Patterns For Resin Casting - What Is Feasible?

[Dog Star]

I have no experience in either casting parts in resin or making patterns for resin casting so I am starting this topic from a very definite point.... zero, lots of nothing, no map and no direction. The objective is to make some patterns for a 7mm carriage and have those patterns cast in resin; the prototype is the GWR Passenger Luggage Van of diagrams K14, K15 and K16. I intend to make the carriage sides by cutting and rejoining the sides of a V5 kit and the ends from the same source (and before anyone asks... David White is aware of this project and is supportive of the intended approach).

In preparation for cutting and joining there are a few questions which have come to mind and I invite comment / guidance on those concerns. The overall length of the K14 carriage is 40' (prototype) or 280mm (7mm scale), thickness of side circa 3mm - 4mm... and there is turnunder to the sides and to the ends.

[1] what shrinkage is likely in the resin castings compared to the patterns?
[2] what works best.... an one-piece body or separate sides and ends?
[3] what works best.... a body with integral roof or a body with integral floor?
[4] with or without the guard's lookout? (which projects about 8mm from the body side)
[5] what is the minimum thickness which can be cast for a lap joint? (as in the Slater's approach to the joint between 7mm sides and ends where the lap on the sides forms the outer piece of panelling on the ends).

The difference between the K14, K15 and K16 coaches is in the ends - K14 was built without a gangway and hence follows the typical Dean "non-corridor" arrangement for panelling of the end whilst K15 was built with a gangway connection and hence the end panelling is more cramped in layout (K16 was a K15 end without a gangway). If the way forward is separate sides and ends then the provision of alternate ends is not an issue. However, if the preferred approach is an one-piece body then seems to me that the body needs a dummy end (a la Jenks) so that the appropriate panelling can be applied. Hopefully Steph has a view on this aspect of carriage construction!


thank you for your comments and guidance, regards, Graham

 

[Wagonman]

The objective is to make some patterns for a 7mm carriage and have those patterns cast in resin; the prototype is the GWR Passenger Luggage Van of diagrams K14, K15 and K16. I intend to make the carriage sides by cutting and rejoining the sides of a V5 kit and the ends from the same source

In preparation for cutting and joining there are a few questions which have come to mind and I invite comment / guidance on those concerns. The overall length of the K14 carriage is 40' (prototype) or 280mm (7mm scale), thickness of side circa 3mm - 4mm... and there is turnunder to the sides and to the ends.

[1] what shrinkage is likely in the resin castings compared to the patterns?
[2] what works best.... an one-piece body or separate sides and ends?
[3] what works best.... a body with integral roof or a body with integral floor?
[4] with or without the guard's lookout? (which projects about 8mm from the body side)
[5] what is the minimum thickness which can be cast for a lap joint? (as in the Slater's approach to the joint between 7mm sides and ends where the lap on the sides forms the outer piece of panelling on the ends).

Graham

It's a good few years since I did any resin casting – and that in 4mm scale – but the first point is that there will be shrinkage. I suspect the amount will depend on the resin used, but I found typical shrinkage rates of around 8% which would be noticeable if your master was exactly to 7mm scale: it would be almost an inch too short! Alec Tiranti (or whoever) should be able to give you info on shrinkage rates.

Also you are dealing with some quite large chunks of material so I would recommend a modular approach with separate sides and ends, floors, roofs etc. Perhaps even do the sides in three pieces with alternate centres with/without duckets. On reflection I would suggest using something other than resin for the floor and possibly the roof too.

Remember that with RTV silicon rubber moulds you can have a limited amount of undercut which should be able to handle the panelling etc on the sides of the ducket.

I wish you the best of luck.


Richard

 

[Dog Star]

.... there will be shrinkage.... the amount will depend on the resin used.... typical shrinkage rates of around 8%....

Interesting shrinkage figure. I have been given circa 1% from a customer of CMA, I need to ask CMA.

.... I would recommend.. ..separate sides and ends, floors, roofs etc. Perhaps even do the sides in three pieces with alternate centres with/without duckets.

I favour the separate sides and ends approach as I feel this gives more control over model assembly - hence my question about the minimum casting thickness for the lap of the side / end joint. I do not like the idea of making the side in three pieces as joining the parts of the model is time consuming and the joint is difficult to hide. I appreciate your suggestion about a separate section for the lookout area.... maybe I can make the lookout a separate one-piece casting and hence the side could be finished either with lookout (add casting) or without lookout (add plastic sheet overlay). What started out as a project to do a K15 got broadened to include the K14 and K16... you have have just expanded the horizon by adding further diagrams (those which are carriages without the lookouts).

.... I would suggest using something other than resin for the floor and possibly the roof too.

Clearly if the body is an one-piece with either floor or roof as an integral part then the floor / roof is going to be resin. A separate floor can be 100 or 120 thou plastikard, a separate roof is not yet thought through. When I was intending to do just one coach, in the style of Richard De Camin, then the K15 roof was going to be made by joining two roofs from the V5 kits. Now that the project seems to have taken on the possibility of a mini-production (and not by me) then a separate roof has several options:-
[1] join two shorter roof (eg. V5 parts).
[2] cast in resin (need a pattern).
[3] mould in styrene (um, cost?)

In passing, what do fellow WTers consider to be an appropriate method of joining resin sides and ends?

regards, Graham

 

[Buckjumper]

Read Maurice Hopper's article in MRJ199 which explains how he went about building a master for an LSWR standard van in S scale which was then cast by CMA Moldform (I have one of these castings - superb!). The fact that Maurice was able to use altered cast bodies as further patterns for van variants (meat, fruit, butter) suggests the shrinkage using this firm is minimal, and possibly non-existent.

I'd suggest emailing CMA with your questions and ask for a copy of their guidelines which will be pertinent to the resin they use.

Edit: cross-posted with Graham's above.

 

[Buckjumper]

In passing, what do fellow WTers consider to be an appropriate method of joining resin sides and ends?

Zap superglues.

 

[Dog Star]

I'd suggest emailing CMA.... ask for a copy of their guidelines which will be pertinent to the resin they use.

Good idea. I have searched the CMA Moldform website and cannot find any reference to guidelines.... what am I asking for? (given that I do not know what resin is going to be used.... ).

 

[Simon]

The shrinkage has nothing to do with CMA, it's a function of the resin used. Richard's 8% relates to that evil smelling polyester stuff. Polyurethane resin(s), vastly superior and those used by Maurice's wagons, Pete Waterman's mob, Fred Phipps kits and my odd pieces show virtually no shrinkage as Adrian says above. That said there may be different grades with marginally varying shrinkage rates I suppose.

I have a fair bit of experience in a "kitchen table" sense and have produced some good results, but am still learning. When designing/building patterns, one of the most important things is to fill in all the "gaps" so that the moulding rubber doesn't flow into the interior spaces of the pattern.

No amount of advice will beat trying it for yourself!

Simon

 

[Buckjumper]

The contact I was given back in May was Dave Buttress: d.buttress at cmamoldform dot co dot uk

Edit: Deleted repeat info due to cross-posting with Simon. I'm getting good at this today...

 

[28ten]

In my experience I would steer clear of sides made from sections, they are a devil to keep straight and the joins are difficult to hide, but that may be because I am not the best with plastic. Separate sides and ends sounds the best plan to me as you can reuse parts and it is more economical.

 

[Wagonman]

The shrinkage has nothing to do with CMA, it's a function of the resin used. Richard's 8% relates to that evil smelling polyester stuff. Polyurethane resin(s), vastly superior and those used by Maurice's wagons, Pete Waterman's mob, Fred Phipps kits and my odd pieces show virtually no shrinkage as Adrian says above. That said there may be different grades with marginally varying shrinkage rates I suppose.

I have a fair bit of experience in a "kitchen table" sense and have produced some good results, but am still learning. When designing/building patterns, one of the most important things is to fill in all the "gaps" so that the moulding rubber doesn't flow into the interior spaces of the pattern.

No amount of advice will beat trying it for yourself!

Simon

Thanks Simon. Yes it was stinky old polyester – I don't think anything else was readily available at that time. I'm glad to hear there are better alternatives now and might even try it again!

Graham's best bet would be to ask CMA for their advice, but a bit of practice at home would also, as Simon says, be the best tutor.


Richard

 

[marsa69]

With regards to CMA then speak to John aka dennis-rs as I think this is the company he uses for his fire engine kits

 

[dennis-rs]

hi Yes its the company i use for my models the best on i have used hardly any or no srinkege
give them a call and ask for dave very nice guy and also very helpfull i have his no if req
heres my latest master ready to go yo them for casting

 

[Dog Star]

Yes its the company i use for my models... hardly any or no shrinkage

So you get minimal shrinkage with the castings by CMA, that is good news. I visited the company several years ago and I was shown different resins that CMA considered suitable for 7mm models.... which resin is used for your castings?

give them a call and ask for dave.... i have his no if req

Yes please, thank you.

regards, Graham

 

[Dan Randall]

Hell's teeth Dennis, that master looks excellent!



Regards

Dan

 

[Dan Randall]

Hell's teeth Dennis, that master looks excellent!


Regards

Dan

 

[Dog Star]

Thinking a bit further on this one, yes I know that it is often dangerous when I start thinking.... what is the normal method of incorporating windows into a resin casting such as a coach side? All that I read on the subject of patterns for resin casting into RTV moulds is that there must be no openings. If I include "open" windows in the sides, as per the original styrene mouldings to be used for the new sides, then how is the mould formed such that the rubber does not "go through" the window and thereby trap the pattern?

regards, Graham

 

[Simon Dunkley]

Hi Graham,

Same way as the two parts of the mould do not trap the pattern around the edges: before pouring in the fubber for part two of the mould, part one and the pattern are sprayed/brushed with a release agent, to stop the new sticking to the old.

Regards,

Simon

 

[Tony West]

Thinking a bit further on this one, yes I know that it is often dangerous when I start thinking.... what is the normal method of incorporating windows into a resin casting such as a coach side? All that I read on the subject of patterns for resin casting into RTV moulds is that there must be no openings. If I include "open" windows in the sides, as per the original styrene mouldings to be used for the new sides, then how is the mould formed such that the rubber does not "go through" the window and thereby trap the pattern?

regards, Graham

Graham,
I can only echo the responses re CMA....excellent firm !. I have also attached a couple of photos of one of 'our' 7mm GCR cattle wagons as a demonstration of what can be acheived with a one piece casting !.
Cheers , Tony.

 

[Dog Star]

I have attached photos of one of 'our' 7mm GCR cattle wagons as a demonstration of what can be acheived with a one piece casting !.
Cheers , Tony.

Welcome Tony and thank you for sharing the photos. Not too much GC-speak on here so please try to square the circle with relevant contributions.... under Area 51 maybe. Just watch out for the banter!

regards, Graham

 

[dennis-rs]

Regards to windows on models cma will also do vac forming in clear to fit inside, again speak to dave hes best to advise the no is 01213335805. The office has moved and are now just of the motorway at birmingham just near to the old dunlop building.
hope this is of help.
John.