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Sink fold is one of the basic folds in origami, even though it is considered an advanced technique. To be honest, the idea behind the sink fold is simple, but the implementation itself could be tricky, especially in the so-called closed form.
Generally speaking, sink fold is a process of removing or collapsing a corner’s tip. Or, in other words, the main purpose of a sink fold is to smooth out sharp corners.
Since the picture is worth a thousand words let’s begin with a simple example. Let’s stage a situation where we have a corner that should be smooth out. As you can see, the example is super simple (figure 1).
Open sink fold
The simplest form of a sink fold is the so-called open sink fold. But, to perform one, a simple example in figure 1, should be rearranged a bit. Namely, we have to refold it by performing an inside reverse fold (figure 2).
Now, it is easy to do an open sink fold (figure 3).
As you can see, the fact that paper layers could be easily spaced apart, allows open sink to be formed equally easily. Only what is supposed to be done is to space layers apart and push a corner tip down. This is a main reason why open sink fold is considered a reasonably simple maneuver.
Here, it would be interesting to study a corresponding crease pattern (figure 4). Specifically, it is interesting to notice that all axial creases are mountains (creases that form a triangle). That is important to realize since that property is a crucial feature of the open sink fold.
Closed sink fold
Another common sink fold is the so-called closed sink, but this one is not that simple to perform. Problem with this fold is the fact that it should be performed on a cornet tip whose paper layers could not be separated easily. The layers are said to be closed. As a consequence, the sinking itself is quite hard, very often involving a lot of force, since it involves pushing the corner tip down without sufficiently separating paper layers.
One such closed sink could be performed using an example shown in figure 1.
As you can see, the fact that paper layers could not be spaced apart, results with a sink that is equally tightly packed. It could not be open either. In other words, upon performing close sink paper layers are so intertwined that they could not be open. Thus the name – closed sink.
If we look at a corresponding crease pattern (figure 6), we will realize that not all axial creases are mountains (creases that form a triangle). That is important since that feature is a key difference between open and close sink when shown on a crease pattern.
Finally, there is a fold known as unsink fold. Strictly speaking, this fold does not belong among sink folds at all. Nevertheless, I have included it here due to its name.
So, the unsink fold is completely opposite to the sink fold. In other words, instead of sinking a certain tip or a point, we are doing exactly the opposite. We are trying to pull it out if it is already sunken. Conceptually, it is not hard to understand what we are supposed to do, but practically this manoeuvre is hard to perform since in most cases there is nothing we could hold onto while we are pulling the sunken tip out.
A nice and simple example of an unsink fold is shown in figure 7. As you can see, a rabbit ear fold is performed first, only to stage a situation in which we have something to pull out. The idea is to pull out the back layer of a rabbit ear fold (figure 7C). The final result is presented in figure 7D.
As you can see the whole idea is quite simple but the maneuver itself is a bit problematic. If you do not believe me try it for yourself. You will see that pulling the back layer out is almost impossible without unfolding the model to some extent. And this is exactly what is a problem. Here, due to the model simplicity, something like this is possible. Unfortunately, on more advanced models, especially in later stages of the folding, such an approach is not an option. In such cases, unsink fold is very problematic. But then again nobody ever said that origami, especially advanced one is easy.