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Mould Making and Resin Casting

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Mayner
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I decided to have a go at producing castings from my own masters. The rtv rubber and polyurethane resins are Barnes products from Australian but similar proucts should be available in Ireland and the UK.

 

I used Pinkysil rubber which is basically non-hazardous and flexible enough to deal with undercuts but sufficiently rigid to avoid distortion. These products are usually two part that need to be mixed in accordance with the manufacturers instruction. I use small disposable measuring cups and hand mix with a wooden spatula or tongue depressor.

 

After producing a couple of one piece moulds I have had a go at producing two part moulds.

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The first step is to make the mould for the exterior of the casting, in this case a narrow gauge open wagon. The inside of the master is filled with modelling clay and then placed on a bed of modeling clay, the indents are for aligning the two sections of the mould.

 

 

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Some people use Lego but I use plasticard offcuts to build the mould box, the black plasticard is intended to form a sacrificial strip for air bubbles to collect at the top of the casting.

 

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The RTV rubber is mixed by hand in two parts until you get an even colour/texture. The rubber is non toxic and was used for skin casting in the Hobbitt.

 

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Filled the first stage of the mould.

 

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Here is one I made earlier fuel tank mould

 

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Gravity filled 2 part mould venting is very important otherwise you loose half the model:SORRY:

 

 

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Casting separated from the sprues, in this case the master was 3D printed and sanded down to remove the stepping/ridging.

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24 Hours later!

Outer mould and pattern separated from base

 

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Pattern removed from mould.

 

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Rebuild mould box and place pattern inside mould.

 

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Axlebox pattern placed on base for 1 piece block mould.

 

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60 Minutes later Voila 2 complete moulds!

 

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Pattern extracted from completed mould

 

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1st shot from mould.

 

The resins tend to be pretty hazardous I used Barnes Easy Flow Polyurethane resin part A contains an Isocyanate which gives the resin its strength is highly flamable and toxic, needs careful handling and good ventilation, others contain styrene which has similar properties. Less hazardous resins are gaining ground in the UK.

 

 

Besides the side effects mix proportions are critical otherwise the resin sets too quickly or wont set at all, pot life and shrinkage is tied up with mixing, temperature and humidity, results are quite un-predictable.

 

With this method you basically part fill the mould and push in the plug which displaces the resin. The volume can be worked out by calculation of trial and error basically pushing in the plug and adding some weight!

 

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At this stage the main problem seems to be trapped air in the base and sides, probably from pushing in the plug. I will try some plasticard shims on top of the mould to create an air gap when two parts are pushed together.

 

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The wall and floor thickness are probably a bit thin for resin the floor and headstocks are only .4mm. I need at least 10 of these wagons for a coal train so it looks like resin casting should save a lot of time over scratchbuilding individual wagons or assembling kits.

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Very interesting, and interesting information about the resin too. I had always thought that resin and styrene were less hazardous than white metal, which I'm trying to avoid using.

 

Is there a working temperature range for the rubber mould? Prince August do a lead-free casting alloy and, if it could be used along with a non-hazardous mould, it might provide a route to non-hazardous casting.

 

Nice result on the wagon casting.

 

Alan

 

 

Edit: Also meant to say that's a great explanation, and makes the whole thing clearer than other explanations I've seen in MRJ. Thanks.

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Very interesting, and interesting information about the resin too. I had always thought that resin and styrene were less hazardous than white metal, which I'm trying to avoid using.

 

Is there a working temperature range for the rubber mould? Prince August do a lead-free casting alloy and, if it could be used along with a non-hazardous mould, it might provide a route to non-hazardous casting.

 

Nice result on the wagon casting.

 

Alan

 

 

Edit: Also meant to say that's a great explanation, and makes the whole thing clearer than other explanations I've seen in MRJ. Thanks.

 

Alan I would not get too hung up on the hazardous aspect of the resins, the main thing is to have good ventilation and to avoid contact with skin and eyes. Everything is a poison to some extent and the average modellers exposure levels pretty insignificant though a friend managed to poison himself with cadmuim through soldering.

 

Artists and Special Effect suppliers like Alec Tiranti http://www.tiranti.co.uk/ are probably the more useful for some one getting started than commercial fibreglass/composite industry suppliers.

 

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I use disposable syringes for measuring the resin and hardner and use small plastic measuring cups for mixing, metal or styrofoam is not recommended as mixing the resin starts a chemical reaction, part A is incompatible with water, brass and other metals.

 

I use plastic takeaway containers acts as a spill trays for mixing and storing the syringes, its probably better to replace rather than clean the tools with acetone or other solvents.

 

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One I made earlier. Fuel tank from a 2 part mould showing sprues and locating spigots. The master was 3D printed and had a half thickness section for a Kadee Coupling pocket.

 

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Fuel tank from underside. The 3 D master was cleaned up to remove the stepping. This was tricky to cast as there is a tendency for air to be trapped around the flange at the top of the casting and in the spigot holes.

 

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Wagon body mould showing resin extruded through joint between two sections of mould, the resin is solid at this stage the actual caasting was done in aplastic takeaway container.

 

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Out of the mould. Air becoming trapped in the mould cavity seems to be the biggest problem with plug or squash moulds. I drilled a number of holes in the top of the mould to allow air to escape as the two sections are pushed together, I am also experimenting with painting the inside faces of the mould with resin, the slowly pushing the two sections together.

 

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Completed casting. A few air bubbles the mould is from a brass master with a minimum wall thickness of .4mm.

 

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A few castings! A days output trying different pouring techniques.

 

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At last a coal train the Master and its clones. The first batch of castings basically uses up my stock of C&L wagon underframes. Another rake or two would ring the changes.

 

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Close up. I first tried resin casting wagons with block moulds from plasticard patterns years ago, the main problem was with controlling casting thickness and the old resins that was always tacky to touch and warped and twisted all over the place.

 

I am reasonably happy with my first attempts at 2 part moulds, though care is needed both with the planning of the pattern and design of the mould.

 

Having tried a simple open I am tempted to have a go at something more complex maybe a long cattle van or KN for those shipping specials from the West to Dublin or Waterford Ports.

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I'm wondering if it would be better to split the mould half way up the plank side of the wagon rather than at the top for a stronger finish. It seems the top part is very brittle. I have no experience in this but just a thought. Otherwise they look very neat and well done.

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I'm wondering if it would be better to split the mould half way up the plank side of the wagon rather than at the top for a stronger finish. It seems the top part is very brittle. I have no experience in this but just a thought. Otherwise they look very neat and well done.

 

The section of the top was based on an idea in RM Web to allow the air bubbles to collect at the top and break off, which did not quite work out in practice, probably becuse the neck was too thin. T

 

he castings are surprisingly strong considering the wall thickness is less than .5mm in places.

 

I will probably do a re-design on the next mould, which will be for a different variety of open, to allow the resin to be topped up and avoid the excess spilling down the side.

 

The main modification will be to the master, I will aim for a minimum wall thickness of around 1mm with the master in plasticard with brass strapping.

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  • 2 weeks later...

I managed to assemble and paint a number of the wagons over the Easter and fitted a few with couplers. I assembled the chassis for these wagons in a Guest House in Carrick while working in the area about 15 years ago and prepared the master for the body in 2010 slow even by my standards.

 

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Typical C&L 3T running in with an empty special for the Arigna mines.

 

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Assembled wagon resin body Backwoods Miniatures chassis, Kadee N Scale couplers, Blackham snail. needs running numbers, tare and loading, weathering & coal load.

 

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Mixed leaving town, no tail lamp or board. The C&L was ahead of the Broad Gauge lines from an early stage with American style bogie coaching stock and vacuum braked wagons there was no need for a brake van at the rear of mixed trains.

 

I need to do another batch of bodies to complete the rake, the main problem has been air bubbles in the castings and the thin walls in the master, temperature and humidity are an important factor basically a dehumidifer and a constant 20-25° is needed in the workshop.

 

I have a pair of ex-Clogher Valley opens to build in plasticard and a pair of ex T&D ballast wagons to build to use up the current stock of underframes.

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  • 2 weeks later...
  • 1 month later...

Coal trains are about to start running with 10 wagons painted lettered and most fitted with couplers and work in progress on coal loads with real coal.

 

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Weshty produced a sheet of custom decals for solid snails and lettering used on the C&L section.

 

 

The station also got a carriage shed to balance the loco shed at the country end of the station.

 

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The shed is framed in box seection brass, the corrugated roof from a Wills Timber Yard kit and the wall cladding Evergreen planked styrene. The building was spray painted with a cheap car primer with a nice weathered look.

 

This one neeeds some serious weathering, the GSR removed all the C&L carriage sheds in the 30s as an economy measure and the carriages gradually fell to pieces stored outside in the Leitrim weather.

 

Somehow or other the shed at Keadue was forgotten, but there is no money in the maintenance budget to repaint a shed that does not exist.

 

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Ceramic soldering pad a very usefull tool from Micro-Mark, no its not asbestos.

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Guest hidden-agenda

Lovely work as usual John.

 

Can you get Easi flo 60 resin in your neck of the woods?

Edited by hidden-agenda
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Lovely work as usual John.

 

Can you get Easi flo 60 resin in your neck of the woods?

 

Hi Gareth

 

I seem to be having a good run of modelling at the moment with a lot of progress on differnet fronts, holding back on starting the layout until I have finished a lot of unfinished projects has been a great motivator.

 

Polyurethane resins are much of a muchness, looking at the data sheets Barnes Easy Cast has similar characteristics to Easi Flo 6 but slightly longer pot life.

 

The main drawback with polyurethane are the air bubbles and brittleness, I am looking at using a liquid epoxy the nexxt time I have a go.

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Guest hidden-agenda

The reason i asked about resin 60 is its very very strong compared to some others i tried John except as you rightly point out its pot life. I tried the the 80 type which lasts longer in the pot but the draw back i was told by the gents at MB fibreglass is because it takes longer to cure you end up watching paint dry as bubbles appear for no reason. I personally have stayed with 60 as i found it ok for most moulds but again i have found bubbles appearing in the likes of floor areas which i have sheeted with thin plastic if need be. I look forward to your use of liquid epoxy let us know how you get on.

Re G.

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