Post by smmcginnis on May 19, 2015 20:32:02 GMT -5
I searched, perhaps I didn't do it correctly but could not find anything on here about this.
Not to make light of a bad situation, but I'm having a Shrinkage problem.. Both with my Silicone molds, and Water Clear castings..
My Question is this.. Do people normally follow the "Oven" curing recommendations both for the new silicone molds, and thin castings?
Here is how it all started..
I have a pretty precise machined mold which I took a silicone mold of with an Alumilite product sold from a local hobby store. Let it setup and cure 24 hours, and then poured a Water Clear casting from it. It was the correct size that it should have been, but was a little flexible a few days after taking it out, and the finish was funky, not smooth. So I was reading around, and found that I did not "Oven" cure the silicone mold, and figured it was a surface reaction from the new silicone. I also found the information about oven curing thinner pieces. At this point I also had gotten my order and made another High Strength 3 mold from the same negative, so I popped them both in the oven to cure them as recommended.
I also cured the first castings I made (that were the correct size) @ 150 for 5ish hours, as recommended.
So why now did everything shrink about 2%?
For instance.. What was 4.425" is now 4.337" and is completely useless to me.
What am I doing wrong? And is there any way to "re-hydrate" silicone molds? I can live without having to oven cure the castings, but the molds now are too small as well..
Good questions. No quick way of answering so let me just begin ...
When shrink is critical we recommend using a platinum base silicone rubber like any of the Plat series on our website (10,25, 40, or 55). Platinum silicone rubber from Alumilite have a shrink of "Nil". Because of the base chemistry of tin base silicones, some shrink is expected. Those tin base silicones would include the High Strength series rubber and the Quick Set. However, platinum base silicones are susceptible of inhibiting (not curing) when they come in contact with some specific materials such as tin, vinyl, super glue, and sulphur (found in many clays) to name a few. Tin base silicones cure against anything and do not inhibit which is why they are so widely used in every industry (especially for do it yourselfers). The down side is that they will shrink but most times the extremely small amount is not even noticeable. Platinums don't shrink but will inhibit or not cure when they are contaminated which makes them difficult to supply for wide spread applications. Retailers like Hobby Lobby and almost every other retailer want the silicone to cure regardless which is why they tend to carry the tin base silicones and worry less about small shrink that most people will never notice.
Now if that isn't enough ... there is also shrink in almost all non filled casting resins (resins without physical particles like microballoons, atomized aluminum powder or other fillers). Clear resins are never filled as they would not be clear and almost all of them shrink.
So how much of the shrink is the mold and how much is the resin. You'd have to use calipers and mic the mold to see what dimension that is then subtract the difference in the master and cast part to see how much each shrank.
Best question ... so how in the world were you suppose to know this? These are the intricacies or fine details of mold making and casting in which entire books are written. Yes, the info is on the technical data sheets and different areas within our website however it is unrealistic for us to expect someone walking into a retailer to gather that much info from a box. We simply don't have the real estate on the box or even the instruction sheet to educate people on every aspect of mold making and casting.
Bottom line is that we need try to figure out a way to add critical details into our presentation of our products which could effect the use or application of someone's project.
Please give us a call and I'd be happy to discuss your application to see what we need to do to make a good part with the dimensions you need out of a clear resin. That may involve us sending you some Platinum rubber as well as require you to make a master slightly larger than what you need to account for the shrinkage (not of the Plat because it doesn't shrink) but of the Clear that will.
Please give us a call and ask for me. We'll get you taken care of. I will more than likely have you email me a pic of your current mold and cast piece so I can see what you are trying to do.
Post by smmcginnis on May 21, 2015 10:54:03 GMT -5
Thanks for the info, and reality check.. I'll try and give you a buzz on Friday once work settles down this week, I do still have a few questions. Though I think I have reworked my idea somewhat to avoid this. I generally do cnc machine work, so I thought I had figured in the correct shrinkage when I made the mold (Aluminum), and remaking it at this point probably isn't going to happen for some time , the mold really was just to save me some time mostly, so I didn't have to machine the parts to size. But I now think that is going to be a wiser idea in the long run.