Friday, 6 May 2011

Undercuts and what the hell they are...

Been planning to do this one since the beginning but it required me to prepare some simple images so I got distracted by other shiny topics. Today I want to talk about that big miniatures buzzword: undercuts. This is apparently a rather mysterious phenomena that is not entirely understood. Generally people seem to be vaguely aware that metal and resin minis can typically have undercuts but plastic ones can't. This is broadly true due to the processes in which they are cast. The more accurate truth is that it's not the casting material but the material that the mould is made from that makes the difference. But let's not get ahead of ourselves. This is a big topic so lets keep to the basics. What the hell is an undercut. To explain this simply I need to take you through a vastly simplified explanation of moulding a component.

This rather unremarkable shape is our component. It's designed as a zero undercut component though this doesn't need to be clarified yet. The above image is actually a cross section of the component.

Now we see the component in a one part mould (shown in grey). Not a complicated thing to mould at all.

And here we see the component taken out of the mould. It can be slipped out easily and a casting could do the same.

Now then, we move onto the meat and potatoes of this explanation.

Here we have a new component cross section. Similar to the first except for the indents on either side of the lowest section. These indents into the sides of the component are called undercuts (yay).

And here, once again, we have the component in it's one piece mould. Many of you will know what's coming next...

Marked in red on this picture are the points where the mould material enters the undercuts. The bottommost section of the component is therefore locked into the mould and it can't be easily pulled out.

Now we get onto the difference between the mould materials.

Rubber - Metal and Resin miniatures are typically cast in rubber moulds which flex. In the above example you could free the component from the mould by flexing the rubber and pulling the component. Over time, depending on how deep the undercut is, this will put wear and tear on the mould so it's best to keep your undercuts on a sculpture quite shallow. But, all in you're okay. Your sculpture with undercuts can be cast okay.

Metal - Plastic Miniatures are typically cast into moulds made from steel and there's no way you can flex them at all. So, in the above example the component would literally be locked into the mould and the only way to remove it would damage the component . So, sculptures for plastic production can have no undercuts on them at all. Not even shallow ones. Not even a panel line.

So, that's the very dry and dull explanation of what undercuts are. It's the areas around the side of a miniature or component that go inward and would trap the mould. Limited undercuts are okay for metal or plastic miniatures but plastic figures can't have them.

And thus we come to how sculpting miniatures for plastic production is a whole other ballgame. It becomes a massive puzzle where you're trying to find a way to get all the detail you want on a figure without ever having a single undercut. The ways to do this are many and varied. Sometimes you go with multiparts to minimise the problem. Other times it's just about being bloody clever with how you design the component, perhaps designing it to lie in the mould at a strange angle or thinking your way around a visual design that hides the limitations.

I've been studying plastic miniature sprues for years. I have a bit of a mental disease where I'm constantly looking at plastic figures to understand just how they were achieved. It's incredibly clever stuff.

Well, I hope that made some sort of sense. It's a brutal subject to explain properly and I think I shall follow up with another post where I look at a few minis in different materials and point out exactly what I've been talking about in more practical examples.

And I deeply apologise if this suddenly felt like an extremely dull lecture at college...


  1. That wasn't dull at all, that's the way to go with an explanation, wich IMHO can't leave anyone who reads it with doubts about the subject so great job both about the writting and the images.

    Introduction to undercuts for dummies.

    I can see how that will go on, with the multi component models and multi part molds...

    I guess people who have ever sculpted anything was vaguely aware of this at some moment of the process, even if there wasn't any intention of going into production (imagination / what-if... mindset required tho) so it's nice that people can get info about this phenomena (pain in the ass I'd say) and get it right.

    "Now you know, and knowing is half the battle"

  2. A solid explanation for a concept that I'm not sure why people don't get. Perhaps too many figure painters are used to metal models made with rubber molds, but haven't pondered the process much and have been "spoiled" by the 3-D forms that it allows.

    I grew up painting plastic model airplanes, and believe me, some cheaper kits really compromised on final form to save money on molds. So I figured out how it all worked quite a while ago. What I didn't know about was the sheer sophistication of multi-part metal molds with sliding parts etc. that can be used to ALLOW undercuts in the final piece. This tends to be outside the cost range for making figures though.

    A good example of skilled plastic molding is the GW plastic "Fellowship of the Ring" box set. All the figures are in amazing detail, and most of the "side" detail loss is carefully concealed by the figures' angles in the mold, and a few separate limbs etc. No streaky areas on faces etc. to be seen!

  3. Very interesting and informative. I suppose you've seen what Dragon Models Limited have been doing with their Generation 2 plastics - slide molding is wonderful when properly applied.

  4. The tech on those Gen 2 plastics is pretty awe inspiring. I bought one of the sets to have a good look over and I think the coolest thing was that the slide punch had detail on the end. Crazy.

  5. Exactly! Rifle barrels with tiny holes, perfect detail on tiny pieces of individual gear... I am sure molds for Gen 2 figures cost DML pretty penny, but the results, apart from the heads maybe (Hornet still rules in this regard), are very close to the quality of resin stuff. I have a MIG Productions USMC infantryman, and he is actually less detailed than, for instance, excellent late war Germans from the "Last Battle" set. Of course, the miniatures are far more complicated to build, since they have more parts than Gen 1 ones, but that's probably the only drawback of the whole idea. Plus, the prices are really good, compared to resin.