Split Mould Casting Technique

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Split Mould Casting Technique

Postby mangozac » Sun Oct 10, 2010 11:35 am

After some recent discussion on WargamerAU, I decided it would be best to write up a tutorial with images in order to easily explain the single part split mould. So here goes:

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
These advantages do come at a cost of slightly shortened mould life. Note however that the overall cost in silicone is pretty much the same for both techniques. The split mould technique lends itself best to parts that are relatively small in at least two dimensions (say, less than 20mm x 20mm) but the parts can still be fairly long.

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.

splitmould1.jpg

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.

splitmould2.jpg

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.

splitmould3.jpg


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.

splitmould4.jpg


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!

splitmould5.jpg



Casting
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!
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Re: Split Mould Casting Technique

Postby blind pig » Sun Oct 10, 2010 12:31 pm

Great tutorial Mangozac!

I should PM you more often when you're drunk at 1am. :lol:

Split moulds are my prefered method of casting for ease of manufature. Other companies - such as Forgeworld - use split mould for a great deal of their casting, for the reasons you state.

FW use split moulds for quite large model parts, including Reaver Titan legs!

Here is a pic of the backpacks from the FW ORK KOMMANDO CONVERSION SET. As you can see, they use the same process as above for *very* detailed items. The overall length of the casting is a bit under 120mm.

Image
Image
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Re: Split Mould Casting Technique

Postby Kennui » Wed Oct 13, 2010 2:03 am

Now i know why FW stuff have such huge tabs which seem like a waste of resin.

I popped into barnes the other day, lady was busy with others so i didnt have a chat but saw some cool stuff. Like FOAM!

Foam looks like a ton of fun. Foam marines. Yummmmmm.

Or in your case, foam plasma pistol. :lol:
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Re: Split Mould Casting Technique

Postby blind pig » Wed Oct 13, 2010 10:57 am

Kennui wrote:Foam looks like a ton of fun. Foam marines. Yummmmmm.

Or in your case, foam plasma pistol. :lol:


Foam also expands the moulds and distorts the model. Tricky stuff to work with.

And a foam pistols and weapons were made by Heble and I for last years GenConOz.
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Re: Split Mould Casting Technique

Postby josh76 » Tue Nov 02, 2010 10:22 am

Hi mangozac

I found this guide and have been playing around with split moulds for the last few days.

Previously I'd been making 2 part moulds with pinkysil and only had limited success, there would be a lot work to do once id poured and a substantial amount of resin/pinky wasted, also really severe flashing.

I've found the split moulds to work incredibly well, especially with no pressure/vacuum (something i may progress in to in the future).

Initially I had some air bubble problems due to the relatively small openings, i tried using the technique you described of squeezing the mould to try and fill the cavity but still had some issues. Then I had the thought of needling out these bubbles, essentially just sticking a needle in to the cavity to get it fulled up and it worked really well. For the first time I have two pours that I can use everything on. Very pleasing.

Thanks for the great explanation.

Cheers
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Re: Split Mould Casting Technique

Postby mangozac » Wed Nov 03, 2010 8:34 am

Hi Josh, congratulations on being our 100th member! Thankyou for giving such great feedback, it's really great to hear that people are finding this site to useful and improving their skills.

I'm not sure I quite follow you on this "needling" technique. Any chance you could elaborate a bit more? Would it be possible to see pictures of your results?
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Re: Split Mould Casting Technique

Postby amphoterik » Fri Nov 05, 2010 9:27 pm

I made a split mold based on this tutorial, and was gonna send pictures of my results. Unfortunately, my results came out horrible. A little more practice I think.
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Re: Split Mould Casting Technique

Postby mangozac » Fri Nov 05, 2010 9:29 pm

What went wrong? The mould or the cast? It would still be educational if you could share pics anyway...
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Re: Split Mould Casting Technique

Postby amphoterik » Fri Nov 05, 2010 11:03 pm

Well, the problem is twofold. The first is that I failed to make a cut along the molded object, but beside it. This has resulted in problem 2: air bubbles like crazy. I haven't given up on the mold yet, I just am practicing with it to see what I can do better.

I will see about pictures a little later.
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Re: Split Mould Casting Technique

Postby factor40 » Wed May 04, 2011 11:28 am

Is it possible to get some insight as to how your example would apply to making a mold of, say, 5 guns? My question is...how do you deal withe the silicone that is in between each piece?
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Re: Split Mould Casting Technique

Postby mangozac » Wed May 04, 2011 12:40 pm

factor40 wrote:Is it possible to get some insight as to how your example would apply to making a mold of, say, 5 guns?

The secret there is the use of a weak connection somewhere between the part and por channel. Once the silicone has set and you are ready to cut/demould then the pour channel bonds are carefully broken, leaving the 5 guns embedded in the silicone. With some skill it's not hard to then cut the splits between the parts. Does that make sense? If not I'll draw up a diagram.
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Re: Split Mould Casting Technique

Postby factor40 » Thu May 05, 2011 1:34 am

Ah...no need for a drawing, the light just went on :)
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Re: Split Mould Casting Technique

Postby Seb » Wed May 11, 2011 11:33 pm

Great tute Zac, clearly you've come back from your trip refreshed :D

I should get back into basic level casting again, have several ideas that really are best suited to reproduction of key parts.

Bookmarked!
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Re: Split Mould Casting Technique

Postby mangozac » Thu May 12, 2011 7:12 am

Haha you know this tute is about 7 months old now Seb? No matter ;)

You know if you do need any odd parts reproduced drop me a line and I can sort you out!
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Re: Split Mould Casting Technique

Postby Seb » Fri May 20, 2011 6:13 pm

Aargh!.. seven months? :shock:

Was browsing 'New Posts', this must have been one I missed from my big catch up session a while back.

Looks like I need a holiday then. I'll get my bags.

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