Thermally conductive plastic or diecast aluminum 2

[berkshire]

I have been asked to draw up a heat transfer pad for a medical device.This is a first draft, subject to external peer review later. Right now having drawn the thing up, the question "What are you going to make it out?" of comes up.

Most of the device will be injection molded plastic, and the suggestion has been made, that this be used for the heat transfer pad. I had suggested Diecast Aluminum, but the feeling was that for a 500 part run that the dies would be much more expensive than a conventional injection molding die. Thermally conductive plastic has been suggested, however I know very little about this other than an article I found on google. Can anybody out there point me in the right direction.
B.E.

http://www.designnews.com/article/165-Thermally_conductive_plastics_beat_the_heat.php

 

[patprimmer]

Thermally conductive plastic actually means por instead of extremely poor conductivity.

Plastics injection moulds are also quite expensive. cast zinc or aluminium would be my choices or maybe even pressed metal.

Regards
Pat
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[Pud]

We are currently working with these on a project:

http://coolpolymers.com/

H

http://www.tynevalleyplastics.co.uk

 

[MacGyverS2000]

Does the aluminum have to be diecast? If it's a simple design, CNC of the item itself is often quite cheap.

Dan - Owner

http://www.Hi-TecDesigns.com

 

[lumenharold]

I believe the coolpolymer site Pud refers to also has a moldable metal material that is an aluminum / zinc alloy. It is supposed to be compatible with standard injection molding tooling and equipment although it will wear the tool faster (probably not a concern at 500 per year). It does not however work in a co-mold application.

I would think the first issue to resolve is how much heat do you need to dump and let that determine the interface.

Harold
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[KENAT]

There are other casting processes to consider. For instance Rubber Plaster Mold Casting

http://en.wikipedia.org/wiki/Plaster_mold_casting.

I'm currently looking at getting a small electronics enclosure, with heat management requirements, made this way.

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[berkshire]

Thank you for the input Gentlemen,
Based on this it would appear that my first thought of diecast or lostwax metal would be the way to go.
I get the idea that the thermally conductive resins have a high metal filler load and can be abrasive on the dies.
I am meeting with the customer and an injection molder next Monday. By then I should be able to ask intelligent questions ( I hope.)
B.E.

 

[KENAT]

Berkshire, take a look at the RPM process. Based on the quotes I received the tooling is quite a bit cheaper for RPM (1/2 price in my case) than investment casting/lost wax - though this was a small sample.

Given it's from an RPM company I'm not gonna claim it's truely objective but take a look at this for more general cost comparison.

http://www.aljcast.com/advant.html

It may not be an issue for your part, but I think the RPM process, or at least some variations of it, can give better tolerances than investment. Not sure if that's a concern for you though.

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[Demon3]

I was surprised to read that thermally conductive plastics, although not as conductive as metals, can give the same cooling performance when you have active cooling (i.e. a fan) and compare metal and thermally conductive plastic.

I know this is counterintuitive and I am not an expert but I read a very convincing article explaining this in detail. I've seen the same effect when I worked at Electrolux, we made a water condenser out of plastic and it was better than the same thing made in aluminium and we didn't understand why.

See Compounding World February 2010

http://cde.cerosmedia.com/1X4b792136ec3ad436.cde

Free download.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[ornerynorsk]

Investment cast? Then you could match the metal to the heat transfer required. Copper, silver, whatever.

 

[patprimmer]

Pud

Great info.

I have used aluminium flake filled nylons and acetals in the past for electric motor housings that needed electrical insulation of some sort but good thermal conductivity. It ended up several pieces with a metal housing and plastic insulators where required instead of he original one piece hoped for. It was a submersible pump for garden fountains. The nylon also allowed to much water to diffuse through it.

In testing they where too conductive of electricity and not conductive enough of heat and their performance was erratic and physical properties where absolute rubbish. That experience might have tainted my views, but my conclusion was plastics are not very useful at conducting heat. A wax model from which a silicon rubber tool is made which is used to injection mould wax plugs for lost wax casting is a relatively cheap method for very small numbers. It is commonly used in the jewelry industry.



Regards
Pat
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[berkshire]

Kenat,
The RPM process looks similar to the method used for casting wax preforms for lost wax molding.
Pud.
That is an interesting material I will mention this when we meet with the molder. It looks like its thermal conductivity is approaching that of aluminum.
Demon 3
This is the thing that started this question. This heat transfer pad is a small dome that fits in the palm of the hand with a labyrinth on the under side with water circulating in it. Not unlike some heat sinks.
Patprimmer
When I see a molding expert like you shrug off thermally
conductive plastics, I pay attention. If the local molder has the same opinion, I guess I know which way we will go.
I am familiar with silicone rubber molds for wax preforms and agree that that lost wax or CNC machining may be the way to go.
MacGyverS2000
I ran up a tool path for a cnc router, it would take two operations, one for the inside and one for the outside about 15 mins per part. I have to see how that compares to molding.
All
Thank you for the inputs I will see how the meeting with the molder and customer turns out.


B.E.

 

[Demon3]

If you do contact CoolPolymers ask for Jim Miller. He knows his stuff.

Chris

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[Compositepro]

I agree with Pat. The thermal conductivity of pure polymers is about 0.5 W/mK compared to 800 for copper. Conductive fillers can increase this to maybe 5 and still be electrically non-conductive. If the filler particles are fibers or packed so tight they touch the you can bring conductivity to 20 or so, but the material will be electrically conductive. Then why not use metal which is cheaper and has far better conductivity?

Keep in mind that all materials resist the flow of heat so having a heat path that is short and wide in area is best. Heat-pipes can often be an elegant solution. They have can achieve thermal conductivity equivalent to 50,000. But you still have to get heat in and out of the pipe.

 

[Demon3]

Imparting thermal conductivity is not synonymous with providing electrical conductivity. Many electrically insulating fillers are good at improving thermal conductivity. Two prime examples are hexagonal boron nitride and diamond. The latter is the most thermally conductive material there is and yet is an electrical insulator. You may think that diamond powder is amazingly expensive but actually it's not. Diamonds too small for jewelry are quite cheap. The larger ones are artificially expensive due to De Beers' virtual monopoly.

As mentioned, to get very high conductivity requires percolation (a pathway of touching particles). That is facilitated by high aspect ratio fillers with fibrous or platy structure.

Chris DeArmitt PhD FRSC CChem

http://www.phantomplastics.com

Consultant to the plastics industry

 

[HDS]

Look at an off the shelf AL heatsink that will be machined to fit you plastic part (or design the plastic part around it). You will find plenty in an electronics distributors catalog like digikey. Depending on the performance sheet metal can work as well.

 

[KENAT]

berkshire, I actually visited an RPM place last Thursday. The process is essentially similar to sand casting but using plaster instead of 'sand'. They use rubber molds to make the plaster molds.

For lower volume work they have some advantages over investment casting, so depending on your volumes might be worth looking at.

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[berkshire]

Kenat,
Ok so they just use the rubber to make the mold and skip the wax.
I got foiled a little today, My customer took my drawings to the molder without me, and has not reported back yet.
B.E.

 

[KENAT]

No wax involved at all. Maybe one of these will help.

http://www.aljcast.com/rpm.html

http://www.trilore.com/RPM Process.htm

I feel like I'm maybe pushing this a bit hard, sorry if that's the case, however given the 500 production run and the relatively low tooling cost it may be appropriate.

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[berkshire]

Kenat.
Looking at these two sites is blowing the cobwebs off.
Before I left the UK and came over here, the company I was employed at, as a drafter, had a 10 ton foundry. We mostly used to sand cast iron, but we also did some brass and bronze work, and for that we used to, use plaster, to get fine detail.The last time I drew casting details was in BS 308 (pre1968) and I think that standard has been dead and buried for many years.
Any way this is getting away from the point, I will pass this info on to my customer, and see if he wants to follow up. Thank you again for the information.
Brian.