Fab-in-a-Box Casting & Molding: Deep Dive – SCOPES-DF

Lesson Details

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Author

Abigail McCune
Abigail McCune
Informal educator
Abigail McCune is an Educational Outreach Consultant at the Fab Foundation, where she plays a pivotal role in the integration of digital fabrication into education. Abigail leads the SCOPES-DF project, overseeing its comprehensive website, curating and developing engaging content, and… Read More

Summary

Learners will design a two-part, two-step mold from scratch using a blank xDesign template.

What You'll Need

Materials:

Computer with CAD software

3D printer

PLA filament 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

 

Facilitator Considerations:

Adjustments:

Instead of hydrostone or silicone, learners can cast hot wax (with the appropriate supervision) and add wicks to create custom candles.

For young learners, it can help to run a pre-activity creating custom ice-cubes or popsicles. This can expose them to the idea of a liquid being poured into a rigid form and hardening into a solid, taking on its shape.

 

It’s tempting to run an edible activity here, like molding custom candies. However, note that in order for an activity to be food-safe, EVERY STEP of its process must be food-safe. That means the 3D printer (nozzle, bed, and all parts) would need to be food-safe; the PLA would need to be replaced with a food-safe medium, etc.

 

DO NOT let learners pour casting materials down the drain or into any plumbing. When mixed together with their activators (or in the case of hydrostone, with water) they will cure and solidify. This will cause extensive damage to pipes and plumbing. We advise you to run this activity far away from plumbing to prevent accidents.

 

Many curing processes are exothermic! That means the casting medium releases heat as it solidifies. This can potentially be dangerous if it occurs in a small, confined space or near something flammable. Ask learners to leave excess mixed mediums alone in their cups to cure fully before disposing of them.

Learning Objectives

Learners will be able to:

Design a two-part, two-step mold from scratch in xDesign, including all necessary features for alignment and material flow.

Apply knowledge of mold design to solve real-world casting challenges (e.g., air entrapment, material overflow).

Analyze the chemical and physical changes in casting materials during the curing process.

Independently compare and reflect on the volume, weight, and quality of cast parts versus their liquid precursors.

Communicate design decisions and iterate based on casting results and peer/facilitator feedback.

 

 

Reflection

Each step of this process is time-consuming: design, printing, and casting.

The Instructions

Setup and Preparation

Prepare Fab-in-a-Box for this 3D printing lesson.

Use heavy-duty surface protection (like plastic sheeting or silicone mats) for advanced casting work. Provide blank xDesign templates and ensure learners have access to design tutorials or facilitator guidance.

 

Set up dedicated stations for:

CAD design

Mold printing

Casting and curing

 

Stock advanced casting materials (two-part silicone, resin) and ensure safety data sheets are available. Equip stations with mixing tools, scales, and timers for precise measurements. Reinforce drain protection, and provide spill kits or absorbent materials for emergencies. Encourage learners to wear eye protection and gloves, especially when working with reactive materials.

Welcome and Overview

Welcome class and introduce them to the activity with context and key terms.

Welcome:

Welcome the class, and introduce today’s challenge: learners will design a complete two-part, two-step mold from scratch using xDesign. Show advanced examples of multi-part molds and discuss how these are used in professional manufacturing to produce complex parts. Review the Engineering Design Process and how it applies to mold-making, planning, prototyping, testing, and refining. Emphasize the importance of precision in CAD design, especially when creating features like registers, sprues, and vents. Encourage learners to think critically and creatively as they take full ownership of the design and casting process.

 

Context:

Casting takes advantage of the properties of liquids; specifically, their ability to conform to the shape of a vessel into which they’re poured.

 

The rigid form into which a casting medium is poured to cure is called a mold.

 

The special liquids we pour 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. 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.

 

Examples of casting and molding in the real world:

Ice cubes

Chocolates

Sandcastles

Specialty soaps

 

Key Terms:

Mold: The rigid frame into which a casting medium is poured

 

Casting Medium: This is the material poured into a mold. It undergoes a chemical or physical change, to harden from a liquid into a solid, and often gets mixed together from two parts.

 

Cast: This terms describes the act of pouring a casting medium into a mold; it is also the name of the solid form that comes out of a mold.

 

Curing: The chemical process a casting medium undergoes while hardening

 

Single-part molds: Molds for simple shapes without undercuts

 

Advantages:

Straightforward to design, produce, and use

Low requirement for material and production time

Easy to pour and demold

 

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.

 

Advantages:

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

 

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.

Introduction to CAD Software and Design

Demonstrate the basic functions of the chosen CAD software. Focus on essential tools like shapes, text, and path editing.

Ideate

Choose an object you wish to cast. This can be something you will model in CAD (or download from the internet, with requisite copyright permissions), or a real-world item you want to replicate. Consider the object’s geometry: Does it have undercuts that will necessitate a multi-part mold?

 

If making a mold of an existing object:

Determine what the object 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.) Check 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.)

 

Design Mold

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

 

For a two-part, two-step mold:

Create a center plane between two faces of your object, splitting it in half along your desired seam.

Use the plane you just created to “cut” the object, dividing it into two halves. Rotate one of these halves so that both are side-by-side on the same plane.

For each half of your object, create a “bed.” This should be a box around it with ~3mm-5mm-thick walls.

Add a sprue and vent. Choose the side of the mold you wish to pour your casting medium into. Add two small, parallel cylinders (no smaller than~3mm in diameter) running from the object to the outside of the mold. Make sure these are in the same spots on both halves.

Use these cylinders as tools to “cut” both halves of the mold. You should be left with small channels.

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 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.

Create an offset line approximately 3mm outside of the rectangle you just made. This should create a margin all the way around your object.

Extrude the margin a distance slightly greater than the object’s height (~3-5mm).

Extrude the back of the box (margin included) ~3-5mm.

Join all of the bodies together. You should now have a hollow box with your object on the bottom.

 

If making a mold of an existing object, you will need a small container slightly larger in all dimensions than the object itself. Pay attention to what your container is made of, and medium you will be casting with.

Remember: some polymers will meld with others.

 

For xDesign Steps Click Here

 

xDesign steps can also be found:

In xDesign under Content

3D Printing and Production

Explain how the 3D printer works and its role in the casting and molding process. Demonstrate how to set up the 3D printer and load the filament. Show how to transfer a design from the CAD software to the 3D printer for printing. Provide a brief overview of the next steps in the project, such as refining designs and preparing for printing.

Prepare & Slice Files

Open your slicing software: Bambu Studio

Import your design into the slicer: This is easy: you can just drag and drop!

Select the type of printer you’re using (P1S).

Select the bed type.

Select the filament type being used (PLA).

Select the slicing settings.

Click “slice.” This will create a .3mf file and take you to a preview window that shows you what your finished design looks.

 

Print Mold

You have two options to launch your print:

1) Send it wirelessly.

2) Use an SD card.

The printer will likely run an automatic leveling check before printing. This usually takes a few minutes.

 

Once the printer is done, pop your mold off the bed. If it seems stuck, gently use a soft prying tool (a 3D printed one works well!), or remove the magnetic bed from the printer and gently flex it to help the mold release.

 

Prepare Mold:

Although not required, you may wish you spray a mold-release agent into your mold. This can help the finished product slide out more easily.

If using a multi-part mold, fit both halves together. Use clamps or rubber bands to keep them firmly joined. Make sure your sprue and vents are clear.

 

Note: If you have many tiny details, you may wish to wait until after mixing your casting medium before assembling you mold. Painting a thin layer of the medium into any nooks and crannies before sealing it up can help prevent air bubbles.

 

Cast

Prepare workspace:

This is a messy process! Lay down mats, newspapers, paper towels, or another protective layer before beginning.

 

Choose your casting medium:

Choose your casting material based on the properties you want your finished piece to have. Hydrostone yields a hard, white, brittle finish. Oomoo yields a malleable, soft, bluish finish. Sortaclear ranges in firmness and malleability but is considered food safe (assuming the rest of the equipment used was also food safe).

 

Mix your casting medium:

Following the instructions on your casting medium’s data sheet, mix together Parts A and B in the ratios directed. These may be measured by volume (using measuring cups or graduated cylinders) or by weight (using a scale). Note: You will have a limited amount of time to fully mix the material and pour it into your mold once the two parts combine. Check the “working time” on the data sheet.

Once you’ve combined parts A and B, use a stirring tool like a popsicle stick or tongue depressor to gently mix the two together. Do this slowly to avoid introducing air bubbles. Try to avoid folding the material over onto itself, as this also mixes air in.

You will know you’re done when the medium looks homogenous, meaning the same throughout. You shouldn’t have any chunks, or marbling of colors.

 

Pour:

Slowly and carefully, pour your casting medium into your mold through the sprue. If your sprue is large enough, you can also use a funnel.

Keep filling until your mold begins to overflow. Give it a few taps on the table to help any air bubbles rise, and pour a bit more material in, if space allows. Repeat this until no further air bubbles surface.

 

Clean up:

Clean up any spills before they have a chance to cure.

Leave excess casting materials in their mixing cups to cure before disposing of them.

NEVER put any casting materials down the drain!

 

Wait to cure:

Now, simply wait. Your data sheet will tell you the approximate curing time for your casting medium.

 

Demold

After waiting the appropriate curing time, remove any clamps or rubber bands from multi-part molds, and carefully open them.

Trim off any seams or overfill from the sprue or vent.

 

For multi-step molds:

You have just used a mold to cast another mold! This new mold is likely more flexible than a rigid 3D print, allowing for easer removal of finished objects.

To cast your finished object, repeat the instructions outlined above.

Wrap-up and Reflections

Open a discussion about the full design-to-cast process. Invite learners to share insights or challenges they encountered while designing their molds from scratch. Summarize the advanced skills they practiced: designing a two-part, two-step mold, incorporating functional features like registers and vents, and evaluating the performance of their cast. Reinforce the connection between digital design, physical prototyping, and real-world manufacturing.

Discussion Questions:

What was the most challenging part of designing your mold from scratch?

How did your mold perform during casting?

What would you do differently in a future iteration?

 

Optional Tie-ins:

Art & Design: Students can experiment with patterns, both organic and geometric, incorporating intentional uses of positive and negative space to elevate form and function. Whether designing a decorative tile or a functional grip, the iterative process teaches design thinking and spatial awareness. These concepts naturally lead to conversations about aesthetics, form, and creative problem-solving, showing how industrial processes can still allow for artistic individuality.

 

Science (Chemistry): The chemistry of casting materials is key to the success of any mold. Students can learn how different compounds, such as resins, silicones, or thermoplastics, cure and interact based on environmental factors like temperature and humidity. These lessons can touch on polymerization, chemical reactions, and the properties of materials at a molecular level. It’s a hands-on gateway to applied chemistry, where students observe and control variables in real time.

 

Manufacturing & Product Development: The ability to mass-produce identical parts through casting and molding is a cornerstone of modern manufacturing. Students will gain insight into scalability, quality control, and iterative prototyping, all fundamental to product development. Exploring how molds can be reused, modified, or upgraded introduces the concepts of production efficiency and lifecycle. It’s a great way to show how one creative idea can evolve into a reproducible product ready for market.

 

Career Connections:

Mechanical Engineering: Mechanical engineers are deeply involved in the design, analysis, and optimization of molds and cast components. They apply principles of physics and material science to ensure that parts meet structural and functional requirements. Through CAD modeling, stress analysis, and prototyping, engineers refine designs for manufacturing. This lesson introduces learners to key engineering concepts like tolerances, thermal properties, and the mechanics of fluid flow. These skills are essential in industries ranging from aerospace to consumer electronics.

 

Fine Arts: Many artists use casting and molding techniques to bring sculptural visions to life, whether creating gallery pieces, wearable art, or set elements. The process encourages a blend of form and functionality, teaching learners how repetition can serve aesthetic goals while also exploring how material choices impact texture, color, and finish. Whether working in resin, plaster, or mixed media, this career connection opens the door to creative experimentation and craftsmanship.

 

Product Design: Product designers shape the look, feel, and functionality of everyday objects, from tech gadgets to furniture to packaging. In casting and molding, they use 3D printing to prototype parts quickly, test ergonomics, and fine-tune design details before mass production. Learners exploring this career gain insight into how creativity meets usability, how shapes, textures, and materials impact the user experience. This lesson demonstrates how iterative design, prototyping, and feedback loops drive innovation in products we interact with daily.

 

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