The material choice is much more important for combustion-driven actuators than for others. No company actually tests their silicones under simultaneous stress (i.e. thermal and mechanical). Therefore, silicone data sheets only tell half the story. However, we found several silicones that are able to withstand thermal and mechanical stress up to 30,000 combustion events. We'll start with some information about our selection criteria.
We use room temperature vulcanizing (RTV) silicones from Altropol GmbH*. They offer a wide range of two component silicone systems.
*Note: Like all of our guides, this was written by contributors from their own experience. Users in other countries may have to search for alternative suppliers.
The following points largely influenced our material selection:
- Handling Viscosity (HV) – Since we will later press the uncured silicone mixture inside our lost-wax-like molds, we need a mixture with a viscosity below 60 Pa·s (that of thick Maple Syrup). Otherwise, we need to apply larger pressures in order to fill the molds within the given handling time.
- Handling Time (HT) – The mold filling process takes, depending on the size of the mold, up to 80 minutes (for some silicones you will reach the handling time limit well before this). A rule of thumb from our experience: you need about 10-15 minutes per 100 cm3 including all preparation steps.
- Maximum Elongation at Break (MEB) – This value indicates if a material actually is able to withstand a combustion event. We suggest choosing a material with at least 500% MEB. Since thermal stress seems to decrease this value, a higher MEB would be preferred.
- Shore A Hardness (SA) – This value indicates weather a silicone is stiff (Shore A = 100) or very flexible (Shore A = 0) by measuring the depth of metallic cylinder being pushed into the surface with a force of 25 N. The higher the Shore A hardness the more violent the actuation.
Recommended Material Types
- RTV 1701 (HV = 22 Pas, HT = 80 min, MEB = 1000%, SA = 23)
Note: Condensation based systems will shrink with time. This is because the condensation product, a short alcohol, evaporates from the silicone.
We achieved over 1,000 combustion events using the RTV 23 and over 30,000 for the RTV 1701. The exact handling of the presented silicone types is described in the fabrication section.