Atlantic's works: Portable layout - Scenic details next

[Atlantic 3279]

I am wondering on where to make my cuts

Advice sent by PM.

[Atlantic 3279]

After several days of fatigue and distractions I managed to have another look at the A1/1 resin test-build tonight. The problem of the exposed splasher tops was solved by fairly simple means, much as I had hoped and expected. The situation at the front was remedied by filing a little more of the flat area towrds the rear of the new smokebox base, and towards to rear the problem was solved by nipping the top of the running plate S-curve more tightly into the top of the corresponding cut-out in the firebox. I've temporarily used a couple of tint screws to achieve the latter effect - it wasn't necessary with the brass master running plate pieces of course as they were more rigid.

I shall look into the possibility of producing stiffer running plate castings when time permits.

A couple of other little points I've dealt with include hinging the fall plate (after I broke it off with one inadvertent bending episode too many), doing what I readily could to shorten the conductive drawbar (whose length had been emphasized by the shortening of the cab sides) and applying a couple of coats of gloss varnish to eliminate a "solvent fog fingerprint" that I had managed to put on one of the cab windows .

[Atlantic 3279]

A small piece of encouraging news regarding cast resin cylinders for the A1/1: I've now found what seems to be a safe and effective way to lever off the Hornby cylinder rear covers undamaged. I believe I have also worked out how to make a mould for, and then to cast, in one, the main body of the two cylinders and the stretcher. I've even prepared the insides of my A1/1 cylinders to facilitate the moulding and casting processes. I hope therefore to achieve a good blend of simplicity, strength and relaibility. The one piece main body will give strength and the necessary A1/1 "look". Re-use of the Hornby rear cover removes the need for another casting process which would have to capture both the rather thin valve crosshead guide and (ideally) the holes / slots for the piston rod, slidebars and valve rod. The projecting valve crosshead guide would have little strength in resin and could well be be an infuriating bubble-trap in the casting process. Projections within the mould designed to form the holes for the rods would have a tendency to be torn off when removing the cured resin part, spoiling the mould fairly early in its life, so re-use of the Hornby item is in my view the winning answer.

[Atlantic 3279]

A1/1 with cylinders off again (save for the rear covers which are still holding the various rods in some sort of order to save me some bother when reassembling). I've laso had a go at filling and smoothing the joints in the running plate, painting the valances green, and lining these out. I'm not fully satisfied with the bow-pen lining yet - it is a little too high up below the S-curve section, ragged in places, and the colour isn't quite right. Humbrol No18, which was fine for the Bachmann based A2/3 would have been quite obviously too garish compared to the fine Hornby lining, so I went for Humbrol No8 which is better but if anything is just a bit too dull and subtle. I shall have to try a mixture.

[Atlantic 3279]

Here's what has been happening to the cylinders:

I'm not going to deliver an exhaustive treatise on resin casting, but in response to a recent request I'll cover this example in fair detail since it involves an interesting two-piece mould. {Simple, flat-backed pieces are far easier to cast - just stick the master, flat side down onto the base of a moulding box, spray with release agent, pour on rubber, wait 24 hours, separate, turn mould over, fill with resin, add a top cover (flat acetate sheet or similar, or plastikard protected by a smooth layer of sellotape), wait for the resin to set....QED}

As well as adding material internally to thicken the cylinder walls for strength and to facilitate resin flow into the mould (when made), I added some pieces of plastic rod around what would be the top rim of the casting - these create pasages in the top half of the mould through which resin can be poured and air bubbles released. These can insetad be drilled through the set rubber afterwards, but I find it easier to do things this way providing the necessary positions for all of the holes can actually be anticipated correctly! The cylinder master in this picture has been bedded in well work-softened plasticine within the Lego moulding box, pressing the master down carefully (without distorting it or rocking/wriggling it and thus creating gaps around it) until the plasticine came up to, and closed off, all potential "undercut" faces. The exposed top face was then given a spray with cheap furniture polish, allegedly containing bees' wax.

And here's the first pour of rubber.

More later, when time is on my side.......

[Atlantic 3279]

After 24 hours for the rubber to cure I pulled down the sides of the moulding box sufficiently to make it easy to lift out the un-disturbed combined block of plasticine and rubber - note the piece of waxed paper that I used at the bottom to stop the plasticine from getting really "keyed in" to the Lego base.

Some complex casting shapes would make it very important that the master remain undisturbed from its seat in the first half of the mould until the second half had also been made - it would be just about impossible to accurately re-seat the master in those cases. Great care may be called for in removing the plasticine bit-by bit whilst keeping the master in place. My cylinders (I hope) were still simple enough for me to wade in and pull the lot apart in a hurry. I appear so far to a have made a success of re-seating the master in the first half of the mould - but the final truth will not emerge until I make the first resin casting - but I'm getting out of sequence here, so I'll carry on with the story.

Some of the plastic pins just break off as the rubber is parted from the master. No problem, as they've done their job already and need to be removed before the first half of the mould can go back into the box, upside down, for the next phase of production. The five dimples in the plasticine, which have produced little cones in the joint face of the rubber, were produced with a pencil, and may help to align the two mould halves accurately for casting.

[Atlantic 3279]

The top surface of the first half of the rubber mould has a raised meniscus around its edges, and around the holes made by the plastic pins. Little leaves of rubber that have leaked into joints in the sides of the moulding box also cling to its edges. In order to ensure that it goes back neatly and undistorted into the moulding box I peel off any thin pieces of waste, and trim off the raised areas of rubber meniscus with the scissors to leave flat faces and chamfered edges.

The countersink around the potential resin-pour holes is, I believe, of obvious relevance/benefit, again produced using scissors.

Plasticine removed:

Add-ons removed from master:

[Atlantic 3279]

Clean undamaged master wriggled back fully into place in first half of rubber mould and laid in first course of moulding box. Vaseline then lightly but thoroughly smeared / brushed over every scrap of the joint-face of the rubber. Good luck pulling the mould halves apart if you don't use vaseline or some other thoroughly effective barrier / release substance. Any vaseline that gets onto the master (and it WILL) needs to be very thoroughly wiped away and/or brushed out so that there is no clogging of detail on the surfaces.

Box sides built up to full height again.

Rubber mixing. Should be 5% catalyst by weight. I've found success, without expensive complication, using home made measuring cylinders (spare parallel sided transparent or translucent pots or tubes) graduated by calculation in cc, so that I can add 6 to 7% catalyst by volume to the liquid rubber. Sometimes a little less also works satisfactorily, although setting times can be delayed, especially (it seems) if not thoroughly stirred in or if very thin layers of rubber are created.

[Atlantic 3279]

Second half of mould, freshly poured. Note all those air bubbles rising back to the surface. You MUST arrange for the rubber pours to be ABOVE the master for each stage of making a two-piece mould, or you risk having these air bubbles trapped against the underside of the master, ruining the impression of the surface that you are trying to take!

The mould halves are due for separation later this evening. We'll then see how good they look, and how a resin casting turns out......

[Atlantic 3279]

Here are the mould halves as first separated, the ragged one of course being the (as yet) un-trimmed new half, which will in due course be the bottom half:

Master removed, the capture of detail seemed to be good:

Time to prepare for a trial casting. I've put the two mould halves back together to check that they fit neatly, and have double checked that the inside of each mould half is clear of any waste. The lump of plasticine will be used as a bit of top pressure to keep the moulds closely united against any tendency for the resin filling to make the top half buoyant. The correct amount of weight is a matter of trial and error - you don't want so much that it squashes / distorts the rubber! The two resin components stand ready, along with equal-volume measuring pots and a mixing tub, plus a couple of screwdrivers as broad-bladed mixing paddles! Several layers of newspaper provide a disposable, drip safe working surface, thin disposable latex or PVC gloves protect hands whilst preserving "feel", and plenty of tissues or kitchen roll should be on hand for wiping tools, mixing pots and spills before the resin hardens.

[Atlantic 3279]

I estimated how much resin to mix so as to be sure of having enough to over-fill the mould in a single attempt. If you have had to create holes to allow resin to be poured in to the closed mould, and to allow air out, it is no use expecting resin to flow through them faultlessly if you find yourself having to "top-up" with a second mix after the first one is already curing!
once the resin is mixed you must work as quickly as possible, but even so, I find it pays to fill the bottom half of a mould while it is open, and use a tiny screwdriver or similar to wriggle resin in to, and air out of, deep holes such as tail rod recesses. I later discovered that I should also have gone around the edges of the valve chest ends.....

It is then a matter of putting on the top mould and pouring resin in through a feed hole until resin is seen to be coming out of all of the other holes. You can direct the resin towards the feed hole if necessary by pouring it down the length of a screwdriver blade or other slim stirrer. Mould full to overflowing, I then pop the weight on, and leave well alone until the resin overspill has turned thoroughly opaque and hard. Get cracking on the job of cleaning up tools etc at this point!

[Atlantic 3279]

If possible I like to leave an hour before opening the mould as the resin casting is then likely to be hard enough to retain its shape during handling, unsupported storage, and cleaning up. I think it is worth breaking the flared ragged ends off from any feeds before opening the mould so that these are not dragged through the holes in the rubber. I've done that here, so the mould is ready for opening:

Mould top off - somewhat obviously.....

There's always a thin sheet of flash across the joint face, as emphatically seen on first lifting the casting from the mould:

The flash is so thin, and at this stage still so soft that it peels away to leave an almost perfectly clean casting:

Despite the many breather holes I provided, trapped bubbles have still created a couple of nicks in the rear (was top) edge, but only at points that cannot be seen on the finished loco and which do not affect the fitting of the rear covers for the cylinders. As mentioned above, one or two tiny bubble defects are also present around the fronts of the valve chests, but I can eradicate these from future castings. Once the resin is harder the last traces of the moulding seam can be filed / sanded from the flat faces of the cylinder wrappers.

[Atlantic 3279]

Better images substituted for originals 24/12/11, thanks to the benefits of daylight and some primer on the cleaned-up castings to give the camera sensor something to focus on! Overall, I reckon the cylinder casting is a success, and the interiors are just as required - clear of any obstructions to the fitting of the covers or the movement of the rods.

All of the mionor defects that you can now see in thses better images can, I believe, be eliminated by casting technique or by susbsequent user-application of tiny amounts of filler.
Images of cylinders fitted to the loco on next page.

[S.A.C. Martin]

Absolutely brilliant Graeme!

Makes the whole process about a hundred times easier for the ham fisted among us (ahem!)

[mick b]

Very interesting