What Is The Best Silicone For Mold Making?
Silicone mold making refers to the process of creating reusable molds out of silicone rubber for applications like resin casting, baking, chocolate work, soap making, and more. Silicone is a thermoset material that can be mixed, poured, and cured into flexible molds that offer excellent release properties and high heat resistance.
Silicone molds are valued for their ability to reproduce fine details and withstand multiple castings. They are commonly used to make detailed novelty chocolates, bakeware, soaps, candles, jewelry, and prop replicas. Silicone molds offer a cheap, reusable alternative to machined metal molds.
The process involves mixing two liquid silicone components that chemically react and cure into flexible rubber. Choosing the proper silicone material and following a precise mixing process is key to creating quality, long-lasting molds. The cured silicone can flex and bend but will return to its original shape, enabling repeated castings.
Types of Silicone for Mold Making
There are two main types of silicone used for mold making – tin cure (condensation cure) silicone and platinum cure (addition cure) silicone. The main differences are:
- Tin cure silicone uses a tin catalyst and crosslinks via a condensation reaction. This reaction produces a byproduct, so molds have shorter shelf life. Tin cure silicones generally offer better tear strength and flexibility. They have a lower durometer or Shore A hardness reading.
- Platinum cure silicone uses a platinum catalyst and crosslinks via an addition reaction, so no byproducts are formed. This gives a longer shelf life. Platinum cure silicones have better chemical and heat resistance. They have a higher durometer or Shore A hardness.
Some of the pros and cons of each type include:
Tin Cure Silicone
Pros: More flexible, better tear strength, lower viscosity, cheaper
Cons: Shorter shelf life (6-12 months), can inhibit platinum cure silicone, limited heat resistance
Platinum Cure Silicone
Pros: Long shelf life (1-2 years), excellent chemical/heat resistance, less likely to inhibit other materials
Cons: Less flexible, weaker tear strength, higher viscosity, more expensive
Overall, platinum cure silicone is preferred for high quality, durable molds, while tin cure works well for simpler, shorter term applications. The specific project needs will determine which material is optimal.
Key Properties to Consider
When selecting a silicone for mold making, some of the most important properties to consider include:
Tensile Strength
Tensile strength refers to the maximum stress a material can withstand before breaking or permanently deforming. A silicone with high tensile strength is important for mold making, as the silicone must be able to withstand the forces involved in demolding without tearing or deforming [1].
Shore Hardness
Shore hardness is a measure of the stiffness of the cured silicone rubber. Silicone rubbers range in hardness from very soft and flexible to medium and hard. The shore hardness required depends on the application. Softer, more flexible silicones are often needed for mold making to allow for easy demolding [2].
Tear Resistance
Tear resistance refers to the ability of the silicone to resist tearing or ripping during demolding. High tear resistance is critical to prevent damage to the mold and allow for repeated use [1].
Flexibility
Flexibility is critical for easy demolding of complex geometries and undercuts. Silicones maintain flexibility over a wide temperature range, allowing mold making with both cold and hot casting materials [2].
Pot Life
Pot life refers to the amount of time the mixed silicone remains usable before hardening. Longer pot lives provide more working time for complex molds or large pours.
Cure Time
Cure time is how long the silicone takes to fully harden after mixing. Faster cure times increase mold making productivity. However, extremely fast cure times can make large or complex pours more challenging.
Best Silicones for Different Applications
When selecting the best silicone for mold making, it’s important to consider the intended application. Different types of silicone have properties that make them better suited for certain projects.
Best for Food Molds
For food molds, platinum-cure silicone is the best choice. Platinum silicone does not contain solvents that could leach into food, making it food-safe. Look for silicones marketed specifically for baking and candy making, like Smooth-On’s Mold Star 15 Slow, which has a low viscosity for capturing fine details.
Best for Candle Molds
High heat resistance is key for candle molds. Platinum or tin-cure silicones with heat resistance above 400°F, like Smooth-On’s Mold Max 30, work well. The Mold Max series offers tear and heat resistance needed for candle making.
Best for Resin Casting Molds
Molds for resin casting require high tear strength. Opt for condensation cure silicones like Smooth-On’s Dragon Skin series, known for its extremely high tear strength. The Dragon Skin 10 Medium is an excellent choice for detailed resin molds.
Best for High Detail Molds
Reproducing intricate details requires a low viscosity silicone. Platinum-cure silicones offer the best flowability and surface reproduction. Smooth-On’s Mold Star 15 Slow or Mold Max Stroke are great options for optimizing detail.
Brand Recommendations
When it comes to silicone mold making materials, there are a few major brands that consistently produce high-quality products:
Smooth-On
Smooth-On (https://www.smooth-on.com/product-line/mold-star/) is one of the most popular brands for silicone mold making supplies. Their Mold Star line offers a wide range of shore hardnesses, cure times, and viscosities to suit any project. Some of their top products include:
- Mold Star 15 Slow – a tin-cure silicone with a 15A shore hardness and 45 minute working time
- Mold Star 30 – a condensation-cure silicone with a 30A shore hardness and 7 minute working time
- Ecoflex 00-30 – a platinum-cure silicone with a super soft 00-30 shore hardness and 5 minute working time
Alumilite
Alumilite (https://www.aeromarineproducts.com/product-category/mold-making/) is known for their rigid urethane resins, but they also produce quality silicone rubbers. Their platinum-cure silicones offer excellent tear resistance and come in shore hardnesses from 20A to 40A. Some popular options include:
- RTV Silicone Rubber – an economical 20A silicone with a 45 minute working time
- Silicone 30 – a 30A silicone with good tear and abrasion resistance
- High Strength 2 Silicone – a 35A silicone with improved tear strength
Reynolds
Reynolds Advanced Materials (https://www.reynoldsam.com/) manufactures tin-cure, platinum-cure, and addition-cure silicones. They have a wide range of shore hardnesses from 5A to 80A. Some of their best-sellers include:
- GI-1000 – an inexpensive tin-cure 10A silicone
- CS25 – a tin-cure 25A silicone
- XSIL555 – a semi-rigid 80A silicone
When selecting a brand, consider shore hardness, working time, cure time, tear strength, and price to find the right silicone for your particular project.
Storage and Shelf Life
Proper storage is crucial for maximizing the shelf life of silicone molds. According to Cast-Tech[1], silicone rubber molds that are properly stored can last 1-2 years. Silicone molds should be cleaned thoroughly after each use, making sure to remove any residue from the casting material. Store cleaned molds in a cool, dry area away from direct sunlight and extreme temperatures. Avoid storing molds in garages, attics or sheds where temperature fluctuations can be significant.
Humidity and moisture should also be avoided, as condensation can form inside the mold causing deterioration over time. Seal molds in plastic bags or airtight containers with desiccant packs to maintain dryness. Do not stack molds or place heavy objects on top of them during storage, as this can lead to deformation. Check regularly for any cracks, tears or changes in mold flexibility. Well-cared for silicone molds can produce dozens if not hundreds of castings over their lifetime.
Safety Considerations
When working with silicone mold making materials, there are some important safety considerations to keep in mind:
Proper personal protective equipment (PPE) should be used, including nitrile gloves, eye protection, and a respirator mask or proper ventilation. According to one Reddit user, “The rubber itself won’t harm you, but you do need nitrile gloves. Chemicals in uncured resin can give you contact allergy rashes and eczema when exposed to skin” (Source).
Ensure the area is well-ventilated when working with silicone mold making materials. Inhaling fumes or dust from certain products can be harmful. As Smooth-On notes, “Avoid breathing in plaster dust, mist from aerosol sprayers or fumes of any kind, etc. When using any material associated with mold making and casting, you must observe appropriate health and safety precautions” (Source).
Opt for non-toxic, food-safe silicone options when making molds for items that will come in contact with food. Do your research to select the safest materials for your particular application.
Following basic safety precautions allows you to enjoy silicone mold making while minimizing risks.
Releasing Agents
When molding with silicone, using a release agent is crucial for cleanly demolding your finished piece. There are two main types of release agents to consider:
- Water-based release agents like EconoMIST are economical and environmentally-friendly. They leave no oily residue behind.
- Silicone-based release sprays like Universal Mold Release provide the best release, especially for intricate molds. They contain volatile organic compounds so require ventilation.
Only apply a very thin layer of release agent to avoid inhibiting cure. Spray the release agent into the air and let it settle gently onto the mold surface. Apply multiple thin coats rather than one thick coat for best results. Allow the release agent to fully cure for 15-30 minutes before pouring your silicone.
Mold Making Techniques
There are several key steps involved in silicone mold making:
Preparing the Master Model
The model to be molded, known as the master, needs proper surface preparation to allow clean mold release. Models should be made from clay, wax, plaster, concrete, wood, or other rigid material. Smooth, non-porous surfaces like metals or plastics may need a release agent. Porous surfaces can be sealed with paint or lacquer. Dust and debris should be cleaned off the model before molding [1].
Mixing and Degassing
The two silicone components must be precisely mixed together per the manufacturer’s instructions, usually at a 1:1 ratio by volume or weight. Thorough mixing and degassing are critical to avoid bubbles in the finished mold. Most silicones require a vacuum chamber to fully degas [2].
Pouring and Curing
The mixed silicone is gradually poured over the master in a mold frame or other container. Care is taken to avoid trapping air. The silicone then cures via addition or condensation chemistry. Cure times range from 30 minutes to 16 hours.
Demolding
Once fully cured, the flexible silicone rubber peels away from the master model. The mold is now ready to make resin castings.
Troubleshooting
Common issues like bubbles, inhibited cure, or mold deformation can be prevented with proper measuring, mixing, vacuum degassing, and adequate cure time.
Conclusion
The choice of the best silicone mold rubber depends on several factors, including the complexity of the mold, desired durability, heat resistance needed, and cost.
Here are some key points to summarize:
- Platinum-cure silicones offer the highest heat resistance and dimensional stability, but they are the most expensive.
- Tin-cure silicones strike a good balance between performance and price for most makers.
- Condensation-cure silicones are the most budget-friendly but have lower tear strength.
- For simple molds, a mass market silicone like Oomoo or Mold Max works well.
- For complex molds, a specialized silicone like Dragon Skin provides the highest quality.
- Test different silicones on small projects first before committing to large volumes.
- Always follow safety precautions and use mold release agents.
For beginners looking to make their first silicone molds, a tin-cure silicone offers the best value. Look for brands that have low viscosity for easy pouring and de-bubbling, as well as adequate working time. Carefully follow all directions for mixing and curing the silicone. Start with simple projects first before attempting complex molds with undercuts.
