Casting Resin - Fine Detail Question

[Aurora Mark]

Folks,

I'm going to start making masters for some of the parts I plan on casting in resin for my 1/48 CP-140 Aurora. The larger parts (like spinners, landing gear wheels, and so on) aren't much of a concern with this question. But for pour casting smaller details, how small is too small, and would best be left to adding scratch built pieces afterwards?

A good example would be the rims of the tires for example. I plan each wheel being 3 parts; the main rubber, and each rim half. On these rims are nuts and bolts that would be cast at about 1mm or so. Would it be realistic to make these on the master, or better to just have them glued on after the resin piece is cast.

Any help, advice and input is welcomed.

Cheers,

Mark.

[chuck540z3]

I've never done it myself, but resin wheels and rims can be very detailed. Here's a pic of the 1/32 Sierra Hotel A-10 resin wheels, which are fantastic and very detailed.

[spejic]

Mold material will reveal the smallest details of your master part. A detail can be too small if it breaks off from the master and stays in once the mold is cured. It can also be too small if the resin won't flow down into it. I don't think a nut detail on a wheel rim will have either of those problems.

[Aurora Mark]

Yes, there are some incredible details on a number of resin parts out there. By concern though comes from many of these being made in a manner that involves many more pieces and tools than I currently have.

Without the use of vacuum pots, or pressure pots, and having just the pour technique available to use, I'm worried about partial castings or bubbles rearing their ugly heads.

[Triarius]

Use a low viscosity resin with the longest pot life available, and the softest mold-making material you can. Avoid undercuts in the mold.

[Aurora Mark]

Thank you. This here is the resin I was thinking of using. Thoughts?

http://www.sculpturesupply.com/detail.php?id=796552&sf=subcategory&vl=Smooth+Cast+322&cat=Liquid+Plastics

[Triarius]

Thank you. This here is the resin I was thinking of using. Thoughts?

http://www.sculpture...Liquid+Plastics

That looks like a good choice.

[MoFo]

There are really two questions rolled together here.

Any decent, hobby-grade RTV and resin will cast detail as fine as you can make. Nuts, bolts, rivets and the like? No problem. You can often see glue marks on commercially available resin sets, because the RTV accurately casts the microscopically rough surface of a plastic part and the glossier, smoother finish. So from that perspective, you're not going to have to worry about size.

What is more likely to be a problem with a tabletop set-up, is air bubbles/air locking. Both from creating bubbles when you mix the resin, and trapping bubbles when you pour into the molds. There are a few ways to minimize the risk, but yeah, without a pressure pot and vacuum chamber, it'll be a bigger potential issue than if you had those tools.

First up, as Triarius says, low viscosity and long pot-life. You're not doing production casts here, so you don't need rapid turnover. A lower viscosity will help flow into details, and a longer pot life will give you more time to work out bubbles.

Second, if you have a particularly detailed surface, rather than just pouring into the cavity you can 'prime' the mold by brushing on a coat of resin. It'll help you work the resin into all the nooks and crannies and push out any bubbles; once you're sure the details are filled, you can continue to fill the rest of the cavity. This is the process I'd use to cast the wheel hubs in Chuck's pic, for instance.

Third is by properly designing your parts. For finer, raised details, you'll want to make them wider than they are deep. If you've got a bolt that's, say, 1mm diameter and 3mm deep, it's going to be harder for the resin to flow into that hole, and you're almost certainly going to get an air bubble. If it's 3mm diameter and 1mm deep, you shouldn't have any problems with flow.

Finally, you can help yourself with careful mold set-up. Use larger casting blocks for detailed parts, so you've got more mass pushing into the mold (same principle as pressure casting, just way less pressure). You can add pins (a length of, say, 1mm styrene rod) and risers (basically a small reservoir) on the ends of longer, trickier-to-fill molds, to give excess air a place to collect and compress. You can even add a small reservoir on the end of hard-to-fill parts and use a sort of pipette effect to help fill it. (squeeze on the reservoir, expelling some air; pour resin into the mold; release reservoir so the vacuum pulls the resin all the way into the mold)

Most importantly though, remember:

**YOU WILL GET BAD CASTINGS**

Even the big guys do, so don't expect every pull to be perfect. Particularly not when you're just starting out. It's a unique skillset, so if you've never done it before, there will be a learning curve.

[Aurora Mark]

Thank you guys for the fantastic advice. I really appreciate you all taking the time to help out with this.

Cheers,

Mark.