1.Setting the part up
This component gave a natural parting line and this should be expected. The component would have started life as some sort of moulding; mouldings need draft to release from the mould tool. Draft produces the parting line.
The parting line can be extended with Magic Tape (Scotch product name). The edge of the tape is coloured so it can be found more easily when cutting the tool later.
This technique helps avoid cutting in to the mould surfaces, in error when opening the tool.
but in this case the part was not my own and being unsure of the materials used strong washing liquid and water was trusted to ensure the silicone wouldn’t react.
2.Runners and Risers
In this case the part is set fairly flat; the item will be cast without vacuum by my friend. Being flat will help lift air bubbles of the “A” face. The A face is in this case a face! The A face will often be the lower surfaces of the mould.
The runner; resin goes in here, is a piece of 6mm rod and the risers; air and resin flow out here, are some 2x2mm plastic off cuts.
The runner is often best placed at one side of the part and the risers
at the opposite side. This part being fairly flat in the mould gave three “high” spots; these would trap air and need risers. The feed in placed centrally between these risers makes good sense – imagine poring a thick liquid on to a flat surface, a circular puddle is formed. The runner at centre will flow out equally within reason to the risers. At centre the part has a boss and rib feature for connection to the body of the figure; these make good flow features and give further reasoning to the runner position.
If the part were to be vacuum cast; I would have lifted the chin and
have used a single riser.
3. Set the tool
The runner is bonded to a support and the set is placed in a box (the bottom of a milk carton – I am cheap!).
Ensure a good wall will be produced around and under the part to be cast. 15mm minimum for this size object.
Note – the tape parting line has been trimmed; it was too close to the edge of the box initially.
4. Mixing and Degassing
The silicone needs to be carefully measured and mixed.
The ratio must be correct, 10:1 in my case. But for small volumes difficult,
use some good scales with appropriate resolution. When pouring, I prefer to pour catalyst first; so not get unmixed rubber in the bottom.
Mix in a oversize pot to aid degassing.
When degassing the trapped air expand and the volume increases to a large extent; the quickest way to degas is allow full expansion to take place (reaching low pressure before overflowing the pot). The air bubbles expand and burst; the silicone shrinks back to its starting volume in the chamber with vacuum.
When mixed the pot life to pour might be 1hr or more; full cure in 24hrs. A good 10min degas is possible; no rush.
5. Pour and degas
The technique for pouring; pick one spot and stay at that spot. I generally pour the silicone in to a deep section and then try to stay at that position,;
this allows the silicone to flow around the surfaces of the part pushing air our, minimising trapped bubbles.
If working with delicate supports or parts it is often necessary to pour
equally around the master part. The silicone is very viscous and will break
masters from the support if heavy handed or flowing at one side.
The issue with moving the pour point around is over-layering the silicone; trapping more air between layers and the need to degas further.
I always degas again, this can be more difficult as the tool boxing / shape might not be as tall as the first
degassing pot. It will be harder to achieve full vacuum without overflowing the box; the box is lower just to keep the runners and risers short.
6. wait and cut open
The full cure with most RTV silicone materials will be around 24hrs at a decent temperature and the outside cures first.
The material is an insulator so the inside and mould surfaces might not be cured even if it feels good at the outside. Be patient.
A good technique to aid final cure (after 24hrs) is to use an oven – a resin warming oven or such at 30 or 40 degrees C; not your cooker! The metal runner will conduct heat towards the centre of the silicone tool assisting the cure.
Have a good look at the split line (that black edge has some meaning now). Check the tape is still attached to the master.
Initial cutting of tool should be a wavy (but smooth) single cut all the way around the four outer edges of the block. This cut should be one scalpel blade deep and can be (just about) seen in the photo.
Parting pliers can be used to gain access so that the cut from wavy edge to tape edge can be made, this cat should be smooth without jagged "stabs". Any jagged cuts now will hold the tool apart when in use.
When cut the tool can be cleaned, remove all bits of cyano, tape, bits runner etc.