Heat Resistant Plastic

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lpd

Member
Joined 2003
Hello there. I have a Stax DA-80 amp that was damaged on transport. The damage is to where the heat shields mount to the chassis. The heat shields mount to a piece of heat resistant plastic which is mounted to the chassis. I am guessing the heat shields can't be mounted directly to the chassis due to grounding??

Anyone here know of a suitable plastic that could be used? The strips are about 6 inches long, 1/2 inch wide, and half inch deep.

Thanks,

Peter
 
J

Jim Leach

Member
Joined 2010
drivetheglobe.com
The highest temp 'plastic' I am aware f right now is PEEK.

Just be sitting when you get the quote...

We use it a lot for non-conductive parts of plasma guns. It takes 400F (if memory serves) continuous pretty well.

Of course Teflon is quite a high temperature insulator as well. Plus it doesn't melt- it sublimates to gas. It is a bit soft however. PEEK is much more solid- similar to Delrin (maybe a bit harder?)

Now, if you want a thermally conductive plastic that does not conduct electrically, that is something I am not aware of. This may be what you need for this application?
 
A

AndrewT

R.I.P.
Joined 2004
Many of our modern plastic are "thermo-plastic". They soften as they get warmer.
A lot of our older plastics (bakelite) were "thermo-setting". They set when heat is applied. They say set once that cure has been achieved.
When heated they retain much of their mechanical properties, but as with all organics, heat them sufficiently and they burn/smolder/give off noxious fumes. By then the vast majority of the thermo-plastics will be a puddle in the bottom of your chassis.

There may be some grades of formica/phenolic composite that meet your requirement.
 
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wrenchone

wrenchone

Member
Joined 2004
A picture of the part in question would work wonders. Since the part carcked in transit, I would guss that it might be something like bakelite, which tends to be brittle. It also hold up to heat fairly well. I assume that this is a heat shield around some tubes - correct me if I'm wrong.

I looked this thing up, and the Stax DA-80 appears to be a SS amp, in which case you might be talking about a heat sink suppport. Am I right? In that case, I would use glass/epoxy composite of necessary thickness, unless the part is some tricky molded piece of nonsense. Glass-epoxy is strong, reasonably cheap, and will stand up to the temperature of a properly designed heat sink. It can be drilled and tapped. McMaster-Carr and other suppliers sstock a wwide variety of forms and thicknesses.Anyway, pictures first!
 
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L

lpd

Member
Joined 2003
Thanks guys. I can post pics of the plastic pieces tonight and the damage. The amp looks electronically perfect, no damage to the circuit just the mounts. I was concerned that the amp was grounded through the heat sinks and that they were mounted on the chassis to isolate it from the chassis? Looks good if they are just for dampening and to raise and support...I can easily rebuild something if that is the case.
 
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lpd

Member
Joined 2003
Here is a pic of the amp with the broken channel. As you can see the bakelite mounting bars are broken. I removed the heat sinks for better view. The heat sinks have three wires two input into the transistor, one onto the heat sink. Maybe this is why the heat sinks are raised from the chassis? Grounding?

An externally hosted image should be here but it was not working when we last tested it.


I may be over my head on this one. Looks like the heatsinks are isolated from the chassis.
 
wrenchone

wrenchone

Member
Joined 2004
Now that you have the amp in house, the most expedient approach would probably be to take the amp apart just enough to get to that mounting bar, and glue it back together with a good quality epoxy. This should hold for day to day use. As far as I can see, the bar would be a simple enough thing to fab from some glass-epoxy sheet (much stronger), but glue is even more simple.
 
jacco vermeulen

jacco vermeulen

Member
Joined 2004
lpd said:
Grounding?
Click to expand...

The output transistor's case in the Stax is the collector, which connects to the rails (plus or minus) in your amp.
If, as you say, each 3d wire connects to the heatsink, that one would carry a live voltage.

I can give you a couple of examples of power amplifiers that do have live heatsinks, mounted inside and isolated from the chassis.
The reason for this is to make the output transistors run cooler.

Though Class A, the live heatsink approach isn't necessary for the Stax, plus the heatsinks are mounted to close to eachother to be considered safe.
Each of the output transistors in the DA80 draws a little over 20W during idle operation.
With insulation between the output devices and the heatsinks, the power transistors still run cool enough.

You likely mean that two wires connect to the pins of the output device, the 3d wire is attached to one of the bolts that mount the power device to the heatsink.
Afair, the output devices in the DA80M monaural had common mica insulation, no reason to be different in the stereo model.
When i look at your amp picture it appears to have mica between output device and heatsink, no ?
 
unclejed613

unclejed613

Member
Joined 2006
some Pioneer receivers used "live" heat sinks mounted on plastic standoffs, so it's not uncommon, just a bit unusual to have live heat sinks..... epoxying it migt work ok.... the best stuff i've found is the epoxy putty, it's fairly easy to work with and sets as hard as a rock and sticks to just about anything. and you don't have to worry about getting the proportions right, just break a piece off the stick, knead it until it's an even grey color and put it in place. it sets in about 5 minutes
 
L

lpd

Member
Joined 2003
jacco vermeulen said:
The output transistor's case in the Stax is the collector, which connects to the rails (plus or minus) in your amp.
If, as you say, each 3d wire connects to the heatsink, that one would carry a live voltage.

I can give you a couple of examples of power amplifiers that do have live heatsinks, mounted inside and isolated from the chassis.
The reason for this is to make the output transistors run cooler.

Though Class A, the live heatsink approach isn't necessary for the Stax, plus the heatsinks are mounted to close to eachother to be considered safe.
Each of the output transistors in the DA80 draws a little over 20W during idle operation.
With insulation between the output devices and the heatsinks, the power transistors still run cool enough.

You likely mean that two wires connect to the pins of the output device, the 3d wire is attached to one of the bolts that mount the power device to the heatsink.
Afair, the output devices in the DA80M monaural had common mica insulation, no reason to be different in the stereo model.
When i look at your amp picture it appears to have mica between output device and heatsink, no ?
Click to expand...

Not sure if there is mica between the devices. What does it look like? There are three wires that connect to the heatsink. Two of them attach to the device, one directly to the heatsink itself. I purchased a strip of teflon for rails to mount the heatsinks on. I think that should work ok.

Peter
 
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