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RTV Silicone Rubber Sectional or Split Mould

RTV Silicone Rubber Sectional or Split MouldRTV Sectional Split Mould

Two piece moulds are needed when you have an undercut like seen in the picture to the right.

A split point must be established and effected. To the right you can see the piece partially submerged in the box. Essentially this can be done using plaster, clay or alginate. Note the four location pins to ensure the two halves mate correctly every time, use dowel or upvc or pu rods for this.You can drill out the plaster also.

Smaller items can also be attached to the side of the mould box. Instead of bedding the item in, you fill half way in the box with silicone before applying the release as below and keyways and continuing with the second half.

Mix your silicone as per the ratios marked on the cans. In case of a kit for the job add one part to the other and homogenize well to a solid colour with no streaks. ACC Silicones have a catalyst marker to show when mix is complete. Use a folding action rather than a whisking action avoiding air entrapment. If you have a vacuum or degassing chamber, use it now to remove any air from the mix.

Now start to fill the case from the lowest point allowing the silicone to flow up and over the former. If you couldn't degass the mixture then make the stream as thin and as slow as possible as this will aid air release.Fill the box to the top or at least 15 mm above the top of the former.

Once the first half has reached its demould time deconstruct the box and remove the "bedding material" (clay/plaster etc). Clean the piece but try not to disturb the silicone. At this stage you must apply a release agent. By far one of the best is Vaseline. Melt it over hot water, add a little white spirit to keep it liquid when it cools and paint it on, being careful not to paint the former or disturb the silicone. You can also add some IPA (Iso-Propyl Alcohol) to the mix to assist with viscosity, this flashed out quickly after application.


Other methods we have been told of but not tried (and therefore do not yet endorse) include DRY PTFE Spray.

Return to the box and fill to the top once the release agent has cooled. Once the demould time is reached for the second half you can start to deconstruct the box. Carefully separate the two halves, be careful of the location pins and not to tear or damage them, without these the mould is nigh useless.

Clean up the mould ready to accept a casting product and apply release system if needed. Bind both halves of the box together using tape or similar ensuring the location pins are correctly engaged. Some prefer to build a support case for this, we cover this in a later section. If you have a support case use that to hold the two halves together. Slowly pour in you casting media and allow to set/cure.

Smaller moulds can be clamped between thick card, thin pieces of 5mm ply or plasticard. Use rubber bands to hold the halves together and keep flashlines to a minimum.

Demould the finished cast after the published cure times have passed.

Image courtesy of and with permission from ACC Silicones. We also have trawled You Tube for the best videos and have decided to embed this one directly onto the page for viewing.