Working the Properties of the Mold

When I first got into casting I basically followed factory guidelines.  A mold contains 2 halves. But those are steel injection molds.  As my experience has progressed, I have found that maintaining a 50/50 mold split is making for ALOT of extra work because of where the cast line ends up.

We all know how much I hate cast lines.  With steel the molds are unforgiving so it has to be a 50/50 split.  We are using silicone.  which is flexible.  We can bend and stretch the mold carefully to remove pieces from the mold that would be impossible with steel.  Not only that, by varying the surface contour, we are helping to ensure positive alignment of the mold halves.

These following pictures are the new molds for Homeworlds upcoming Anniversary Edition Crystal Roboid.

This is a sprue mold.  The left side would be the bottom of the mold.  By being able to bend the mold to aid in the release of the cast piece, I can now cast the holes in the boots so I dont have to manually drill them.  I have moved the Mold break line so that it falls on the edges of piece instead of in the middle of the detail.  So Detail is no longer affected by sanding.  This is HUGE news for me.

Here's a shot of the mating part of the mold.  At the top of the mold you see a small channel and at the bottom you see both the vent and spout.  We could not do this if these were steel molds

The unconventional mold split also helps to guarantee a perfect mold alignment along with the locators.

Im looking forward to doing this more and more.

OOMOO 25 VS OOMOO 30

I have discussed this once before, but I think it needs refreshing.

I use Smooth On products.  Its all I have ever tried.  Having a distributor 10 minutes away makes it that much more attractive.

The Smooth On Starter Kit comes with OOMOO 30.  This is a medium setting Silicone.  It has a 7-9 minute pot life and cures in roughly 6 hours.  This is what I use the most.

OOMOO 25 is a much faster setting Silicone.  It has a pot life of 2-3 minutes and cures in roughly 2-3 hours.  It says 90 minutes, but its still tacky after 90 so I leave it a few more hours.  The benefit of using the OOMOO 25 is that it is a quick setting silicone.  I use it for what I call Stage Molds.  Stage molds are basically one time use molds.  You make the piece in stages.  You make a mold of a built up area then carve/sculpt it.  Then you make another mold of the sculpted piece and either use that or rework the piece some more.  Its like saving your work in case you slip with a knife or cut too much off in one area.  Thats why I use it.

Its also good if you are making only 1 or 2 small molds.  Remember you have to mix it and pour in under 3 minutes.  Proper pouring procedure of silicone is a nice, SLOW steady stream to help break up air bubbles introduced while mixing.  Plus you need to get it into the pressure pot.   If you are mixing alot of product or have several molds to pour, I dont recommend using the OOMOO 25.

I prefer the OOMOO 30 except for the 6 hours cure time.  But thats the time it seems to take for a good solid mold so I live with it.  I like the detail the OOMOO 30 provides over the OOMOO 25.  Because it has a longer pot life it doesnt thicken up as quick.  So I can pour several molds properly without fear of the silicone curing before I can get it into the pressure pot.  Also, because it stays in liquid form longer, it can get into all the little tighter areas nicer and cleaner.

Smooth On Oomoo 25 VS Oomoo 30

I have now had the opportunity to use a new silicone for molds.   OOMOO 25.  Its technically the same as OOMOO 30.  By that I mean it has all the exact same properties as OOMOO 30...

EXCEPT.....

OOMOO 30 has a pot life of  30 minutes.  OOMOO 25 has a pot life of  about 3-4 minutes.  OOMOO 30 has a 1:1 volume ratio and cures in 6 hours.  OOMOO 25 has a 100:130 mixing ratio by weight or 1:1 ratio by volume and cures in 75 minutes.

I like to leave my molds in the pressure pot for the entire cure time.  The OOMOO 25 offers me the ability to make molds in a much shorter time.  That doesnt mean I can make more molds in a day.  Because the OOMOO 25 has such a short pot life, I can only make 1 maybe 2 mold halves at a time and they have to be small molds.

When you pour your silicone, you want to do it from a respectable height and in a controlled small stream.  This breaks up most of the air bubbles caused by mixing.  This takes some time.  And if you are using the OOMOO 25 the silicone is starting to set by the time you have poured the one mold.

With the OOMOO 30 you have the luxury of a 30 minute pot life.  So you can make more molds at once and depending on the size of your pressure pot and depending on how you utilize the space in that pressure pot, you can actually make more molds in a day with the OOMOO 30 even with a 6 hour cure time.

The more relaxed you are the less likely you are to make a mistake or have a mishap.  With the OOMOO 25 you feel rushed and you have to pour your silicone into the mold as fast as possible.  Mixing air bubbles and all.  You dont have the time frame to afford an error.

Each product is made for a specific purpose.  And if you use them for the proper purpose they will both work fine.  My recommendation is if you are only going to make one mold or a couple of relatively small molds...then use the OOMOO 25.  It will be faster and the shortened pot life shouldnt affect you if you are organized.

But if you have a few molds you want to make at once or are making larger molds, then I highly recommend using the OOMOO 30.  You can follow all the procedures without being rushed and the extended pot life means you can also make more molds without having them start to cure at an accelerated pace before you get them into the pressure pot.

*** When I say a small mold, I am referring to a mold that would make a 3 3/4" figure part or an accessory for a 3 3/4" figure.  When I say larger mold, I am referring to a mold that would or could cast say a Magno Leg or anything larger or even a 3 3/4" figure if it were all in one piece. ***

Reworking the Mold PART 2

Side View of reworked mold with old mold cured to it.

 When I first started casting, it was mentioned to me that some people have been experimenting recycling their old molds.  Grinding them up and reworking it in with fresh silicone to help 'a' recycle and 'b' cut down on costs.

This is something I have been wanting to try, but without a method to grind up the silicone I wasn't sure how.

Reworked mold with old mold cured to it.

Then I became concerned chunks of silicone would trap air against the piece being molded.  So I had been toying with the idea of using only a thin layer of fresh silicone followed by a thicker layer of recycled and reworked silicone.  With no effective grinder, there was no way to try...

I JUST LOVE LEGO.   Lego provides a standard.  It gives one a way to remake the mold frame to exactly the same  dimensions EVERY time. 

Picture of reworked mold

The urethane is very hard on this mold.  I am asking a little "tit" less than 1/16" in diameter to resist being pulled out of a small hole.  Well that "tit" ripped out again.  I have 4 chest canopies left to make...then it hit me.  I am almost out of silicone and I didn't have enough to make a nice beefy mold.  But I did have other crapped out molds I wasn't using.  So I poured some fresh silicone to remake the 2nd half of the mold again.  But the layer was thin.  I am almost out.  Then I grabbed an old worn out mold made with a LEGO frame that has the exact same dimensions and seated it on top of the fresh silicone and let the 2 cure to each other.

 This is NOW, the top of the mold.  Not the casting cavity.

It worked.  I have beefed up the mold using one of my old worn out molds.  This is a VERY cool discovery for me.  Very Cool.

Reworking a Mold

Well what I suspected was going to happen...happened.  All of my molds held up and I should be able to make a few more quite easily...except for the Chest Canopy Mold.  Urethane is so hard on the molds.

One of the advantages to using a pressure pot is that the pressure forces the silicone into all the little nooks and crannies.  The back of the chest canopy has a small hole for the screw to tighten it on.  The silicone even gets into this hole and if you can believe it even molds the threads inside the hole.  Well the urethane plays murder on the mold and after 9 Clear chest canopies, the little "tit" that makes the hole, tore off.

I was a little put out and then I thought about it.  Only HALF of the mold is trashed.  Not the entire mold.  So I put the Chest canopy back into the good half of the mold and made sure it was seated properly and firmly.  Then I poured silicone over it.  It worked.  Hmmm...I have a second mold...same problem.  So I tried it again.  And again it worked.  So now I have 2 molds to use and 8 canopies left to make.  So 4 from each mold.  For the second mold however, I didn't put the head or drill back into the mold so the silicone filled up those spaces and took on their form.  It just makes for a more firm mold lock is all.  The molds with the hole in them with the little "tit" inside it are the reworked molds.  The ones on the top are still in good shape, so there was no need to remake the entire mold.  Thankfully...I am almost out of silicone.

I am only making a small amount of resin, but with 1 mold I still had lots of resin left over.  So I didn't want to waste it.  I will be making a separate post for this, but I now have quite a few sets of Clear Pharoid Wings and also some Blue Pharoid wings that will be for sale shortly.

Reworking the mold is only possible if you can reseat the original piece perfectly.  Otherwise the pressure will force the silicone into whatever airspace is there and create an extra thin layer of silicone around your piece you are using to make the mold.  This 'flap' or thin layer of silicone will play havoc on your finished cast.  If the 'flap' comes out, it will make your cast now out of true shape and it will be thicker and bigger.

8 more chest canopies to go...then full bore assembly.  :~)  YEAH!!!!

One More Reason To LOVE Sprue Molds

As if my love affair for sprue molds needed more reasons to keep me happy.

Here is one more reason to use a sprue mold if you have the choice.  Smush Mold...resin in 1/2 of the mold and resin in the other half of the mold.  Smush em together, pressurize if possible and cure. 

How many things can go wrong here?  You can trap air in the mold.  You inadvertently orient the mold wrong and in your panic, all the resin runs out.  You wait too long to put the 2 halves together and the resin has already started curing and now you cant get the 2 halves of the mold tightly together giving you an oversize and distorted cast.  Once cast, if there is an air bubble somewhere, its very hard to fix.

Those are the most common things to go wrong.  Here's is how a Sprue Mold overcomes these issues.

The mold is already together when you pour the resin.  Guaranteeing you a close to accurate cast size.  So your biggest worry here is The Shrinkage Rate.  Expect 3%-5% of total size lost.  Since the Mold is preassembled prior to pour, there is no chance of improperly orienting the cast halves. 

But here is my favorite.  A sprue mold relies on the basic concepts of physics.  Gravity.  gravity pulls things down.  So when you pour a cast, gravity pulls the fluid resin down.  It will follow the channels pushing the air out in front of it.  IF something does go wrong, its usually at the end of a pour. 

Recently after I poured a sprue mold, when I took it out of the pressure pot, the air pressure had pushed the resin deep into the mold forcing the air out via the channel.  I had thought the mold full when I put it in the pot only to find that because of the air pocket the cast was no incomplete.  There wasnt enough resin poured into the mold now.

Because of the expereince I have had curing 2 separate colors together, I simply made a little bit more resin.  Carefully put the mold back together with the cast still in it and poured the resin into the mold ontop of the previous cast.  It filled it in beautifully.  You wouldnt know if I didnt tell you.  And if I didnt know, I wouldnt be able to tell.  Its a PERFECT fix.  Try doing that with a smush mold.  Most likely you would scrap the piece and try to make a new one.

Also, in areas that might trap air in the cast, you simply create (cut) a small air pocket where the problem is.  It pushes the air up into the pocket so that the resin can fill in the mold.

Yup, there are so many plusses to sprue molds.  But Smush molds are still easier and involve less trimming and are VERY goos for hi detail parts.  But you will have more scrap if you arent careful.