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Plug, Core and Planter Moulds

Making a Planter or Core Mould

This mould type is often referred to a Plug Mould or Core Mould. They essentially have an outer and an inner side that need moulding together.

In the case of a plant pot or planter, this is not too difficult to achieve!

Gather a baseboard and put your planter onto the board. Do not fix at this stage, we need to construct the top half. Measure up and construct a shaped mould box that follows the shape of your planter. Why? Because you will save a ton on mould materials! A square box for this is pointless (unless the planter is square of course!). Dry construct the top box and seal with either clay or hot glue.

Now on the baseboard, put a layer of clay, around an inch thick. Make sure it is FLAT! Borrow the rolling pin from the kitchen and if its getting sticky, use some french chalk (like flour to dough, french chalk to clay!) and place your planter firmly onto this. Make a keyway for the second half now, it saves cutting cured rubber later, a paperclip works well for an oval shaped like keyway. Now apply a release agent. Wax will work well for most things, silicone may not even need one.

Offer up the top mouldbox. Trim clay as required so it will seat to the bottom board and seal.

You are now ready to pour stage one. Mix your RTV or Urethane Elastomer and pour into the STOP SHORT as this will become your pouring point when casting. Allow the material to fully cure, 72 hours advised with tin cures to lose the shrink!

Now turn over the assembly and remove the base board. You will find this fairly easy. Remove the clay and clean up.

In the video below they recommend  cutting in a keyway for the top plug. I would recommend 2 or more. Apply a release agent, in the case of silicone, a mix of vaseline and white spirit work well. Cover all the silicone, and in the case of using PU, make sure you have good cover!

Here is where our method differs a little. Primarily due to using a Silicone.

Pour the second half to the top of the mould box. Allow to cure. Now replace the baseboard which will be the plug holder and adhere it to the rtv with some acetic silicone (similar to bath sealant). For a commercial tool this isnt likely to last too long. In that case, before the rtv sets on the second pour, you can add some nuts, bolts and washers through the baseboard, exposed on the other side for the rtv to flow around and lock into. Same end result really!

Once everything is cured, remove the mould boxes and expose the former by slitting the rtv. RETAIN the mould boxes as they are your support cases and plug guides.

Finally put everything back together and choose a casting media. Plaster, Polymer Modified Plaster, Cementious Mixes and Fibrocem are just some of the options. You could even make a cold cast resin version to simulate a cast bronze planter! The choices are endless!

We have a brilliant movie to watch for this, made by Smooth On and published on You Tube. They use a Polyurethane Elastomer, whereas we would recommend use of a fairly hard silicone, but this is pure choice! The PU will outlast the RTV in a commercial environment!