Single part split moulds have a few advantages over two part moulds:
- Simple and easy
- Faster to make than a two part mould
- Minimised number of mould lines
For this tutorial I've drawn up some nice little pictures to demonstrate the process of casting a gun - I will add some actual pictures at a later date when I'm back home. The technique I'm describing is only for a single part being moulded, however in production I scale the technique up to include several parts on the one sprue.
Step 1: Preparation
In a two-part mould, the master part would usually first be embedded in some plastecine clay to create the part line. Into this clay would also be placed the major resin pour and air channels. With the split mould there is generally just the one pour channel through which the resin enters and air escapes (secondary air channels can be created but this will be discussed later). In order to make the mould, the master part is temporarily glued to the pour channel, which is attached to a cone shaped piece into which will create the funnel into which the resin will be poured.
It is important that the join between the pour channel and the master part not be too strong, as they will need to be separated during the demoulding process. I often use a bit of sprue from a styrene kit as my pour channel.
Note that in the picture I've shown an alternate configuration, where a large block is placed instead of the cone shape. I use this configuration because I use vacuum as part of my casting process: when you pour the resin into the mould and then place it under a vacuum, the resin temporarily expands up out of the mould due to the degassing process. This reservoir hold the resin during the degassing, preventing it from spilling out of the mould. Note that the reservoir has to be sized appropriately according to the volume of the part.
Step 2/3: Mould Box and Silicone
With the master prepared and attached to the pour channel it's now time to build a mould box around it. Lego is the best medium for making mould boxes. Period. I build up the Lego box to leave at least around 10mm around each side of the part. Note that making the mould wider in the direction perpendicular to how it will eventually be cut will help minimise issues with misalignment during casting.
Mount the mould box on something at least semi-rigid. I then stick a layer of clay around 3-5mm thick in the bottom of the mould box. This is sealed up against the Lego bricks to prevent any silicone leaking out the bottom. I then push the base of the pour channel into the clay, making sure it won't easily come out.
Another trick I like to do is build the Lego mould box in two sections: the first ring that is only two bricks high, with which the clay is placed and master part attached. The remainder of the mould box height is built as a second ring and placed to the side. By having the mould box in two parts like this it makes it easier to do the silicone painting, as described next.
Now the silicone is mixed up. I degas the mixed silicone too, but only because I'm not casting at room pressure. Next, using an old paintbrush the silicone is "painted" into all of the little detail spots of the master part, ensuring that no air or bubbles are trapped. Once this is done I fix the rest of the Lego mould box on top of the first ring (ensuring that you apply a lot of pressure to get a good seal between the bricks), then slowly and carefully pour the rest of the silicone in.
Pour the silicone into the lowest corner of the mould box, not onto the master part.
Step 4: Demoulding
Once the silicone has cured (I wait 12 hours for my Ultrasil, just to be safe) and the master part entombed in silicone, it's time to demould. Break off all of the Lego bricks and peel off the clay from the bottom. Then carefully bend the mould around where the master part meets the pour channel in order to break the join, and remove the pour channel.
Step 5: Cutting the Mould Split
Now's the scary part: cutting your master part out of the block of silicone. Use a fresh knife blade to get a clean cut. Start at the funnel shaped section and cut down through the pour channel until you reach the part. Then carefully, using a prying motion on the two halves with one hand, cut down along the edge of the master part.
Like I said, it's cary the firt time you do it but you get used to it. Yes, you will get a couple of scrapes and scratches on the master, but nothing that can't be fixed simply with some sandpaper/files/knife work. Plus, once you have duplicates the master won't matter so much anyway...
It's important to only cut down as far as is necessary to remove the part from the silicone. This means effectively cutting past any undercuts. The higher up you can stop cutting the less issues you will have with mould alignment.
Step 6: Mould Alignment Boards
With a two part mould, alignment keys are specially created as part of the claying process, so that alignment problems will generally be minimal. With the split mould however, there are no alignment keys, so without any help the casts are going to suffer from terrible mould slip (although despite what I have just said, it's pretty common to use this technique for two part moulds anyway as an extra bit of help).
I use scrap 3mm MDF but 2mm styrene or anything like that will do. If the mould is much longer than it is wide, boards are generally only required on the long sides, however if the mould is more square in its top profile then boards will be required on all sides. Try to match them as closely to the width of the side they are mounting as possible.
The boards are then held in place using elastic bands. You want things to be fairly tight - try to use thicker bands when possible. Make sure when doing this step that the mould halves are nicely aligned - there's not point adding the bands and board if you don't ensure the alignment is correct while placing them!
The casting step is pretty straightforward. Assemble the boards around the mould. Mix the resin and pour. I use both vacuum and pressure on mine, so I rarely have issues with bubbles.
If casting at room pressure I would do the following:
- Dust the mould with talcum powder/baby powder before each cast. The talc helps prevent bubbles from sticking to the silicone.
- While pouring the resin into the mould, repeatedly squeeze the two sides of the mould parallel to the split cut. This helps force the resin and air around the mould. I always do this, even when using the vacuum and pressure casting.
- You may have to cut extra air channels into the mould. Just use a knife to cut small V-shaped channels into one side of the mould, from the point where the air gets trapped to the top of the mould.
Well I think that just about covers it. If you have any questions please feel free to post them!
Happy casting, and if you get a chance we'd love to see your results!