Silicone Moulding Rubber RTV and Addition Cure

Oxford Round silicone mould

ACC manufacture three main groups of silicone moulding rubbers. They have easy mix ratios, cure at room temperature and offer fine detailed reproduction.

    Mouse Click 2 for High Tear Silicone Condensation Cure Range

    • High tear
    • Well suited to fine details and extensive undercuts
    • Durability
    • Excellent release (no need for release agents)

    Mouse Click 2 for Low Tear Silicone Condensation Cure Range

    • Lower costs
    • Suitable for less inticate work

    Mouse Click 2 for Addition Cure Moulding Rubbers

        • High tear
        • Very low shrinkage
        • Ideal for reproducing masters
        • Can be heat accelerated

        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.



        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.


        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
        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
        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 EffectsTA2 thixotroping agent silicone rubber

        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 MouldingSilicone mould with plaster building

        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 Silicone tampo print padopen 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.


        ACC Silicones Ltd, Amber House, Showground Rd, Bridgwater, TA6 6AJ
        Tel: +44 (0) 1278 411400 Fax: +44 (0) 1278 411444