fog\" that plagues modern headlights come as
a result of UV damage? Or, fine-particle abrasion
(the lights being on the leading edge of the vehicle)?\"

but, I know the sun *eats* things, here (and, I\'ve no experience
with the performance of these types of lights in other climates
as I\'ve lived here since before they were common).

Our sunlights became *incredibly* brittle from decades in the sun,
to the point that a good hail storm would have cracked them
(hail being infrequent, here). When I replaced them, they shattered
in my hands!

 

[wmartin]

On 2/26/23 18:22, Don Y wrote:

On 2/26/2023 6:55 PM, wmartin wrote:
On 2/26/23 11:29, Don Y wrote:
On 2/26/2023 10:09 AM, wmartin wrote:
I drive a box van - so I\'m quite happy to have it look like a box.
I\'m
most definitley NOT happy to have to polish and re-coat expensive
stupidly-made headlamps each year for the sake of \'style\'.

yeah, I\'m actually with you on that one. That and the stupid clear
coat that seems designed to fail if left in the sun. I have a now
retired 1985 van with an aftermarket paint job (cheap, no clear
coat!) that is still not flaking off, but my 2007 Tacoma has lost
most of it\'s clear coat already.

Not garaged?

Sad to say, it won\'t fit in the garage, alongside lathe & mill & few
racing toys.

Ah, you\'re lucky enough NOT to have a basement?  :

Yes, not common in So Cal where I am located.

(I wonder if I could *dig* a basement under the house?  I had a friend
who did that -- but starting from a crawlspace...)

Well if you were in Florida, you might get one for free....sink holes
abound.

Lately, the trend, here, has been to cover parking lots with solar
arrays
(which has the pleasant side-effect of keeping cars cooler AND out of
sun).

Sadly, the shopping centers haven\'t caught on, yet.  Likely because the
occupants (who pay the utility bills) aren\'t the property owners (who
would incur the cost of installing the panels)

 

[Don Y]

On 2/27/2023 7:58 PM, wmartin wrote:

Not garaged?

Sad to say, it won\'t fit in the garage, alongside lathe & mill & few racing
toys.

Ah, you\'re lucky enough NOT to have a basement?  :

Yes, not common in So Cal where I am located.

Here (So AZ), they are very rare. I think a few businesses may have
them (one of the local hospitals has a building with a basement)
but I don\'t know of any residences.

I don\'t think it is because of things like water table as even the
homes in the foothills are built on slabs.

It may be because of soil qualities (e.g., in Denver, there\'s a type
of clay in the soil that expands significantly as it takes on water;
enough so that it can rupture a foundation -- building requirements
insist that the soil on a site be tested before construction).

[Yet, I had a basement in Denver]

Or, it could be an overall \"cheapness\" in construction. I really haven\'t
been able to understand why there aren\'t more below grade/earth-bermed
homes here -- even *now*, after they\'ve \"learned\" the cost of energy!

Regardless, it has a very definite impact on how you live (i.e., store
your sh*t) as it means living space often gets reallocated in unplanned
ways.

[I\'m very careful as to what I store in the garage as it gets very
warm in there, even with an insulated door. And, outdoors is only
good for storing *rocks*! :-/ ]

(I wonder if I could *dig* a basement under the house?  I had a friend
who did that -- but starting from a crawlspace...)

Well if you were in Florida, you might get one for free....sink holes abound.

*Pretty* sure that\'s not what I\'m looking for...

 

[Martin Brown]

On 26/02/2023 03:56, Don Y wrote:

On 2/24/2023 3:28 PM, Martin Brown wrote:
On 24/02/2023 20:43, Don Y wrote:
On 2/24/2023 2:13 AM, Martin Brown wrote:

If you can scratch the surface with a shard of bottle glass then it
isn\'t made of glass (may devalue your fitting). Rear light clusters
are almost always some sort of polycarbonate I think. Water clear
epoxy will repair them but UV damage turns it yellow after a while.

Any organic solvent will quite likely turn a minor scratch into
serious deep cracks through the material so your only option is
physical removal of the surface crazing by rubbing compound followed
by a new protective UV coat (or leave well alone and live with
cosmetic damage).

You might get away with a high viscosity silicone oil as a way of
hiding the hazing by filling the gaps rather than removing it.

Cosmetic defects on my headlights doesn\'t really bother me that much.

I\'m not interested in headlights (our car is garaged, driven very little
and probably sees less than a few hundred hours of sunlight -- including
driven time plus time spent sitting in unprotected parking lots -- each
year).

But, I am concerned about the characteristics of the *material*.

OK. So is the problem to make a clear optical envelope of some sort to
contain a device and protect it from the elements outdoors.
How important is it that the thing remains optically clear?

I could use a metal case if I didn\'t care about light transmissibility!  :

But does it have to transmit light well enough to make an image.
IOW is fogging actually a problem in this application?

If it is then you will have to provide heating to avoid dew forming on
the cold sky facing surface when the air cools and relative humidity
rises after dark. That is a problem astronomers face all the time.

The main downsides are being much softer than glass, prone to damage
by the wrong solvent cleaner (remember the plexiglass aquarium failure?).

Yes, any \"special\" material would need \"special\" cleaning
equipment/solutions.  Like NOT using ammonia-based glass cleaner
on your monitors (yet, folks *do*!)

Metal polish like Brasso works reasonably well on many soft plastics.
Organic solvents can be instantly lethal to them.

I suspect in your conditions they go downhill pretty rapidly once the
UV coating has worn off due to being sand blasted by desert sand and
grit.

Many materials degrade quickly outdoors, here.  The sun is unrelenting.
Leave a piece of PVC pipe outside and it turns brown, then black.
But, only on the top, of course!  :-/

I think car headlamps are a particularly difficult case. Hopefully your
kit will not be driven at 50mph inches away from the ground for most of
its life. Once the UV protective coating has failed all bets are off.

Or, an alternate material.

Perspex or polycarbonate are the two materials of choice for this.
Which one is better for your requirements is another matter.

What sort of working lifetime are you aiming for?

If it was a piece of glass, I\'d target 10-20 years.  If a piece
of fog-able (yes, folks, you heard it here!) plastic, I\'d hope
for \"several\" years.  Not that the cost of replacement *parts*
would be a problem but, rather, that customers would get annoyed
with having to maintain it.  (or, pissed at the cost of hiring-out
that service)

Cheap and nasty solar panels on garden lights (some polyester resin or
other by the look of it) craze to opaque after about 3-4 years here. It
affects efficiency. The clear plastic with the LED in which is properly
UV stabilised generally outlasts the PV cell assembly.

How exposed to direct sunlight?

As long as the sun is above the horizon.  :

I think you really need to speak to a specialist in clear engineering
plastics for outdoor UV exposed applications. Headlamp shells failing
may be because of the appalling conditions that the face on the front of
a car being driven around on gritty sandy roads.

--
Martin Brown