Waste Mold time!

To stay on track for having a cast model ready for tomorrow’s Live Show in Cheshire, I needed to make a waste mold last night so I can cast tonight.  I’ve been working late this week too.  That, combined with my studio efforts, is starting to take it’s toll on my mind and body, so please excuse me if some of this isn’t as clear as it should be.

I wanted to document this process as I think there are some tips that other artists might find useful, along with, of course, a few “trial and error” issues that I don’t think I’ll ever stop having (and learning from)!

So, preparations:  I took my time over this, I didn’t have much time to cast a good copy out, so I needed to think ahead and plan the mold as much as possible.  Normally a couple of trial castings help me work out where to cut sprues* in but this time I wanted a good cast first time out.  I found a base for the model that could act as the pour-chamber and overflow when I vacuum the resin, it’s a photo paperweight I found online, it has an acrylic top with a thin metal plate that attaches using magnets, you would normally put a photo between them and use it to weigh down your papers, but I saw a whole different use for it – I was feeling very smug about this as it would give me more flexibility later on.  I glued three sprues (acrylic rod 3mm) to the belly, the end of the tail (the tail is attached to the back leg so would also vent that way) and to the muzzle, as per this photo:

*sprues are the clear pillars in the photo above.  Once the rubber is cured, these pillars leave an opening for the air to push out of when you pour the resin into the mold, which stops air pockets from leaving you with something like this. Remember that what you see above will be turned up-side-down for casting.

Then I got out the Lego to build a box around her… but I had another idea:

I took a little ADDIS type pot, the sort you keep pasta or your lunch in.  I’ve used these before for medallions and they work beautifully for several reasons: firstly, perhaps because I got most of my Lego from the toy box at my parents, so it’s probably between 20 and 40 years old, my Lego mold boxes ALWAYS leak, I’ve literally had to scoop and pour the rubber back in until it gels before now (for over half an hour), I always end up in a sticky mess!  With these pots there is no leakage.  The second reason I like these is that they have slightly tapered sides, this means that getting the rubber mold out of them once it’s cured is easier, as you’ll see shortly.  These pots usually have a leak-proof lid, so if you chop off the bottom of the pot, it means you can use the air-tight seal to ensure no rubber leaks out from the top (or the bottom, as the pot is now up-side-down) and then easily unclip it to get access to the cured rubber mold.  As you can see from the photo above, this time I didn’t use the airtight lid, the reason is that the lids aren’t perfectly flat on the inside so I normally spend a while pouring resin or rubber into the lids and waiting for it to cure to give a nice flat surface to work with, but I just don’t have the luxury of time this week.  Finally, these pots tell you how big they are, which helps work out how much rubber to use (I always get it wrong!) – this pot is a 1 litre pot, so I knew that it was going to take about 800ml of rubber to cover the model, allowing about 50ml (a guess) of volume for the mare herself.

So, instead of using the leak-proof lid the pots come with, I used my trusty little hot glue gun as you can see above (every home should have one!) to both stick and seal the up-turned-bottom-chopped-off pot to a piece of plastic offcut I bought off eBay recently.  For the record, I normal take more time over chopping the bottom off the pot, making it level and neat and smooth, but as it was the middle of the night I had to keep Dremel usage to a minimum (cutting with  knife would have swiftly lead to a finger amputation or two!) so that is why this pot has a pretty rough top edge.

In the photo below, I’ve painted a small amount of mixed rubber on the underside of the model (you can’t see that here but I didn’t take a photo of that bit), placed the model into my newly-made mold casing, and poured the rest of the small batch of rubber in (about 40ml in total).  A couple of reasons for this: one, if the hot-glue gun didn’t completely seal the pot onto the plastic base, the rubber will leak out.  By pouring in a small amount of highly accelerated rubber (ie I added a chemical to make it cure super quick), this will seal any holes before it has chance to leak everywhere (using such a small amount also means there is less gravitational pressure which would force the liquid rubber out of any gaps before it had chance to gel – causing a big sticky mess!).  This rubber also “fixes” the model and the sprue ends to the base so I know it won’t move about (or float!) when I pour in the rest of the rubber mix.

Now, you may notice in the picture above that the lovely acrylic oval base I was going to use has disappeared!  You might also notice that the mare’s withers are only about half an inch from the top of the pot – I ran out of room!!  Doh!  At least I had the flexibility to remove the acrylic and cut down the muzzle sprue to fix the problem.  Normally I would just wait and go buy a bigger pot, but with time of the essence I removed the base completely.  This means I won’t have an overflow chamber when I pour the resin, but for a single cast I can deal with that.

Bit of a random photo, but there is a point.  Becky Turner once told me she recycles broken molds by chopping them up and mixing them in with the rubber on future molds.  With the high price of rubber, especially the platinum cure that I use, I thought I’d give this a go for my waster mold as, if it caused any issues it would only affect the one model.  A while ago I spent a few hours chopping a used mold up into teeny tiny pieces for this very purpose, but I’ve not had the nerve to really go for it and try this.  I’ve added pieces of rubber to top-up a mold, but not mixed the rubber pieces right in there.

Here you can see the first pour, I mixed the rubber in 200ml batches as my mixing pots are 600ml and when vacuumed, the rubber expands to fill the pot.  I mix and pour quickly so that lamination doesn’t occur (where the mold might split at the point where the separate pours were made).  You can see the little bits of recycled rubber in the mix here too.

Slowly filling up, here is 400ml of rubber BUT with 100ml of added recycled rubber, that’s saving me 20% on my rubber costs for this model!  I didn’t dare add any more chopped rubber to the mix, not until I knew what effect it would have on the final mold.

After the third pour, I had to chuckle.  I had anticipated 800 ml of rubber would make this mold – seemed reasonable to assume this, so after three 250ml pours, ie 750ml of rubber mix including the chopped bits, you can clearly see this is way too little!   THIS is where I would have been (ok I’d have had 50ml more but that wouldn’t have fixed this problem) had I gone with my calculations alone.  I over-mixed the rubber as I had another mold I could pour if I had extra rubber.  Thanks to the recycled rubber pieces, I still had another 200ml of rubber and 50ml of recycled rubber to add, so I would have enough this time!

I’m a sucker for an experiment (as you can see by my experimenting with recycled rubber even though I’m up against a tight deadline) and one thing that has bugged me recently is the fact that I find it hard to locate the exact mold I want due to them all being very similar looking from the outside.  So, I set this up to add a strip of purple rubber to the top (what will be the base) of the mold so that I can find it easily when I need to, amongst the dozens of other molds I have.

As it turns out, this was the only bad reaction I had with the mold and I’m very dross with myself for not realising it would happen.  As you know I use Platinum cure rubber.  The other kind of silicon is called Tin cure.  They are both very good for different reasons, but they should never be used together.  Now, what sort of rubber do you think rubber bands are made from…. yep, tin cure!  So, when I cut them and removed them this morning they have caused a tiny little pocket of stickiness in the cavities they have left.  Thankfully this won’t affect my casting as the rest of the mold is sound, but it could have.  Disaster averted ONLY because Lady Luck decided to take pity on me today!

So, today is a new day and I woke to find a lovely cured mold waiting for me.  Not for the tricky, manual, nail-breaking, finger cutting, painful bit – getting the rubber mold from the pot, and cutting the mare out from the mold…

This is where those tapered sides on the plastic pot are useful.  I use my mixing sticks to push down the sides of the mold, breaking the suction-grip between the rubber and plastic.  As you can imagine, rubber doesn’t smoothly glide on the plastic, it grips and grabs, so the sticks jus separate the two surfaces enough so that I can slide the rubber mold out from the plastic.

And here is the result.  I’ve removed the metal plate I stuck the mare down onto and now I have a mare and some sprues encased in a block of rubber.

I plan out the cuts I’m going to make to get her out without damaging the mold (or her) too much.  As this is a waste mold I can make more cuts than you would if you cast a production mold this way – I don’t make production molds like this (I take the Plasticine route), but I had a good idea of where the cuts should go.

Using a scalpel and a Foam Knife, a whole lot of care and a fair bit of brawn, I slowly made the cuts.  Ideally there should be no more cuts than absolutely necessary, as each cut is one more seam on each cast, and an opportunity for the mold to displace slightly causing misalignment issues.  Although is appear from this photo that I cut along the underside of her jaw, round her muzzle and up the centre of her face, the cut stops at her nostrils.  I had to cut this far in order to pull her hear and ears out, but stopped as short as possible so that her head should cast perfectly aligned, which is the most important place to be lined up!

Tada!!  She made it out in one piece thankfully!  I can breathe easy now as, even if a disaster stops this mold from producing anything at all, the hours I invested in her are safe and there ready to make a new mold if needed.

One final little job – I had put a sprue on her belly but it was very thin.  I cut out a larger hole so that I can pour the resin in.  As I lost the acrylic bas and therefore now don’t have a chamber to pour the resin into, this hole needs to be bigger so I can pour the resin fast enough that it doesn’t cure before it is poured and vacuumed.

And here is the finished article.  It appears that my recycling experiment has worked (possibly due to that first coast of super-fast curing rubber giving a nice even surface, perhaps) and now I just have to get home tonight and get casting!  I’m yet to pack for the show too, so when I get one good cast out I’ll stop there, and hopefully I’ll have something pretty, all one colour and with crisp sharp details to show tomorrow.