week 13

Molding and Casting

Silicone Molds & Resin Casting

Exploring silicone molding techniques and comparing resin casting materials
with 3D-printed and CNC-milled mold approaches

Individual Contributions

👩‍🔬 Micaela Córdova - Three Contributions

1. Chavín de Huántar Silicone Mold & Resin Casting: Creation and documentation of the workflow of a silicone mold using a master model found in our laboratory: a Chavín de Huántar "Cabeza Clava" — an iconic piece of ancient Peruvian art. We then performed casting tests with two different resin materials: SILIKAST EPOXY CRAFT A + B (crystal-clear epoxy for jewelry-grade finish) and Resina Silikast 70D (honey-amber polyurethane for structural strength). We compared the material properties, curing times, and final aesthetic qualities of both resins cast from the same silicone mold.

2. 3D-Printed Molds Experience: Documented the complete 3D-printed mold workflow using SLA resin printing to contribute to the comprehensive comparison of 3D-printed molds versus milled molds.

3. Technical Documentation — Resins & Silicones: Contributed to the laboratory documentation of all silicone and resin products used during the week.

👨‍🔧 Andrés Mamani - Three Contributions

1. Brain Figure Silicone Mold & Material Testing: FDM-printed a small brain figure mold, then created silicone molds using two different materials: Silicone Rubber (Silicone Rubber with catalyst) and F20 Plus (two-part RTV silicone). Documented handling characteristics, mixing ratios, flexibility, surface detail capture, curing times, and demolding ease.

2. Milling Mold Method Experience: Documented the complete CNC milling mold workflow to contribute to the comprehensive comparison of 3D-printed molds versus milled molds.

3. Technical Documentation — Resins & Silicones: Contributed to the laboratory documentation of all silicone and resin products used during the week.

Technical Documentation

Complete Material Comparison & Safety Guidelines

As part of our group contribution, we documented the properties of all silicones and resins used in the Fab Lab. Since these are Peruvian-origin products, formal international data sheets are not widely available — the information below is based on regional product specifications and manufacturer guidelines.

📊 Complete Material Comparison: 4 Silicones + 2 Resins

Comprehensive comparison table of all 4 silicone molding materials and 2 resin casting materials used in Week 13. Includes the two resins compared in the Chavín de Huántar casting tests.

Property SILICONE MOLD MATERIALS RESIN CASTING MATERIALS
RTV Type 6 F20 Plus Platinum 1510 Epoxy A+B Polyurethane 70D
Type / Curing Condensation (Tin) Condensation (Tin) Platinum-Cured Chemical Reaction (Epoxy) Chemical Reaction (Polyurethane)
Primary Use MOLD MOLD MOLD CAST CAST
Mixing Ratio 2–2.5% catalyst 2–3% catalyst 1:1 by weight 1:1 by volume 1:1 by weight
Pot Life 15–25 min 15–20 min 15–20 min 10–15 min 2 min (FAST!)
Cure Time 2–6 hours 4–12 hours 4–6 hours 10–24 hours 5–10 min
Hardness (Shore) 20A (Flexible) 20A (Flexible) 10A (Very Flexible) Hard/Brittle 70D (Rigid)
Visual Appearance Opaque White Opaque White Semi-Translucent Crystal Clear Honey Amber
Detail Capture Excellent Very Good Excellent Excellent Excellent
Food Safe No Yes Yes No No
Cost Economical Moderate Premium Moderate Moderate
Best Applications Cost-effective molds, high detail Food projects, precise detail Extreme flex, high temp, jewelry Jewelry, decorative, transparent display Functional, structural, fast cycles

🛡️ Equipment & Personal Protective Equipment

🛡️ Personal Protection — Critical for Safety

Nitrile Gloves

Essential for all handling. Silicones and resins are irritating to skin and can cause dermatitis. Always wear when mixing, pouring, or handling cured materials. Double-glove if handling for extended periods.

Safety Goggles

Protect eyes from splashes and aerosol particles. Especially critical when mixing resins and pouring into molds. Resin splashes can cause chemical burns. Mandatory for all resin work.

Lab Coat or Protective Apron

Protects clothing from stains and spills. Silicones and resins are nearly impossible to remove from fabric once cured. Use disposable aprons or dedicated work coats that can be cleaned.

Hair Tie / Head Cover

Keep hair back to prevent entanglement with machinery or mixing equipment. Hair in curing resin/silicone is difficult to remove. Hairnets or headcovers recommended for long-term exposure.

📐 Before Molding — Measurement & Preparation

Precision Scale (0.1g accuracy)

Critical for catalyst ratios. Even 0.5g difference affects cure time and final hardness. Use digital scales with ±0.1g precision minimum. Calibrate before each use.

Measuring Cups (Graduated)

For volumetric measurements when using 1:1 ratios by volume. Mark at key volumes (50ml, 100ml) for quick reference. Use separate cups for silicones vs resins to avoid contamination.

Mixing Containers (Disposable)

Always use disposable cups for mixing. Never reuse containers that have cured silicone or resin residue — they interfere with new batches. Plastic cups work for silicones; use stronger containers for resins.

Stirring Rods / Sticks

Non-metallic preferred (wood or plastic). Stir slowly and deliberately to avoid trapping air bubbles. Vigorous stirring introduces bubbles that ruin molds. Scrape container sides thoroughly to incorporate all material.

Release Agent (Desmoldante) — CRITICAL ⚠️

ALWAYS apply before pouring silicone. Without release agent, the master model becomes permanently bonded to curing silicone. Results: trapped model, damaged mold, broken master. See Desmoldante section below for details.

Mold Box / Containment

Prevents overflow and contains spills during pouring. Use plastic containers, cardboard boxes, or dedicated mold frames. Ensure containment is at least 10mm taller than the tallest feature to accommodate silicone expansion.

🔥 Molding & Casting Process — Environment Control

Ventilation / Fume Hood

Critical: Work in well-ventilated area. Silicone and resin fumes (especially during curing) can be irritating to respiratory system. Use fume hoods or open-air work stations. Never work in enclosed spaces without ventilation.

Thermometer

Monitor ambient temperature throughout the process. Cold temperatures (<15°C) significantly slow curing times. Heat can accelerate cure (check material specs). Ideal range: 18–25°C for consistent results.

Timer / Clock

Track working time (pot life) and curing time with precision. Set alarms for critical time windows. Record times for each batch to identify patterns and optimize future projects.

Vacuum Degasser (Optional but Recommended)

Removes air bubbles trapped during mixing. Highly recommended for jewelry-grade molds and fine-detail work. Creates vacuum that pulls bubbles to surface before they become permanent. Expensive but transforms quality.

Heat Source (Optional)

Can accelerate platinum-cured silicone curing (check specs). Some polyurethanes benefit from gentle heat (not exceeding 40°C) during initial cure. Use heat lamps or warm water baths — never direct flame.

Waste Container

For safe disposal of unused material. Never pour down drains. Cured silicone/resin can clog pipes. Allow waste to cure in container, then dispose as solid waste per local regulations.

Section 1: Casting in Epoxy & Polyurethane Resins

Chavín de Huántar Silicone Mold Creation & Dual Resin Casting

🗿 The Cabeza Clava — Master Model

The Cabezas Clavas are stone sculptures belonging to the Chavín culture of Pre-Inca Peru. Originally embedded in the walls of the Chavín de Huántar temple, they represent human-feline transformations and are iconic symbols of ancient Peruvian lithic art. Having one as our master model was a meaningful cultural connection.

The piece was used as-is from the lab's collection, making this a pure mold-making exercise without a prior design or print stage. This presented an excellent opportunity to test silicone molding on a complex, detailed, and culturally significant artifact.

Cabeza Clava master model from the lab
Chavín de Huántar original temple reference

Chavín de Huántar temple in Peru — the original context for these sculptures

🧪 Creating the Silicone Mold — RTV Type 6

To replicate the Cabeza Clava, we used SILIKA CAUCHO SILICONA RTV TIPO 6 combined with its specific catalyst. This material was selected for its high fidelity in capturing textures and its ease of use for manual casting.

01
Volume Calculation & Measurement

Measured mold capacity by filling with water to estimate the required silicone volume. Calculated the amount needed, intentionally adding surplus to ensure complete coverage of the master model.

02
Precise Catalyst Ratio (2–3% by weight)

The RTV Type 6 requires precise mixing: 2% to 3% catalyst by weight. For every 100g of silicone base, we used a precision scale to add exactly 2–3g of catalyst. This precision is essential for proper curing and full mechanical resistance.

Weighing silicone base on precision scale Adding catalyst to silicone

Precise weighing and catalyst mixing — the foundation of consistent results

03
Thorough Mixing (3–5 minutes)

Mixed both components slowly and thoroughly, scraping sides and bottom to ensure no unmixed base remained. Unmixed material prevents proper curing and creates weak spots in the mold.

04
Careful Pouring (15–20 minute window)

Once mixed, had approximately 15–20 minutes to pour silicone before it began to thicken. Poured from the lowest point of the mold box, allowing the material to rise naturally around the piece to minimize air bubbles.

05
Degassing & Curing (4–8 hours)

Tapped the mold against the table so trapped bubbles would rise to the surface. Left to cure at room temperature for 4–8 hours depending on ambient conditions.

⚠️ Critical Lesson Learned: We forgot to apply the release agent (desmoldante) before pouring! When we tried to demold, the master model became stuck inside the silicone. We managed to extract it safely, but this serves as a critical reminder: always apply release agent before pouring. This ensures clean separation and protects delicate master models.
🔬 Resin Casting Tests — Two Materials Compared

Once the silicone mold was complete, we performed casting tests with two different resin systems to compare their properties, aesthetics, and structural characteristics. The same silicone mold was used for both casts, providing a true apples-to-apples comparison.

Cast 1: SILIKAST EPOXY CRAFT A + B — Crystal-Clear Finish

Mixed Part A and Part B at 1:1 ratio by volume. Poured into the silicone mold and let rest for 10 hours. The result was a beautiful, crystal-clear, glass-like piece that captured every detail of the original Cabeza Clava with exceptional optical clarity.

SILIKAST Epoxy Craft A+B product
Mixing and pouring epoxy into the Chavín mold

Pouring epoxy resin into the completed silicone mold

Epoxy cast result — front view Epoxy cast result — detail Epoxy cast result — side view

Epoxy cast: Crystal-clear finish captures every cortex fold and surface texture with optical transparency

Cast 2: Resina Silikast 70D — Honey-Amber Polyurethane

Mixed Part A and Part B at 1:1 ratio by weight. Poured into the same silicone mold. Cure time: 5 hours — notably faster than epoxy (10 hours). The result was a solid, vibrant amber-toned replica with excellent structural strength (70 Shore D hardness).

Silikast 70D polyurethane resin product
Pouring polyurethane resin into mold Polyurethane resin curing in mold

Polyurethane resin pour and curing process — note the distinctive honey-amber color

Final polyurethane resin cast result

Polyurethane cast: Solid amber-toned replica with structural strength for functional applications

📊 Epoxy vs Polyurethane — Final Comparison

Property SILIKAST Epoxy A+B Silikast 70D Polyurethane
Visual Finish Crystal-clear, glass-like, transparent Honey-amber color, opaque
Mix Ratio 1:1 by volume 1:1 by weight
Pot Life 10–15 minutes 2 minutes (very fast)
Cure Time 10 hours (long) 5 hours (fast)
Hardness Hard but brittle 70 Shore D (impact resistant)
Detail Capture Excellent Excellent
Best For Jewelry, display, decorative, showcase pieces Functional, structural, durability required
Epoxy Advantages
  • Crystal-clear optical transparency
  • Ideal for jewelry and display
  • Beautiful glass-like finish
  • Captures finest details visually
Polyurethane Advantages
  • Faster curing time (5 vs 10 hrs)
  • Higher impact resistance (70D)
  • Distinctive honey-amber aesthetic
  • Better for functional parts
Key Insight: Both materials produced faithful replicas of the Cabeza Clava with excellent detail capture. The choice between epoxy and polyurethane depends on the final application: use epoxy when optical clarity and beauty matter most (jewelry, display), and polyurethane when structural strength and durability are priorities (functional parts, repeated use).

📸 Final Comparison: All Casting Methods

Final comparison: 3D print original, Silikast 70D, and Epoxy Craft cast side by side

From left to right: SLA 3D-printed master model, Silikast 70D polyurethane cast, SILIKAST EPOXY CRAFT A+B epoxy cast. All produced from the same silicone mold.

Section 2: Silicone Mold Casting

Brain Figure FDM Mold & Silicone Material Comparison

🧠 Brain Figure Master Model

For the individual molding and casting assignment, We designed and FDM-printed a small brain figure (cerebro) to use as a master model. The goal was to create two different silicone molds using different materials and compare their handling characteristics, flexibility, surface detail capture, curing times, and demolding ease.

🧪 Silicone Mold Testing — Two Materials

We created two silicone molds of the same brain figure using different materials to conduct a direct comparison:

Mold 1: Silicone Rubber (Silicone Rubber with Catalyst)

Material: SILIKA CAUCHO SILICONA RTV TIPO 6
Mixing Ratio: 78g silicone base + 2g catalyst (2.5% by weight)
Cure Time: 6 to 8 hours at room temperature
Visual Result: Opaque white silicone
Flexibility: Very flexible and easy to demold

The Silicone Rubber captured the brain cortex folds with good detail. The material's high elasticity makes demolding very easy — the brain figure pops out cleanly without stress.

Silicone Rubber brain mold result

Mold 2: F20 Plus (Two-Part RTV Silicone)

Material: Silicona RTV F20-Plus
Mixing Ratio: 1:1 by weight (equal parts A + B, no catalyst)
Cure Time: 4 to 6 hours at room temperature
Visual Result: Opaque white silicone
Flexibility: Slightly firmer than Caucho, 20 Shore A

The F20 Plus produced slightly sharper detail on the smaller cortex folds compared to the Silicone Rubber. The two-part system (no catalyst to measure) is more convenient and the faster cure time (4–6 hrs vs 6–8 hrs) is advantageous for production workflows.

F20 Plus brain mold result

📊 Silicone Rubber vs F20 Plus Comparison

Property Silicone Rubber (RTV Type 6) F20 Plus (RTV-2)
Mix Type Base + catalyst (must weigh precisely) Two-part A+B (1:1 by weight, no catalyst)
Ratio 78g silicone + 2g catalyst 1:1 by weight (A+B)
Food Safe No Yes
Flexibility Very flexible (easy demolding) Slightly firmer (20A, still flexible)
Surface Detail Good (captures cortex texture) Very good (sharper on fine folds)
Cure Time 6 to 8 hours 4 to 6 hours (faster)
Ease of Demolding Very easy — high elasticity Very easy — proper tear resistance
Best Use Case Economical choice, maximum flexibility When food safety or speed matters
Brain molds comparison — Silicone Rubber and F20 Plus placed side by side

Both brain molds after demolding and curing. The rectangular cavities show the brain figure captured in both Silicone Rubber and F20 Plus materials, demonstrating the difference in visual opacity and detail sharpness.

Silicone Rubber Advantages
  • Most economical option
  • Extremely flexible (20A)
  • Easy demolding due to high elasticity
  • Good detail capture
F20 Plus Advantages
  • Faster cure (4–6 hrs vs 6–8 hrs)
  • Sharper detail on fine features
  • No catalyst weighing required (1:1)
  • Food-safe certification
Key Learning: Both materials produced high-quality molds of the brain figure. The choice between them depends on priorities: Silicone Rubber wins for cost and maximum flexibility, while F20 Plus excels in speed and detail sharpness. For production runs where demolding ease and cost matter, Silicone Rubber is unbeatable. For food-related or precision jewelry work, F20 Plus is superior.

Section 3: Mold Production Methods Comparison

3D-Printed (SLA) vs CNC-Milled Molds

This week provided hands-on experience with two distinct approaches to mold creation: additive manufacturing (SLA 3D printing) and subtractive manufacturing (CNC milling). Both methods successfully produce molds ready for silicone casting, each with unique advantages and trade-offs.

🖨️ 3D-Printed Molds (SLA Resin)

Advantages: Excellent detail resolution (0.05mm layers), ideal for organic complex shapes, easy design iteration, no tool wear concerns, jewelry-grade aesthetics.

Limitations: Single-use master model, post-processing required (support removal, curing, sanding), print time 12–24 hours, resin fragility.

Key Issues Encountered with 3D-Printed Molds:

SLA Resin Printing Issues:
  • Support Mark Cleanup: After printing, support attachment points leave visible marks and small pits on the model surface. Requires filling with residual resin and careful sanding to achieve a mold-ready surface.
  • Post-Processing Time: The refinement phase (filling, sanding, spot-curing) adds significant time beyond the 12-hour print. Total timeline: 12 hours print + 2–3 hours post-processing = 14–15 hours.
  • Resin Brittleness: SLA resin parts are harder but more brittle than silicone. While excellent for master models, they require careful handling during demolding to avoid cracks.
SLA-printed surface with support marks and pits before refinement

Before: Raw SLA print with visible support marks and pitted surface

Filling and sanding SLA surface to achieve mold-ready finish

After: Filling pits with resin and sanding for smooth, mold-ready surface

Surface refinement is essential before silicone mold pouring. Both SLA and FDM prints require post-processing to achieve mold-ready quality.

FDM 3D-Printed Molds (General Issues):
  • Surface Finish Not Perfect: FDM prints have visible layer lines and rough surface texture. This imperfection transmits directly to silicone molds and final castings. The resulting cast pieces show layer-line artifacts that FDM cannot hide.
  • Model Fragility: FDM plastic parts are more brittle than SLA resin. Thin features can break during support removal, mold insertion, or silicone pouring. Cultural artifacts or irreplaceable masters are at risk.
  • Complex Post-Processing: Removing support structures from FDM can be tedious and time-consuming. Sanding to smooth layer lines requires careful attention or the texture will appear in all castings.
  • Adhesion Issues with Desmoldante: FDM surface porosity means silicone grips even tighter. Without proper desmoldante application, extraction is nearly impossible.
Final SLA 3D-printed master model — polished and ready for silicone mold

Final SLA 3D-printed master model after surface refinement (filling pits, sanding). Ready for silicone mold casting with desmoldante applied.

⚠️ Desmoldante is ESPECIALLY Critical for 3D-Printed Molds

FDM and SLA 3D-printed models have inherent surface porosity and micro-texture that silicone can grip into deeply. Without release agent, the bond is almost unbreakable. Post-processing (filing, sanding) makes the surface even grippier. Always apply desmoldante generously and wait for it to cure before pouring silicone. This is non-negotiable for 3D-printed masters.

⚙️ CNC-Milled Molds (Subtractive)

Advantages: Superior surface finish (direct from machine), dimensional accuracy, reusable mold material, durable polycarbonate, multiple casting cycles.

Limitations: Slower total time (6–10 hours), limited by tool geometry, CAM setup complexity, tool replacement costs.

CNC milling process and milled mold documentation

CNC-Milled mold workflow and finished mold demonstrating superior surface finish and precision.

Key Issues Encountered with CNC-Milled Molds:

CNC Milling Challenges:
  • Tool Deflection on Deep Cuts: When milling deep into polycarbonate blocks, cutting tools can deflect slightly, resulting in subtle geometry errors (±0.2–0.5mm). This is acceptable for most molds but problematic for tight-tolerance functional parts.
  • Chatter and Surface Vibration: At high spindle speeds, the material can vibrate against the cutting tool, creating visible spiral marks or waves on the milled surface. Requires careful spindle speed and feed rate tuning.
  • Tool Wear and Breakage: Carbide tools are expensive and wear with use. A single dull tool produces poor surface finish or can break mid-cycle, requiring immediate tool change and potential restart of the operation. Tool inventory is critical.
  • CAM Software Complexity: Creating efficient tool paths requires skill and experience. Inefficient paths waste machine time and increase tool wear. Manual optimization is time-consuming.
  • Polycarbonate Brittleness: Unlike metals, polycarbonate can crack or chip if too much force is applied. Requires careful parameter selection to prevent damage. Thinner walls are especially vulnerable.
  • Design Constraints: Undercuts and deeply recessed features cannot be milled in a single setup without multi-axis programming. Complex geometries require tool repositioning, adding time and complexity.
⚠️ CNC Milling Lesson: While CNC produces superior surface finish compared to 3D printing, the process has its own challenges. Tool selection, spindle speed, and feed rate are critical parameters. A single wrong setting can ruin hours of machining time. Always run tests on scrap material first, and maintain detailed records of working parameters for future reference.
📊 Complete Comparison: Both Methods Side-by-Side
Criterion SLA 3D-Printed CNC-Milled
Detail Resolution 0.05mm layers (excellent) 0.10–0.20mm (very good)
Total Time 12 hours print + 2–3 hours post-processing = 14–15 hours 6–10 hours machining (direct, no post-processing)
Design Flexibility Excellent — modify STL and reprint immediately Good — requires CAM rework
Surface Quality Good — requires post-processing to remove support marks Excellent — direct from machine, no refinement needed
Mold Durability Single-use (resin fragile) Reusable for 10+ casting cycles
Release Agent (Desmoldante) Essential — resin can stick to other materials if not applied Recommended — polycarbonate is slightly more forgiving but desmoldante ensures clean release
Best Applications Jewelry, one-off art pieces, intricate organic geometry Functional parts, production runs, high-precision tolerances
⚠️ Critical Learning: The Importance of Release Agent (Desmoldante)
One of the most critical lessons from this week: always apply release agent before pouring silicone. Whether using a 3D-printed resin master or a CNC-milled polycarbonate mold, the master model can become permanently stuck in curing silicone if no release agent is applied. This happened with our Chavín de Huántar mold — the master became trapped and required careful extraction, risking damage to both the silicone mold and the cultural artifact. Release agent (desmoldante) prevents adhesion between the silicone and the master, ensuring clean, safe separation after curing.
🧴 Desmoldante (Release Agent) — Essential for All Molds

What is Desmoldante? A release agent applied to the master model surface before pouring silicone. It creates a barrier preventing the curing silicone from bonding to the model, ensuring clean separation after cure time.

Available Forms: Paste, spray, or liquid. Common types in Peru: Silicon Peru brand paste (shown), or spray formulations. Apply generously but evenly over the entire master model surface.

Cost: Economical — a small jar lasts for many molding sessions.

Desmoldante release agent (Silicon Peru paste)

⚠️ What Happens When You DON'T Use Desmoldante

Master Gets Trapped

The silicone cures around the model with no barrier, creating chemical adhesion. The master becomes permanently bonded to the silicone mold. Extraction requires force that can damage both the mold and the master model.

Model Damage Risk

FDM 3D-printed parts are especially vulnerable. Forced extraction can snap thin features, break delicate geometry, or tear away surface details. Cultural artifacts or irreplaceable masters can be destroyed.

Mold Damage

Attempting to force out a trapped master can tear or rupture the freshly-cured silicone mold, ruining weeks of planning. Support marks and damage become permanent.

Model Pieces Left Behind

If the master model fractures during extraction, pieces may remain embedded in the silicone, contaminating the mold and making it unusable for clean casting.

🎯 Final Recommendation

Choose SLA 3D Printing when:

  • Creating one-of-a-kind jewelry pieces with intricate details
  • Exploring organic, sculptural geometries that are difficult to mill
  • Rapid prototyping and design iteration is important
  • Jewelry-grade aesthetics and surface smoothness matter

Choose CNC Milling when:

  • Production runs requiring multiple casting cycles
  • High precision and tight dimensional tolerances are required
  • Functional parts with structural integrity are needed
  • Direct surface finish without post-processing is desired

Document Files

Brain Model STL — Master model for silicone mold creation (FDM 3D-printed original)



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