RTV silicone
RTV Silicone (Room Temperature Vulcanizing silicone) is a type of silicone rubber made from a two-component system (base plus curative; A+B) available in a hardness range of very soft to medium - usually from 15 Shore A to 40 Shore. RTV Silicones can be cured with either a platinum catalyst or a tin catalyst. Applications include low temperature overmolding, making molds for reproducing, and some optically clear grades have lens applications.[1]
RTV silicone rubber applications
RTV Silicone Rubber will reproduce fine details and is suitable for a variety of industrial and art related applications including prototypes, furniture, sculpture and architectural elements. RTV Silicone Rubber can be used to cast materials including wax, gypsum, low melt alloys/metals and urethane, epoxy or polyester resins (without using a release agent).
RTV Silicones industrial applications include aviation, aerospace, consumer electronics and microelectronics. Some aviation and aerospace product applications are cockpit instruments, engine electronics potting, and engine gasketing. RTV silicones are used for their ability to withstand stress and temperature extremes.[2]
Types of RTV Silicones
- 1 Part Acetoxy Cure
- 1 Part neutral
- 2 Part Alkoxy ( Condensation ) Cure
- 2 Part Catalyst
- 2 Part Addition Cure
- Thermally Conductive
- Silicone Gels
- Encapsulants
- Mold Making Silicones[3]
Features of RTV silicone rubber
- Good characteristics of easy-operation
- Light viscosity and good flowability
- Low shrinkage
- Favorable tension
- No deformation
- Favorable hardness
- High temperature resistance, acid and alkali-resistance and ageing resistance
Operation instructions for making a mold by RTV silicone rubber
Correct mixing ratio
Component A is silicone rubber, and component B is curing agent or vulcanizing agent. Usually the mixing ratio will be 2-3%, e.g. to 100g silicone rubber should be added 2g catalyst and then mix them thoroughly. (Note: the weight of the silicone and catalyst must be weighed accurately). The following table gives suggested proportions at various temperatures:
| Temperature °C | Mixing ratio % | Notes |
|---|---|---|
| -5 | - | no reaction |
| 10 | 5 | - |
| 25 | 2 | - |
| 38-45 | 1 | - |
Preparation of the original molds
Because RTV silicone reproduces the surface texture of the original model exactly, the surface of the original to be molded must be finished: all faults must be removed, and it must be sanded or polished to the desired smoothness. If a smooth finish on the final casting is desired, the original must be polished. For a matte finish, polishing is not necessary.
Vacuum deairing
Vacuum deairing is a method of removing entrained air bubbles from the mixed silicone and catalyst. Air can be mixed in during the mixing of the silicone and hardener and, without Vacuum deairing, the bubbles may not be released from the mixture. If a large number of bubbles are present internally in the mold then the mold will have a lower tensile strength which, in turn, affects the reproduction times.
Considerations for making a mold with RTV silicone rubber
- RTV Silicone Rubber is white flowing liquid with general hardness of 22-40A° after curing.
- If the mold makers want to make large products and large size molds, the mold should be made of silicone rubber with high hardness. On the contrary, if the products require fine and precision pattern reproduction, it should be made of silicone with low hardness.
- The amount of the curing agent depends on the actual requirements of the customer. If molding requires fast drying and demold, more curing agent is needed. Contrarily, for molds requiring slower drying and demold, less curing agent is needed.
- It doesn't hurt to add silicone oil into silicone rubber. If a special soft silicone mold or fine detailed and complex patterned products are required, 5%-10% silicone oil can be added to soften the mold for easy demolding.
Advantages and Disadvantages of RTV Silicone Rubber
Advantages
RTV Silicone Rubber has the best release properties of all the mold rubbers, which is especially an advantage when doing production casting of resins (polyurethane, polyester and epoxy). No release agent is required, so there is no post-production cleanup. Silicones also exhibit good chemical resistance and high temperature resistance (400°F / 205°C and higher). High temperature resistance makes silicone the only mold rubber suitable for casting low melt metals and alloys (e.g. zinc, tin, pewter, and Wood's metal). The combination of good release properties, chemical resistance and heat resistance makes silicone well-suited for production casting of resins.
Disadvantages
RTV Silicone Rubber are generally high in cost - especially platinum-cure. They are also sensitive to substances (sulfur clay for example) that may prevent the silicone from curing (referred to as cure inhibition). Silicones are usually very thick (high viscosity), and must be vacuum degassed prior to pouring to minimize bubble entrapment. If making a brush-on rubber mold, the curing time factor between coats is long (longer than urethanes or polysulfides, shorter than latex). Silicone components (A+B) must be mixed accurately by weight (scale required) or they will not work. Tin catalyst silicones will shrink somewhat and do not have a long shelf life.
Certain types of RTV release acetic acid during the curing process, and this can attack solder joints, causing the solder to detach from the copper wire.