Mold making

Digital substitution

In this agenda-packed project, we are creating work that:

• Embodies a collage sensibility of construction.
• Expresses implied motion through static form.
• Explores, directly or indirectly, classes of sculptural production including additive, subtractive, assembly, and substitution.

That’s a hefty agenda for a simple carving! To get to the last criterion — substitution — we’re going to create an object that is a means toward an object: a mold.

The image above is a clear silicone rubber casting. It used a mold created exactly how we intend to make ours, by using a CNC router to carve high-density foam. The foam creates a surprisingly durable mold for plaster, resin, rubber, and other kinds of mold-able materials.

This exercise will introduce you to techniques that will ensure your mold will be effective if you decide to use it!

Modeling the “model”

In traditional mold-making, we create a mold by casting an original form, a physical object called the model. The model is often a found object. However, it can be cast from a live person as well: the casting of a face or arm is a common exercise in a basic sculpture class. Artists will often create a mold from a model that has been developed using some other class of production. The clay used as an additive means, for example, is quite common. 

These physical models often lack durability, so the casting process substitutes a more permanent material, like bronze or steel, for a less durable one like clay or wood. This is what puts the substitution in Substitution.

In a digital world, our models are not real. They are just data, interpreted as 3D models in a user interface. To make a casting of our digital model, instead of substituting one material for another, we are instead substituting an actual, physical reality for a virtual, digital illusion.

So if the digital model we design is a mold, we need to emulate the rigors of best practices for mold-making, which we review below. Our goal will be to create the simplest kind of one-part mold, but even this requires attention to detail.

Undercuts

An undercut is a protrusion or indentation in a model that will prevent the removal of a model—and, consequently, the casting—from the mold.

We illustrate an example of a model with no undercuts using a truncated 4-sided pyramid and a stepped pyramid form, seen in the diagram from Instructables below.

To help visualize this a bit better, the rendering below shows the truncated pyramid model in a mold. The front half is rendered transparent to help us see how the object is easily removable because no undercuts are present.

But the drawn diagram also shows a kind of square hourglass or peanut shape. Notice the area in red. This indicates the presence of an undercut in the mold, which prohibits the model from being removed. We highlight the same in our rendering below.

Undercuts can be avoided through analysis of the positive shape before applying a Combine. It is especially useful to isolate orthographic side views for inspection. Look at the Right view in the embed below and you can see the red “warning zone” for undercuts aligned with the hourglass form.

Model by the author.

Notice also that in this model, you’ll see some other forms that illustrate potential issues and opportunities with mold-making. Let’s explore these below.

Texture in the mold

When making a mold from a real object with texture, you might notice low-relief details on the surface. Canvas fabric or even wood grain can develop a small texture. This kind of texture will create micro-undercuts. 

Do such tiny textures matter? It depends on the kind of material you create the mold from. Some molding materials, like urethane rubbers, are quite flexible and can easily bend off. Other, more rigid materials, like our rigid high-density foam, are quite stiff and will only allow the removal of a cast by destroying the mold.

It’s unlikely you’ll model such fine-grain textures, but you might end up with larger-scale relief manipulations that the resolution of our ¼-inch bit will recognize and carve. In our rendering below, we created such details but did not micro-bevel them for clarity. The horizontal texture seen at left creates a tiny series of undercuts that, when you add them together, creates a huge amount of undercut.  That same texture, applied vertically at right, does not.

However, this vertical texture still presents a problem: it increases the surface area by about 1.5 times that of a smooth cube! This significantly increases the amount of friction that we feel when we try to remove a mold from a model. This doesn’t matter in an abstract digital 3D world, but if you use the mold you fabricate, you may need to destroy it to free the cast. However, this fate can be avoided with draft angles.

Draft angles

A perfectly vertical surface makes a very challenging removal situation! If you look at ice cubes removed from an ice cube tray, you don’t see perfectly vertical surfaces for a reason: the tray (a mold) is sloped to allow the ice cube (a cast) to easily pop out. This is an example of a phenomenon known as a draft angle.

The draft angle on an ice cube is rather aggressive. in our case, we want to create a less severe draft angle, if we want our object to appear to have vertical faces. As the draft angle develops under 5 degrees, it becomes harder to remove the cast from the mold, but 5 degrees is also quite detectable. So degree of difficulty for removal is at odds with a desire for a perceived vertical plane.

In our illustration below, we modeled a cube from a square Sketch using Extrude with a Taper Angle set to -3 degrees (that is, 3 degrees facing inward). On the left, the non-beveled model does not appear to be a truncated pyramid, and from certain perspectives, this draft angle is not detectable at all.

The model at right contains the same draft angle but also incorporates an ⅛ inch fillet into the workflow. Up until now, we’ve used fillets to create micro-beveling to help our renderings feel more authentically “real.” But in this case, the fillet has a utilitarian function.

Beveling corners

In the illustration above, we set the fillet at ⅛ inch so the ¼ inch router bit will recognize and cut it accurately. If we don’t include a fillet, you’ll still get one as a function of the bit diameter, but it will potentially contain unwanted texture and obscure detail you think the router will carve. Proactively including a fillet whose radius is identical to the radius of the bit — a ¼ bit is the diameter, so ⅛ is the radius — will allow us to visualize the outcome better.

We use a standard bit size in the shop to economize on tool time, but as you become more proficient, you might want to experiment with bits that can capture finer detail. You’ll still want to develop fillets! Why?

The kind of casting materials appropriate to this kind of mold material include plaster of Paris, resin, and silicone rubber. Metals are molten and will simply melt the foam! The materials listed tend to be quite brittle — even silicone, which is a bit jiggly, tends to fracture at a super-sharp corner. When you create a fillet at every corner, you relieve the stress on the material before it becomes too thin to handle it. This helps not only to release the cast from the mold but also allows the cast to live a long and healthy life in a harsh world, with corners that have built-in durability because they are already “worn down” in a deliberate, designed manner.

Your turn

Don’t create a new model for this exercise!

Instead, analyze the model you created for implied motion modeling in the previous exercise, and do the following:

• Copy your model file first to save a historical state for the first exercise.
• Analyze each form in your model for undercuts. Are any of these forms moldable? If not, how can you modify the form? Before modifying the form outright, consider reorienting the form through rotation. Otherwise, using a cut plane to halve the model, moving a face, or other mod strategies can all be considered fair game. Example: the bagel shape of a torus has an undercut, but if you slice the bagel in half, the half does NOT.
• Apply an ⅛ inch fillet to every corner of the forms, except for the base (see the illustration above and note NO fillet at the base of the cube). The base represents where the model will intersect with the top of the mold — remember that you cast upside-down! Notice where any detail gets compromised in the form when you introduce a fillet, and understand where you can revise the form can eliminate a conflict.

Time won’t permit an actual cast during the project but this link at Instructables shows you how if you want to cast from your mold at some future date.