Casting and Molding (Plastics)

Made by Michelle Kyin, David, Rachel Nakamura and Ruben Markowitz

Learning about casting and molding processes as well as where on campus we can perform these processes

Created: January 21st, 2016


How the Process Works

There are a million different ways to mold plastics. One of the simplest is to cast a positive item in a harmless degradable casting material, and then form identical positives from the negative mold.

To begin this process, one must select an object capable of being casted in this way. For something to be castable in this way, the surface of the object really only needs to have minimal "undercuts". An undercut is a section of the surface where the material turns in from the surface, and then turns under. If there are undercuts that are over an 1/8" approximately, when you remove the object from the mold, the casting material will get stuck under the surface of the object and tear. If you want to cast an object with an undercut, you can place a small amount of molding (non-drying) clay to cover the orifice.

To do a simple 2D mold, start by making a mold box. Scrap foam-core works best for this. Construct a 5-sided box out of the foam core, about 1 inch larger than the item you are trying to mold. Use Hot Glue to make the seams water-tight. Affix the item to the bottom of the box, if possible. If not, a way of suspending the item in the mold will be necessary.

To do a simple single-use mold, an algae-based mold making materials. This is also skin-safe. More durable molds can be made out of silicone rubbers. Mix the material according to package instructions. Fill the mold box with the liquid material 3/4 full. Suspend the item in the mold if necessary. To make a 3D mold, use pencils or other cylindrical objects and place 3 or 4 surrounding the object, being careful not to interfere with the mold. Let the material dry, according to package directions.

Once dry, carefully remove the mold box. To make a 3D mold, leave the object inside the mold you have already made. Remove the cylindrical objects. Make a new mold box that will contain the mold you made, the object, and then space to make another mold of the other side. Repeat the process.

Carefully remove the master object, and mold parts, and any other extraneous bits and bobs used in the mold making process. If making a 3D mold, stand the mold halves upright, and carefully cut a hole to the base of the object you are casting. This will allow you to pour the plastic into your mold. Spray the inside of your mold or mold halves with a release agent. Then place a piece of foam core on either side of the mold, parallel to the split in the mold and rubber band it together with as many rubber bands as you can find. The foam core will keep the rubber bands from slicing your casting material in half, but the rubber bands need to apply enough pressure to keep the split in your mold water tight. 

Mix the ABS (or preferred casting material) according to the package directions, and slowly pour into the molds. Gently shake the molds to remove air bubbles. Let set per package directions. Gently peel out of the mold to make a plastic replica of an object.


Why 2-Part Mold Making?

2-Part mold making is an extremely efficient way to get replicas of the exterior of an object, in 2 or 3 dimensions.

Organic shapes can be hard to replicate in virtual space. In order to get replicas of organic object, or objects that already exist, it is easy to mold them with harmless plastics and rubbers. Depending on the quality of the mold, it is also an easy way to make several consecutive identical replicas of an object. You can also pretty easily mold people, which is something very hard to replicate in other ways.


Why Not 2-Part Mold Making?

2-Part Mold Making is hard to scale, and requires some tending.

Although 2-Part mold making is fairly easy to use, one must develop some ingenuity when it comes to making good molds. This can be tricky. Suspending originals is tricky to do properly.

2-Part mold making is also a fairly rapid process, but it is not advised that leave the molds unattended for too long. The negative molds especially can split and ruin the mold if they aren't maintained.

The hardest part of 2-Part mold making, though, can be finding a positive original to cast. One popular way is to mold an existing object. If you are trying to make a plastic original, however an original item must first be sculpted before it can be cast. For the artistically inclined, this can be easy. 

2-Part mold making could also be combined with 3D printing to easily and rapidly duplicate complex plastic parts.


How Much Does it Cost?

2-Part Mold Making is fairly cost effective.

All of the components can be purchased online. Together, to mold a 5" cube model, the cost is approximately $45 Each additional piece is about $25.

Negative mold making material costs between $20-$25 depending on the quality. This only needs to be purchased once.

The pieces can be made from a variety of plastics, but standard 2-part plastics cost around 25$ for this size.


Where can I make this stuff?

2-Part Mold Making can happen anywhere! Kind of...

2-Part Mold Making can be a bit messy. All the components can be purchased online, most of the other bits can be purchased from an art store or scrounged. You will want to work on a work surface (not your dorm room carpet), and you will want buckets, sinks, and paper towels nearby. Beyond that, you can really work with this stuff anywhere. Most molding materials that are easy to work with are also safe in terms of fumes and chemical dust, although a dust mask is still recommended.

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Learning about casting and molding processes as well as where on campus we can perform these processes