Resin casting advice needed

[Edward Pinniger]

Over the last couple of days I've tried casting some copies of the parts for my 1/700 Monitor M-33. The hull turned out OK, with only a few air bubbles:


However, the "sprue" with the smaller parts didn't go quite so well.


This is the best of 3 attempts at casting this part. One of the 6-inch turrets cast perfectly, the rest are all mis-cast due to air bubbles. The other turret shattered due to a large bubble in the centre, the funnel is about half cast, and the smaller parts (vents, searchlight and high-angle AA gun minus barrel) are just stubs.

All of the master parts are glued onto a "sprue" of 1mm thick styrene sheet, and most of them have a small square of 0.5mm sheet between them and the sprue to make it easier to remove the finished castings. It's a single-piece mould.

Basically, the problem seems to be that the parts, or the points where they are connected to the sprue, are too narrow to allow resin to enter and air bubbles to escape. I have tried using a pin to dislodge bubbles from the mould before the resin cures, and have also tried tapping the mould gently in an attempt to release the bubbles, neither have had much effect.

Other than using (expensive) vacuum/pressure moulding techniques, is there any way I could get this master to cast properly? Would it help if I made the "sprue" part thicker, to provide more weight of resin above the parts? Alternatively, is there any way to cut air channels without the resin leaking out of them?

I've also thought of re-arranging the parts so that they are on their sides rather than vertical. The problem with this is that it would make it much harder to cut the castings off the sprue without damage.

Any advice from anyone with more experience working with resin would definitely be appreciated!

[ModelMonkey]

This is a common and very annoying problem when casting parts, especially small parts.

You may have to increase the size of the attachment point on the casting block for your parts to allow for a larger opening to insert a larger probe.

Even the big commercial casting companies suffer from bubbles and malformed casts due to entrapped air in the mold resulting in voids and malformed parts. Commercial casting companies avoid this problem by placing their molds into either a pressure chamber or vacuum chamber where entrapped air is either driven deep into the resin or pushed out of the resin. That's why you find that when you sand the hull of your new resin kit you expose bubbles. But even with pressure or vacuum equipment bubbles can result and waste is high. High waste is one reason resin casting is costly.

For those of us who can't afford expensive pressure or vacuum chambers and cast at room temperature and pressure, use a probe to drive out the air bubbles. The probe can be a simple, common, disposable object like a toothpick. Have some round and square toothpicks on hand when you cast. Just after pouring the liquid resin into the mold, use an appropriately sized probe and insert it into the mold cavity and work out any bubbles by hand.

Also, be advised that the resin components you mix are susceptible to chemical change when exposed to moisture in the air. Keep your resin component bottles capped when not being used, cast in a dry room, and use your products soon after purchase. The older your resin products and the more moisture they absorb, the more problems you will have with bubbles.

Another technique that works, especially for large parts, is to cast them inverted so that any air bubbles entrapped in the resin will naturally rise to the casting block or to the bottom of the part where it won't be seen.

Hope all this helps.

[Admhawk]

Two options.

1: use a fine needle to fill the mould from the bottom up, driving the air up and out as it fills. Toss the needle away when done.

2: Use a very sharp Xacto knife or scalpel and slice the mould in half along the center line, don't go much deeper than the part(keep the mould together at the bottom) and dont cut right to the ends. This way when you pour resin, you can hold the mould open along the split and ensure no air is inside, then it will close naturally when releases. If it doesn't close all the way, you can make a small jig to slide it into to keep it closed( a channel for it to sit into) The parts will have a fine film along the parting line that can be scraped away.

Darren

[Devin]

I have a friend who does a lot of casting and he gets rid of air bubbles by using a super-slow curing resin. Skin times are usually 1 hour plusand every 5-10 minutes he'll simply shake the mold around to help move out the air bubbles. If you're casting a line of kits that method isn't really feasible, but for small personal projects it works fairly well.

-Devin

[Edward Pinniger]

Thanks to everyone for the advice. I've already tried the toothpick/pin method, but will try it again in combination with increasing the depth of the sprue/pour stub (it's now about 6mm thick).
I will also try Admhawk's cutting method on the old mould for the M-33 fittings. As I'll be casting a new one with a thicker sprue, it doesn't matter if the old one is damaged.

The parts are already cast inverted, the bottom of the casting block is at the top of the mould when they're being cast - the problem seems to be that there is insufficient pressure for the resin to force the air out of the moulds, especially with small or narrow parts.

A couple of notes - firstly the age of the casting materials shouldn't be an issue, I bought them only a few days ago, from a major supplier of casting/sculpting supplies (Tiranti) who have a rapid turnover of stock. Secondly, I'm using a fast-curing resin which takes about 15-20 minutes to cure. As I'll sometimes need to cast large numbers of identical small fittings from moulds containing only 1-2 masters - e.g I need to make about 12 deck hatches and 8 steam valve covers for HMS Velox - I don't really want to wait a couple of hours between casting each one!


I've had a bit more luck with casting 1/96 scale fittings. These are casts of the capstan and navigation lights from Cerberus and the binnacle from Velox. The former two turned out very well, with only small surface bubbles (easy to fix with filler) but the binnacle has a very large bubble in the middle which resulted in it breaking in two when removed from the mould. All my attempts at casting this part turned out like this, I'll probably try making a new mould with a larger pour stub.

[Ron Smith]

Use a deeper pour block, that helps when you probe the bubbles out as you can steer them to the sides away from the pour "sprues" so they don't get sucked back in. On finicky castings I have two tricks I use, the first is taking a microbrush and painting the inside of the mold with resin. The second that works for especially deep molds is a very large pour bock and I flex the mold from various axes while probing to get all the air bubbles out.

Using those methods for gravity casting I've gone from a 60% reject rate to a 25% reject rate. The only easy way to improve that would be pressure casting but I don't do enough to justify it.

There are three other things you can do. Switch to a slower resin. Use the thinnest resin you can get. Add small extensions to the ends or bubble prone spots on your parts to have someplace for bubbles to go.

[Karsten]

This is a little trick I learned when working with JB Weld, it might help resin and might not.

I mix the JB Weld (2 part liquid steel) and heat with a hair dryier while mixing to turn it into a liquid rather than a paste. Pour or fill the area I am working on (not models) than heat agian to level and bring the air bubbles to the surface.

Again, just a thought.

Karsten

[Dustermaker]

This is a little trick I learned when working with JB Weld, it might help resin and might not.

I mix the JB Weld (2 part liquid steel) and heat with a hair dryier while mixing to turn it into a liquid rather than a paste. Pour or fill the area I am working on (not models) than heat agian to level and bring the air bubbles to the surface.

Again, just a thought.

Karsten

OMG i never thought about using jb weld, that stuff would be perfect for the small project i have in store!

[Edward Pinniger]

I've had a bit more luck with casting the parts after trying the tips suggested in this thread - the mould-cutting technique has been particularly useful. I'll post some more photos later.

Bubbles are still a serious problem, even with larger parts. With the M-33 components, I can now get the turrets and funnel to cast reliably, other than a few small surface bubbles, but the smaller bits are either partly cast or are broken by bubbles. I think these parts may just be too small and thin to cast reliably without using vacuum or pressure casting.

[Foeth]

I use a bit of rod and poke the mould. I also squeeze the mold, removing bubbles. When released, the mould draws in resin, not bubbles. Combined poking and squeezing results in a very low failure rate. You can only do this for very small batches, the resin cures quickly.

The thickness of the resin slows down the formation and release of bubbles, but I think it's the surface tension that determines if bubbles release or not and that surface tension is more difficult to lower...

[Edward Pinniger]

Sorry for the lack of photo updates (again) but I've tried using Foeth's mould-squeezing method and it seems to work very well, especially when combined with cutting the mould (as suggested by Admhawk). The only problem is that you can end up squeezing too much resin out of the mould and have to refill it (a large pour stub helps, as it acts as a reservoir). But it certainly works better than any other method I've tried for getting bubbles out of the resin. A toothpick/pin also helps to dislodge the bubbles, but I found that squeezing the mould removes most of them anyway.

I can now get the M-33 hull to cast without any significant surface bubbles (even on the searchlight platforms), similarly the 1/96 castings have turned out virtually bubble-free. I've managed to cast one complete set of M-33 fittings, but have not yet tried any others, so can't say for sure that I've solved the problem of casting these.

I'll post some photos of the castings soon.

[Edward Pinniger]

Here's the hull casting and parts sprue. The thin "flash" is very easy to remove. I will probably modify the fittings sprue with a better representation of the AA gun, as the current part is overscale and doesn't cast very well.


This is a quick and basic example of an assembled "kit", the gun barrels and mast are made from brass wire. It is not assembled very well and the funnel is too far aft - I just assembled this in 5 minutes from some spare castings. I'm currently working on a better one which will also have WEM PE railings and ladders.

There isn't much that can be done with multiple M-33 castings, as there weren't many ships in the class and I don't think they varied much - this model is just a test piece for trying out casting techniques and seeing what works and what doesn't! In future, I'm planning on scratchbuilding and moulding various small WW2 warships, such as a Flower-class corvette, which should allow me to make different ships and variants from the cast copies.

One question: would anyone be interested in purchasing a M-33 "semi-kit" and/or any of my 1/96 fittings?