[BFG] Corvette

[Islacrusez]

Right, until I receive further feedback on the overall state of the ship, I can't really call it complete or progress much. As a result, I have plenty of time to think about how to cut it up for casting. And think I shall!

If I remove the two aft wings, I'll be able to easily cast most of the ship as a split mould, cutting it in half vertically. I can find a decent spot to separate the fore winglets from the hull, but am still struggling with a spot to remove the prow altogether. The engines on the back of the hull and keel are also giving me trouble. If the mould splits vertically down the middle, the engines are rather deep undercuts, especially those on the join line. The mould would likely suffer serious wear there.

So:

Remove wings at wingroot; use the edge on the engine as alignment slot.

Remove aftmost section of the hull together with the engines.

Remove keel at root, use side braces and the spine under engine section to align position.

Remove fore winglets at hull, easy alignment from what's left behind.

Cut prow off just below main gun, just behind bay doors.

These would be cast with mould split into fore and aft. While what's left, the hull and bridge, would be cast in a mould split into port and starboard sides.

This does mean that the ship comes in 7 pieces, with the guns as another 2. 9 Pieces for an escort? Ouch! Would anyone be willing to build that?

[Islacrusez]

Ended up thinking as a two-part mould (in fact was thinking that originally, except got my terms backwards).

This way, the main engines come away with the main wings as a single part. The keel has to be removed as a separate part.

That brings our pieces down to 8 (I counted wrong earlier, too!)

Still bouncing ideas on the prow, but it's not looking good.

[connahr]

i think it would be possible to build

[Islacrusez]

Fins added, forward vent replaced on rear wing inboard engine.

[captPiett]

It's a beautiful ship, nice job.

If you're really looking for things to change, I can only suggest this: there are virtually no curvy lines on it except for what I assume are the point defense turrets. Aesthetically, they would fit better into the overall design if they were boxy.

Here's the disclaimer though: I don't play BFG anymore and will be unlikely to try and acquire one of these. So, an appropriate amount of salt should be taken with the above advice. 

[Islacrusez]

Noted, good sir! Though, there are a few other curvy shapes - the engines mainly, but also two domes atop the bridge. Really there should've been more up there, but I think they got lost. While I appreciate that the general aesthetic of the ship is a boxy one, and this is indeed mirrored by the boxy main guns, the main problem is one of scale. The turrets are about 0.8mm across, and a blocky turret would need detailing to not look like a box. As such, the turrets are spherical, because I can get away with a lack of detail then

[Islacrusez]

Funny colours for a corvette, eh? Those are the proposed component splits, draft 1.

[horizon]

The prow might be difficult to assemble if it gets slightly warped I think because it is three layered.

Why not combine yellow + blue as complete prow, no orange on top. Use pin like structure in middle of body extending from prow into the hull body?

[Islacrusez]

The prow might be difficult to assemble if it gets slightly warped I think because it is three layered.

Why not combine yellow + blue as complete prow, no orange on top. Use pin like structure in middle of body extending from prow into the hull body?

My main concern is undercuts which occur between the wings, as well as some which occur further down. The prow section will have to be done as a 2-part mould, in parts fore and aft. They have to separate roughly in the middle of the wings, and thus the prow bay would be buried deep in the mould, and has its own undercuts to consider (as does the weapon platform topside).

I might be able to get away with casting the prow bay with the wings, but my thoughts are that it will be a lot easier to mould them together if they're in pieces, than it would be to cut them up if they're a single piece to begin with.


How does warping get introduced into the cast? Is it during the casting process due to the mould becoming skewed, or other factors after being removed from the mould? More to the point, can it be controlled?

[unseelied]

Hi, just joined in the thread so I might have missed something.  I happen to be head of the moldmaking dept where I work.  We make silicone rubber molds of stuff so that we can make resin castings.   Is that what you are planning to do? 

To answer your question. Casting distortion comes from several things.  First is in a production environment you use heat or addititives or both to accellerate the cure of the material and you are trying to get the things out of the mold with a quick of a turnaround time as possible.  Often they won't be quite ready so they will distort when you pull them out.  The other thing that can happen is mold shift.  Thats when the two flexible halves of the mold don't quite line up right.  This gives you a shift in the casting like a little shelf all around the parting line.  Mold spreading sometimes occurs when the mold isn't closed all the way an it makes the casting thicker in places than it should be.  Lastly as a mold gets older pieces of it rip out and that leads to blobs on the casting.  This is often in areas of high detail and around the parting line.

This is an escort sized ship I gather?  I think that in order to get a better mold you might want to simplify some of your detail.  For instance all of the holes, engines, vents and such could just be shallow intents rather than deep holes.  At this scale no one is really going to notice and the rubber is going to want to rip at those points.  All of your gun barrels should be backfilled.  Instead of a small cylinder they would be a wall with a rounded top. If you are planning from the beginning to make a mold keeping in mind "moldability" or how well it can be molded is a good idea.  No mold lasts forever but the better planned it is the longer it will last.  I'd print this out at actual size and give it a look over to see exactly what you can actually see.  I think that you can easily lose some undercut detail and still keep the overall look/feel of the ship. 

[Islacrusez]

Welcome to the thread! Make yourself at home, we have copious amounts of tea and large amounts of stupid

It is indeed an escort ship, clocking in at 37mm at this moment in time. I am also indeed intending to make silicone moulds to make resin castings. Not in very high numbers, I imagine, so less of the hurry of a production environment... Will the freedom to take my time help reduce casting problems?

For the engines, I appreciate that I'm asking for trouble, but if needed I can always fill them at a later stage and make a new mould. Most of the windows are fairly shallow, and the direction of moulding should help keep those areas intact. Only the fore wings (blue section) have undercuts at the mould line, which I expect will cause the most wear. (I may be wrong?)

For the guns... I'm really, really, hoping to get away with having barrels on the small turrets. They're as close to a cube as it goes, so I'm hoping they'll be ok. I don't want to live in a reality where I can get better details scratchbuilding than casting! This one will be tougher to fix post-print if it does cause issues, but at these proportions, will it? The domes are about 0.8mm diameter, while the barrels are in the 0.2*0.2*0.2 range if I'm not mistaken. Accurate dimensions in this thread somewhere...

The larger QF turrets are going to have barrels added during assembly, because they're the wrong sort of shape (long thin cylinder shape) to cast reliably and without damaging at some point. They'll just be ordinary styrene rod stock. The main guns with actual barrels... Possibly also going to be stock, though thicker. Not sure if those barrels would cast any better (not done the measurements for those yet).

I'd absolutely love to print this actual size and get a good look at it, and I'd probably go back in and strip out certain bits of detail, but unfortunately this is a luxury that simply isn't available to me. I don't have direct access to any 3D printer, let alone the exact type/make/model of printer that it will eventually come out of. I'm kind of willing to write the first print+cast off as an experiment; though I do hope it won't be a complete waste. If some of the details are wasted, that's fine by me. If the mould doesn't live as long as it could have, that's fine by me. If it doesn't come together when it comes out of the mould, or if the mould is destroyed after the first or even fifth, that's less cool. Do you think I can manage to meet those sorts of goals? 

Your professional input has been very helpful, and is much appreciated!

[unseelied]

I think I have enough stupid but thanks for the tea. 

Neatness counts in making molds so taking your time will aid you in getting both better molds and better castings.

Your engines will be a problem.  Thin fingers of rubber will flow into the negative space, grab onto all that detail inside and will tear out very quickly.  Like imediately. I was calling it vents but it could be struts on the front of the wing will also be a problem.  I think the material there is too thin.  Resin is like tomato soup in consistancy and it is not going to want to go into that thin of an area and if it does it it tends to be brittle when its thin like that so its going to be fragile.  Long and thin is not good.  The gun barrels are super small and thin but they are kind of stubby so they might come out now that I look at them.  Still there is a danger of them snapping of during demolding.  The undercut in the wing isn't really so bad.  The upside of a rubber mold is that it stretches so you can spread the mold to get slightly undercut stuff out of it. 

Not sure what a QF turret is but if it is a long thin piece that stick out into space and you are OK with using metal rod than I'd use the rod.  Hard to find rod thinner than 3/4 of a mil so keep that in mind.

I was suggesting you print it out 2D on a piece of paper, to the correct size.  Place it on your gaming surface and look at.  A lot of your detail, I think, is needlessly tiny.  If I did my conversion math right,( we don't use milimeters here in the states) your gun barrels are the same thickness as two pieces of printer paper.  You are not even going to be able to see them when the ships are on the table. I think you'd be good if you stuck to nothing smaller or deeper than half a millimeter.  It will ease the molding process and give you a better final product.  Its going to look very clunky on the screen but it will look good actual size.  I honestly doubt that as the ship is now that you will be able to get a successful mold out of it but its going to be a fairly tiny mold so if it doesn't come out then you've only really lost your time and a small amount of rubber.

Well I think its a cool design and I wish you good luck with it. 

[Islacrusez]

Humm. Tomato soup, or tomato purée?

I guess I'll have to experiment as to what it'll fill and what it won't, and if some casting techniques can't be applied to help the process... I'm going to have to go with the knowledge that anything that is too deep can always be made shallower.