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27 April 2016
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Silicone Technique
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What is RTV silicone?

 

Excerpt From Wikipedia

Introduction

RTV Silicone (room temperature vulcanization 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 a catalyst consisting of either platinum or a tin compound such as dibutyltin dilaurate. Applications include low-temperature overmolding, making molds for reproducing, and lens applications for some optically clear grades.

Applications

To produce the material, the silicone rubber is mixed with the curing agent or vulcanizing agent. Usually the mixing ratio is a few percent. In order for the RTV silicone to reproduce the surface texture, attention is paid to the cleanliness of the original. Vacuum deairing removes entrained air bubbles from the mixed silicone and catalyst to ensure optimal tensile strength, which affects reproduction times. In casting and mold making, RTV silicone rubber reproduces 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 mechanical and thermal stress.

Features

Good characteristics of easy-operation
Light viscosity and good flow-ability
Low shrinkage
Favorable tension
No deformation
Favorable hardness
High temperature resistance, acid and alkali-resistance and aging resistance

Advantages and disadvantages

RTV silicone rubber has good release properties compared to mold rubbers, which has an advantage when doing production casting of resins (polyurethane, polyester, and epoxy). No release agent is required, obviating post-production cleanup. Silicones also is good chemical resistance and high temperature resistance (205 °C, 400 °F and higher). For this reason silicone molds are suitable for casting low melt metals and alloys(e.g. zinc, tin, pewter, and Wood’s metal).

RTV silicone rubbers are however generally platinum-cure. They are also sensitive to substances (sulfur-containing modelling clay such as Plastilina, 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 do not work. Tin catalyst silicone 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.

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