Platinum-cure silicone is a widely used material for mold making. It offers several advantages over tin-cure silicone, such as low shrinkage, low viscosity (resulting in fewer bubbles), odorlessness, and safe skin contact, making it the preferred choice for many users. Nonetheless, platinum-cure silicone has a significant drawback - it cannot cure upon contact with substances like acids or bases. If you have carefully followed all the operating instructions and the liquid silicone in a specific area fails to cure, inhibition may be the underlying cause, which is also referred to as silicone poisoning.
sticky silicone mold
1. Inhibition Principle
Addition-cure silicone relies on platinum as a catalyst to drive the curing reaction. When Part A and Part B are combined, the platinum catalyst initiates a reaction that affects the curing process. However, platinum can lose its activity when exposed to certain chemicals, leading to a condition known as inhibition, where the affected area may remain sticky or tacky. The platinum content in different addition-cure silicones may vary, and silicones with lower platinum content are more susceptible to inhibition caused by specific chemicals, materials, and elements.
Once addition-cure silicone becomes contaminated and undergoes a poisoning reaction, it is unlikely to fully cure. While addition-cure silicone can be rapidly cured through heating, contaminated silicone cannot be cured even at elevated temperatures. This is due to the permanent and irreversible effect of contaminants adhering to the platinum catalyst.
2. Inhibition Characteristics
The characteristic of inhibition is that the surface of the cured silicone mold may appear completely solidified. However, when demolding, the area in contact with the model remains tacky if it has been contaminated. Once this occurs, the poisoned area will not cure, requiring the mold to be removed and remade. Typically, the effects of inhibition on the silicone curing process are limited to specific contaminated areas and do not spread extensively into the liquid silicone itself. If other areas of the mold, aside from the surface in contact with the model, are uncured or tacky, it may be due to other factors such as inaccurate mixing ratios or inadequate mixing.
3. Inhibiting Substance
When working with addition-curing silicone, it is essential to prevent the use of materials that can cause platinum inhibition, as this can hinder the proper curing of the liquid silicone. Below is a list of chemicals, materials, and elements that may inhibit the curing of addition-curing silicone. If your addition-curing silicone fails to cure or remains tacky even after the specified curing time, please check for any contact with the substances mentioned below.
- Sulfur (S): Sulfur is likely the most common poison for platinum, so avoid using sulfur-curing materials such as sulfur-based clays, latex chloroprene rubber, butadiene rubber, and natural rubber.
- Tin (Sn): Found in organic silicone adhesives, condensation-cure silicone (tin-cured silicone), certain types of PVC plastics.
- Nitrogen Compounds: Such as amides, amines, nitriles, cyanates, etc.
- Certain metals like silver, tin, lead, and mercury.
- Some paint products and residues of soldering flux.
- 3D-printed UV curable resin models.
- Chlorinated solvents.
- Acetone, Butanone（MEK）.
- Adhesive tape (e.g., duct tape).
- Coatings, paints, solvent carriers.
- Gel coats.
- TDI urethanes, polyester paints, urethane caulking.
- Composite pre-preg.
Please note that the substances listed are provided for reference purposes only. If you are uncertain whether specific materials can cause inhibition in your silicone, it is recommended to conduct tests. These tests should be carried out in inconspicuous areas on the master model to check for any potential inhibition of the curing process. If the liquid silicone cures properly during these tests, it indicates that no inhibition has occurred.
4. Experience Suggestions
If you are using both condensation-cure silicone and addition-cure silicone, it is crucial to keep them separate. Despite both belonging to the RTV-2 silicone category, they have distinct chemical compositions, and addition-cure silicone is particularly susceptible to contamination from condensation-cure silicone, leading to curing inhibition. To prevent curing inhibition, it is recommended to use dedicated tools and containers for each type to prevent cross-contamination. It is also important to store different silicone types in clearly marked, separate areas to prevent any mixing or contact between them.
5. How to Prevent Cure Inhibition
1. Ensure that the pigments, thickeners, diluents, curing-slowing agents, curing-accelerating agents, and other additives you are using are free from substances that can cause platinum poisoning.
2. If your project's application is suitable for tin-cured silicone, it is advisable to use tin-cured silicone for mold making. This type of silicone rubber offers broader material compatibility and is less susceptible to the effects of cure inhibition.
3. Thoroughly clean and allow the master model to air dry before using it.
4. If it's not possible to avoid contact with inhibiting substances, restrict their direct interaction by applying a primer to form an isolating layer, preventing any contact that could lead to inhibition. For example, applying one or multiple layers of clear acrylic varnish on the surface of the model is often an effective method.
