Silicone Moulding Rubber RTV and Addition Cure

Silicone rubber has been in use as a moulding material for several decades. These elastomers have natural properties which make them ideally suited for use as a moulding medium:

ACC Manufacture four main groups of silicone moulding rubbers

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   0. Click for High Tear Silicone Condensation cure Range
   0. Click for Low Tear Silicone Condensation Cure range
   0. Click for Addition Cure Moulding Rubbers

General Features

• Fine detailed reproduction
• Excellent release (no need for release agents)
• Durable
• Good tear strength and elongation
• Low shrinkage
• Simple to use

RTV silicone rubbers use two different types of silicone chemistry to form an elastomer: condensation cure systems which use a tin based catalyst and addition cure systems which use platinum based catalyst.  Although the end products can be very similar they do have some key differences which should always be considered when making your product selection.

Addition Cure vs Condensation Cure

The differences between the two systems should not be referred to in terms of positives and negatives but rather properties that will provide benefits for different applications.

Key differences:       

Addition Cure          

• Low shrinkage, below 0.1%
• Marginally higher tensile strength
• Slightly tougher rubber
• Need for careful and accurate mixing
• Platinum catalyst can be poisoned (*see note below)
• Good abrasion resistance
• Can be accelerated using heat 
• Tolerant to the addition of silicone fluid as a softener

Condensation Cure  

• Lower costs
• Broader product range
• Less sensitive to exact mix ratio
• Easy to use thixotroping agent available
• Accelerator catalysts available to speed up cure
• Mixing of grades possible to achieve desired hardness                                                                  

Where the application demands good dimensional stability, such as precision moulding, prototyping, making masters or very large objects, then addition cure would be the preferred choice however, the shrinkage levels for a good condensation cure are still very low at about 0.5%. The reproduction and release properties of both materials are excellent as is the transfer of very fine detail.

*The platinum catalyst used in all addition cures is susceptible to attack from certain chemical compounds which in turn will lead to inhibition of cure and results in a partially cured product.  Bringing the uncured material into contact with the following chemical compounds should be avoided during the manufacturing process: nitrogen, sulphur, phosphorus, arsenic, organotin catalysts, PVC stabilizers, epoxy resin catalysts, sulphur vulcanised rubbers and condensation cure silicone rubbers.

Catalyst's

Platinum

As already explained, addition cure rubbers use a platinum catalyst and the A and B parts are manufactured together as a balanced kit.  For this reason only use the A and B parts from the same kit and always weigh out and mix to the correct ratio.  The catalyst can be contained in either the A or B part of the system, as this may vary from supplier to supplier, it is important to check first if using a new material with automated dispensing equipment.  We strongly advise purging and cleaning equipment before changing to a new material to avoid cure taking place in the pump and pipe work.

Tin

Condensation cure or tin catalyst systems are not manufactured as a balanced kit so it is possible to use different catalyst from separated batches provided they are recommended for each other.  To adjust the cure speed it is possible to use catalyst with a higher tin content which will increase cure speed or you can add a booster or accelerator to a standard catalyst, but there are some precautions.

All ACC Silicones data for their condensation cure systems are based upon using a standard speed catalyst.  If faster catalysts are used there may be some changes in the long term physical properties of the cured rubber.  This may result in some post hardening and loss of tear strength over a period of months or years.

We do not recommend using higher amounts of catalyst than stated on the data sheets as this will result in result serious long term damage to the rubber.

New Code	Description	Feature	Colour	Mix     Ratio	De-Mould Time Hrs	Pot Life mins
HIGH TEAR CATALYST
MM TA2	Catalyst	Thixotroping Additive	Clear	100:2	N/A	N/A
MM Cat L8W	Catalyst	Slow Cure Clear	Clear	20:1	<24	120
MM CatL6W NT	Catalyst	Standard Cure Clear	Clear	20:1	<24	45
MM Cat B5 NT	Catalyst	Standard  Cure Blue	Blue	20:1	<24	45
MM Cat R5 NT	Catalyst	Fast Cure Red	Red	20:1	2	15
MM Cat W	Catalyst	Booster	Clear	100:1	1-2	15
LOW TEAR CATALYST
MM Cat W	Catalyst	Booster for rapid cure	Clear	100:1	1-2	15
MM Cat L5IV	Catalyst	Shoe moulds low hazard	Green	20:1	<24	45
MM Cat VEI	Catalyst	Shoe moulds fast cure	Green	20:1	2	15
MM Cat VE	Catalyst	Shoe moulds standard cure	Green	20:1	<24	45
MM Cat L5I	Catalyst	Leather reproduction fast cure	Clear	20:1	2	15
MM Cat L5	Catalyst	Leather reproduction Std cure	Clear	20:1	<24	45

Low Tear Rubbers

Resistance to tear is normally an important feature of any moulding rubber as it will enable the rubber to be stretched and pulled and retain is original shape, it also will ensure a longer usable life of the mould. To produce high tear resistant rubber requires careful selection of quality silicone polymers and fillers which are generally more expensive.  Some applications, which are very basic in shape and may only be used a few times, do not require high tear strength rubbers.  Two typical examples of these are the manufacture of shoe sole moulds/masters and the reproduction of leather.

In order to keep production costs to a minimum low tear silicone rubbers, such as the ACC 800 series have been formulated.  These are low cost rubbers with low tear strength but will provide high quality reproduction.

Special Effects

Film and theatre use silicone moulding rubbers to create props, special effects, animations, prosthetics and stage scenery.  Condensation cure rubbers, such as ACC 900 series, are used extensively to create plaster mock ups of interiors.

For example, a replica of a wooden panelled wall or plaster cornice in a stately home can be reproduced using ACC 900 series rubber with TA2, thixotroping agent or MM4400 moulding paste.  Plaster is then poured into the mould and after setting is finished to replicate the original wall on a stage set.  A similar process may be used to reproduce artefacts, sculptures, natural rock etc

General Moulding

Silicone moulding rubbers find applications for moulding a variety of materials, such as:

Material                      Application

Wax                            Candles and use in the lost wax process for bronze casting

Polyurethane                Mirror and picture frames, architectural mouldings

Plaster                         Covings and cornices, film studios, figurines

Polyester resin              Giftware, figurines, sculptures

Ceramics                      Earthenware pottery

Low melt alloys             Toys, models

Glass                            Auto windscreens

Stone filled resins          Replication of works of art

Rapid Prototyping

The fast and cost efficient production of accurate prototype parts is an essential ingredient for the effective introduction of new products in today's rapidly developing markets.  Technologies used to produce masters from 3D CAD images have revolutionised the prototyping industry over the last 10 to 15 yrs.  Once a master has been created there is usually a need for further copies to be made for ongoing evaluation and development.  A fast cost effective method uses a silicone mould into which quick curing resins are injected.

The silicone rubber used for this application is normally a translucent addition cure, such as ACC MM240TV.  A translucent material is used because it enables the master to be completely encased in silicone then using a knife, the silicone can be cut open to remove the master.  The mould can be reassembled and resin injected to recreate the original.  Being translucent it not only enables the cutting open of the mould but also provides a visual check to see if the resin has been effectively injected.

Prototypes are often used to assimilate the fit of several components, the use of a low shrinkage addition cure is essential.

Tampo Print Pads

Tampo print pads can be produced using either condensation cure or addition cure rubbers.  For a detailed discussion of the process please refer to the specific Tampo Print Pad page.