This issue is known as cure inhibition. Cure inhibition happens when a surface contaminant prevents the material from curing properly.
Inhibition When Making a Mold
Cure inhibition occurs when contaminants on a model’s surface interfere with the liquid mold rubber’s curing process. While the rubber may appear fine on the outside, it will remain sticky at the interface between the rubber and the model’s surface, indicating that it hasn’t fully cured.
For instance, if you apply latex, urethane, or silicone rubber directly over a model made of sulfur-containing clay, the sulfur in the clay will prevent the rubber from curing. While the rubber may look fine externally, any areas where it contacts the clay will stay sticky and uncured.
Remedy: Apply a suitable sealer.
Common Causes of Cure Inhibition
- Not Allowing a Sealing Agent to Dry: If a sealing agent isn’t fully dried or allowed time to “flash off” before applying the release agent or mixing and applying mold rubber over your model, it can cause issues with curing.
- Solution: After applying the required coats of sealing agent, let it dry thoroughly (at least 30 minutes) before applying a release agent (if needed) or mold rubber.
- Humidity Impact: Humidity affects the drying time of solvents. In a humid environment (80% RH), the solvent in sealers can take a few days to dry or may never fully dry. If you use a solvent-based sealer like shellac or acrylic, and then apply a liquid release agent, the solvent in the release agent can “re-liquify” the sealer, extending the drying time.
- Incompatible Mold Rubbers: Some mold rubbers are not compatible with others. For example, condensation cure silicones will not cure when applied over addition cure silicones, and vice-versa.
Preventing Cure Inhibition
To minimize the risk of cure inhibition, always consult the technical data sheet for the products you’re using. It will typically provide guidance on conditions that may cause inhibition and offer recommendations on how to prevent it. For instance, applying a proper sealing agent and ensuring it dries fully can help avoid issue