Things You Must Know About Silicone Rubber

1.   Silicone Type is Foundational (Condensation Cure vs. Addition Cure)

Condensation Cure (e.g., RTV-2):

• Curing Mechanism: Cures by reacting with atmospheric moisture, releasing by-products like ethanol.
• Pros: Typically lower cost, better material compatibility (less sensitive to inhibitors), longer pot life and tack-free time.
• Cons: Higher shrinkage (0.5%-1% or more), generally inferior physical properties (tear strength, elongation, abrasion resistance) compared to addition cure, lower long-term stability (may become brittle or shrink further over time), cure depth affected by ambient humidity.
• Common Uses: Less demanding one-off molds, large molds, architectural/decoration molds, applications requiring compatibility with challenging materials.

Addition Cure (Platinum Cure Silicones):

• Curing Mechanism: Cures via platinum-catalyzed hydrosilylation reaction, releasing no by-products.
• Pros: Extremely low shrinkage (typically <0.1%), excellent dimensional stability, superior physical properties (high tear strength, high elongation, high rebound, abrasion resistance), cure unaffected by humidity (but highly affected by temperature), capable of fine detail (food-grade, precision casting), deep section cure, excellent long-term stability.
• Cons: Higher cost, extremely sensitive to "inhibitors/poisons" (sulfur, tin, amines, certain plastics like Bondo/body filler, uncured epoxies/polyurethanes, latex gloves, components in some clays), which can cause partial or complete cure inhibition. Pot life is usually shorter.
• Common Uses: Precision replication (jewelry, models, dental, microelectronics), food-grade molds, molds requiring high longevity and performance, applications demanding minimal shrinkage.

Selection Key:

   High precision, long life, fine detail -> Choose Addition Cure. Ensure the master model and environment are free from inhibitor contamination!

   Limited budget, lower precision/life requirements, complex master materials -> Consider Condensation Cure.

📏 2.   Shrinkage - Impacts Dimensional Accuracy of Castings

   All silicone rubbers exhibit some degree of shrinkage during cure.

   Addition cure shrinkage is significantly lower than condensation cure (<0.1% vs. 0.5%-1%+).

   For high-accuracy replication (e.g., engineering parts, jewelry), low-shrinkage addition cure silicone is essential, and shrinkage compensation (usually minimal) should be considered in the master design.

   Condensation cure molds may undergo slight additional shrinkage (post-cure shrinkage) over days or weeks after initial cure.

⚙ 3.   Physical Properties Determine Mold Life & Use

   Hardness (Shore A): Ranges from very soft (Shore A 00-10) to quite hard (Shore A 50+). Softer rubbers facilitate demolding complex shapes (high draft angles or undercuts) but offer poor support and are prone to distortion; harder rubbers provide better support, dimensional stability, and longer life but make demolding complex parts difficult. Common range is Shore A 20-40.

   Tear Strength: Resistance to the propagation of a tear. High tear strength is crucial for mold longevity, especially when demolding parts with sharp edges or thin sections.

   Elongation (%): Percentage stretch before breaking. High elongation indicates good flexibility and ease of demolding.

   Tensile Strength: Resistance to breaking under tension.

   Abrasion Resistance: Affects longevity under repeated use.

   Selection: Choose silicone with appropriate overall properties based on the casting material (does it have abrasive fillers?), demolding difficulty, and expected number of casts.

🔬 4.   Pot Life & Cure Time Dictate Your Workflow

   Pot Life: Time after mixing when silicone remains flowable and pourable. Higher ambient temperature shortens pot life. Ensure you have enough time for mixing, degassing, and pouring.

   Cure Time: Time to reach a usable state. Full properties are typically achieved after complete cure (e.g., 24 hours). Addition cure silicones are temperature-sensitive; increasing temperature significantly shortens cure time (e.g., using an oven).

   Plan your workflow! Especially critical for products with short pot life.

💨 5.   Degassing is Crucial - Bubbles are the Enemy of Good Molds

   Mixing silicone and catalyst entrains air, and pouring can also create bubbles.

   Bubbles at the mold surface create bumps, resulting in pits on the casting, severely affecting quality.

   Common Degassing Methods:

       Vacuum Degassing: Most effective method! Place mixed silicone in a vacuum chamber; bubbles expand, rise to the surface, and burst. Essential for high-quality molds.

       Centrifugal Casting: Uses centrifugal force to drive bubbles towards the center or a specific area.

       Vibration: Gentle vibration helps bubbles rise.

       Pouring Technique: Pour slowly and continuously from one corner, allowing silicone to flow naturally over the master to minimize air entrapment.

       Defoamers: Some silicone formulations include defoamers, or small amounts can be added externally (confirm compatibility).

🧰 6.   Master Preparation is Paramount to Mold Success

   Clean & Dry: The master must be thoroughly cleaned and completely dry. Any dust, grease, mold release residue, or moisture can inhibit silicone cure (especially addition cure) or adhere to the mold.

   Sealing: Porous masters (e.g., wood, unsealed 3D prints, plaster) must be sealed. Otherwise, silicone will penetrate pores, making demolding difficult and potentially damaging the master. Common sealers include shellac, dedicated model sealers, acrylic lacquer.

   Mold Release: Even though silicone has inherent release properties, using a compatible mold release agent is strongly recommended. It significantly eases demolding and protects both master and mold. Choose a release agent compatible with both master and silicone (water-based, solvent-based, solvent-free waxes), and apply it in a thin, even coat. Excess release can pool in details, affecting casting surface finish.

   Parting Line Consideration: Design the mold's parting line location to ensure easy demolding. Use parting walls/shims (plastic sheets, wood, modeling clay) to define the parting plane if necessary.

   Support & Fixing: Ensure the master is securely fixed within the mold box and won't shift or float during silicone pouring.

🧱 7.   Mold Thickness & Support - Ensuring Strength & Preventing Distortion

   Silicone molds require sufficient thickness to ensure strength and stability, preventing distortion when casting heavier materials (e.g., resin, concrete).

   Complex or large molds often need support from a rigid shell (Mother Mold). Shells are typically made from rigid materials (plaster, fiberglass, plastic, resin) and encase the silicone mold, providing structural support and preventing the silicone from bulging or distorting under casting pressure. Thin-walled silicone molds especially require a shell.

🧴 8.   Mixing Ratio & Uniformity are Critical for Proper Cure

   Strictly follow the manufacturer's specified weight or volume ratio! Incorrect ratios cause incomplete cure (tackiness) or cure that's too fast/slow.

   Mix thoroughly and uniformly! Use a spatula to scrape the sides and bottom of the mixing container, ensuring the catalyst is fully dispersed. Incomplete mixing causes localized cure failure. Mix for at least 3-5 minutes; "box mixing" (pouring between two containers several times) ensures more thorough mixing.

   Using an accurate digital scale for weight-based mixing is the most precise method.

⚠ 9.   Inhibitors/Poisons (Critical for Addition Cure Silicones)

   This is the biggest risk factor when using addition cure silicones.

   Certain substances (sulfur, sulfur-containing clays, uncured epoxy/polyurethane, tin-cure silicones, amine compounds, some plastics like Bondo/automotive filler, some rubbers, talc on latex gloves, some wood preservatives) can poison the platinum catalyst, causing areas in contact or the entire mix to: not cure at all, cure incompletely (remain tacky), or have insufficient cure depth.

   Prevention:

       Isolation: Ensure masters, tools, containers, and work surfaces are completely contamination-free. Avoid masters/mold boxes made from sulfur-containing clays. Use disposable plastic cups/buckets and plastic spatulas (PP, PE are generally safe). Avoid latex gloves (use nitrile or powder-free vinyl gloves).

       Testing: Always perform a small spot test on uncertain masters or materials! Apply a small amount of silicone to an inconspicuous area and check for proper cure.

       Cleaning: Thoroughly clean masters and tools with solvents like isopropyl alcohol (IPA), ensuring solvents are completely evaporated and dry before use.

       Ventilation: Avoid using addition cure silicones in areas recently used for condensation cure silicones or polyurethane products.

📦 10. Storage - Extending Silicone Shelf Life

   Unopened silicone (especially catalyst) should be stored in a cool, dry, dark place (e.g., cupboard).

   Avoid high temperatures and freezing.

   Condensation cure silicones are moisture-sensitive; reseal containers tightly after opening.

   Keep addition cure Part A and Part B components strictly separate to avoid cross-contamination.

   Be mindful of the product's expiration date.

🛠 11. Mold Making Methods

   Poured Block Mold: Master is fixed in a mold box and silicone is poured over it. Suitable for most shapes.

   Brush-On Mold: Layers of thicker/viscous silicone (or specialized thixotropic brush-on silicone) are brushed onto the master. Each layer cures before applying the next. A rigid shell is made after reaching the desired thickness. Suitable for large objects, objects hard to box, or to save silicone.

   Envelope/Pour-over Mold: Master is pressed into/wrapped within a block of silicone (often combined with vacuum bagging).

📌 Key Takeaways Summary

1.  Choose the Right Type: Addition Cure (precision, performance, low shrink) vs Condensation Cure (economy, compatibility).

2.  Guard Against Inhibitors: The Achilles' heel of Addition Cure – ensure environment, master & tools are "clean" (no S, Sn, amines, etc.).

3.  Degas Thoroughly: Vacuum degassing is best for high-surface-quality molds.

4.  Mix Precisely: Strict ratios, thorough and uniform mixing.

5.  Perfect the Master: Clean, dry, sealed, release agent applied, securely fixed.

6.  Provide Adequate Support: Ensure sufficient mold thickness; use a rigid shell for complex/large molds.

7.  Understand Properties: Select hardness, tear strength, elongation, etc., based on needs.

8.  Control Time: Be mindful of pot life and cure time (temperature is key).

9.  Test: Always spot test with new materials or uncertain conditions.