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School Master

Silicone Mold Issue

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This is my first attempt at a lead silicone mold. Made a 2 piece bladed jig  mold using Smooth-On Mold Max 60. Everything went to plan and the mold turned out really nice. I attach the blade to a link before I pour it, so no opening eyes and it's never coming off. 

My issue is I'm getting LOTS of gas voids all over the jig as seen. I made a spinnerbait mold as well that isn't pictured and I'm getting the same result.  Any ideas? Will vents fix this? 

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some talc powder might help. take a rag with some powder on it and dab each side including the gate running in the cavity, turn it upside down and tap on the mold to get ride of any small piles of powder in the cavitity, then put it togeather and pour.

maybe letting set a little longer before demolding.

 

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Did you degass your silicone when you made your mold?  

Venting might indeed help, but ..........

I am not a Smooth-On guy, more of an Alumilite guy myself, but a lot of the things are interchangeable to a point.   If they can help at Smooth-On, best to get the information straight from the horse's mouth; but, if not, then come back to us and perhaps we can work it out.

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I did not, due to the non-degassing capabilities of my shop!! I figured it was just to remove air bubbles to avoid voids in the mold. 

Interesting though, I wonder if they trapped gas is causing a reaction? Totaly makes sense. Will it get better with time?

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use a candle let it smoke and turn the mold black it makes release eaiser and pre-heat the mold the same time.

 

I mix in powder chalk 20% in the silicone rubber

(RTV Silicone Moulding Rubber Shore A15)

 

Edited by OIR
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3 hours ago, School Master said:

I figured it was just to remove air bubbles to avoid voids in the mold. 

It is, and most of the time you do not need to degas silicone.  In the case of lead molding, it is a little different.  The hot lead causes the silicone to expand.  When the silicone expands, pores/voids that were otherwise not seen or even matter, can open and create similar things.  Most lead silicone molds don't last long so keep that in mind.

19 minutes ago, OIR said:

use a candle let it smoke and turn the mold black it makes release eaiser and pre-heat the mold the same time

Remember that hot flames are higher than silicone can take so be careful.  Also, I have never found removing anything other than silicone from silicone to be difficult.  Lead just drops out so I don't see this is necessary at all.

22 minutes ago, OIR said:

I mix in powder chalk 20% in the silicone rubber

This is new to me.  OIR, what does the chalk do to the silicone rubber?  Time for me to learn something new here, awesome.

4 hours ago, School Master said:

I wonder if they trapped gas is causing a reaction?

Not so much a reaction, but if air cannot get out, then it will create bubbles.  Moisture will also do this.  My concerns would be that most of the time the air would be pushed to one spot, so........ the reason I asked about degassing the silicone.

 

10 hours ago, School Master said:

Will vents fix this

This would be the first thing I would do, and it is the easiest to do.  A sharp razor knife and you can cut a vent in the silicone.  If it fixes it, great, if now, then it will only leave a small sprue to remove.

10 hours ago, dlaery said:

some talc powder might help. take a rag with some powder on it and dab each side including the gate running in the cavity, turn it upside down and tap on the mold to get ride of any small piles of powder in the cavitity, then put it togeather and pour.

This reduces the surface tension allowing the air to move out better.  Without a vent, it still has not place to go, but this is also a great test to do.  I have seen this work on resin as well.

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25 years ago.... I was told that chalk makes the mold more hard and  +10-15% resistant to heat

to the left : 25 years old lead  (2 part mold

to the right : 4 days old (1 part mold)

 

25years.jpg

Edited by OIR
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I had a few silicone molds made by InShore tackle Company several years ago.  They recommend using talc as a release agent.  They also vented every mold they made for me.

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UPDATE:

The baby powder worked like magic gypsy dust!! Poured a couple of each and they came out really nice! Thanks for the tips everyone, now we will see how long it holds up. 

I will get a couple pictures up when I finish some

Edited by School Master
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Talcum powder is kind of magic.  I use a flux brush myself to lightly dust molds.  I buy the flux brushes in bags of 36 I think at HF for a few dollars.  They work great as resin brushes and I just throw them away when used for that.  I try to keep two or three bags on hand so I never run out.  Like popsicle sticks. 

Remember that talcum powder is like any other easily airborne powder.  It can get in your lungs, and long term use has been linked to health issues.  I'd strongly suggest wearing a dust mask while using it.  Johnson and Johnson recently lost a very large health related lawsuit regarding their baby powder products. 

 

Edited by CNC Molds N Stuff
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