Continuing from our previous post about mold making…
11. Checking the mold is set.
Once you’ve waited long enough return to your mold and examine the dregs in your mixing pot. You can see in the photo I’ve peeled almost all of the dregs out in a single piece so it’s clearly fully set. Once set RTV is a very flexible vulcanised rubber that you can stretch out quite some distance before it snaps.
Since ‘Ultrasil’ has an 8 hour recommended curing time and I left this mold overnight for about 22 hours, I would expect it to be set.
12. Removing the pour box.
After rolling the Klean Klay back and saving it for another mold, peeling back the masking tape and carefully tearing the foam card walls away from the mold I’m left with this: a block of RTV clinging tenaciously to my glued down masters.
Click on the photo and you’ll see some RTV has snuck under the edges of the foam card and was stopped by the masking tape and/or Klean Klay. This shows you how easily RTV can escape from an incompletely sealed pour box. I usually trim this off with a sharp Xacto before peeling the mold away from the base.
You can also see there’s a lip of RTV standing proud from the mold bottom as well as several set drips. I cut these away when I bevel the edges of the mold with the same Xacto to make the mold bottom reasonably flat.
13. Removing the mold.
You need to be careful removing the mold from the masters because if you’re going to damage anything it’ll be now. I slowly lift the first edge away from the base and then work at rolling the mold off the masters from each side a little at a time as in the photo. Eventually the mold should release itself from your masters, popping right off. Don’t be afraid of bending the RTV mold back 90 degrees or more while doing this as set RTV is very flexible.
Some people will recommend you coat your masters with a mold release product before pouring the RTV to aid removing the final mold. As I brush on the first coat of RTV in all my molding (and this tutorial) I believe the use of mold release may be a little redundant.
However if you want to try it I’d suggest picking up a spray can of mold release. Avoid using any kind of brush on product (some tutorials I’ve seen use vaseline) as it may unintentionally add an additional brush stroke texture to your masters! We use RTV for molding because it is very good at picking up minute surface details after all.
14. Tidying up the mold.
Ah the perils of tutorial writing! While removing the mold two of the smaller masters snapped off the base, remaining in the mold as you can see.
Click the photo and you’ll also see RTV has got under a fair amount of one master edge as well. This highlights the point that there’s a trade off between really gluing your masters down to your base, sealing all their edges and actually getting them off the base ever again.
As I mentioned in the first of these posts I opt for a weaker bond with the base as I often reuse my masters later on, possibly in different layouts. However that does mean that this particular problem can occur. Fortunately it can easily be solved by popping the stubborn pieces out and then trimming away the excess RTV with a pair of fine scissors. I use both straight edged embroidery scissors (borrowed from my wife’s sewing box, ssssh!) and a curved set of nail scissors for snipping away excess RTV.
15. The finished mold.
There you have it! Your mold is ready for casting and I hope you’ve enjoyed this rather long tutorial. Feel free to post questions or comments and I’ll answer them as best I can.
If you’re considering casting your mold in a hard plaster product like Hydrostone or Ultracal 30 you may be interested in a much earlier post about the plaster casting technique I use to produce the 15mm wargaming terrain for sale on this blog. I may also publish a photo tutorial of plaster or resin casting in the future so keep an eye out.