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RTV Silicone rubber mold check list ?

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RTV Silicone rubber mold check list ?
Posted by dragonriversteel on Saturday, April 07, 2018 11:05 AM

Hiya guys & gals,

 

  Currently working on building a mold for scratch built HO overhead crane girders. It's a fairly large mold. Measuring 8" X 16" inches.

 This is my first time working with RTV silicone rubber. Trying my hardest to to screw this mold up.

So far my mold base is poly-clay 1"1/2 thick because of protruding structure member. Then used those round head nuts (forget what they're called)for location .

Just got done making sure the master is snuggle fit and no gaps between either master or poly-clay.

The only thing left to do is hot glue the mold walls together and spray mold spray.

How do you measure how much RTV rubber to use in a mold this size ????

Sound like I have everything covered ?

Any tips would be greatly appreciated.

 

Patrick

I

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Posted by SeeYou190 on Saturday, April 07, 2018 11:18 AM

Assuming your master is thin, I like to work with RTV molds 1 inch thick, so you need 128 cubic inches of mold material. The total displacement of your master is probably negligible and can be discounted from the calculations.

.

You will need a bit more than a half gallon (5 pints) of mold material.

.

This is by far larger than any mold I have ever made. I am afraid I cannot answer any questions about molds this big.

.

Good luck!

.

-Kevin

.

Happily modeling the STRATTON & GILLETTE RAILROAD located in a world of plausible nonsense set in August, 1954.

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Posted by RR_Mel on Saturday, April 07, 2018 1:54 PM

I’ve been working with Silicone molds and two part Epoxy Resin for about 12 years and I wouldn’t attempt anything that large.  Is it possible to make it in sections and glue it together?  You would have to use at least 30 minute pot time epoxy and work it quickly to get the bubbles out of the Epoxy.
 
My largest mold is 4” x 6” x ¾” and it’s difficult to get the bubbles to rise to the top even helping them with toothpicks.  I can’t imagine trying to get the bubbles out of that much resin.  Even to get a good through mix with that much resin will be difficult.  If the resin isn’t mixed perfect the casting will not cure properly.
 
Unless you have found a 1:1 Epoxy for under $30 per 32 ounces that’s a chunk to toss if it doesn’t work.
 
I made five fairly large casting last night and I missed one mix and had to toss that casting.
 
 
 
The Chair Seats are 3¼” x 1⅛”.  I had quite a few bubbles in the five castings.
 
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
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Posted by 7j43k on Saturday, April 07, 2018 2:07 PM

A search for:

"removing bubbles from epoxy"

might be helpful.

 

If'n I was going to start doing this sort of thing, I think I'd start with small experiments before going to the biggy.  For example, you can find out if it's worthwhile to embed reinforcing into the casting.

 

 

Ed

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Posted by dragonriversteel on Saturday, April 07, 2018 2:49 PM

SeeYou190

Assuming your master is thin, I like to work with RTV molds 1 inch thick, so you need 128 cubic inches of mold material. The total displacement of your master is probably negligible and can be discounted from the calculations.

.

You will need a bit more than a half gallon (5 pints) of mold material.

.

This is by far larger than any mold I have ever made. I am afraid I cannot answer any questions about molds this big.

.

Good luck!

.

-Kevin

.

 

 

Hi Kevin,

 Yup, the horizontal crane girder and vertical structural girder. Both about 1/4" in thickness . Flat as a board mushed into poly-clay. Sculpted as perfect as humanly possible.

Trouble is still how do I measure RTV rubber ?

Found a You-Tube video of using Rice as a filler . Dump the Rice into mold, level rice to your limit, dump Rice into clear mixing container, lastly level Rice in container,mark the line. Thus giving one the much needed measurements.

Sounds like a crock but it's all I have so far...

Thank you for your input.

Patrick

I

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Posted by dragonriversteel on Saturday, April 07, 2018 3:04 PM

RR_Mel

I’ve been working with Silicone molds and two part Epoxy Resin for about 12 years and I wouldn’t attempt anything that large.  Is it possible to make it in sections and glue it together?  You would have to use at least 30 minute pot time epoxy and work it quickly to get the bubbles out of the Epoxy.
 
My largest mold is 4” x 6” x ¾” and it’s difficult to get the bubbles to rise to the top even helping them with toothpicks.  I can’t imagine trying to get the bubbles out of that much resin.  Even to get a good through mix with that much resin will be difficult.  If the resin isn’t mixed perfect the casting will not cure properly.
 
Unless you have found a 1:1 Epoxy for under $30 per 32 ounces that’s a chunk to toss if it doesn’t work.
 
I made five fairly large casting last night and I missed one mix and had to toss that casting.
 
 
 
The Chair Seats are 3¼” x 1⅛”.  I had quite a few bubbles in the five castings.
 
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
 

 

 Hi Mel,

   Thank you for your valuable insight. The RTV Silicone rubber I'm using is 1 to 1 mixing food grade. 

Learned that when pouring rubber into mold. One pours a very thin stream of Silicone,letting the mold fill up. Hopefully not getting the amount of bubbles by not vacuum degassing. Just don't have the degassing equipment. My Wife will have to measure the RTV. She's not a modeler,nor has she messed around with mold stuff. 

Kinda scary using this much material hoping for success. Won't know till I try !

This project started three years ago with the Masters being built. Present day. I needed these overhead crane girders made before scratch building my BOF. Chunky piece of HO real estate .

Thank you Mel.

Patrick

I

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Posted by dragonriversteel on Saturday, April 07, 2018 3:12 PM

7j43k

A search for:

"removing bubbles from epoxy"

might be helpful.

 

If'n I was going to start doing this sort of thing, I think I'd start with small experiments before going to the biggy.  For example, you can find out if it's worthwhile to embed reinforcing into the casting.

 

 

Ed

 

 Hi Ed,

    I've made HO heavy equipment wheel molds using 1to1 mold puddy but nothing like RTV Silicone.

Totally new territory.

As for resin casting. A couple of years. Trial & error type goofing around. 

I like your idea with re-enforced mold. Thinking along the lines of nylon screen ? Have you ever used nylon screen ? 

Thank you Ed.

Patrick

I

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Posted by RR_Mel on Saturday, April 07, 2018 5:39 PM

Patrick,
 
You won’t have any problems with the Silicon rubber, it goes on easy . . . . a bit messy but easy.  The problem will be with the Epoxy, as it cures it creates internal heat and bubbles.  The bubbles can be filled with plastic putty but on a piece as large as you’re doing the bubbles will be massive.
 
Rice is the way to measure both the rubber and the Epoxy, works great!  I always use 1:1 mix rubber and epoxy, it’s the easiest way to get the perfect mix.  A non perfect mix is bad news, especially at a dollar an ounce.
 
If you did do a Google search on bubbles you saw plenty fixes.  I’ve tried all of the fixes, good luck and if you happen to find a fix that works please post it on the Forum.  I’ve been looking for a fix for 10 years.
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
  • Member since
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Posted by 7j43k on Saturday, April 07, 2018 5:57 PM

dragonriversteel

 

 

I like your idea with re-enforced mold. Thinking along the lines of nylon screen ? Have you ever used nylon screen ?  

 

I don't recall doing reinforcing, it just seems like it might be a good idea.  For something long, I was thinking more like steel rod.

 

Edit:  I just realized we're talking about different things.  I'm talking about reinforcing the casting, not the mold.  As noted above, you'll likely want the mold itself to flex for removal.

 

Ed

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Posted by Little Timmy on Saturday, April 07, 2018 7:29 PM

dragonriversteel
I like your idea with re-enforced mold. Thinking along the lines of nylon screen ? Have you ever used nylon screen ? 

 A few year's ago ( 15 or 20  ... I don't recall ) I lined my mold's with "Panty-Hose".

Yea ... waiting in line with them made for a few Raised eyebrow's.

I found the mold's lasted a bit longer with the added re-enforcement. And the mold will still be "Flexable", so you can remove Stubborn / Stuck part's.

Rust...... It's a good thing !

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Posted by dragonriversteel on Sunday, April 08, 2018 9:16 AM

RR_Mel

Patrick,
 
You won’t have any problems with the Silicon rubber, it goes on easy . . . . a bit messy but easy.  The problem will be with the Epoxy, as it cures it creates internal heat and bubbles.  The bubbles can be filled with plastic putty but on a piece as large as you’re doing the bubbles will be massive.
 
Rice is the way to measure both the rubber and the Epoxy, works great!  I always use 1:1 mix rubber and epoxy, it’s the easiest way to get the perfect mix.  A non perfect mix is bad news, especially at a dollar an ounce.
 
If you did do a Google search on bubbles you saw plenty fixes.  I’ve tried all of the fixes, good luck and if you happen to find a fix that works please post it on the Forum.  I’ve been looking for a fix for 10 years.
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
 

 Thank you Mel. I'll be sure to share my findings. Whether it be a complete bust or not.

Just sprayed the mold with release and I have enough RTV rubber to cover at least one side of the mold. Fingers crossed...both.

I

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Posted by dragonriversteel on Sunday, April 08, 2018 9:22 AM

7j43k

 

 
dragonriversteel

 

 

I like your idea with re-enforced mold. Thinking along the lines of nylon screen ? Have you ever used nylon screen ?  

 

 

 

I don't recall doing reinforcing, it just seems like it might be a good idea.  For something long, I was thinking more like steel rod.

 

Edit:  I just realized we're talking about different things.  I'm talking about reinforcing the casting, not the mold.  As noted above, you'll likely want the mold itself to flex for removal.

 

Ed

 

 

 

   Gotcha ! Hopefully this will work but granted my modeling track record. Something always goes wrong. Hope for the best,prepare for the worst.

Thanks Ed.

Patrick

I

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Posted by dragonriversteel on Sunday, April 08, 2018 9:28 AM

Little Timmy

 

 
dragonriversteel
I like your idea with re-enforced mold. Thinking along the lines of nylon screen ? Have you ever used nylon screen ? 

 

 A few year's ago ( 15 or 20  ... I don't recall ) I lined my mold's with "Panty-Hose".

Yea ... waiting in line with them made for a few Raised eyebrow's.

I found the mold's lasted a bit longer with the added re-enforcement. And the mold will still be "Flexable", so you can remove Stubborn / Stuck part's.

 

 

 That's a great idea . Would have never have thought it could be used mold making.

Thanks for the tip Little Timmy.

Patrick

I

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Posted by khier on Tuesday, April 10, 2018 5:28 AM

Hi,

It has been a while since I made an RTV mould. I used to make moulds for 1/24 car shells. However, what I strongly advice is mould release agent, especially if you have a multi part mould. Dedicated products are bloody expensive. You can use petroleum jelly instead. Do not spare the release agent. When you think you applied enough, add more. Mix a tiny amount first an apply on the fine details to ensure covering them. Then you can pour the mixture. Tilt the box slightly and pour in the lowest point to avoid air bubbles. If you are not sure about the quantity mix a little at a time and add to the mould. It takes relatively long to harden. During this time you can mix and add. At some point you will have regular volume to fill. you can estimate the required amount by measuring length, width and depth. The product of the three is the volume needed to complete the mould. Allow 15-25 mm (something like 3/4 to 1 inch) wall thickness. You can it thicker of course if you like spending money. Remember to have the mould always in a supporting box when you cast similar to the box you made for making the mould. You can use LEGO bricks for the purpose. The reason is to avoid distorsion of the case parts since the mould is not 100% solid and can deform a bit.

Good luck

Walid

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Posted by khier on Tuesday, April 10, 2018 5:32 AM

One thing more, Talcum powder helps the resin to flow, at least the PU resin I used.

Get the plane powder from the pharmacy, not the Baby products.

Aluminium oxide can be also used with PU resin as a filler. Perhaps it works also with Epoxy.

 

Regards

Walid

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Posted by dragonriversteel on Tuesday, April 10, 2018 7:12 AM

khier

One thing more, Talcum powder helps the resin to flow, at least the PU resin I used.

Get the plane powder from the pharmacy, not the Baby products.

Aluminium oxide can be also used with PU resin as a filler. Perhaps it works also with Epoxy.

 

Regards

Walid

 

 

  Thank you Walid. Haven't heard of using powered for resin viscosity flow. Will give that a try.

Patrick

I

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Posted by khier on Tuesday, April 10, 2018 7:39 AM

Patrick, again, I have never done anything with Epoxy resin. Talcum powder reduces the surface tension of the liquid PU resin which helps it to flow inside small gaps.

Regards

Walid

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Posted by dragonriversteel on Tuesday, April 10, 2018 9:34 AM

khier

Patrick, again, I have never done anything with Epoxy resin. Talcum powder reduces the surface tension of the liquid PU resin which helps it to flow inside small gaps.

Regards

Walid

 

 

 I'm using Polyurethane resin . Wouldn't mess with epoxy resin,to figgety . Thanks again for the tip Walid.

Patrick

I

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Posted by Overmod on Tuesday, April 10, 2018 10:51 AM

I have never done RTV casting without vacuum degassing, and in my opinion you will go crazy trying to get all the bubbles and blisters out of the materials at any workable viscosity.

This is not an application involving turbomolecular pumps or getter flashing -- I started pumping down with an ordinary conmercial tank vacuum and then finished with an ex-high-school Hyvac.  It's astounding how much more bubbling comes out after there are no visible voids in the agitated RTV after the pours ... especially when you have woven sheets or tapes as the reinforcement.

Be sure to make a vacuum-sealed 'pot' with relief to the back sides of the mold both when mold-casting and making actual casts.  That will help keep some parts of the mold from deflecting before all the material settles in the cavities.  Remember that there's a certain amount of hydrostatic 'head' depending on depth; that's not critical for most Clouser-style modeling but some large pieces oriented vertically might need more reinforcement either built-in or clamped on nearer the bottom.

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Posted by RR_Mel on Sunday, April 15, 2018 9:28 PM

I found a site on how to reduce bubbles in resin and gave it a try, it worked pretty good.  Their suggestion was to thin the resin with Acetone to reduce the viscosity and let the bubbles rise to the top faster.
 
I went with their 40% Acetone and it seemed to work much better.  It doesn’t appear to reduce the strength of the resin.  It wasn’t totally bubble free but much better than unthinned resin.  I did three pours and all three were much better.  
 
It changed the pot life from 90 seconds to close to 3 minutes.  I didn’t try helping the bubbles to rise with toothpicks.  I have quite a few more castings to make and I’ll give that a shot to see if that helps, using a toothpick did help the bubbles rise in the unthinned resin.
 
I made three interior molds for my Athearn streamline passenger cars and they work for my newly acquired Athearn Heavy Weights too, coach, sleeper and lounge/diner cars.
 
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
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Posted by RR_Mel on Wednesday, April 18, 2018 7:49 PM

I’ve made a half dozen castings and determined that the amount of Acetone need to rid the resin to 99% plus bubble free is 10%.  The last three casting I made using 10% Acetone are bubble free.
 
I did help rid the bubbles using a toothpick before the resin started to cure.  
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
  • Member since
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  • 920 posts
Posted by dragonriversteel on Thursday, April 19, 2018 9:30 AM

RR_Mel

I’ve made a half dozen castings and determined that the amount of Acetone need to rid the resin to 99% plus bubble free is 10%.  The last three casting I made using 10% Acetone are bubble free.
 
I did help rid the bubbles using a toothpick before the resin started to cure.  
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
 

 

  Thank you Mel. I have some acetone on the shelf. Will give it a try on one of my small molds.

The PU I'm using has a two minute pot life. Hopefully the acetone will help slow the reaction down enough.

Thank you again Mel.

Patrick

I

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Posted by RR_Mel on Thursday, April 19, 2018 10:42 AM

32 double seats and not one air bubble.
 
 
 
EDIT:
The Acetone almost doubled the pot time on my resin.  I buy 2½ minute pot time epoxy from Specialty Resin & Chemical, Model-Pro.
 
 
 
Mel
 
Modeling the early to mid 1950s SP in HO scale since 1951
  
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 

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