Monday, 31 August 2015

HBK Moulds design, casting and such

So as per last weeks post I decided to keep everything in house.

Back when I was learning to cast 6 years ago the main assumption was you made 2 part moulds casting. The people started to use split moulds because it was far quicker and you could get really close to a perfect cast since the parting seam was only partway around. The technique was new to most people there wasn't much in the way of info and after reading up about it that's how I set out to make my moulds. There were a lot of blanks left to fill in and my technical skill and tool box were basic.

I started with plastic card mould boxes sealed with plasticine but they were iffy and often leaked, so I switched to Plastic cups they were pretty good and fast to make moulds. I also got lazy when it came to adding proper venting to allow air bubbles out because I had vacuum pump I could degas the resin before pressure casting it. And that worked well for the more simple MW4 models they were blocky with surface detail. But I found more and more the MWO models had more detail, undercuts and fins etc that technique was becoming inadequate.

One good thing that came from trying to source a caster. One of the companies I spoke to (a well regarded judging by the list of commercial companies that use their services) came straight out and said he be to expensive to do the work.  But he was happy give some advice so told him some of the problems I had, he walked me through how he'd do it. There was no magic trick on how time consuming it would be to cast but there were lots of ways I could improve my moulds. I then spent a fair bit of time researching what he had told me.

  • No more being a slacker I had to add venting, make sure there's no big spots where bubbles can sit. Time spent here pays dividends over fickle casting later on.
  • Second I had to make square moulds because they align better. Since my earlier attempts I had bought a glue gun and this it a perfect way to quickly seal cardboard mould boxes but does take allot more time than just dropping stuff in a cup.
  • Then I had to thicken my mould walls much more than I had been doing before plus extra thickness on the side to be cut
  • Draw a straight parting line then with the help of someone holding the rubber taught (I always did it by my self) and a fresh thin scalpel blade make a cut waving back and fourth across the line
  • One of the guides I found on line recommended cutting as little as possible at first and using a mould release to help the master out. Then its a case of easing the parting line down till you get a happy balance.
  • Lastly he recommended going with a clear RTV(rubber) so you can land you parting line perfectly around any details. 6 years ago it wasn't that common to hear people talk about clear rubber. I started with the blue stuff like most and stuck with what I knew worked. I did some research into the clear RTV. It's not as easy to work with its allot harder and it can react badly to certain substances such as super glue. I ordered some samples of which one company never got back to me and the other one will send me something but unfortunately the guy (apparently there is only one) who decants the rtv is away on holiday till Tuesday. bummer
Any way I had some blue RTV and I decide to mould the masters that had simpler parting lines. So I followed all the above advice and waited for the moulds too cure. I had to wait 2 days for the kids to be out and the wife to be in a helpful mood to cut open the moulds. The resin also has to be cast on a dry day but we had 3 days of torrential down pours to wait out.

HBK foot mould

Any way the wait was worth it, the parts came out pretty much exactly how the masters went in, a few some small bubbles but you can't vent everything, the parting lines were really tight you couldn't even where tell there were but for some thin flashing that just brushed off, I'm so happy how they came out. Since I've used mould release the parts went into the sonic cleaner with some organic degreaser, they'll get a soap bath and a rinse after that (you don't want some grease spoiling a nice paint job).

As soon as I can get some more RTV I can finish of the rest of the moulds and get a whole model cast.

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