Silicone mold for concrete: tear resistance, Shore hardness and release agent
Concrete casting is tough to mold: heavy, often undercut parts, alkaline material, and numerous impressions. A silicone mold must therefore be, above all, tear-resistant. This article shows what matters when choosing materials, hardness, and release agents.
To the point
Choose a tear-resistant mold silicone with medium hardness (approximately Shore A 20–40): soft enough to gently demold even undercut concrete parts, yet strong enough for durability and detail accuracy. A release agent will significantly extend the mold's lifespan.
What a concrete form must withstand
Concrete is heavy, abrasive, and alkaline, and textured or undercut geometries put stress on the mold with each demolding. Three properties determine its service life:
- Tear strength and tear resistance – the most important property; it determines how many impressions the mold can withstand, especially with thin ribs and undercuts.
- Shore hardness – too soft causes warping and edge loss, too hard makes demolding difficult. For concrete, the range is usually Shore A 20–40.
- Low shrinkage – for dimensionally accurate, detailed castings.
Choose the right hardness
| component | hardness | Products |
|---|---|---|
| Intricate details, many undercuts | softer, Shore A ~20 | MF-Flex 20, PC-Flex 20 |
| Standard concrete elements | medium, Shore A ~30 | MF-Dura 30, PP-Flex 12 |
| Large, heavy parts, long service life | firmer, Shore A 35–40 | MF-Dura 35, MF-Dura 40 |
Release agents and processing
Even though concrete doesn't adhere to silicone, a release agent protects the mold from leaching and adhesion and extends its service life – for example, PVA film release agent or a liquid release agent; the selection section covers release agents for mold making. Pouring should be bubble-free (vacuum or thin initial coating), and the mold should be allowed to cure completely. Basic information on material selection and processing can be found under Silicone for Mold Making; for application in model making, see RTV-2 Silicones in Architectural Model Making.
Frequently Asked Questions
What Shore hardness is needed for concrete molds? Usually Shore A 20–40, depending on the level of detail and component size – softer for undercuts, harder for large, heavy parts.
Why does my mold tear so easily? Usually, it's due to insufficient tear resistance or the material being too soft, especially for delicate geometries. Using a tear-resistant mold and a release agent will extend its lifespan.
Do I need a release agent? It's recommended – it protects the mold and significantly increases the number of impressions.
Consultation and samples
Tell us the component, level of detail, and quantity – we'll recommend the appropriate hardness, type, and release agent, and provide a sample. Contact us or write to info@silitech.ch.
