Resin Casting Detail?

[JimG]

Just a question on the level of detail that can be reproduced by resin casting. As might have been seen in another thread, I'm investigating making masters for an S scale model of a 12T BR van with the intention of making resin castings from them. I'm not intending doing the casting myself but probably using a firm like CMA Mouldings to do the work.



What is concerning me is whether the steel section used on the sides of these vans will reproduce well in resin if made to scale sizes. The "T" shaped section on the prototype is 4" x 3" x 3/8". In S scale this means that the protruding 3" part of the section will protrude by 1.2mm/0.47" with a thickness of 0.15mm/.006" if made to scale. Would something like this be filled properly when moulding?

The diagonal "L" shaped section is even smaller, being 2 1/2" x 3/16" on the prototype, which gives a protrusion of 1mm/0.039" with a thickness of 0.75mm/0.003" on the model which could be even more difficult to fill.

I'm not intending to make a master of the complete body, but to have the sides and ends cast separately so that I can ring the changes on the sides (planked/plywood/mix) and ends (two panel/three panel/vent options) to make many versions of the prototype.

Jim.

 

[mickoo]

I would of thought so, I've done some resin kits before in military modelling and the detail is amazing. I understand modern resins are very viscous, seeping into the smallest gaps and cracks. I dimly recall there are different types of resin though, some for larger robust things like brick walls and buildings, but also some very thin ones for ultra fine detail.

Rather than running the risk and costing a fortune, why not just get a basic resin kit for yourself, add these parts to a sheet of Plasticard, build a bund around it and run a test on that small piece, obviously it won't be perfect and might have the odd bubble here and there, but it'd give a good feel for what it can do.

Or, have a word with CMA and ask their advice, would such small pieces cast ok and be strong enough, I'm sure they would know.

Mick D

 

[Locomodels]

There should be no problem with fine detail. I have even seen castings showing my fingerprints on the surface!

If trying this for yourself it is best to have some method of degassing the mix before pouring the moulding medium.
RTV silicon rubber is quite expensive put probably worth the cost as it accepts the finest detail and, with care, last for many many castings. I am sure that you will get a lot of good advice shortly. I know that 'Overseer' has a lot of experience of casting and I expect him to arrive soon!

 

[JimG]

Thank you both for your responses. It looks as though it will be OK. I don't really want to get into doing my own casting at the moment - I've got too many things on the go and gearing up to cast in resin means sourcing even more bits and pieces. I'll send the masters off to CMA and see what they say.

Jim.

 

[lankytank]

Jim

As Paul says the detail reproduction from CMA is damned good. I've seen (built) resin kits where the 'stain' from the MEK used to produce the masters has been replicated.

And having dealt with CMA in the past, they are very, very helpful - or they were about 15 years ago (a previous life.....!)

Good luck
Baz

 

[JimG]

I've seen (built) resin kits where the 'stain' from the MEK used to produce the masters has been replicated.

Baz,

You've just raised another matter I was thinking about - the finish on styrene. I was thinking about giving the styrene a thin coat of primer to show up any problems which are not readily apparent when the styrene is in its white state. Will this affect the mould making in any way? Otherwise I would go over the styrene with a soft glass fibre stick to get an even finish.

Jim.

 

[Simon]

Paint won't matter at all, spray away!

As Baz says, the mould will replicate everything, fingerprints, Mekstains, the slightest surface imperfection.

In my experience, the resin will cope very well with long thin sections such as the details you are concerned about.

What are trickier are bolt/rivet heads, but then CMA are masters of their art.....

Simon

 

[lankytank]

Jim

I, personally, wouldn't prime the masters, Finishing with a glass fibre brush will 'score' the surface, even this will be replicated - the resin castings will be exact clones of your master. The level of finish is quite phenomenal, especially if the 'milky bar' resins are used (my terminology, they really look like the milky bar kid has produced them).

Mike Williams (of this parish) is more up to date with CMA & the differing types of resin used today, the grey stuff seems particularly hard, my dealings with them were some considerable time ago.

Baz

 

[lankytank]

And Simon responded as I was tryping........ good man

What you do have to watch for are the undercuts (deep undercuts shorten mould life) & make sure your detail is stuck fast.

You can no worse than talking to CMA they really are bl**dy helpful.

Baz

 

[Mike W]

Jim,

You should be OK. The only part which I question is (and I think this is what you are saying) a piece of Plastikard standing on edge and sticking out from the sides 1.2mm, only 6 thou thick. That should cast OK, but will it be strong enough in service and handling?

Priming is fine and used to be recommended by CMA on their website, but I can't see it there now and its not necessary, though I do prime wooden bodies to seal the grain before sending them to CMA. The scratching from a glassfibre brush might be a good representation of wood grain in your scale?

The main thing to watch is that detail is stuck down firm. Where strapping spans across board marks, if there is a tiny gap at the back the rubber from the mould will go through it and could pull the strapping from the body if not firmly fixed.

As others have said, CMA have vast experience yet remain extremely helpful.

Mike

 

[JimG]

Mike,

You should be OK. The only part which I question is (and I think this is what you are saying) a piece of Plastikard standing on edge and sticking out from the sides 1.2mm, only 6 thou thick. That should cast OK, but will it be strong enough in service and handling?

My first worry was that the resin might not fill very fine detail and I hadn't fully considered whether the final result might stand up to handling. I might have to consider making the steel sections 0.25mm/0.010" instead to give a bit more strength. I'll hav eto give it a fair bit of thought and I might make both versions to see if I can live with the thicker section.

Priming is fine and used to be recommended by CMA on their website, but I can't see it there now and its not necessary, though I do prime wooden bodies to seal the grain before sending them to CMA. The scratching from a glassfibre brush might be a good representation of wood grain in your scale?

One of the glass fibre brushes I have has very fine fibres and is extremely soft - I got it in Buck & Hickman's shgop in Tottenham Court Road over forty years ago. Its green tape cover started disintegrating a year or two ago so I re-packaged it in heat shrink and it's had a new lease of life. I've used it for years to finish off Plastikard to a matt surface to get rid of MEKPAK marking, etc. If I have problems making the masters with a good finish from styrene, I could move to brass.

The main thing to watch is that detail is stuck down firm. Where strapping spans across board marks, if there is a tiny gap at the back the rubber from the mould will go through it and could pull the strapping from the body if not firmly fixed.

I could get round that problem by machining the "T" section and "L" section as solid which will avoid the problem of thin strips of styrene stuck on edge coming adrift. Thanks for the tip on strapping on planking. I'll make sure that all small openings are sealed.

Jim.

 

[Overseer]

Jim,
All good advice. The concern is the fragility of thin sections being damaged by handling over time. I know military modellers often cast very thin sections but they don't usually send the resulting model hurtling around behind a locomotive and then put it in a stock box to transport it to exhibitions. Angle irons have a radiused end (half radius to inside of angle) so you can hide some extra thickness. I do like to spray my masters with a very thin coat of paint, I have used the small Tamiya spray cans, usually in strange military colours bought on sale because nobody else wants them. As well as hiding mek marks the paint forms a very small meniscus in corners which seems to make casting easier using basic processes, though this won't be your concern.

I don't know how much experimentation JLTRT did while producing the masters for their 7mm BR standard vans but they have quite heavy angle irons which spoil their appearance in my opinion. Maybe someone involved could explain whether it was for durability or casting issues.

I have some open wagons to do with external angle iron side knees and have decided to do them as a one piece body but with separate brass strips for the protruding part of the angle irons to ensure they are strong enough. They only have two angle irons per side so not nearly as complex as your vans. Whatever you decide we have great expectations having seen the subtlety of your pressed ends which are better than any of the 7mm pressed ends so far.

 

[unklian]

I have been casting resin for many years and would like to echo a couple of the points made so far. I would paint your masters as Overseer says it adds a slight fillet to internal corners which will help mould release and casting strength. It also helps to show up any defects in the pattern, white plasticard hides all sorts of sins grey primer will show up . The best finish for a pattern to cast in silicon rubber and resin, in my experience is a high gloss! But really the smoother the better. I would have said that although 10thou thick angle will cast OK, it will be pretty fragile in most types of resin. If you are machining it, could you not put a slight taper on the flange that projects, so that it is say 15thou at the base and 8thou at the edge. With a little rounding of the edge it should look fine enough ? Casting sides and end separately should be a big help in casting as the moulds could be used detailed face downwards. Otherwise as per all the previous advice , and look forward to seeing the results.

 

[JimG]

I've been pondering the matter over the weekend and taking on board all the comments so far. Another problem not discussed so far is that the "T" sections project below the sides of the van and they could be extremely delicate. So I am now considering doing a mixed kit, with resin cast ends and etched sides which will give much stronger body detail. The kit will be designed to fit on the S scale 10ft wheelbase underframe kit supplied by Justin Newitt of Rumney Models. One advantage I might have with etching is that it might be easier to reproduce the heads of the 3/8" bolts - I was pondering how to do it in styrene.

Jim.

 

[unklian]

Way to go Jim, horses for courses, the best material for each part of the job, etc. Good luck with coordinating it all though .

 

[JimG]

Good luck with coordinating it all though .

Ian,

That's what I'm thinking about right now.

Jim.

 

[Mike W]

Could you move the join twixt body and chassis from the bottom of the side to the bottom of the solebars? i.e. include solebars, T irons etc etc in the one-piece body casting. You would have to thicken the solebar flange beyond scale and also the headstock flange, but makes it very easy to assemble if you want several of them.

Mike

 

[JimG]

Could you move the join twixt body and chassis from the bottom of the side to the bottom of the solebars? i.e. include solebars, T irons etc etc in the one-piece body casting. You would have to thicken the solebar flange beyond scale and also the headstock flange, but makes it very easy to assemble if you want several of them.

Mike,

The kit is intended to fit on a 10ft wheelbase etched underframe provided by Justin Newitt at Rumney Models and this underframe provides everything up to floor level. It's actually the LMS variant of the underframe but I believe it will be suitable for a BR prototype, or Justin might make adjustments if required. Making the body include the solebars would mean a complete re-think on the rest of the underframe. And the more I consider etched sides for the van, the more I like it. At present I'm working on a good method of holding the etched sides and the resin ends together.

Jim.