Prototype Encapsulation and Sectioning

[craigj]

Dear Group,

I would like evaluate a new plastic prototype assymbly by cutting sections in the product so that I can see how internal parts are mated. Someone told me of a method where you cast (encapsulate) your prototype in an clear RTV resin. Once it has dried you take a diamond saw and cut sections thru the assymbly thereby exposing the insides. You can then polish the sections to get a nice clear cross section view...then you can measeure, etc...

Has anyone out there done this? Can you share any particulars like resin type, cast size, saw blade type, cutting equipment, cutting rate, poilishing technique?

Cheers
CJ

 

[MikeHalloran]

RTV is not rigid; I'd expect the force of the saw to mess things up, and the rtv to gum up the blade.

Try some clear casting acrylic. You'll probably need to put the assembly in a bell jar and evacuate it to remove all the gas.

Once you've got it encapsulated, you can probably use a cheap tile saw from the builders supply to cut into it.

Or a Rem-Grit blade in a hacksaw, followed by filing and wet sanding. A large flat file that's _never_ been used to cut anything but plastic can generate a pretty decent surface.




Mike Halloran
NOT speaking for
DeAngelo Marine Exhaust Inc.
Ft. Lauderdale, FL, USA

 

[CoryPad]

What you are describing is called materialography. You can learn more about materials, equipment, and techniques at:

http://www.struers.com

http://www.buehler.com

I agree with Mike - RTV is not suitable. Acrylics are used, but they have high thermal contraction and low adhesion, both of which cause gaps to form between the part and the encapsulating material. Epoxy has much better properties and is my recommendation.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[craigj]

Cory and Mike,

Yes RTV would be a bad choice...I wasnt thinking. Thanks for the hints and the name of the process, you both have been very helpful.

Cheers
CJ

 

[NickE]

materials scientists do this all the time. Some additional notes.

1: be sure to use vaccumn on the samples. other wise you wont get good structure, and things could move during the sectioning/grinding/polishing phase.

2: Use a slow speed diamond saw. This way you will take a very thin kerf (~0.010-0.020"). and have the nicest surfaces for polishing.

3: To planarize your sample use wet SiC paper, start at 220grit. After about 60sec of grinding wash the specimen and check for flatness and clarity. Depending on how hard your specimen is you may need to change the paper often.

4: Double the grit # each step (220, 400, 600/800, 1000/1500, 2000). Use water and wash the specimen really well each time.

5:when you are about as smooth as 2000grit can get (no large scratches, the epoxy should be translucent when clean and dry. You will have to figure out if the surface is clean enough. IF so then add a light oil film and the epoxy will turn clear.

nic

 

[SMF1964]

Be careful what oil you use, NickE, as some oils will induce stress cracks in some polymers.

Also, be aware that some epoxies used for materialography require an elevated temperature cure (150°F) which may result in distortion of the plastic prototype. Be sure to select an epoxy that will cure at room temperature.

When grinding/polishing plastics, don't use a lot of pressure as that can cause gumming up of the sample.

If you don't have access to a diamon wafer saw, a fine toothed blade on a bandsaw works well, provided you use a slow speed and slow feed rate for the cutting (to prevent gumming up and such). You will need to spend more time on the early stages of grinding/polishing to get rid of the damage from the bandsaw blade.

If your prototype is all plastic, I would advise starting on 320 or 400 grit paper. Even 220 will remove a lot of material from a polymer sample.

I've used this process on PVC mounted in Struer's caldofix resin material and had very good results.

 

[craigj]

Thanks very much for the input!