I have this light: http://www.fasttech.com/product/1206500 and want to to some mods to it, but would like some advice on how to improve the heat handling.
What can I do besides adding thermal compound? Could I fill up the hollow space in the pill with copper?
What can be done to this particular light to greatest improve heat handling without having to machine own parts?
This is the pill:
you took pic of wrong side of the pill, this side has nothing to do with heatpath, make sure your led star sits on flat surfice, and pressed hard, your pill is firmly screwed into the body, you could wipe off the lube that i see on the treads, and use thermal compaund instead, but it will not make day and night difference, only will sligtly improve heatpath. i see many people here use thermal compaund incorrectly, you do not fill large hioles\gaps with it, you want to have surfice you mount your star onto be as flat as possible, and use THIN layer of compaund between.
Of course that has something to do with the heat path…that solid aluminum shelf in the pill IS the heat path. (be thankful that shelf wasn’t drilled out and made into a “hollow” pill…bleh) What he wants to know is if filling up that area between that back shelf and the driver will aid in heat control
The short answer is yes, slightly
Long answer is how much
If you look close at pill the threads more or less are the thermal path between the pill and the body of the flashlight, that’s not alot of path, but it is a path, the biggest hindrance is the mass of the flashlight, heat transference is one thing…where the heat goes once it migrates away from the emitter is the next…and there ain’t alot of places for that heat to go, no fins for heatsinking, no heavy mass to heat from ambient temperature, yadda yadda
Does the OP have cheapo digital calipers to know how wide the hollow behind the emitter shelf is inside the pill?
I agree that a good bonding agent is needed to help pull the heat from the emitter to that shelf to the pill thru the threads to the head of body
Recommendations:
Sensor safe RTV from lowes called ultragray, little dab under the star bonding to shelf, or some sort of epoxy, artic aluminua or even JB Weld will work (just remember a little dab will do ya)
Then some cheap computer CPU grease on threads to aide heat transfer in thread zone, without substantial beefyness and heatsink fins the heat has nowhere to go, the flashlight even with good thermal conduction will eventually “heat up”
Another problem is the design, the zoomie head (where the heat sink fins are located) is thermally isolated from the body by a rubber o-ring so it can slide back and forth to zoom, so all that cool design and whatnot really doesn’t aid in heat removal from the pill
only 2 places there that matter as far as heat transfer goes, front side of the pill that star sits on, and threads, which is the spot where heat moves from the pill, into the body, or a head, like in this particular case. that is it. the rest is irrelavant. back of the pill has no heat transfering function.
i hope pill surfice that star sits on is smoother than the back side that we see on the pic. cuz if it is as rough, you need to sand it, you want mettal to mettal contact to have as much area contact as possible.
very true, some people have gotten pills that have machine marks that rival the Himalayas or are beveled 
Back of the pill/space between that shelf and the driver can be used as a place to build up for extra thermal mass (bigger heavier object takes longer to heat than smaller lighter object [but will retain that heat longer on the other end as well])
Screws would be good.
With 2 screws you have 2 big heat paths better than paste or some threaded ring.
You’ll need to drill holes though…
Aluminium pop rivets would even be better, or real copper ones.
But might get in the way of the reflector, if you have one.
I have a similar zoomie.
Pill has 2 O-rings for the head movement.
Because the head is a big chunk of aluminium, it would be nice to use it as heat sink, but the O-rings are practically all the contact between body and head…
I have been experimenting with copper wire O-rings and sheet copper ring in between the O-rings, but it was horrible moving the head…
I may give it another go some time.
Your correct adding mass in the back there won’t improve the thermal path but it will help in overall thermal management, more mass will take longer to heat soak which means better overall performance.
Here’s the one I’m working on now, it’s the pill from a TR-9XT6, alone it weighs <80g, the copper slug weight ~250g.
Here’s the slug by its self
hah! Press that plug in…nice!
Thermal paths aren’t ideal in every light. Add some copper behind the driver, just try to mate both surfaces the best you can. That machine work is very bumpy, if you can, sand it down and lap it to whatever you put in there.
Keeping the led cooler for longer is sometimes the best option if the pathway isn’t ideal.
And don’t forget the copper mounted emitter with a direct thermal path such as Sinkpad or Noctigon.
The front side of the pill where the LED sits is thankfully much smoother than the back side. I will be using a LED on a Noctigon MCPCB and have Artic Alumina thermal adhesive two component mix to fix it to the pill shelf (will use very little). I don’t want to use screws as this light has a nice little screw in retaining cap for the LED. It doesn’t serve any purpose with a glued on LED but I like the look of it 
I have thermal grease I can use for the threads, but as the threads are also leads for negative pole from battery I would have assumed that it might increase resistance.
I didn’t think about the o-rings being the only contact with the head. Makes me wonder why they made the head so big and heavy, it could have been a little slimmer as it is not much good as a heat-sink.
Good thought on the added mass will retain the heat longer, but in my case the longer it takes to heat up the better as this particular light will be on for long durations. I guess I’ll have to run some tests and see how hot the light gets on different levels. I’ve just received the ATiny programming kit and have a few QLite drivers with stacked chips for about 4,4A so I should be able to find a sweet spot.
Do the drivers produce heat also, or is it mainly the LED? The drivers themselves do not have any heat-sinking, but as they are sitting in the pill they obviously will get hot. How sensitive to heat are they? Providing that the wires used are good enough I’d assume the driver is the weakest link. Has any tests been done on how hot a pill can get before heat causes some kind of failure?
djozz tests 6 different stars, found that direct thermal path whether aluminum or copper work almost the same (until you get VERY high power)
That is the biggest thermal gain you can expect in emitters under say 5A
Screws are no miracle cure and in a lot of cases just make things worse. If you use screws large enough to be able to apply enough clamping force to make the thermal paste work as designed, that force is enough to deform the MCPCB so that the bottom turns concave and you only get contact out at the sides right around the screws.
If you can make the bottom of the MCPCB convex before assembly, and then use the screws to pull it down to flat or nearly flat, you'll get a much better interface where you need it, in the center under the LED.
Yes, i have similar dodgy experience with screws, but that was with a cheapo pill and thin 16mm star (disc)…
Works good though.