# Making a Mold - some questions



## jimtyp (Jan 2, 2008)

I have a master part that I want to make a bunch of. I know I need to start by making a master mold. I'd like to make the castings in pewter.
I want to keep things as simple as possible but I want a very high quality mold. The part I'd like to cast is very small - a tie rod washer with nut bolt detail.
First question is what material to use for the mold? Would you use RTV, or something else? My understanding is pewter melts at about 600 F. It appears Alumilite starts softening at 200 F. My understanding is RTV can handle up to 1200 F.


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## Burl (Jan 2, 2008)

I think what you will want to make your mold out of will be vulcanized rubber, not RTV. If I'm not mistaken, it cures under heat. RTV stands for "room temperature vulcanizing".


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## d_sinsley (Mar 29, 2011)

I know that in the fishing lure making business RTV is used in the making of lead parts. I have first hand knowledge of a lure maker that uses RTV with lead. With that said these are not high detail parts and I know they have to reproduce the molds every so often as they don't last for ever under high heat. I am looking to do the exact same thing except I am going to use resin and I am looking at RTV. I have some experience with alumalite and it is not as rigid as RTV.


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## Michael Glavin (Jan 2, 2009)

Jim, 

RTV Silicone Mold rubber is widely used for casting low temp metals such as lead free pewter and others. 

Michael


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## Burl (Jan 2, 2008)

I didn't realize this was out there, but have you looked at MoldMax 60?


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## bnsfconductor (Jan 3, 2008)

Posted By jimtyp on 09 Mar 2012 03:42 PM 
The part I'd like to cast is very small - a tie rod washer with nut bolt detail. 
Got any pictures of your tie rod? I'm looking to do something similar, and cast the points of the turnout along with the point bars. I'll be watching and waiting to see how this turns out for you.


Craig


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## Mr Ron (Sep 23, 2009)

Some very good information here. www.smooth-on.com.


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## jimtyp (Jan 2, 2008)

I checked out the MoldMax 60, but it says it can stand up to 560 F, a little under the temp that pewter melts at. 
Thanks for the smooth-on link, they have a lot options, just not sure what would work best and easiest. 
Thanks for confirmation that RTV would be good for pewter.


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## Michael Glavin (Jan 2, 2009)

Jim,

Lead free Pewter melts at about 575*F, the RTV Silcone I have is rated for 600*F. I think I go it from Tap Plastics.

Micro-Mark has supplies for casting low temp metals.

Michael


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## SteveC (Jan 2, 2008)

Jim

I don't know just how much you may desire getting into the low temperature metal casting hobby, but... Have you considered looking at sites that specialize in miniature metal figure casting, like the one below? Check out the Quick-Sil RTV Putty.

Miniature Molds - Castings[/b]


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## Matt Vogt (Jan 2, 2008)

I've used Quick-Sil a few times and have been very happy with it using white metal and resin castings. Pretty cool stuff that sets up quicker than anything else I have used.


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## spincaster (Mar 10, 2012)

RTV silicone mold making material for casting pewter can withstand about 625degF or higher for a short period of time. Look for a red colored, condensation cure (aka tin cure) RTV. Dow Corning 3120 with type S catalyst is the best I have found. Micromark also has suitable material though not red and not as durable. 

You will also need some mold release like parafilm and sulphur free clay like Kleen Klay for making a two part mold. 

You need to have a long enough runner and gate to allow some gravity to assist the pour. 


92/8 Pewter melts at about 480degF but casting temp should be about 550degF to 650degF depending on the size of the part It needs to be fluxed with salamoniac 

You need to preheat the mold before you cast parts. 

Talc powder is applied to the mold prior to each pour. 


Casting is fun but be careful!


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## jimtyp (Jan 2, 2008)

Thanks for all the input gents. I appreciate it. Lot of information I need to look through. 

Spincaster, thanks for the info on RTV specifics. Does the Dow Corning require a vacuum chamber to remove air bubbles?


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## jimtyp (Jan 2, 2008)

Steve, the link to miniature molds looks great, and the RTV putty looks like a great way to get started. Thanks!


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## SteveC (Jan 2, 2008)

Jim

I've never used the stuff so I have no first hand experience with it, hopefully I've not steered you wrong.


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## docwatsonva (Jan 2, 2008)

Jim,

Here's a posting I made about the use of molding putties. Maybe this will help.

http://www.mylargescale.com/Communi...spx#226349

Doc


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## spincaster (Mar 10, 2012)

Posted By jimtyp on 12 Mar 2012 11:46 AM 
Thanks for all the input gents. I appreciate it. Lot of information I need to look through. 

Spincaster, thanks for the info on RTV specifics. Does the Dow Corning require a vacuum chamber to remove air bubbles?


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## spincaster (Mar 10, 2012)

Posted By spincaster on 18 Mar 2012 08:30 PM 
Posted By jimtyp on 12 Mar 2012 11:46 AM 
Thanks for all the input gents. I appreciate it. Lot of information I need to look through. 

Spincaster, thanks for the info on RTV specifics. Does the Dow Corning require a vacuum chamber to remove air bubbles?


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## Torby (Jan 2, 2008)

Always seems to me that mold just makes itself.


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## jimtyp (Jan 2, 2008)

Doc, excellent write up.


Two questions, 
- I assume since it's a putty consistency you don't have to worry about air bubbles?
- Will this product hold up to pewter, ~600 degrees F?


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## docwatsonva (Jan 2, 2008)

Jim,

I guess it's possible to trap air bubbles while working the two components together. I haven't experienced bubbles, but I was careful and took my time. I think you would be more likely to trap bubbles when you were forming the putty around the master.

I doubt it would go over a couple of hundred degrees. This stuff was developed for the food industry and is food-safe. You could always cast chocolate versions of the things you model for your significant other.

Doc


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## spincaster (Mar 10, 2012)

Jim 

Condensation (aka "tin") cure RTV like Dow 3120 can be poured w/o a vacuum. Just pour a thin ribbon or RTV into one corner of the mold and let it slowly work it's way across the pattern. It isn't 100% every time but darn close. If you are trying to do production casting as a business this isn't the way to go but for the odd few casting s for your own use it is more than adequet. I use this method for casting patterns and they usually only require very minimal clean up.


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## jimtyp (Jan 2, 2008)

Thanks Spincaster! Good to know I don't need a vacuum if poured thin and slowly. Can you help me by explaining the "Condensation (aka 'tin') cure? I've seen "tin" and "platinum" cure in my mold research, but not sure what that means. It sounds like you add tin or platinum to cure the mold?


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## spincaster (Mar 10, 2012)

Hi Jim, 

Precisely what addition and condensation cures mean is irrelevant in your case. Just stick with condensation cure (aka tin cure) RTV which doesn't require a vacuum and is much less prone to cure inhabition. Addition cure (AKA platinum cure) RTV has to be vacuumed and reacts with various paints and materials making it difficult to use.


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## tractionfan (Mar 22, 2012)

I have been making molds for 20 years, must have tried everything out there. I spun cast zinc and pewter. I have made hundreds of molds
THE BEST ARE VULCANIZED SILICONE . Black organic rubber is cheaper. I have had very good success with a new low temp silicone,

This product also will produce quality castings from styrene ,acrylic, and rapid pro typing. masters . Leave me a E mail address or phone and we can talk RAY


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## spincaster (Mar 10, 2012)

Ray, 

I have to agree with you about vulcanized silicone being the best casting rubber BUT Jim was asking about RTV. I use both RTV cavities in vulcanized rubber molds and low temp vulcanized silicone molds but those are usually outside of the ability and cost of hobbyists.


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## Grimm (Oct 5, 2009)

I also have used quick-sil and like it a lot. It can withstand up to 800 degrees and I have not had any problems casting pewter with it. All you need is a strong form that you can put some pressure on and you will not get any bubbles with it. I dust the mold with unscented talcum powder and put lots of vents in the mold to keep air bubbles from forming when you pour. I also used a large sprue to help keep the shrinkage down. The large sprue also keeps pressure on the casting so it better fills the little parts. I know I have got a good cast if I see some pewter dribble out the vents. It's easy to use, just mix both parts 50/50, like working with clay or silly-putty. I used modelling clay to make two-part molds and liquid soap as a release. Once cured the molds are very flexible, but stiff. The molds will release baby oil when they sit for a while, but you can just wipe it off and it's fine. Let me know if you would like more (and better) explanation.









Jason 


Here is a picture of one of the molds I made with Quick-Sil:


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