Mold Max 14NV 25 or 30?

[mangozac]

Yep you can use the pressure pot for vacuum, as shown in my old setup (I've since done a lot of tweaking of my setup to be a bit more efficient, but the basic principle is the same).

It's fine squeezing the moulds when they have the MDF boards (all of your moulds should have boards around them when casting anyway). I just use rubber bands - the trick is to get a feel for how tightly to set them. Too loose and you get lots of flash. Too tight and you get warping/mould slip.

[ChickenMacNugget]

Oh sweet, well at least that's something I can think about. The pot was the expensive part for me anyway. I can get the fittings and a vacuum pump perhaps... The only downside would be that I can't see inside the pot to see how everything is degassing. It'd be a good start though.

Do you have any recommendation for the kind of vacuum pump is good for this sort of thing?

EDIT: I should probably elaborate on that actually.

What I mean is, I've heard 29mmHg is what is needed to degas Silicone properly. So I'm assuming that's a must?
Is there a good CFM rating that's a sweet spot? I guess it doesn't need to degas super fast but I'm sure there's a minimum reasonable rate of vacuum.
Is a two stage pump needed to get these pressures?

[mangozac]

You don't really need to be able to see the resin during degassing so it's not a big deal, however for degassing silicone you really need to be able to view it so it doesn't overflow everywhere! You can still do your moulds with just pressure though.

You want a refrigeration/aircon mechanics pump. They will all go down to the vacuum level required for resin or silicone. We recommend nothing below 6CFM. I run 8CFM and it's great - I can evacuate my 10L pressure pot in 30 seconds. I think most of these such pumps are 2 stage.

[ChickenMacNugget]

Damn that kind of pump will set me back atleast €250. Looks like vacuuming won't be on the cards for at least a couple of months!

Thanks for all the tips. Hopefully I'll be able to improve my results with some of the stuff mentioned already

[mangozac]

You should be able to get decent results with just the pressure system. Definitely give the squeezing the moulds a go

[ChickenMacNugget]

Well you've saved my (resin) marriage

I tried out squeezing the molds tonight while they were between the mdf and the ammount of bubbles that I got out were immediately noticeable. Before I was relying on the pressure pot getting rid of them (which it wasn't). I went on to cast a few pieces and already I'm noticing huge improvements in the reduction of huge air pockets in the casts.

Thanks for your help doc!

[mangozac]

That's great to hear!

What people need to understand when learning casting is the difference between bubbles and air pockets. Bubbles are only small (<2mm in size) and caused by mixing and reaction of the resin. Air pockets are larger cavities caused by failure of the resin to force the air out of the mould as it fills.

Pressure is only able to crush bubbles, not air pockets. The solution to this is either to use a vacuum to remove the air pockets, separate gates and vents (so that the resin is able to displace the air smoothly as it enters the cavity) or squeezing the mould like you've just tried.

[ChickenMacNugget]

Yeah I think if possible, I want to avoid using intricate pour channels, as it just makes life a lot harder and wastes more silicone. If I can get good results with this method often enough, I'll stick with it. Obviously there are some cases where in/out channels are advantageous though.

On another note. I'm noticing that my smooth-on 305 is staying sort of pliable, even after a few days. I suppose it's because I'm not doing a post cure at 65C for 4-6 hours like they recommend. I have an ultrasonic cleaner that has a heating element in it, and I might try using that to maintain the 65C temperature for a few hours, and submerge the pieces I've cast in it. I'm thinking I'll either place the parts in a ziplock bag and submerge them (to keep them dry) or, if moisture doesn't have a negative effect, I might just submerge them in the water directly. It seems like a good plan as water is great for uniformly heating things, rather than a heat lamp or hot plate.

I'll keep you posted!

[RageofAchilles]

I have used 3 smooth -on resin products including 305. I do find that they are quite volatile compared to other resins I have used. a slight imbalance in the mix can cause expansion or retard the curing process. They also eat through my silicon moulds like no other resin ever has.

As far as post curing goes I would have thought that some parts might droop or distort at those temps. Here is a pic of some 305 resin parts I had fun with in my vac chamber. : Note I had tried using an anti foaming agent with the resin to stop air bubbles, which it did, but at a cost...

[mangozac]

[ChickenMacNugget]

[mangozac]

A small amount of flexibility in the cured part is certainly desirable (since brittle parts are more likely to be broken), but that's more so a property of the resin itself. I guess this is just an issue of temperatures. I suspect when casting in Summer the post curing won't be necessary.

Really interesting that the bag that leaked had such a drastic result!

[RageofAchilles]

There are so many issues with resin casting , it's crazy. things were simpler for me before getting a vac chamber. Sure I had a few bubbles here and there but nothing like the disasters I had under vacuum.

it probably would solve things if I had a pressure pot and changed my silicone, but at this point I would have to sell 40+ Mecha kits just to break even. that's not going to happen so it's standard atmosphere gravity casting for me from now on.

There are a great number of threads on this forum concerning casting issues, which just goes to show how tricky it can be. I feel for anyone having problems and I salute those that have managed to get good results

[paulson games]

I use both the 300 and 305, with my molds being the OMOO 25.

The soft resin that's leaking I've experienced from a couple of influences. When the temperature is too cold is slows or evens tops the curing process, the exterior will typically be somewhat solid but the inside is very soft/bendy and it continues to sweat resin for hours or even days. If you break the piece open it'll crumble easy and there's a squishiness to the insides.

I ran into this when I'd gotten some resin during the winter and while it'd been in my house for two days it hadn't reached room temp yet. Sometimes you may need to wait a few extra days if it's been cold.

It can also be caused by the resin settling, make sure to shake everything very well before you pour it into smaller containers. I always shake the bottles before each run of casts to make sure that it hasn't separated while sitting. The A half tends to separate quite quickly and being that it's clear it's very difficult to tell by looking at it. Separation of the B half tends to be easier to spot. Play it safe and shake the heck out of the big jugs and smaller bottles everytime you need to use them.

The third instance where it can occur is when it's past it's shelf life, resin and silicone do go bad after 6-12 months depending on the brand. Typically they just harden up but they would tend to lose curing strength first.

The last area where I've run into non-curing silicone and resin was using stuff from TAP plastics where their products can't be used with wood stirring elements as the natural resins in the stir sticks cause the chemical reaction to fail. It was a bit of a WTF moment when I encountered that as I had no idea it could be a problem but if I'd actually read the label I would have seen that before I started.



When it comes to the OMOO I never vacuum my molds, I just place it under pressure while curing and it's fine. I never get any bubbles or cavities as it self degasses quite well and the pressure crushes anything that manages to stay behind. (I still vacuum the resin though)

You can still get air pockets depending on how the piece is positioned, that can be addressed by how the piece is orientated and also using a slow pour with the silicone as it allows more time for the air to be pushed out then if you just dump it in rapidly. Use an elevated pour so that it helps break any large bubbles by making them travel through a narrow stream and as you pour go around the parts in a circular pattern as it'll help the material flow more evenly. If you pour from all one direction it can knock pieces loose and also trap bubbles under undercuts more frequently.

How you place the parts in the mold can really impact where potential air traps form, it can also greatly increase or reduce your molds overall lifespan. You may find that simply changing the angle the parts or rotating them can save you a lot of headache with both bubbling issues and also where undercuts and tear points appear.



The crusty bits in the resin is from air, and it can also occur if you are using dye. Even when I'm not actively using my resin I try and make sure I shake it up every 24 hours that way it help keeps the material from settling and reduces the formation of flakes especially when there's dye in the mix. In minor amounts the flakes don't tend to harm the mix, but it does look odd when you have little dark spots from dye flakes.

Also make sure that you aren't working in high humidity, resin reacts with water and causes it to boil and form all sorts of bubbles. There's some people that like to wash their molds out which I'd advise against unless you make sure they have ample time to dry. Also make sure you have a moisture trap on your air compressor line to help avoid any unwanted humidity being introduced into the process.


I've run into issues when using different brands of silicone and resin, some are very destructive towards each other and can bind together or eat away at the surface. So if you are using Smooth-On resin make sure to use Smooth-On brand Silicone as well and if tehre are still issues I'd check with one of their sales reps or support staff to find out if tehre are any reported issues of using moldmax with smoothcast. (I use the smooth cast with omoo all the time without any issues).

[RageofAchilles]

A lot of great info there . it's good to know that someone else has encountered the same problems and overcome them.

Thanks