Rubber O-Ring-Silicone RTV Rubber Moulds And Mould Making Compounds

RTV-2 silicone rRubber Gasketubbers are pourable， spreadable or kneadable compounds， which will crosslink in the presence of a second component to form flexible rubbers。 They are outstanding as mould-making materials because of their excellent properties:

• Easy to use

• Excellent release properties

• Accurate reproduction of surface details

• Good heat resistance

• Little or no shrinkage
ubbers can also be use for encapsulation and sealing， or for making flexible parts。
ubbers always consist of two components， namely the rubber base itself and a catalyst， or the components A and B。 They are cured or vulcanised at room temperature (RTV= room temperature vulcanising)。

Different grades are available， with different properties suitable for a wide range of applications。 They can be classified according to the following criteria。

1。 Vulcanising system (condensation-curing or addition-curing)

2。 Consistency (pourable， spreadable or kneadable)

3。 Mechanical properties of the curedCustom Mold Rubber Parts rubbers (e。g。 tear resistance and elasticity)

Condensation-curing RTV-2 silicone rubbers

These cure at room temperature after addition of catalyst， volatile alcohols being liberated during the reaction。 Shrinkage occurs， which is completed after about a week。 Although not very effective， the vulcanising process can be speeded up by heating。 The temperature should under no circumstances exceed 70oC。 It is also most important that freshly made moulds， i。e。 which still contain the alcohol produced during the curing reaction， are not subjected to temperatures higher than 80oC， since this would cause a loss of elasticity and softening or stickiness of the silicone rubber (see also Mould pre-treatment)。

Addition-curing RTV-2 silicone rubbers

These vulcanise without the formation of reaction products， so that there is practically no shrinkage。 The cured rubbers can therefore be used immediately after they are fully cured。 Vulcanisation can also take place at higher temperatures (up to 200oC) which greatly reduces the required curing timeRubber Seal。 In this case however， one must take the high coefficient of thermal expansion of silicone rubber into account。

Cleanliness is especially important when processing addition-curing RTV-2 rubbers， since the platinum catalyst can lose its power through contamination。 Catalysts used for condensation-curing silicone rubbers， for example， can interfere with vulcanisation already in small amounts。 Furthermore， vulcanisation of addition-curing rubbers can be delayed or completely prevented if they are in contact with certain materials such as natural and synthetic rubbers， PVC， amine-cured epoxy resins， plasticine， adhesives， soldering tin etc。 Preliminary tests are therefore vitally important。

Storage stability

RTV2 silicone rubbers will remain in usable condition for at least 12 months， provided they have been stored cool and dry， in tightly closed containers。 The T series of catalysts have a shelf life of at least 6 months in closed containers， and should be protected from heat and moisture。 All containers should be tightly re-sealed every time some of their components have been removed。
ubbers and food regulations

Finished products made from addition-curing silicone rubbers are suitable for food contact applications in compliance with food regulations。

Condensation-curing RTV-2 silicone rubbers

These are vulcanised by adding a liquid or paste-like catalyst。 The pot life of the resultant mix， as well as its vulcanising time， will vary between a few minutes and several hours， depending on the type of catalyst and the amount used， the ambient temperature and relative humidity， as well as the amount of movement in the air。 Optimum characteristics of the cured rubber can only be achieved if a suitable catalyst is used in the prescribed amounts。

Addition-curing RTV-2 silicone rubbers

Here the mixing ratio for components A and B is fixed and must be adhered to precisely in order to achieve the desired end product properties。

Preparation of components

All pourable components should be stirred thoroughly before use to ensure even distribution of the filler。 Stirring also improves flow in higher viscosity systems。

Mixing of components

Even mixing of the rubber base and catalyst (in the case of condensation-curing rubbers) or the components A and B (in the case of addition-curing rubbers) is easily achieved for pourable and spreadable compounds， using a spatula。 Larger batches are best mixed with a mechanical stirrer。 Kneadable compounds can be mixed by hand， on the rolls or in a compounder， depending on the size of the batch。

Removal of entrapped air

To ensure bubble-free moulds， pourable mixes should be deaerated under vacuum。 The catalysed mix is poured into a vessel so that it is no more than a quarter full。 A vacuum of 10 - 20 mbar is then applied which initially causes the mix to rise， usually collapsing before the vessel´s rim has been reached。 If， however， there are signs that the mix will run over， a small amount of air is introduced and the process is repeated until the mix collapses。 Evacuation is discontinued immediately afterwards， in other words the mix should under no circumstances be evacuated until no more bubbles form， since certain substances essential for vulcanisation to take place could escape under these conditions。

Application to the pattern

A thickening of the catalysed mix marks the start of vulcanisationRubber O-Ring。 The material should be used well before the pot life has expired， i。e。 whilst it is still in a free-flowing state which makes application that muchRubber Parts easier。 Every increase in temperature reduces the pot life。