Introduction to Molding and Casting
Molding and casting is a versatile fabrication process that allows us to create complex shapes and forms that would be difficult or impossible to achieve through other methods. The basic principle involves creating a mold (negative) and then casting a material (positive) into it. This week focused on multiple applications:
- Silicone Casting: For flexible, soft parts with excellent detail reproduction
- Resin Casting: For rigid, transparent or colored components
- Rubber Molding: For durable, elastic parts
- Waterproof Encapsulation: Sealing PCBs and electronics for water resistance
Materials and Safety
Silicone (Mold Star, Dragon Skin, Smooth-Sil): Two-part platinum or tin-cure silicone rubber. Mix ratios typically 1:1 or 10:1 by weight. Working time varies from 15-45 minutes depending on formulation. Cures at room temperature in 4-24 hours. Key properties include flexibility, tear strength, and shore hardness (00-50 for soft, A10-A30 for medium).
Casting Resin (Smooth-Cast, Crystal Clear): Two-part polyurethane resin. Mix ratio usually 1:1 by volume. Fast-curing formulas set in 10-30 minutes. Can be tinted with dyes or pigments. Important to mix thoroughly but slowly to minimize bubbles. Ideal for transparent or rigid parts.
Rubber (PMC-121 Dry, Rebound): Two-part urethane rubber. Mix ratio 1:1 by weight. Provides excellent durability and impact resistance. Shore hardness typically A20-A50. Good chemical resistance and longer lifespan than silicone in high-wear applications.
Safety Considerations: Always wear gloves and safety glasses. Work in well-ventilated areas, especially with resins. Use precision scales for accurate mixing ratios. Keep materials away from moisture before mixing. Store unmixed components in sealed containers.
Mold Making Process
3D Printed Mold Design: Started by designing the mold in CAD software, ensuring proper draft angles (typically 3-5 degrees) for easy demolding. Added registration keys to align multi-part molds. Created pour spouts and air vents to prevent bubbles and ensure complete fills.
3D Printing the Mold: Used resin 3D printer for high-resolution molds with smooth surface finish. Printed at 0.05mm layer height for minimal post-processing. Cured prints under UV light for full polymerization. Washed thoroughly with IPA to remove uncured resin.
Post-Processing: Sanded interior surfaces with fine-grit sandpaper (400-800 grit) where needed. Applied mold release agent or buffed with paste wax to prevent casting materials from sticking. For resin molds casting resin, multiple coats of release agent are essential.
Casting Techniques
Mixing: Measure parts A and B precisely using digital scale. Mix slowly and deliberately to avoid introducing air bubbles. Scrape sides and bottom of container thoroughly. Mix for 2-3 minutes until completely homogeneous - no streaks should remain.
Degassing: Place mixed material in vacuum chamber to remove air bubbles. Pull full vacuum (29+ inches Hg) for 3-5 minutes. Material will rise significantly as bubbles expand - use container 3-4x larger than material volume. When bubbles stop rising and material returns to original level, degassing is complete.
Pouring: Pour slowly from one corner of the mold, allowing material to flow and self-level. Pour in thin stream from height of 6-12 inches to break surface tension and release trapped air. For complex molds, rotate or tilt mold while pouring to ensure all details fill completely.
Curing: Allow to cure at room temperature (70-80°F) for recommended time. Avoid disturbing during cure. Some materials benefit from pressure casting (40-60 PSI) to compress remaining micro-bubbles. For faster curing, some resins can be heated to 150-180°F in oven.
Waterproofing Electronics with Resin and Silicone
PCB Preparation: Clean PCB thoroughly with isopropyl alcohol to remove flux and contaminants. Mask off any connectors, buttons, or components that must remain accessible using tape or silicone putty. Consider conformal coating on PCB traces before encapsulation for additional protection.
Resin Encapsulation: Clear casting resin provides excellent waterproofing and allows visual inspection of internal components. Mix resin carefully and degas to eliminate bubbles. Place PCB in mold or container, ensuring proper positioning. Pour degassed resin slowly, filling around all components. Resin has excellent electrical insulation properties (dielectric strength >400V/mil).
Silicone Encapsulation: Softer and more flexible than resin, silicone is ideal for applications requiring shock absorption or flexibility. Two-part silicone (1:1 ratio) provides good waterproofing while allowing some component movement. Pour in layers if needed for thick encapsulation. Silicone can be removed later if repairs are needed by carefully cutting and peeling.
Hybrid Approach: For my robot project, I used a combination: resin for rigid structural encapsulation of the main PCB for maximum waterproofing (IP67-68 rating achievable), and silicone for wire exit points and flexible connections to allow movement while maintaining water resistance. This provides both protection and functionality.
Testing: After full cure (24-48 hours for resin, 24 hours for silicone), test waterproofing by submersion in water for extended periods. Check for bubbles indicating leaks. Test electrical functionality before and after to ensure encapsulation didn't damage components from heat or pressure during cure.
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