Mold Making

There are occasions in robot building, as well as general hobbies, where you require a custom plastic or rubber component. While there are a variety of methods to create custom pieces from milling and lathing to 3D printers; for the majority of custom parts none are simpler or less expensive than creating custom molds or castings. Some designed pieces might be too intricate or complex to be moldable, but for the rest, mold making can be an invaluable tool.

At the hobby level, there are two basic styles of mold making that I will cover here: layered and poured. In either method you must first start with a design template, or blank, which the mold will be crafted after. If you are duplicating an existing part, that is your blank. For more customized pieces you will have to create the blank first. 

I have found no easier way of creating a blank than using cheap, oven setting children's "clay". Most of the time these compounds are not clay at all, but rather a soft plastic polymer that hardens when heated, and are available at craft stores. The current clay of choice for my projects is Sculpey, as it is inexpensive and comes in a variety of colors (the bright colors help make it easier to see and thus clean clay residue from completed molds). The clay can also be used for single use one piece molds by pressing the shape of an object into the clay.

To create your mold template piece you just squish, carve, cut and form the polymer clay into the shape needed and then bake in the oven at the required temperature and duration. You can often find needed sub shapes for your blank in other objects, such as a round shape from pushing the clay into a water bottle cap or the middle of a spool of wire. Just don't create it on the kitchen table; a nonporous surface is best to avoid any potential staining of your work area.

 Another nice feature of using polymer clay is that you can fix any problems you might find with your blank easily just by adding a little more clay. Below are two images of a mold blank for a ball bearing holder. The first image is of the initially baked clay, where you can see a chip is missing. In the second image I merely filled in the chip with unbaked clay; it is not permanent, but it does not have to be.

 

Once you have a suitable template, you are ready to get started on creating the mold, but first you must figure out what style of mold making best suits your purposes. 

Layered Molds

The simplest and, generally, least expensive method for creating a mold is through layering the mold compound on your blank. This method is good for simple objects that are flat on the largest side, without any major indentations or thru-holes parallel to the flat surface. Thru-holes are not an immediate disqualifier, however, if you are willing and able to machine the hole in your finished object. Just plug the hole with a little wax or clay prior to creating your mold, and drill it out afterwards.

The easiest and best application of layered molds is for small objects with an outside surface area less than one cubic inch. Layered molds are also useful for when you are creating a mold from only a portion of an object, such as when you only want to use the face from a gargoyle statue, as opposed to the entire head or statue. Larger molds can also be created using the layering technique, but under most circumstances you will want to use a one piece poured mold.

The main choice for hobby level mold making is to use liquid latex as the mold compound. Liquid latex is available through most craft and hobby stores, and is relatively inexpensive given the amount of latex that comes standard (usually enough for several dozen layered molds). Application of liquid latex is as simple as using a small paintbrush and "painting" it onto the your blank. A word of warning, liquid latex has the tendency to crack and bubble while drying when applied in too thick of an application; as such it makes a very poor choice for poured molds.

 To begin, in a well ventilated area, place you blank on a nonporous surface with the largest flat surface area facing down. Ceramic tile makes a very good surface for our purposes. You can, but do not need to, apply a small bit of rubber cement or liquid latex to your blank where it meets the ceramic tile, to hold it in place while you create the mold.

Using a small paintbrush, begin applying the liquid latex as you would regular paint. Work any bubbles you see out of the layer with the paintbrush as you cover the object. You will want to coat not only the blank, but provide a flange of additional latex leading out from the object as seen in this image. This first coat should be applied very thin and appear almost transparent to avoid the possibility of bubbling and ensure you have the entire blank covered.

 Using a small paintbrush, begin applying the liquid latex as you would regular paint. Work any bubbles you see out of the layer with the paintbrush as you cover the object. You will want to coat not only the blank, but provide a flange of additional latex leading out from the object as seen in this image. This first coat should be applied very thin and appear almost transparent to avoid the possibility of bubbling and ensure you have the entire blank covered.

Thin layers of latex will generally dry at room temperature in about 45 minutes. You may use a hairdryer to speed the process up if you are in a hurry, although you should always allow the first two coats of latex to dry naturally. After each layer dries, you will want to apply a new coating of latex to the mold. For objects less than one cubic inch in surface area, 12 to 15 coats will usually suffice. For larger objects more will be needed. Make certain you continue to apply each coat to the flange area as well.

The finished mold should have a deep yellow, almost brown, hue to it with no obvious transparent spots. Allow the finished mold to dry an additional hour or more to ensure the latex has completely dried through all layers. Once you are satisfied with the dryness of the latex, you can remove the mold from the ceramic tile by gently rolling up one edge of the flange with your fingertip. Lift the remaining portion of the flange and entire mold from the tile using this edge. Now just pop your blank out of the mold, and you are done.

 

If you find that there are some "soft spots" in your completed mold, usually seen as nearly transparent areas when the blank has been removed, you can apply additional latex to the outside of the mold. Just be very careful not to warp the mold when doing so or, better yet, place your blank back into the mold prior to applying additional latex.

A liquid latex layered mold, depending on the thickness of the walls, is usually good for a dozen castings or so. After each casting the latex seems to break down a bit, eventually leading to a warped, pitted, or otherwise unusable mold. The more casting resin the mold holds (i.e. the bigger the object you are making), the quicker the break down, mostly due to the heat generated from the casting resins.

There are other choices for materials you can use when making a layered mold. In a pinch I have used rubber cement for very small molds, as well as aquarium sealant, which is one part silicone cement, for midsized molds. For very large castings lacking fine detail, fiberglass can make an excellent, long lasting mold.

Mold Factors

 The finished product you are attempting to create has a lot to do with what material you will want to use when making your mold, as well as the method you will wish to undertake. There are a few primary concerns that have the greatest effect on your finished mold, each is listed below.

  • Object Shape - the number one determining factor in your mold creation. An object with a noncomplex shape with the largest side being flat is a good candidate for layered and single piece mold making. While I do not recommend plaster as a good complete mold, it does provide a suitable litmus test here by asking yourself, "If I shoved this in a gallon of plaster of paris and let it dry, could I remove it without breaking it and/or the plaster?" If the answer is "yes", then the object is a good candidate. If the answer is "no", then you will want to use a two piece mold. If the answer is "maybe", then you will need to consider the mold's flexibility.

    Mold making materials come in a variety of flavors and colors (do not eat it though). The two most important aspects are set-time and flexibility. Flexibility is known as Shore Hardness, and can get a little confusing if you do not know what you are looking at. There are two Shore Hardness scales used by manufacturers (Durometer A, Durometer D) as well as Rockwell rating, each using similar numbers, but each on a totally different level. Generally, the lower the number, the softer the material, just pay attention to which scale is used as a 50 for Shore A is a lot softer than a 50 for Shore D.

    You might ask why this matters, and I assure you I am getting to that. The outer wall strength of your mold is determined by the wall thickness and Shore Hardness. We will get back to that in a minute. The less flexible your mold is, i.e. the higher on the Shore scales, the less of an odd shape design you can put into it. If you mold is very flexible, you will be more likely to be able to wiggle a completed casting from the mold that has complex protrusions and shapes, than from a mold made from a more rigid material.

  • Object Size - the size of an object greatly determines what type of material you will want to create your mold from. As I mentioned above, the Shore Hardness tells you the flexibility of the mold making compound, and as I said the wall strength comes into play with that. The larger an object is, the more resin you will have to pour into the mold to create the duplicate object, the more resin in the mold, the more stress and strain on the outer walls, and thus, the more likely the walls of the mold (inner and outer) are likely to deform. Instead of pouring resin into a mold to make a large cube, you suddenly have something that partially resembles an egg.

    Wall thickness can only compensate to a limited degree, after that you need to create a more rigid mold. "But my object has all these twists and turns that I wouldn't be able to get out of a rigid mold!" Well, there is some hope; you have the option of reinforcing the walls of your mold in a variety of ways. For internal walls, you can use toothpicks or sewing needles or small pieces of plastic or anything else that is more rigid, encased in the mold itself. By inserting these objects into key areas prior to pouring your mold and then covering them with the molding compound, you can in effect get the best of both worlds. For outer walls, you can create a second mold around the first using a more rigid material (in this case plaster is a decent idea).

  • Quantity - the number of objects you wish to create will effect what materials you use for the mold, as well as the method you want to use. Molds made from pressing an object into clay, or made from plaster of paris, are only good for one or two uses (a few more if you are really careful). Layered molds will generally withstand a dozen or so product creations before losing shape. I have one and two piece poured molds that are several years old and still kicking strong.

  • Object Material - the last concern that I will cover is what the object will be made from. Materials that can be used to create a mold can be used to create an object, and vice versa. The catch is you want to make sure that the material the object will be made from is not the same type as what the mold is made from. As a general rule, if your finished object will be a plastic you will want to use a rubber mold, if the finished product is to be rubber, you will want to use a plastic mold. If the material will have both (don't ask me how, but I have done it), you will want to go with a rubber mold.

There are additional factors to consider when creating a mold such as cost, Hazmat concerns, availability, etc. Just keep all of the above in mind, don't break the bank, and read (and follow) all the warning labels and instructions; and you should be fine.