Casting Post-cast

Since the topic came up in another thread I though I might make a post about this specifically.

What I am talking about is all the tings that happens after the cast is poured. This isn't often discussed, it doesn't even have a label of it's own here so i chose "casting" as it's the closest in the process.

So this is just a general thread on the topic. i have taken a bunch of pictures of the finishing process.

A couple of things to keep in mind, though:
*I am semi-professional at best. I do sell dolls ocationally, but I work out of my own studio and I am not at a professional level of casting yet. This is probably not representative of what is done when casting in larger scale, but it might still be useful for the home caster or whoever just want a bit of insight to what goes into making a doll.
*Always use the appropriate protective gear. My routine is to always wear work clothes that are not worn anywhere else in the house, when I am in my workshop. I also wear slippers with closed toe and hard soles, polyurethane flakes can be very sharp and the will get everywhere, including on the floor. In addition to that; gloves, masks and glasses are used when needed. Be an adult about this.
*Pictures are taken mostly with an ancient point and click camera in workroom lighting, picture quality reflects this.
*This is not a tutorial. Please feel free to add your own pictures and experiences and make a discussion about it. I'd love to see how other people go about things.

So let's start.
To get to post casting, first an actual cast is done. There are better places for detailed discussions about this step, but the very short version is that the molds are set up:

And the resin mixed and poured:


So with that out of the way, on to the actual topic!

1. Trim off the overflow:

This step is kind of optional. I don't always do it this carefully.
The most important part is that there isn't so much plastic on top of the mold that it's dificult to open the mold, to get the pieces out or that conects the pieces to each other in a mold that casts multiple part at once. This mold makes a pair of knee joint parts, so separating them from each other is important.
If too much force is placed of the gates or vents when demolding one of two things happen: If the cast is fresh (in it's "green" state, not fully hardened) the piece could deform. If the cast is fully cured the gates and vents could snap off and if that happens close to the doll part, it may snap in such a way that is takes a chip out of the part.
Sometimes opening the mold part way and cut the gates and vents before removing the part is a good idea. It depends on the part and the mold.
All the trimming and rough cutting is done with a sturdy pair of side cutting pliers.

2. Evaluating the parts.

No point in wasting time trying to finish a piece that isn't good enough, so before doing anything else, I will look every piece over carefully. The pices in the picture is a pair of thigh parts, right out of the mold. Over all, this was a good cast, it's complete and doesn't gave any visible bubbles. It does have some slight pock marks as you can just barely see on the piece in the back, but not enough to discard the piece.
Generally I sort piece into three categories. One with the good parts, one with imperfect but fixable parts and one with the true fails.
I also look at the molds. If a cast is bad I need to know why. If a part is constantly coming out of the mold with the same imperfection, that is probably a problem with the mold and I will make a note of that. It may be a simple case of remaking the mold, but it could require other workarounds, such as a different type of mold, a differnt alignment of the parts within the mold or, in extreeme cases, even a revision of the sculpt itself. If i am still in the prototype stage of a doll or still on my first generation of molds for a certain part, this is not etirely uncommon. I am gettin less and less of this as i am getting better, though.
No matter how well made, molds will wear out eventually. This can often be visible to the naked eye, but if not, it will show in casts it produces.
Here is an image of a fresh mold and one of a similar style of mold that has been heavily worn and ready for replacement.


You can clearly see the tell-tale whiteness around the areas that takes the most stress and also that the cuts don't stay closed on their own anymore.
The good casts are set aside, ready for the next step, the true fails are discarded. The middle category get the attention next.

Continued in the next post.

 

3. Fixing what can be fixed.
Depending on how good a finish you require, ther are a few things that can be done to rescue an almost perfect piece. This is never going to be as good as getting it right from the beginning, and it's never going to be less work than just casting a new piece, but it can be a way to keep waste down and to get the last precious cast out of a worn mold.
Here is an upper arm piece that have been cast in a rather tiered old mold. The casting line is clearly visible and it had a few bubbles that has been filled in with resin. This only works if the cast is fresh, you still have the exact colour mixed and the bubble is in such a place where you can let gravity do the job of keeping the resin in place. In this case all those were true and this was a good candidate. The two blobs near the shoulder is the infill. The rough spot near the elbow is where the vent has been roughly cut off.

I always overfill the bubbles a little. If I accidentally didn't fill the bubble completely, I would have to do it again and the next time it would be even harder to get it right and the infill will need sandning anyway so it's better to add just a little too much.
Once cured the overfill is first roughly trimmed. I shave this down using a very sharp thin blade. In this case I will trim the casting line and the vent at the same time. All that is left to do is to sand it smooth and that will be done in the next step.

When the part is all there again, it will be added to the "good pile".

4. Drilling and carving.
When possible I like to use place holders for cannels, but some parts still needs drilling. I do this before any finer finishing, just in case something goes wrong. Accidents are more likely to happen when powertools are involved complared to when doing the finishing, so if I accidentally scratch or otherwise damage a piece I want to have wasted as little time on it as possible.
For this doll, knees and elbows require drilling and so does hands and feet to make a hole for a metal bar to hold the S-hook. The holes for the magnets are in the sculpt, but does sometimes need a little bit of refining in this stage as well.

Sorry about the poor picture quality, but It's exactly what is looks like. A doll part in a vice.
Using a good drill for this helps a lot. Polyurethane can soften with heat, so drilling at a low speed is best. This is especcially important when working with small pieces. Warm plastic will also dull the drill bit faster, so keep it slow.
The joint slits in hands and feet are in the mold, but they do sometimes need a bit of trimming as well, especcially when a mold is getting old and worn.
This is done with a folded piece of sand paper. Since this is not a visible area, a rough grit is fine. Here I am using 240 paper to smooth the slit of a foot.

Here you can also see the casting line, another sign that the mold is either not very well made, or is getting worn.

4. Final finishing.
This step is the most obvious one. This is all about sanding.
This is what a part may look like when this stage starts.

Now, most of the surface of these parts is allready good and way better than the sand paper could ever get it, so I will never oversand. Only where it's doing a positive difference, nowhere else.
All parts will need to have gates and vents fully removed and smoothed out. Even so, if you know where to look they can often be seen, especcially on darker colours. To keep this to a minimum I try to mimic the surface of the original as much as possible. Knowing how it was origianlly finished helps! In this case, I know that every part was finished downs to a 1500 grit sanding, so that is what I will aim for.
I sort out the roughtst pieces and start those with an apropriate grit. I will use whatever I think is the best for the job, but I would rarely start with anything rougher than 400 for this, usually way finer.
Most of the final smoothing of the gate and vent points usually happens at about 1000, with the final stages simply there to make the surface as uniform as possible, while removing as little of the casting surface as possible.
All this is done wet, to keep dust down to a minimum.

After that, all the parts are done. All that is left to do is to install magnets and string the doll.
Here is an example of what the surface of the finsihed doll is like.


So take it for what it is and please do discuss this more if you like. I feel a lot of people forget about all the other steps when they think about casting. Especcially since a lot of people doesn't do their own casting, they just send their doll off an magically recieve copies in the mail, vaguely knowing about making molds and pouring resin, but little else. Much of this becomes invisible.

I hope it's useful to someone.

 

If it's any consolation, many of my dolls over the years never made it this far either. It's a process. Keep at it an you'll get there eventually.

 

Placeholders are great. They only work for some parts, but when they do work they save a fair bit of time and work.
It would be great if you would share you own process here!
I am a firm beliver of the idea that there is no "best way" to do something, or at least not a universal one. There may be a best way to do a thing under a specific set of circumstances, but we all work under different conditions. The more different ways that we get to see, ther better are our chances of finding ways to do things that works for us.