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 Post subject: Molded resistor attempt #3
PostPosted: Nov Thu 23, 2017 5:58 pm 
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Location: Corvallis, Oregon
Hi all and Happy Thanksgiving!

Here is a photo of my 2nd attempt at a molded resistor. The outside form is a 1/4 watt dog bone style. The inner resistor is a 2 watt modern Vishay Pro02.
Attachment:
Molded resistor attempt 2.jpg
Molded resistor attempt 2.jpg [ 61.24 KiB | Viewed 577 times ]

Third attempt is shown below...
Cheers

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Last edited by Bosch's Fault on Nov Wed 29, 2017 1:47 am, edited 2 times in total.

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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Thu 23, 2017 6:17 pm 
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Happy Thanksgiving!

Very nice!

Can you post a series of photos and text of the steps to create the "stuffed" dog-bone resistor?

Chas


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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Thu 23, 2017 7:41 pm 
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+1 on more pics. I'd love to see how you did it.

- Scott


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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Thu 23, 2017 8:25 pm 
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Very nice !

- Leigh

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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Thu 23, 2017 10:47 pm 
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Hi All. The process is still in prototype but...

make solid model
Attachment:
solid view.jpg
solid view.jpg [ 34.38 KiB | Viewed 526 times ]


make machining layout
Attachment:
tool view.jpg
tool view.jpg [ 39.49 KiB | Viewed 526 times ]


I didn't manage to get a shot of the actual machining. Break out the tiny ball endmills!!!

Insert resistor. One problem I am going to fix on the next version is the lead length. I had to cut the leads to fit them in the mold which is just stupid. This particular mold plate is for testing a bunch of different materials so has a lot of stations.
Attachment:
mold 1.jpg
mold 1.jpg [ 54.76 KiB | Viewed 526 times ]


Slather mold with mold release. I used 3 in 1 oil.

Insert mold material. Sorry I cannot give you any recommendation here. I have worked with some of the fast setting repair epoxies but haven't yet tested them for safety. The material I chose is non flammable and non conductive. Have also thought of using high temperature RTV.
Attachment:
mold 2.jpg
mold 2.jpg [ 51.41 KiB | Viewed 526 times ]


The other thing that will be interesting here is to find out how the added "insulator" material affects the resistor's ability to dissipate heat. That is why the 2W replacement inside is 8X the wattage rating of the original dog bone it is replacing.

Extract the resistor. After that. cut away the flashing with an exacto type knife and file as necessary. The better our original mold the less filing required.

Attachment:
mold 3.jpg
mold 3.jpg [ 58.44 KiB | Viewed 526 times ]


My friend owns a game store. He sells lots of miniatures which you would build and then paint for various games. The paint set for Warhammer 40K is really varied and robust in offerings, also non-toxic. I used a base coat type which is naturally flat and made to stick to most anything. Painted image of resistor at beginning of this thread.

Cheers.

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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Fri 24, 2017 12:29 am 
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Be sure to test the paint first. All the water base paint I have tried is slightly conductive and changes the value of the resistor. Oil base paint seems to be the thing to use. But I have not tried every paint made so YMMV.

Rather than molding mine, I put them in phenolic tubing from McMaster-Carr then filled it with electronic grade casting epoxy. Be careful of using silicone, some types contain acid that attacks metal. Also, it is hard to get paint to stick to silicone.

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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Fri 24, 2017 2:29 am 
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Hi Jim - thanks for the tips. Tested the paint and is not conductive.

Cheers

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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Sat 25, 2017 7:00 pm 
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Hi all,

I have ordered an electronics potting compound which is thermally conductive. This should be a good plan for potting resistors. Will pass along the results once available.

The big issue I found with almost all of the electrical potting compounds is paint-ability.

Love to hear any other recommendations from other restorers.

Cheers.

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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Wed 29, 2017 12:21 am 
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Location: Black Hills, SD 57745
Looking good! Very authentic-looking reproduction. Your 3-in-one oil mold release might be inhibiting the paint from adhering.

When I worked at the museum that made cast dinosaur replicas, after cleaning, they spray-painted a base coat on the plastic to help other paints stick. Acrylic craft paints work once you get the first coat on and let it dry thoroughly. You can also tint the casting resin or plastic before you mix and pour it.

-Ed


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 Post subject: Re: Molded resistor attempt #2
PostPosted: Nov Wed 29, 2017 1:47 am 
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Location: Monroe, NC 28112 USA
If you were using silicone molds, you could paint your parts BEFORE molding. Sounds like an oxymoron but it is possible. You spray the base paint on the walls of the mold then pour-in the liquid casting resin... The liquid resin will bond tenaciously to the dry paint on the walls of the mold and the dry paint releases easily from the silicone. It really works, I've done it a number of times.

I used to use thermally conductive potting compounds in switching power supplies. The material does have considerable thermal resistance so don't expect a 2 Watt dog bone resistor replica with a modern 2 Watt resistor inside to dissipate a true 2 Watts.

Robert


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 Post subject: Re: Molded resistor attempt #3
PostPosted: Nov Wed 29, 2017 1:54 am 
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Thanks for the comments. The paint is just really really thick stuff. I think I might try to thin it out and use a better brush.

So resistor version 2 wasn't quite correct to scale. Here is a photo of resistor 2 (upper left), resistor attempt 3 (center), and an original 1/4 watt resistor (lower right). This still isn't perfect but I think I am satisfied with it.
Attachment:
MR try 3.jpg
MR try 3.jpg [ 80.83 KiB | Viewed 215 times ]

Sorry still not the greatest photo.

Here are some other models I am working on molds for currently.
1 Watt resistor - also have a half watt model.
Attachment:
1 watt.jpg
1 watt.jpg [ 32.1 KiB | Viewed 215 times ]


3 dot domino
Attachment:
3 dot domino.jpg
3 dot domino.jpg [ 32.69 KiB | Viewed 215 times ]


6 dot domino
Attachment:
6 dot domino.jpg
6 dot domino.jpg [ 33.58 KiB | Viewed 215 times ]


So right now am using a metal repair epoxy that is non conductive and dries REALLY hard. Have verified that neither the paint or the potting material are changing the resistance values. Will be interesting to see if the same will be true for capacitance.

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 Post subject: Re: Molded resistor attempt #3
PostPosted: Nov Wed 29, 2017 2:05 am 
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Those look great.

I wouldn't worry too much about duplicating a vintage part exactly.
I suspect there was some variation in sizes from one manufacturer to another.

Re the 6-dot mica...
I believe the rectangular form factor was much more common than the square.

What CAD s/w are you using? Those drawings look quite nice.

- Leigh

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 Post subject: Re: Molded resistor attempt #3
PostPosted: Nov Wed 29, 2017 2:18 am 
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Leigh wrote:
Those look great.

I wouldn't worry too much about duplicating a vintage part exactly.
I suspect there was some variation in sizes from one manufacturer to another.

Re the 6-dot mica...
I believe the rectangular form factor was much more common than the square.

What CAD s/w are you using? Those drawings look quite nice.

- Leigh


Thanks Leigh! Is fun stuff. Next time I run across a rectangular form I will be sure to model it.

The CAD software is freecad https://www.freecadweb.org/
As name implies you can have it for free.

A bit of a learning curve but worth it IMHO. The harder part is interpreting the models into machining software and milling all of it out with a .03 ball nose endmill (I broke 2 of them today because I had the wrong rough material remainder). Oh well.

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 Post subject: Re: Molded resistor attempt #3
PostPosted: Nov Wed 29, 2017 2:50 am 
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Yep, .03 is a mighty small endmill.

If you really want small, McMaster-Carr has eight smaller ball endmills.
You need a really high spindle speed for those.

If you're having a problem with breakage, I recommend the cobalt mills.
https://www.mcmaster.com/#ball-end-mills/=1agiww1

- Leigh

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 Post subject: Re: Molded resistor attempt #3
PostPosted: Nov Wed 29, 2017 2:53 am 
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Leigh wrote:
Yep, .03 is a mighty small endmill.

If you really want small, McMaster-Carr has eight smaller ball endmills.
You need a really high spindle speed for those.

- Leigh

My normal supplier has them in carbide all the way down to .005 <-- ridiculous! I can't imagine using those for anything other than wax casting. My bed mill is certainly not smooth enough to keep from breaking one of them at every change of direction.

Now am molding the 6th resistor in this particular mold and wondering how long I will be able to use it before there is a wear issue. So far all is good.

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 Post subject: Re: Molded resistor attempt #3
PostPosted: Nov Wed 29, 2017 2:57 am 
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Bosch's Fault wrote:
My normal supplier has them in carbide ...

If you're having a problem with breakage, I recommend the cobalt mills.
I use those routinely on stainless steel and tool steel with no problems.

https://www.mcmaster.com/#ball-end-mills/=1agiww1

- Leigh

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