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title: 9. Casting & Molding Activity Steps
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3D Printing for Casting & Molding

By Claire Dorsett {: .no_toc}

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Overview

Design and 3D print your very own one- or two-part molds! Use these to cast objects directly, or create a secondary, flexible mold to cast large batches of a design.

Context

Casting is a manufacturing process in which a liquid is poured or injected into a rigid structure called a mold and allowed to cure or solidify. It takes advantage of the properties of liquids; specifically, their ability to conform to the shape of a vessel into which they’re poured.

The special liquids that are poured into molds are called casting mediums. These are typically two-part mixtures that undergo a thermal or chemical change called curing to harden into a solid, creating items like:

• Specially-shaped bars of soap
• Action figures and other plastic toys
• Fancy chocolates

The individual components must be precisely measured, combined at specific ratios, and mixed together fully in order to work correctly. Otherwise, the mixture will not cure properly, resulting in a wet or soft end product.

In the Real World

Sand castles are often made using molds. Wet sand is packed into the forms, which are then carefully removed to leave behind castles in their shapes.

Ice cubes and popsicles are also made made from molds. Water or juice is poured into the forms, which are then frozen.

Plastic toys, clothes hangers, and utensils are also made from casting polymers into specially-designed molds.

Key Concepts:

There are two types of molds relevant to this activity (and many more used in real-world manufacturing!).

1. Single-part molds: Simple shapes without undercuts can be cast in single-part molds.

• Straightforward to design, produce, and use
• Use less material and require less production time
• Easy to pour and demold

2. Multi-part molds: Some shapes are too complex to cast using a single-part mold. These require multi-part molds, in which two or more parts fit together like three-dimensional puzzles, encompassing the object to be cast. Small funnels and air vents allow a casting medium to be poured in, and air bubbles to escape. Once the casting medium has cured, the multiple pieces of the mold are disassembled to provide access to the finished part inside.

• Allow for complex shapes, undercuts, and intricate details
• Provide better access to all surfaces of the object
• Can be designed along natural seams or edges for the object being cast to minimize visible lines on the finished casting

Key Terms:

• Mold: The rigid frame into which a casting medium is poured.
• Casting Medium: The material you are pouring into a mold. This undergoes a chemical or physical change to harden from a liquid into a solid, and often gets mixed together from two parts.
• Cast: To pour a casting medium into a mold; the solid form that comes out of a mold.
• Curing: The chemical process a casting medium undergoes while hardening.
• Registers: In multi-part molds, registers are design features that interlock to keep the mold precisely aligned during pouring and curing.
• Sprue: In multi-part molds, sprues are like small, funnel-like openings into which the casting medium is poured.
• Vent: In multi part molds, vents are small air shafts that allow air bubbles to escape from the interior cavity, preventing empty spaces in the finished product.

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Materials

• Computer with CAD software
• 3D printer
• PLA in a color of your choice
• Casting mediums (any or all)
  • Playdough
  • Hydrostone
  • Oomoo
  • Wax - will also require a crucible or pan and hotplate
• Mold release agent (optional)
• Mixing tools:
  • Stirring sticks (popsicle sticks or tongue depressors)
  • Mixing cups (paper cups) -Measuring tools:
  • Small scales
  • Disposable volumetric measuring cups
• Safety equipment:
  • Gloves
  • Eye protection
  • Lab coats or aprons
  • Close-toed shoes for all participants

Activity Steps

There are a few different options for this activity.

1. Ideate

First, determine what you will cast and how.

1. Choose an object you wish to cast. This can be something you will model CAD (or downloaded from the internet, with requisite copyright permissions), or a real-world item you want to replicate.
2. Consider the object’s geometry. Does it have undercuts that will necessitate a multi-part mold?
3. If you are making a mold of an existing object, determine what it is made of.

• Some plastics will react with or otherwise inhibit the curing process for certain casting mediums. (For example, Sortaclear will meld with Oomoo). Be sure to read your data sheet!
• Determine whether your object floats or sinks in the casting material, and develop a plan for how to keep it in place during casting. (Fine thread or fishing line can help you suspend or submerge an object as needed).

4. Determine how many parts and how many steps your mold will require.

• Single-part molds have only once piece, with the casting agent poured directly into their top.
• Multi-part molds have multiple pieces (usually two halves) that fit together with small, funnel-like sprues and vents left open to accommodate the casting agent and release displaced air.
• Single-step molds are 3D prints that are cast directly into.
• Multi-step molds will require you to 3D print a mold that is then used to cast a second, more flexible mold from. It can be easier to remove objects from this secondary mold, but creating it will require you to think critically about your design geometries along the way.

2. Design mold

You can use any CAD or 3D design software you like for these steps. While specifics will depend on which software you choose, the steps outlined below offer a high-level overview of the design process.

Note: While the initial steps are the same, later steps vary slightly depending on whether you are creating a one-step or two-step mold.

2.1 Model the finished object

1. First, create (or import) a CAD representation of the object you wish to cast. We will design the mold around this.

2.2 Model a mold around the object

Steps here vary slightly, depending on whether you are creating a multi-step or multi-part mold. Please note the subsection labels.

2.2a For a one-part, one-step mold

1. Model a three-dimensional box that fully encompasses the face of the object you wish to replicate and is slightly larger in each dimension.
2. Use the original object to “cut” into this box.

2.2b For a two-part, one-step mold

1. Model a box slightly larger than the object (~1cm in in each direction) with the object centered within it.
2. Select the object and use it as a tool to “cut” the box. This will leave you with a cavity inside.
3. Create a center plane between two faces of the box so that it splits your object—and the box—in half.
4. Use the plane you just created to “cut” or split apart both the object and the box.
5. Add a sprue and vent. Choose the side of the mold you wish to pour your casting medium into, and add two small, parallel cylinders (no smaller than~3mm in diameter) running from the inner cavity to the outside of the mold.
6. Use these cylinders as tools to “cut” both halves of the mold. You should be left with small channels.
7. You may wish to add registers to help the two halves of your mold fit together more precisely.

• Small hemispheres in diagonally opposite corners works well for this. The hemispheres on one half of the mold should be positive (convex) while those on the other should be negative (concave). Consider making the concave hemispheres ~.5mm larger in diameter than the convex ones to add a little buffer.

2.2c For a one-part, two-step mold

1. Create a plane or new sketch on the opposite side of the face you wish to replicate.
2. Sketch a rectangle slightly larger han the object’s footprint.
3. Create an offset line approximately 3mm outside of the rectangle you just made. This should create a margin all the way around your object.
4. Extrude the margin a distance slightly greater than the object’s height. (~3-5mm).
5. Extrude the back of the box (margin included) ~3-5mm.
6. Join all of the bodies together. You should now have a hollow box with your object on the bottom.