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Moulding Plastic Tips

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This is a list of stuff I gave to a forum member back in 2014, I have relocated this in this thread so its easier to find for people wanting to know about Moulding Plastic


I will follow up later and give an update as I have some new tips........ but for now;-


Here is the equipment, it seems like a lot but after you read down through it, it's not. Its like all things- once you have a go, you find how easy it is!


Silicone Moulds;-


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1. Supersil 20 Silicone Rubber Parts A & B. - Mixed by weight 10 parts A to 100 parts B.

2. Weighing scales. - Try and find one that does not switch off after 2 minutes, it's a pain!

3a. Mould casings- I made these from perspex with a 10mm alu angle stuck on, this gives variable sizes for moulding in. Use a perspex or glass base to set-up on.

3b. Mould casings made from plastic card- fixed size

4. Pipette - very important for adding Supersil 20 Part A, don't try to pour it from the bottle

5. Vacuum chamber- not fully necessary, Supersil 20 is quite good at getting air away from the master. This was made from a lunch box and a medical vac pump, though the box is not rigid enough!

6. Plaster-scene for sealing the mould casings sides and to the base with a fillet on the outside - ensure the base extends well beyond the casing for this job.

7. A DART bogie mould and its plastic card master.


A mixing container and spatula is also required- go for a plastic kitchen mixing jug its easier to clean- let the silicone dry and then it just peels off.


Mixing thoroughly is very important- cut the end of the spatula square so that you can wipe the bottom and sides of the jug.

Sit the whole thing on a level surface to dry- it will take a good 6 hours to set completely and then carefully open the casings and trim off the outside edges with sharp knife.


The master parts should be well stuck down to the casing base, ensure there are no gaps under the master or the silicone will find its way under and the the mould may tare when taking the master out- you cannot repair a damaged mould, nothing will stick to it! Just start again.


Store the moulds carefully in plastic bags, do not stack them, they will deform over time if stacked. Put the in a box like this;-


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Two sided moulds are possible but master single sided first. With two sided the plastic needs to be forced in with a syringe and this can distort the mould. I support the moulds with MDF backing boards and light elastic bands. When making this type of mould a release agent (ambersil Formula 5) is required between the pouring of the second side- otherwise you will not get the mould open- silicone rubber will stick to itself.


Plastic Casting;-


I use polyurethane plastic, its not as dangerous as resin, and easier to use


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8. Easy Flow 120 Parts A & B. Mixing is by volume 1 Part A to 1 Part B.

9. Mouldlife resin pigment. Mix no more than 5% of the plastic volume otherwise the plastic will react and bloat, or be very brittle.

10. Asatone- for cleaning syringe and fine Delux points, n other things!

11. ambersil Formula 5 non silicone release agent. Not fully necessary but it does help to let the plastic flow over complex moulds.

12. Gloves. Are necessary- this stuff is toxic

13. Sheet of plate glass for working on, small mixing spoon, chepo kitchen measuring spoons, syringe, and Delux fine feed points. I made cleaning sticks from NS wire to push through these to clean them.

14. Chepo kitchen measuring cup, these came in the same pack as the spoons, I use these for mixing the plastic in. The dirty one is the maximum plastic I use at a time- you only have about 2 mins to get the plastic fully into the mould. Any longer and it will be a mess!

15. Pots to keep your pigment mixes in.

16. Spirit level to set the plate glass working surface level.


Mix the plastic equal and thoroughly, you have less than 2 mins to pour, clean every thing within 5 mins with kitchen towel. I keep a small jar of asatone ready to drop the Delux points into and suck up into the syringe immediately after the mould is filled. Clean the Delux points with the NS wire immediately.


With the open moulds, for larg parts I pour straight from the mixing cup, trying to keep a constant flow into the mould. Its very hard to get the plastic level with the top of the mould because of the air pressure- just wipe the excess plastic across the mould lightly with a spatula onto a kitchen towel or onto the plate glass. If you create a depression doing this just spoon in a few drops to fill again. You can always sand the back on wet n dry paper when the plastic has hardened.


For small parts and deep stuff I use the syringe- place the point deep into the mould and push, try to keep the point in the plastic at all times. With the syringe and fine points you have only about a minute before it gets harder to push. Do not empty the syringe into the mould, there will be air there and you will just push it into the plastic and ruin the cast.


After 2 minutes do not use the plastic, it will only be a mess. The plastic will harden in about 20mins, but best to leave it for about an hour. When removing the parts flex the mould first, carefully pull the edges away from the plastic with your finger nail and slowly work the piece out.



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Excellent, didn't realise plastic was easier than resin casting. Love the DIY vacuum chamber, how did you build it?


Hi scahalane


Yes a bit of a home built thing, goes up to about 12psi vac and the box is about to collapse! that's enough to haul the air out but it would be better at a higher vac.


The vac pump is from a medical vac dressing unit, run on 9volts, and takes about 8 seconds to get to 12psi, this is connected to a Woodie's best lunch box! Though I have a new chamber- its a SS cooking pot with a perspex sheet lid and silicone seal, I've not stuck it together yet but will post a pic when I do....



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Here is the update I promised in the first post;-


I now prefer this material 'Fast Cast' from RPM, it lasts a little longer after mixing and its not as unforgiving as the other stuff if one gets the mix slightly wrong.


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In the first photo one can see the OXO measuring cups, two sets are required. Very handy for getting the correct quantities A & B in the mix. Though keep the acetone away from them- I did not and look at the state of the little cups!! it will melt in seconds.




Watch out for shelf life of the chemicals- use all at least 6 months after purchase, anything older will cause endless difficulty!! Use the old stuff for goofing....


If using colours- mix the colour into part A first, then add part B and mix thoroughly- thoroughly is very important, otherwise one can get oily residue in areas on the casting. Doing it this way gives more time to work after A&B has been mixed.


One will have to play with the pouring the plastic into the mould to get it right- some work straight off but others do not, test it and see does air get trapped!, try different ways of pouring it in!, inject it!, squeeze it! stretch it! or use a blunt pin to wipe over the mould in the plastic to tease the bubble to the surface and lift it out. Keep notes on the mould storage bag as to how the mould works for later reference.


The silicone moulds will degrade over use, small little bits of silicone get pulled away from areas- mainly where the silicone can flow into the glue joints of the master, paint priming the master can assist in filling these areas, which will help to minimise degrading. A complex mould will take about 30 to 40 goes, and then one has to make a new one- keep the masters!!


Quick Moulds for Small Parts;-


For this I use two part Moulding Paste and Supersil 20;-


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Mix two blobs of the stuff and lay it out as the base of the mould, push the master part into it half way and make indent register marks in it (to line up the two half's of the mould), when dry then set up moulding walls around it with styrene card and pour the Supersil 20 to make the second side, use release agent otherwise the mould will not open!


When using this mould, one drops a blob, more than needed, of the mixed plastic onto the base of the mould (blue part) and then the topside is laid on top by kind of rolling it on from one side to the other- hopefully pushing the air out as one goes and squishing the excess plastic out. When the topside is down put a small weight on top and wait till set- a piece of 4mm MDF larger than the mould.




Edited by murrayec
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DART coloured plastic parts production line;-


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This is part of a replacement batch of plastic parts for the DARTs, I got caught out on the shelf life of the chemicals! I made the original batch about 6 months ago and stored it until required, when I went to use it a few months later it had gone all funny and discoloured!



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Hi All


Thanks for looking in....


It is easy and gets easier as one gains more experience of the techniques to make it work!


On the other hand its the time it takes, take the DART chassis parts in photo above;-


There is 38 parts to make up a two car chassis, that's a total of 684 parts for 18 sets being worked on, I work on one mould at a time and there is 5 moulds to make parts for one set, it takes approx 45mins to mix, pour, dry, and clean up each mould = 3.7hours per set = 68 hours for 18 sets.


And then all parts have to be hand processed, clean-up, jigged and glued...... Oh hell I cant think about it any more!




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  • 10 months later...
  • 4 months later...

This €2.00 bargain box SanteFe tank loco came to the workshops missing its smokebox door and front hook sheared off.

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While contemplating what to do American Phil at the Bray Fair had the very thing to make the mould pattern- the same loco for €15.00

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So off with the body and I set about making a mould shuttering around the smoke box door. I used plumbing washers n pipes to do this, as one will find these items all fit into one n another, and with a bit of sealing here n there with Stickitall and Blue-Tak I was pouring silicone rubber.

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The sparkly gold half spheres are there for the second side of the mould if needed- always offset the position of these so that the mould will go together only one way. Yes, they are girly things, found in Hickeys but very handy for this stuff.


The moulds complete and some of the plastic cast tests.

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The hook mould making was not photographed! It was done with the blue moulding putty on the underside of the hook up against the buffer beam with styrene card shuttering all round and carried up well above the hook so the second side of the mould could be poured with silicone on-top of the hook. You can work it out by studying the mould.


A bit of trimming is required on the hook and the smokebox door came out grand the third time, you can see air got caught in the lamp shade in the right hand door- this is removed the second time around with a pin swished around in the mould before the plastic set.

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This project is on the high shelf at the moment, I will post a picture when complete...






I forgot to say- I filled the light hole in the smokebox door with plasticine as I did not want the silicone to go down the side of the bulb in the lamp, this could tear the silicone when releasing the mould off the pattern!! This will be drilled out for the bulb.

Edited by murrayec
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  • 7 months later...
  • 2 years later...

New life for a Lima BR Inter-City 125 missing the front bogie;-

This came to the workshop missing the entire front bogie, ''If the motor works can you fit a front bogie'', the motor works so this is the way I did the repair.

The rear motor bogie was dissembled so that I could make moulds from the bogie sides, this photo shows the bogie sides being set-up to take a mould from the first side, the new bogie frame in the middle is one of my brass etched DART bogies which will suit the purpose.


Two .5mm styrene cards are slotted and fitted to the back of the frame/side, stuck on with 'Sticksall' which seals the join and can be easily removed with white spirit.


I have a support stand made from ply n mdf for this kind of job, plasticine is also used to seal the join on the back of the card- just in case!


This is how the stand works, with the mould box almost ready to pour the silicone mould, just needs some plasticine around the box frame to seal with the base.


Mould poured and set aside to harden, 'Supersil 20' was used.


First side done, bits cleaned up and other side to go the same way.


Both moulds done and a few plastic casts done 'Fast Cast' chemicals were used with a bit of black dye. The lower castings were the first test and you can see they are more grey than black- not enough dye in the mix!


The bogie frame is held on with a snap button so a brass bolster was made, sized to get the loco level which turned out to be 1.5mm thick, which allowed for the bolster to be drilled and threaded M2 to screw fix through the plastic chassis.


The outriggers on the bogie frame above was pushing the sides out beyond the loading gauge, so I used one that had no outriggers and epoxied the new sides on.


And complete.


Electrical pickups were installed on the new bogie and the loco was rewired but this posting is to do with moulding and plastic casting.....


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