|
|
|
|
|
|
|
PROCESSING GUIDE FOR
RHODORSIL® RTV-2'S |
|
|
|
Remix each of the 2 components of the Rhodorsil® RTV-2 every time before using. Mix them thoroughly together in a clean recipient in the recommended proportions, either by spatula or using a mechanical mixer on a low speed setting so as to limit the inclusion of air in the mixture, until a perfectly uniform mixture is obtained.
In order to determine the uniformity of the mixture, it is possible to color the RTV using a color base recommended for Rhodorsil® RTV-2's. It is recommended to ensure that the coloring of the RTV is compatible with the final application of the membrane |
|
|
|
Degassing |
|
|
|
The catalyzed mixture is degassed under a vacuum of 20 to 50 mbar, releasing the vacuum once or twice during the operation; the RTV will expand under the action of the vacuum and it is therefore necessary to have a recipient of an appropriate size, if possible, with larger width than height (l=3 to 5 times h).
Degassing generally lasts several minutes and at least until the bubbles stop rising regularly to the surface when the vacuum is released. However, prolonged degassing can cause the departure of certain components that are required for curing.
The catalyzed RTV is poured in under gravity to the lowest part of the mould (it may be advantageous to tip it slightly) avoiding trapping of air.
We can also, in the case of skin moulding, pour under low pressure in order to reduce the processing time; this is the case when producing membranes in large quantities or that have large dimensions. |
|
|
|
Curing with heat |
|
|
|
RTV-2's shrink least at room temperature. Above 50°C the shrinkage of the membrane increases significantly. This must be taken into account for polycondensation RTV-2's, in cases in which we want to increase the production rate of moulds, by increasing curing temperature.
1- FOR POLYCONDENSATION RHODORSIL® RTV-2'S
Heating to accelerate curing is possible up to around 50°C. Beyond this temperature, the risk of the membrane bubbling is significant. 2- FOR POLYADDITION RHODORSIL® RTV-2'S
There is very little shrinkage of polyaddition RTV-2's (±0.1 to 0.3%). If the production rate is to be increased, we can accelerate curing by heating up to 150°C. This significantly shortens curing time:
|
|
|
|
|
- 24H at 25°C or
- 4H at 65°C or
- 1H at 100°C or
- 1/2H at 150°C |
|
|
|
When heating, it is necessary to take into account, the volume expansion coefficient of the RTV-2 as a function of temperature (see the technical data sheet). In any case, after curing at 100°C and beyond, we can observe dimensional variations related to differences in the expansion coefficient of the silicone and of the materials making up the casting mould and the model. We can therefore lose the benefit of the very small shrinkage of this type of Rhodorsil® RTV-2 at room temperature. |
|
|
|
Inhibition |
|
|
|
The risk of inhibition is generally only seen with polyaddition type RTV-2's. Inhibition is seen as a curing fault of the RTV that is more or less accentuated on contact with certain materials containing amines, sulphur, tin or heavy metals (e.g. barium or cobalt);
natural and synthetic rubbers that are vulcanized using sulphur, polyester resins and sealants, flexible PVC's, polycondensation RTV-2's, chloroprene glues, certain modeling putties, certain adhesive tapes etc. Precautions would be;
|
|
|
|
|
- Clean the model with a decontamination agent diluted in water, according to the manufacturer's recommendations, then wash with a brush and rinse off with water (this treatment may not be effective on absorbent surfaces)
- Prepare the materials to be used (plastiline, brushes, spatulas, etc.)
- If in doubt it is recommended to perform a test on a sample
- If necessary use a protection agent such as polyvinyl alcohol or an acrylic varnish or codex Vaseline grease. |
|
|
|
PROLONGING MEMBRANE LIFE |
|
|
|
Firstly it is recommended to cure in a relatively isothermic room (around 25°C) for dimensional consistency. It is important to comply with a storage time of the membranes after they are demoulded. For example polycondensation RTV-2s' mechanical properties are only achieved fully after 4 days curing at 23°C RH 50%.
This time is only for 24H polyaddition RTV-2's. It also goes without saying that the care taken over producing the mould can contribute to the length of time that the membrane will last. It is therefore right from the mould design stage the quality and accuracy of membranes are determined. |
|
|
|
Protective Agents |
|
|
|
We can classify the materials used for reproduction in terms of increasing chemical aggressiveness relative to RTV-2's |
|
|
|
|
- Non-aggressive: plaster, wax, stearine etc.
- Slightly aggressive: low melting point alloys, cement etc.
- Quite aggressive: semi-rigid polyurethanes, filled polyesters etc.
- Very aggressive: epoxies, rigid polyurethanes, non-filled polyester etc. |
|
|
|
Protection of the membrane is therefore specially recommended for the moulding of epoxies and polyurethanes. Therefore the choice of a polyester or a quick curing polyurethane resin is beneficial for the membrane.
In certain applications, this function is ensured by applying a filmogenic or powder based finishing product before moulding which by transferring to the moulded product (barrier varnish, bronze powder) will protect the membrane. It is also beneficial to leave the membranes to rest at a moderate temperature (around 60°C) for a shift (i.e. the night) to enable volatile substances (e.g. styrene) to evaporate. |
|
Site Map 
