How do I Make a Plug Section Mould?

Tip: before measuring out your mould making material it is helpful to pour water into the vessel first to work out how much you will need bring the mould to the required level (usually 10-15mm covering above your object)

Remember to place your object to be cast in the vessel with the water in order to displace the excess water, we then carefully pour out and measure the volume of water remaining. This is a good way to ensure only minimal wastage occurs.

Of course this method is only possible if both object and vessel are compatible with water, so check with your technician first.

Using silicone for this will ensure we retain complete surface detail.

To demonstrate this I will use a real example, as follows.


1.Place the glass rim down on a board of MDF or similar and draw around the rim with a pencil. Remove the glass. Several millimetres away from the pencil line, using a 10mm drill bit, drill 8-10 very shallow holes into the board. These must be no deeper than 5mm.

2.Using a hot glue gun, glue around the rim of the glass and turn it upside-down onto a board that will act as your base. Ensure there is a seal between the board and the glass. Use a small amount of wet clay to help achieve a satisfactory seal.

3.Using a rigid sheet of either thin aluminium (litho plate) or vinyl, create a loose roll that will enclose the glass leaving a 10-15mm gap between the outer most extremity of the glass and the aluminium roll. Glue this in place with more hot glue around the base and up the side where it is joined. Again ensuring a seal here is very important.

4.Now we can start to mix our mould-making compound. In this case we will use silicone (see, Silicones). We must make sure to give the inside of the mould a quick spray with release agents as glass has been known to stick to silicone on occasion. Once the silicone is mixed we can use the degassing machine to evacuate the air bubbles, giving us a superior quality mould.

5.As always we pour our mould making material from the lowest point of the mould, up. This makes sure we do not trap any air in the moulding. Continue pouring until the glass is completely submerged with an extra 10-15mm extra on top. You can check the level by dipping a stick in and feeling for the glass.

6.Once cured, the aluminium roll may be removed. Peel the silicone (and glass) off the base board taking care not to pull the silicone away from the glass. Turn the whole thing (minus the base board) the right way up and replace the roll of aluminium. Make sure this is fitted as closely as possible to the silicone and that it raises above the silicone by a good 40-50mm.

7.Seal the edge between the glass and the roll with Vaseline. Also seal the edge between the silicone and the glass. Ensure any remaining exposed silicone surfaces have a coating of Vaseline. Take extra care and time over this stage as this will be crucial to the entire moulds’ success.

8.Spray the inside the glass with release agent, then mix more silicone. De-gas it and pour into the lowest part of the glass allowing it to fill the glass completely and run over the top until the rim is covered by at least 20-30mm of silicone.

9.Once fully cured, peel away the aluminium roll and pull apart the two adjoining sections of the mould. Suction will hold the glass in the mould so a little wriggling and squeezing is sometimes necessary to completely remove the glass from the silicone.

10. Notice that the holes you drilled in part 1 now form part of a key to lock the top section of the mould in place when casting. These keys are also the bits that keep the mould distorting, essential for retaining a uniform thickness of material.

11. You are now ready to cast with this mould.

Pour in your chosen casting material and squash down the top (plug) into the lower part forcing the casting material up the sides and out over the edges. Rotate the top section as you do this to ensure the keys lock into position and no air bubbles are trapped.


Plug Section Mould


What is a Plug Section Mould?

A plug section mould is technically a two part mould (or multipart mould) with a specific purpose, made in a particular way where it is possible to cast the inside and outside of an open compound form, such as a glass or cup.

This type of mould making is the best way to reproduce castings with thin or delicate shapes. It is also a great way of capturing surface detail on the inside and outside of an object.


Resin casting in a Plug section mould


Plug section mould of a glass


Inserting the plug of a plug section mould


Inserting the plug of a plug section mould