Styrene sheet moulding - Quintus build - FINISHED

[philipy]

I was planning to ask Father Christmas for a GRS Hunslet body kit, but having read the latest SMT, I'm rather smitten with the idea of scratchbuilding the MW "Quintus" or a fairly close approximation thereof, using battery powered electric propulsion.

That brings me to the the point of this post, I'm slightly puzzled about the best way to produce the characteristic shape of the saddle tank.  My preferred material is Styrene sheet ( Plasticard), but the same uncertainty applies equally to brass.

My thinking is to carve a former from a Balsa block, which shouldn't be too difficult, and then I have two options:

In brass I should be able to wrap sheet round the former and solder in brass 'bulkheads' to hold the shape.

However as I said, I'd prefer to use styrene and I'm thinking of heat forming it and I wondered if anyone had any thoughts/experience on this subject?

The obvious way to start seems to be to pin the top centre of the stryrene sheet to the top of the former, put it in an oven to heat/soften and allow gravity to drape it down the sides. Allow it to cool, turn it over and repeat to get the bottom curves. Is this a practical idea? I've used a similar technique in 4mm, but I'm not sure if it will work well at these larger sizes?

I'm aware that the styrene is likely to shrink, but I can allow for that, plus thickness will play an important part I think ,so I'm open to suggetions on that as well.

Any and all thoughts welcome, but before anyone suggests a solid block, I'll need the tank to be hollow for battery and Rx, etc.

[MDLR]

I've heard of people wrapping Plasticard round a former with rubber bands and putting it in hot water to "encourage" it to take up a new shape..............

[sstjc]

I've been forming styrene for years. Get a suitable container something like a plastic lemonade bottle or whatever has the right diameter. I the wrap the styrene round it and cover with a piece of paper. I use A4 printer paper. Then bind this in place with brown parcel tape. Basically the paper stops the parcel tape sticking to the styrene. I then place this in a bucket of hot wat and fill the lemonade bottle with boiling water from the kettle. Then I leave this until its cooled and cut the tape off. In reality I find its better to have a bottle (or former) a slightly tighter diameter than needed because the styrene will spring out a bit.
Barry

[philipy]

Thanks for the replies, chaps.

Brian,
Yes that certainly works on smallish items and thin sheet, I've done that a number of times in 4mm, but not sure it will work on a piece approx 100mm sq and maybe 1.5/2.0 mm thick with relatively tight bends.

Barry,
Again a variation on the same theme as Brian's suggestion, and fine for, say a curved coach roof, but it isn't going to work on a complex shape like a Manning Wardle saddle tank, I'm afraid.

[sstjc]

Have to admit in a tad surprised you dismiss the idea. Having a closer look at some pictures id say the sharp bend at each side of the saddle tank could be formed using a piece of copper pipe in place of the lemonade bottle. But each to his own....
Barry

[philipy]

Barry,
Perhaps I'm not understanding you correctly then?

The saddle tank has a tightish curve either side at the top corners, plus a shallow curve across the top linking them together. Then two more even tighter curves at the bottom with straight sides in between. I don't see how a single diameter pipe will be able to reproduce all that - are you suggesting that I create a former of various sizes of pipe, with brass sheet soldered to join them together creating a structure to fill with hot water, or something like that?

[Daven]

It is possible to clamp one end of a flat sheet in a vice and use a heat gun (very carefully) to soften the styrene and bend it around a suitable former. This is done in ship modelling for example.


David

[Peter Butler]

I have long admired this loco when looking through my copy of Narrow Gauge Charm..... and if I was going to build it I would laminate the top and bottom of the tank by building up the thickness of Plasticard and sanding to shape. This way you should have more control of the shape as bending might put distortion into the tank and it would always bug me if it looked twisted.
The tank can still be hollow but would certainly be stronger.

[philipy]

It is possible to clamp one end of a flat sheet in a vice and use a heat gun (very carefully) to soften the styrene and bend it around a suitable former.   This is done in ship modelling for example.


David

Thanks David. I guess thats kind of the same idea as my thoughts on using an oven. I don't have a heat gun but I did consider a blow torch but decided that would be too uncontrollable.

[philipy]

I have long admired this loco when looking through my copy of Narrow Gauge Charm..... and if I was going to build it I would laminate the top and bottom of the tank by building up the thickness of Plasticard and sanding to shape.  This way you should have more control of the shape as bending might put distortion into the tank and it would always bug me if it looked twisted.
The tank can still be hollow but would certainly be stronger.

Peter,
Building it up as you suggest is certainly an option. It did occur to me but I may be too impatient to allow the solvent to go off fully before sanding! I've had that problem with myself, before!

I do agree with you about not being able to live with a visible distortion, though.

[GTB]

Personally I prefer to work in brass for this sort of stuff and would form the bends in the tank with a rod and my long v-block. ....but then I like working metal.

Probably quicker for a one off model to form the part as suggested by Peter B. Just takes some self control to leave laminations for several days to dry out before touching them. It helps to have other models on the go, to keep your hands occupied.....

I've heat formed polystyrene in the past, but never had much success using boiling water, as it isn't quite hot enough to fully soften polystyrene sheet.

I got more consistent results by putting it in an oven until the sheet is soft and then stretch forming it over a wooden armature. Works fine most of the time, but you are walking a fine line between soft enough to form, but not so hot that it shrinks. Just resign yourself to experimenting until you get the right temperature for the sheet you use and your particular oven. I've also seen it done with a hot air gun, but never tried it myself.

Something like a loco saddle tank would be a two stage job to form and I'd use 1.5mm sheet, which is thick enough to be self supporting. Anyway, make the core shape in wood, then cut a suitable size rectangular hole in another piece of wood. Put the core in the oven, put the piece of sheet on top and wait for it to start to sag. Quickly pull them out and press the outer former down over the sheet, stretching the sheet over the core to form the top of the tank. Leave the outer former in place, put it all back in the oven upsied down until the sheet softens again and then press the bottom flaps over the armature with a flat bit of wood.

Use an oversize piece of sheet if you go this way, as there will likely be some shrinkage and polystyrene sheet has a bad habit of shrinking more in one direction than the other when heated.

Acrylic sheet (Perspex) gives good results when heat formed, but is harder to glue to polystyrene.

Regards,
Graeme

[philipy]

Something like a loco saddle tank would be a two stage job to form and I'd use 1.5mm sheet, which is thick enough to be self supporting. Anyway, make the core shape in wood, then cut a suitable size rectangular hole in another piece of wood. Put the core in the oven, put the piece of sheet on top and wait for it to start to sag. Quickly pull them out and press the outer former down over the sheet, stretching the sheet over the core to form the top of the tank. Leave the outer former in place, put it all back in the oven upsied down until the sheet softens again and then press the bottom flaps over the armature with a flat bit of wood.

Use an oversize piece of sheet if you go this way, as there will likely be some shrinkage and polystyrene sheet has a bad habit of shrinking more in one direction than the other when heated.

Thanks Graeme, that is almost exactly what I had in mind from a theoretical POV. The only bit that hadn't occurred to me was your use of the outer former to press it down, I can see that would make a huge difference AND prevent things moving for the second stage.

[Peter Butler]

It would be interesting to compare the two methods both for speed and end result. If you are going to the trouble of shaping a wooden former to heat and bend some plasticard over, why don't you laminate some pieces first and wait for them to harden? If one method doesn't work you still have a second option and no wasted time!

[GTB]

It would be interesting to compare the two methods both for speed and end result.

It would, but I don't think I've ever made identical parts using the different methods to be able to make the comparison.

My usual approach is to fabricate one-off parts and use forming methods when I need several parts. I would usually heat form any part that needs to be hollow and has compound curves.

I think the basic heat formed part would only take a day to make, go to whoa. Hard balsa would be durable enough (although pine would be better) for the formers if making only one part, plus maybe a couple of spares/failures. So making the formers would be quick and the actual heat forming would be a matter of minutes per part.

Shaping a laminated part would again be maybe a days work, but the start to finish time would be a lot longer, as it has to sit around while the laminated block dries. I'd probably give something like this a week to dry good and hard before starting to shape it.

Regards,
Graeme

[philipy]

Peter/ Graeme,
I've made a start with a laminated styrene box. It took maybe a couple of hours this morning by the time I'd set out the dims, worked out the best way of laying it up, cutting and gluing, and it is now set aside to dry.

A lump of balsa arrived in the post yesterday, so when I've got nothing better to do, I may have a go at shaping that, and heat forming, just out of interest.

I'll keep you posted if I do.

[Peter Butler]

Great news Philip, please let us know not only which method you prefer, but also which gives the best finish, and..... which was the quickest overall!

[Boustrophedon]

Acrylic sheet; perspex, is much easier to thermoform by hand than styrene.

At work we use a vacuum former with what the suppliers call HIPS: High Impact Polystyrene. Hips is a "graft copolymer" of polystyrene and polybutadiene. The polybutadiene produces little rubbery crystals inside the polystyrene matrix. On heating the polystyrene component softens first leaving the sheet rubbery and stretchy. The trick is not to heat it so much that the polybutadiene melts as well. The sheets are stretched during manufacture so they have to be restrained at the edge on heating or they shrink.

Now I am not sure if the plasticard sold is HIPS or not, but given the vast quantities of HIPS used in vacuum forming, I would guess that it is. Also pure polystyrene melts very suddenly and is very runny when it does.

If I were to make loco parts out of styrene sheet I would use the school vacuum former.

If I have to thermoform plastic by heating and bending, then I use acrylic sheet instead. This is "glassy" it has no well defined melting point, it just gets softer and softer as you heat it up. It is very easy to form by heating in a domestic oven (120C) and, using oven gloves, taking it out and draping it over wooden forms.

If you want non-developable shapes; shapes that require some stretching as well as bending, like hemispheres, then wooden plug moulds can be used, with close fitting male and female parts.

Hope that helps.

[philipy]

Thanks for that, every little helps, as Mr Tesco would say!

I believe that Plasticard, 'styrene' sheet, etc are all nominally HIPS, but I'm also sure that there are various grades and/or possibly other additives, because some cut better than others and some are more brittle, etc.

Your comment re acrylic sheet is interesting. I do have some 2mm clear sheet but I'm not 100% certain it is actually acryllic. It doesn't smell like genuine Perspex when it is cut, but I may give it a try. However, it is a pig to cut with a jig saw, even going slowly, because the swarf drops back into the cut behind the blade and re-fuses itself! Thanks for the idea, though.

[Boustrophedon]

The trick to cutting perspex is to cover the top side in paper glued on with Pritt or masking tape. The little bits of paper or tape get drawn into the cut and stop the edges welding back together. Try it, it really works.

The stuck on paper is easier to mark out on too.

[philipy]

Great news Philip, please let us know not only which method you prefer, but also which gives the best finish,  and..... which was the quickest overall!

Well, the first attempt at hot moulding was spectacularly unsuccessful!

Took about an hour to sand the balsa block to shape. Not difficult just took a while because the original block was about 2mm too wide so not enough to saw and I had to sand it down to size before I could shape it. The actual shaping didn't take long, maybe 15mins or so.

The former is about 20mm over long to allow me to trim off any end imperfections - I just wish it was that simple.
The styrene sheet I used must have some sort of built-in curl which was flattened when the sheet was rolled, because as soon as it started to soften it also started to curl up and inwards! It's a pity really because it has nicely draped over the curves.
If it had worked, I was actually planning to do a half-and-half method,  heat form the top and then glue in a base and hand shape the bottom radii because they would all come out of the thickness of the sheet on the corners.

In the meanwhile, I've cut and laminated the footplate, cut and glued the basic cab structure and found a suitable sized plastic tube for the boiler. The laminated tank structure looks OK, just need to leave it a bit longer to be sure it has cured properly before I start on the shaping.

On a slightly different subject, the drawings of Quintus in the latest SMT, show and describe the steel flap on the RHS of the cab doorway. What it doesn't mention, and I nearly missed, is the fact that actually half of the top of the opening is apparently permanently sheeted on the RHS but the LHS is completely open.
Since the sheeting and flap are obviously subsequent mods, I'm in two minds about doing away with them on both sides.