Silicone mold issue

[harsanyi1]

Hi guys, I'm new here and this is my first post. We are using resing for a couple of years now, but now an issue came across we have never met before. We use Smooth-On silicone molds (moldmax) and Smooth-On resin as well (smootcast 305 or 321). Normally we can do around 45 casts with one mold. The problem is, the difference between the first piece of cast and the last is 1mm in size, which is a lot on a 100mm long piece. I realised this on the last 10 casts. It doesn't metter if it will be a two side mold or a cutted one. I asked the distributor if they have any idea, but they are cluess. Someone told me, the release agent could affect the size of the silicone molds, but we excluded it as some of the molds have never seen release agent...
Most of the faulty pieces has kind of rough surface as well, they are full with little dimples (you can see on the picture), but the mold itsef is nice and smooth.
Maybe some of you have met with this issuse before...
Any thoughts or advices are much appreciated!
Thanks
Adam
The good one

The bad one

The rough surface on the right

[rattrap]

Could possibly be moisture in the resin. Smooth Cast resins are very sensitive especially 305 and do have a limited shelf life once opened.

[harsanyi1]

[rattrap]

1.5 hour curing? do you mean you are leaving the resin in the mould for 1.5 hours? Which Smooth Cast resin are you using? if it's 305, it needs to be pulled at 30 mins recommended by them. I pull after 20/25 mins. As far as resting the mould, I find after 4/5 pulls the mould starts to warp due to the exotherm using 305 and needs to cool to regain it's shape. 305 is well known to be brutal on moulds, but it does give great casts if used correctly, I have been using it for years.

[harsanyi1]

[rattrap]

What rubber are you using? The shrinkage issue could be due to the mould after a certain amount of pulls. Check the data sheet of the rubber you use to see what the shrinkage allowance is.

[harsanyi1]

[Nutso]

Molds don't do well under heavy use. Heavy use meaning you pour new resin into them shortly after pulling a cured cast out. The resin will pull a little of the silicone oil off after each cast. Heavy use does not allow the silicone oil to even out and recoat the surface again.

When this happens microscopic tears and pockets begin to form on the surface and subsequent pours of resin will "catch" more and more often into these tears and pockets. You will notice that parts don't pull out as cleanly after repeated use and the silicone will tend to cling to the parts more. You will notice this on more detailed parts than with smoother parts and it will take longer to appear on smoother parts as well. Another effect is that resin will begin to microscopically embed itself into the silicone and thus causing what appears to be an expansion of the surface of the silicone that is exposed to the resin. This causes the shrinkage and disruption of surface detail you are experiencing. You really notice this on molds that have very thin details. These details get thinner and thinner with each subsequent cast. All these symptoms usually coincide with a discoloration of the silicone exposed to the resin as well.

If you aren't in a hurry, let the molds rest a while between casts. The longer the better. This will give you more casts before this effect begins but will not prevent it entirely. At some point it will happen no matter how careful you are. It's the nature of the chemicals.

[rattrap]

Lots of good points there Nutso...

[harsanyi1]

[Nutso]

It appears that I indeed confused your images and description of the problem.

The vacuum chamber can cause the dimples you are witnessing if the silicone was not thoroughly degassed, before or after, it was poured for the mold. Any tiny bubbles near the surface will expand under vacuum and can disrupt the surface texture like that. Although, you will usually notice that immediately upon the first casts and not only on later casts. It does not cause the expansion of the mold you are experiencing.

How long are you letting the mold cure for before casting resin into it? Are you using any kind of silicone thinner? Any kind of post curing done? Have you used the exact silicone and resin before without the same issues?

[harsanyi1]

[Nutso]

Post curing is not absolutely necessary and the main reason I ask if problems can arise if it is not done properly. In your case, you can't do it wrong since you didn't do it at all, which is perfectly fine. Just checking if any additional factors were introduced.

The initial tackiness is due to the mold still curing and it is still leeching chemicals that inhibit the curing of your resin. The tackiness goes away after a few casts because the resin is absorbing some of those chemicals and therefore less is inhibited with each subsequent cast. Even though you meet the manufacturer's recommended curing time, due to various environmental factors, this might not be enough. If you have varying temperatures where you cure it (for example, left overnight in a garage) the gradual change in temperature can slightly affect the cure time. It is not detrimental though and means you just let the mold cure for longer. You can try leaving your mold to cure for a longer period of time.

I have experienced the tackiness as well in various Smooth-On silicons. Most recently it has been with MoldMax 30. We left the molds to cure for an additional 72 hours and the tackiness was gone. We now leave molds for one week before casting resin into them to be on the safe side. Way overkill but silicon is expensive and we aren't ever in that much of a hurry.

Your issue with the mold actually expanding to make larger pieces is odd though and something I haven't experienced myself. The only instance that I have seen this happen before is when a friend asked me for help figuring out why he was getting distorted casts. His mold had a mold frame (MDF on five sides of an open faced mold) and he was letting the cast cure under vacuum. The frame was not tight against the side of the mold and had a clear air gap in several places. The vacuum caused the silicon mold to pull into these gaps to fill the void left and therefore stretched his mold slightly out of shape. He only did the frame because he wanted to save on silicon and made a very thinned walled mold. I'm going to assume you aren't doing anything similar because it would have been apparent from the first casts.

I'm stumped for a real definitive answer for you. I believe it might have to do with the mold not entirely curing though. Not sure if you can do it but try to let the mold cure an additional amount of time after removing the parts from it.

[harsanyi1]

Nutso, thank you for your answer I will tell my colleague to leave it cure for longer time, I'm sure this tackyness will go!
I just realized, we have the same issue with Rebound 25 brush-on silicone as well! We tried a new way of makeing a mold. We brushed the silicon on the masterpiece, on several layers, untill it got at least 10mm thick everywhere. Than when it cured, we made a frame and poured it out with Acrylic One which is a kind of gypsum with 0% shrinkage, great for mold supporting cover. It is much cheaper way, of makeing molds, but this Rebound expanded very much, when we put the mold back to the gypsum support, it was wiggly or creased as it didn't fit back properly. You could realized the differenc by sight from the first and the last cast! the product was way much bigger, and some lines an sides were wavy. But, it came up quite early maybe at the 10th-15th cast. We were told, the rebound is really sensitive for humidity and the weather was rather rainy those days, but didn't have time for expriments with it, so we came back to mold max, but now we have more or less the same issue...
It seems you are an exprt of silicones and resins, what is your oppinion of Smooth-On? Do you think we should try something else, new silicon and resin?

[Nutso]

We have used many different brands of silicone and resins and found that we get the most consistent and repeatable results from Smooth-On products. Honestly, you are already using some of the best materials available.

Our most commonly used silicone is Mold Max 30 and Mold Max 20 for poured open face and multi-part molds. We are testing Mold Max XLS II right now to see if it really does have more chemical resistance to resins than regular Mold Max 30 and so far it is promising. We will most likely be using it in place of Mold Max 30 in the future.

For brush on molds we use both Mold Max STROKE and Rebound 25 depending on how many undercuts the original has and how much stretching the mold will have to endure to remove it from the cast. The brush on silicone will never perfectly fit back into a mother mold no matter what the manufacturer or others claim. Just the nature of the material and application. Brush on molds have thin layers which saves on material but, with repeated stretching and deterioration of the mold during use, stretch a little due to the same factors I mentioned a few replies ago. They are not good for accurate reproduction.

For resins we use Smooth-Cast 305 for anything needing to be pressure cast and Smooth-Cast 300 for those that not. Smooth-Cast 65D is used in applications where a little give is needed to prevent parts from snapping when slightly flexed like thin walled parts or rods.

If I were to recommend a non-Smooth-On product, it would be from AeroMarine Products. Their AM 128 Silicone Rubber RTV has performed very well with some applications better than any other product that we have used. And the catalyst smells like grapes! Their Polymer Casting Resin is actually really good as well but the pot life is very short (like Smooth-Cast 300) and not well suited for pressure casting.

That being said, the other products we chose from other manufacturer's weren't terrible by any means. They all performed well. Our decision to use Smooth-On boils down to them being readily available from a local distributor (we don't need to have it shipped to us) and a good variety of so we don't have to get stuff from different vendors and add one more variable to production. It doesn't hurt that they also end up being the most economical in the end.