Lei Feng Fab Academy 2025

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Molding and Casting ​

Course Overview ​

This course introduces molding and casting techniques, which are powerful but often underestimated capabilities in the field of digital manufacturing. The course covers various molding and casting methods, including soft mold making, elastic and rigid material casting, and surface treatment techniques. Learning content includes characteristics of various mold and casting materials, safe usage methods, mixing and pouring techniques, strategies to prevent bubble formation, and demolding methods. This week's assignment requires students to design and create a mold with a smooth surface and use it to cast a final product.

Detailed Course Content Introduction ​

1. Molding and Casting Overview ​

1.1 Technical Introduction and Application Range ​

Molding and casting is an extremely powerful but often underestimated technology in the field of digital manufacturing. Compared to technologies like laser cutting, this technique is less familiar to hobbyists but has strong expressiveness and wide application prospects.

1.2 Examples Showcase ​

The course showcases several successful cases of mold making and casting:

  • Adrian's train model: Trains cast in different materials (plastic, candle wax, concrete, chocolate)
  • Balance bicycle tire making
  • Tempe mold made by Hank, a student from Amsterdam
  • Table football project: Using rigid molds to create elastomer molds, then using elastomer molds to cast characters

2. Common Molding and Casting Methods ​

2.1 Injection Molding ​

  • Principle: Heat plastic pellets and inject them into a mold
  • Key terminology:
    • Screw: Where material enters the mold
    • Runner: Channel through which material reaches the cavity
    • Gate: Entrance where molten material enters
    • Cavity: Space that forms the shape of the part
    • Vent: Channel for air to escape
    • Parting Line: Boundary between mold faces
    • Flashing: Material trapped between mold components

2.2 Insert/Overmolding ​

  • Principle: Place an object in the mold, then cast material around it
  • Application: Making perfectly fitting cases for electronic products

2.3 Vacuum Forming ​

  • Principle: Heat and soften sheet material, deform it through vacuum suction
  • Applications: Product packaging, masks, helmets, etc.
  • Limitations: Limited to sheet deformation, limited features

2.4 Blow Molding ​

  • Use: Making bottles and other hollow products
  • Principle: Blow polymer into a mold

2.5 Rotational Molding (Rotomolding) ​

  • Principle: Place material in a mold, then rotate the mold on all axes
  • Feature: Material is pushed to the mold surface rather than forming a solid object

2.6 Vacuum/Pressure Assisted Molding ​

  • Use pressure to force material into the mold
  • Use vacuum to extract air from the mold

2.7 Die Casting ​

  • Use molten metal for casting

2.8 Investment Casting ​

  • Melt a wax model, then cast

2.9 Flexible/Soft Tooling Molding ​

  • Course focus: Using elastic molds for short-term production
  • Examples: Machine controller, cat-shaped mold for making chocolate cats

3. Mold Making Materials and Tools ​

3.1 Main Suppliers ​

  • Smooth-On: Various mold making and casting materials
  • Dick Blick: Art mold materials
  • West Marine: Boat building materials
  • USG: Gypsum-based materials
  • A-Remco: High-temperature mold materials
  • Protolabs: Injection molding services

3.2 Mold Materials ​

3.2.1 Mold Making Principles ​
  • Rigid mold → Flexible mold → Final product
  • Positive → Negative → Positive
3.2.2 Material Selection Considerations ​
  • Application requirements
  • Material compatibility
  • Working time
  • Demolding time
  • Strength and flexibility
  • Surface quality
  • Cost
3.2.3 Hydrostone and Drystone ​
  • Gypsum-based materials
  • Features: Rigid, good for final products
  • Applications: Architectural models, decorative objects
  • Advantages: Inexpensive, easy to use
3.2.4 Epoxy Resin ​
  • Types: Two-part epoxy systems
  • Features: Hard, durable, good chemical resistance
  • Applications: Durable parts, electronic encapsulation
  • Note: Requires good ventilation
3.2.5 Polyurethane ​
  • Forms: Polyurethane rubber and polyurethane plastic
  • Features: Deep curing, high-resolution surface, good strength, good dimensional tolerance
  • Applications: Making tough rubber parts and molds
  • Note: Requires good ventilation
3.2.6 Silicone ​
  • Recommended brand: Oomoo
  • Features: Almost doesn't stick to anything, very inert, doesn't require special ventilation
  • Compared to polyurethane: Not as tough as polyurethane, but easier to use
  • Applications: Making soft materials and molds
3.2.7 High-Temperature Silicone ​
  • Use: Metal casting (such as custom coins)

3.3 Casting Materials ​

3.3.1 Polyurethane Plastic ​
  • Features: Hard, durable, can be machined
  • Applications: Functional parts, prototypes
3.3.2 Epoxy Resin ​
  • Features: Clear, can be colored, good for encapsulation
  • Applications: Jewelry, display objects
3.3.3 Silicone Rubber ​
  • Features: Flexible, good for parts that need to bend
  • Applications: Gaskets, flexible components
3.3.4 Metal Casting Materials ​
  • Types: Low-temperature alloys, aluminum, bronze
  • Applications: Custom hardware, jewelry, art
3.3.5 Food-Safe Materials ​
  • Requirements: FDA-approved materials, clean process
  • Applications: Making custom lollipops, chocolates, etc.

3.4 Additives ​

  • Fibers: Enhance material tensile strength
  • Hollow glass spheres: Make material lighter
  • Nickel or carbon particles: Make material conductive
  • Rubber particles: Increase flexibility
  • Pigments: Change color

4. Mold Making and Casting Process Flow ​

4.1 Work Environment Preparation ​

  • Need a messy workspace
  • Recommendation: Large table, cover with paper, throw away after use
  • Note: Keep containers clean, prevent material from solidifying on lids

4.2 Material Testing ​

  • Always test materials before the main project
  • Test for:
    • Working time
    • Curing time
    • Strength
    • Flexibility
    • Surface quality
    • Color

4.3 Material Mixing ​

  • Read instructions carefully
  • Measure by weight (more accurate) or volume
  • Mix thoroughly but avoid introducing air bubbles
  • Mix in a container at least 3 times the volume of the material
  • Use disposable containers and tools

4.4 Curing Time Considerations ​

  • Working time: Time before material begins to set
  • Demolding time: Time before removing from mold
  • Full cure time: Time before full strength is achieved
  • Temperature affects demolding time (using incandescent lights or low-temperature ovens can accelerate)
  • Curing time indication: Gets hot after about 30 minutes, ready for demolding when temperature drops after about 1 hour

4.5 Strategies to Prevent Bubbles ​

  • Horizontal shear mixing rather than stirring
  • Pour in a thin stream
  • Gently vibrate the mold to help bubbles rise
  • Manually apply a thin layer of resin to complex features on the mold surface
  • Use a vacuum chamber to extract air from the mold
  • Use a pressure chamber to force resin into the mold
  • Be patient, allow time for bubbles to float up

4.6 Demolding Techniques ​

  • Wait until material is fully cured
  • Use release agent if needed
  • Gently pry or flex the mold
  • Use compressed air to help separation
  • For complex molds, consider:
    • Split molds
    • Sacrificial molds
    • Dissolvable molds

4.7 Surface Finishing ​

  • Sand with progressively finer grits
  • Polish with compounds
  • Apply coatings if needed
  • For metal castings, consider:
    • Patination
    • Polishing
    • Plating

5. Safety Considerations ​

5.1 Material Safety ​

  • Always read Safety Data Sheets (SDS)
  • Use in well-ventilated areas
  • Wear appropriate protective equipment:
    • Gloves
    • Safety glasses
    • Respirator if needed
  • Be aware of exothermic reactions (heat generation)
  • Keep materials away from skin and eyes

5.2 Waste Disposal ​

  • Don't pour liquid resin down the drain
  • Best handling method: Mix remaining material, let it cure, then dispose
  • Cured form is more inert, less environmental impact

6. Mold Making Methods ​

6.1 3D Printed Molds ​

  • Suitable printers: Stereolithography (SLA) resin printers
  • Advantages: Can achieve beautiful surfaces
  • Note: May need release agent to prevent silicone from sticking to resin

6.2 Machined Molds ​

  • Material: Machinable wax
  • Advantages:
    • Can achieve high surface quality (depending on machining time and precision)
    • Suitable for high volume production
    • Material can be reused
6.2.1 Machining Techniques ​
  • Roughing: Horizontal cutting, fast cutting speed
  • Finishing: Use smaller step values for better surface
  • Multi-directional finishing: Finish in two directions for smoother surfaces
6.2.2 Tool Selection ​
  • Flat end mills: Flat bottom, good for roughing and flat surfaces
  • Ball end mills: Round bottom, good for curved surfaces
  • Tool length considerations:
    • Short shanks: Can't go deep
    • Long narrow tools: Can enter deep pockets but break easily, need slow machining
6.2.3 Collision Avoidance ​
  • Consider complete tool shape (cutting edge, shank, collet, Z-axis)
  • Check tool against mold shape to prevent collisions
  • Use draft angles to help avoid collisions

6.3 Software Choices ​

  • ShopBot tools: VCarve Pro
  • Fusion 360: Integrated CAM functionality
  • SolidWorks: Has plastic simulation
  • FreeCAD: Has toolpath functionality
  • MODs: Can read STL files to create toolpaths

7. This Week's Project Flow ​

7.1 Basic Process ​

  • Design the mold (note positive-negative relationship)
  • Create rigid mold (through 3D printing or machining)
  • Cast elastomer in rigid mold
  • Cast final product in elastomer mold

7.2 Design Considerations ​

  • Don't be too greedy (not too large, or features with too much size variation)
  • Consider manufacturing method limitations (print resolution or machining steps)
  • Start mold making early (production time is usually the bottleneck)

7.3 Advanced Options ​

  • Multi-part molds: Create complex geometries through nested mold parts
  • Insert molding: Embed electronic components in materials
  • Metal casting: Make custom coins or jewelry
  • Food-safe applications: Make food molds (ensure food safety throughout the process)

Assignment Requirements ​

Individual Assignment ​

  1. Design and create a mold with a smooth surface
    • Can choose 3D printing or machining methods
    • Surface must be smooth (machining needs fine steps, 3D printing needs high resolution)
    • FDM printers require post-processing to meet requirements
  2. Use the mold to cast the final product
    • From rigid mold to elastomer mold, then to final product
    • Recommended materials: Oomoo silicone, Hydrostone, Drystone
  3. Document the entire process
    • Design thinking
    • Production process
    • Finished product display and evaluation

Group Assignment ​

  1. Test mold and casting materials in the lab
    • Review safety data sheets for each material
    • Conduct small test castings
    • Record material performance and user experience
  2. Ensure the lab showcases different techniques and materials
    • Comparison of 3D printed and machined molds
    • Applications of various casting materials (including Serro True metal casting)
    • Food-safe material applications (if applicable)

Learning Resources ​

  1. Material Suppliers
  2. Technical Guides
  3. Material Safety Information
  4. Tools and Processing Resources
  5. DIY Material Recipes

This course combines theoretical knowledge with practical skills. By designing and creating molds hands-on, students will master the basic techniques of mold making and casting, laying the foundation for future innovative projects.