You have done everything by the book: carefully weighed and well mixed. Yet when demolding, you discover that the silicone rubber on the surface of your product is still liquid or tacky. This phenomenon is called cure inhibition. It is a chemical reaction that is blocked by external elements. In this blog we explain how to defeat this 'invisible enemy' and still make your project a success.
Addition silicones versus sulfur and phosphorus
Cure inhibition mainly occurs with addition silicones (also called platinum silicones). These high-quality rubbers are extremely sensitive to certain elements, such as sulfur and phosphorus. As soon as the liquid silicone comes into contact with these substances, the curing process stops immediately at the contact surface. These disruptive elements are often present in:
- Certain types of clay, such as Nakiplast modeling clay.
- Certain sealants or adhesives.
- Latex gloves.
- 3D-printed resins (UV resins).
3 solutions to fix cure inhibition
1. Use a different base (clay or sealant)
The most sustainable solution is to eliminate the source. Use only sulfur-free clay, such as Nakiplast modeling clay, or sealant of which you are certain it will not react with addition silicones. Always check the technical data sheet of your auxiliary materials.
2. Treat the object with spray varnish
Do you still want to work with a model that contains sulfur or has a porous surface? Then apply a barrier layer. By carefully spraying the object with a quality spray varnish (for example an acrylic lacquer), you seal the disruptive elements. The silicone will then no longer come into direct contact with the sulfur or phosphorus, allowing the curing to proceed unhindered.
3. Switch to condensation silicones (tin silicones)
If the problem of cure inhibition persists, switching to condensation silicones (tin silicones) is the most reliable solution. This type is considerably less sensitive to external substances and almost always cures, regardless of the surface. In addition, this type of silicone is often more affordable and less error-prone to use. However, be aware of the downside: condensation silicones have slightly higher shrinkage than addition silicones, are not suitable for food contact and have a shorter lifespan. It is therefore a good alternative, provided that extreme precision and food safety are not your highest priority.
Always test the curing
Prevent wasting a large amount of material by making a small test batch in advance. Mixing just one drop is very difficult and often inaccurate; therefore mix a small, measurable quantity (for example 10 grams of each component with a 1:1 silicone). Apply a small part of this mixture to an inconspicuous area of your master model. Does the silicone cure properly after the prescribed time without remaining tacky? Then you can confidently pour the full mold.
Need help?
View our full silicone rubber troubleshooter for more in-depth information or contact us directly for personal advice by phone, chat or email. Together we will make your project a success!
