Rapid Prototype to Silicone Mold to Lost Wax to Silicon Bronze Casting

[fltenwheeler]

Hi All

Rapid Prototype to Silicone Mold to Lost Wax to Silicon Bronze Casting

I would like to know what others are using for a shrink factor when using rapid prototype parts to make silicon bronze casting. I have been told 2.5% and 3% with mention of using up to 4.5%.

Thanks

Tim

 

[aonemarine]

Tim Its hard to gauge exactly. Silicone molds have some give to them and at higher injection pressure can actually make the part over sized to begin with. With smaller parts 2-3% should be an adiquate allowance, but your going to have to try things out for yourself. Trick is figuring out before you make your pattern where the critical dimensions are and designing your pattern to make sure they come out to where they can be utilized. Machined molds will give you greater accuracy and repeatability over silicone molds.
Think about this, on a 3" part 1% equates out to .03" Now how accurate do you need to be??

 

[abby]

Depends on wax patterns , for high pressure injected wax I use 2% , for hand poured wax 3%.
Why are you using silicon bronze ? It if fine for sculptures but I do not recommend it for model work.
It is difficult to machine unless your tools are very sharp , I find it a poor second to gunmetal (red brass in USA).

 

[fltenwheeler]

I measured the wax and the master. The master is 2.44" and the wax is 2.39". So it looks like I will be needing to redo all of the masters.

The stake pockets where done at 3%.

I have been told that silicon bronze is easier to pour. I will try other metals at some point.

Tim

 

[aonemarine]

Did you mean .244"?? And your off .005"?

 

[fltenwheeler]

Did you mean .244"?? And your off .005"?

The master is 2.44 inches long and the wax is 2.39 inches long. The wax shrank .050 of an inch. Some wear around 2%.

Tim

 

[aonemarine]

You must have another part your working on then? Have you tried using a different type wax? I like the romocast wax, very little shrinkage. It's a filled wax.

 

[cncjunior]

Hi fltenwheeler,

In terms of your question about shrinkage of silicone material I can shed some info on this. I worked in a moulding shop that made pewter and cast resin parts from rtv silicone moulds. The shrinkage of the silicone depends on whether you are using Tin based or Platinum base rtv silicone. Tin based will shrink up to 10% whereas Platinum based silicone won't hardly shrink at all and will give you a very good copy of the master. These are what are available in North America. In the UK there are other options in terms of metal used in silicone.

Daniel

 

[abby]

A good quality Silicone rubber , addition or condensation cure , should have less than 0.1% shrinkage , which can be ignored !
The shrinkage is 2 fold , from the liquid to solid wax and from the liquid to solid metal.
I use metal dies wherever possible , this and high pressure wax injection will keep pattern shrinkage to a minimum , although it cannot be completely eliminated.
For brass , measurements I have taken over several years show a variation of between 2% and 2.25% shrinkage from original pattern to casting depending upon the shape of the casting.
Data for hand poured wax is more difficult to determine as temperature and wax composition can vary enormously but as already stated a filled wax can help.
Read any h&s notes carefully , filled wax may contain pthalates which can be considered a health hazard.

 

[aonemarine]

lots of good advice.

Something you may also want to try for starters is increasing the injection pressure and lowering the wax tempature.