Comprehensive Guide To Casting Wheels

[Stabliofarmer]

Continuing on from here 

 

This is a guide to casting Wheels and Tyres with Rubber Tyres. Very useful to avoid excess cost from breaking models for wheels, the skills are adaptable to other parts and I will try and add some photos of moulds for other parts and methods of how I've done them

The basic idea for the mould for both the rim and tyre is to create an all sided part that would normally require a two part mould from a one part mould. This reduces flash and requires less faffing in preparing the mould before each cast.

Starting with the rim

One short (5mm) length of plastruct tube is glued to the side of the rim. This is then glued to a piece of card. You may notice at this point some writing in reverse on the piece of card. This is to identify the mould once finished as I now have a large number of similar sized moulds this makes identifying the one I need a lot easier as the indent in the card will appear raised in the final mould, the silicone also picks up the pencil so the mould will have its name written in it permanently. 

A sealed box is then built up around the rims, note they have been positioned at an angle with the front of the rim pointing slightly up, this will help avoid an air bubble forming under the lip of the rim when pouring the mould, a common issue I've had.

 

Now for the tyres

A circle approximately 8-10mm greater diameter than the tyre your casting is cut, the writing put on, remember to put it backwards or it'll be in reverse when you finish. Then a plastruct tube and piece of square (5mm long again) are glued to the bottom of the tyre and the tyre then glued on the the card circle.

A sealed box is then built up around it again, it is important that the box is sealed or the silicone will seep out.

I have then draw a circle on the card the same diameter as the centre of tyre, this will be left in the mould and provide a guide for where to cut to remove the tyre later

The method is appropriate for any style and size of tyre, for larger ones you may wish to increase the size of the circle around the tyre to give the mould abit more strength

We're now ready to pour

 

Edited August 15, 2016 by Stabliofarmer

[Stabliofarmer]

Funnily enough I'm just about to run some animal moulds, will try to grab some photos as I go. 

[Stabliofarmer]

See in the link at the start for the method of making the rubber.

A little side note here on the subject of RTV silicone. I have used two diffrent types, the pink one in the link before and a blue one from DWR plastics http://www.dwrplastics.com/product-information/5387c13f01aa9/RTV-Silicone-Moulding-Rubber-104kg-kit The blue is a little bit cheaper to buy and creates just as good of a mould however I have noticed it deteriorates faster. Most places suggest a mould will last 100 ish pours with good maintenance and I am yet to have a mould deteriorate to the point where it can't be used with the pink RTV, some of those moulds are 3/4 years old and have done close to 50 pours. With the blue though I have had moulds tear and break after 10/15 pours so although cheaper to buy in the long run the pink is worth the investment. The pink comes from MB fiber glass http://www.mbfg.co.uk/rtv-silicone/gp-3481-f.html

So pouring the mould is as in the earlier link, a few things to add

• When molding rims tilt the mould to the detailed side up and let the rubber settle into all the details before then topping the mould up completely with rubber, this helps with the previously mentioned issue of getting air bubbles trapped under the lip of the rim.
• Pour from height as all guides suggest, it breaks up air bubble in the mix reducing air bubbles in the final mould without the use of a vacuum cylinder
• Tap the mould on the bench once poured to help air bubbles detach from the part and rise to the surface

Removing the master part from the mould

You may now be thinking that the part is trapped in the mould and you would indeed be correct. Most guides would suggest using a two part mould where the mould can be immediately separated and master part removed. With this style of mould the part must be cut out and thats where the circle I drew earlier comes in, this shows me where the end of the tyre is in the mould, ideally you want to cut roughly a millimeter outside of the circle till you reach the tyre, carry on all the way round, note the plastic sprues have now been snapped of the tyres

I cut by lighlty scoring the entire circle in one go, the mould can now be pulled apart revealing the scar and then cut down with the knife to the tyre with small 3/4 mm cuts, this way you are in more control of the cut and don't risk slicing through the entire mould (or a finger)

The mould can now be turned inside out like one of these little toys that you flip out put on the table and it jumps up into the air

And the master removed

It is an idea to add mould release agent to the mould now, the agent helps prolong the life of the mould, in the case of casting tyres in rubber it is not 100% necessary but I would recommend it for resin casting

With the rims simply make an incision the same length as the rims diameter, prise the mold open and pop the rim out.

 

[Stabliofarmer]

Now we are ready to pour some parts, for the resin rims follow the method shown in the earlier link. Another side note here is that I havent had any issues with DWr resins which again are slightly less than the MB ones however MB has a much larger range of types, at the moment I'm preferring this Axson F32 http://www.mbfg.co.uk/liquid-plastics/axson-f32-fastcast.html which sets super strong after about a week and have found ideal for casting animals

A few photos of the resin pour and parts as they are out of the mould, flash is cut of with a craft knife, be careful, doing it to early and you'll squish or change the shape of the mould, leave it to late and you'll have to use a hacksaw it sets that strong.

 

[Stabliofarmer]

For making the rubber tyres I am using this product from MG http://www.mbfg.co.uk/polyurethane-rubbers/pt-flex-60-rubber.html I have used both Flex 60 and Flex 75, both are good for tyres, Flex 75 you will need to soak in boiling water like Britain tyres to get them on and of the rims, the Flex 60 doesn't and is the one I prefer

The rubbers natural colour is amber and therefore no good for tyres, although it does look quite cool, I have a set of orange britains dunlop tyres that I will post up later. Look like something you'd get on one of there budget models. therefore we have to add a pigment. http://www.mbfg.co.uk/polyurethane-polytek/polytek_pigment.html £7 for 20g! sounds bloody expensive! However when it says a little goes a far way it means it, so far I've gone through 2.5kg of rubber and only used a quarter of a bottle full (5g) 

Okay so to start with we pour an amount of the Part B into a cup, in this case 30g as I calculated by weighing the masters I'd need roughly 60g to fill all the moulds I wanted

Now we mix in a tiny amount of pigment, it says 0.5% of the weight now that's 0.15g, a pair of scales that can measure that will cost a fortune so I just go for a little dollop on the end of a mixing stick, you start to get a feel for how much you need to add after a while.

Mix it in, blimey its strong stuff!

Now mix in 30g of part A

Now its onto pouring the rubber, after a discussion with Joe, catkom, about syringes I gave it a go and they were easily cleaned and have become part of my method. The syringe is placed without the plunger in the first mould and then filled up with rubber

The plunger is popped on and the rubber injected into the mould, as soon as it appears in the air hole move onto the next mould, once the syringe is empty it can be topped up if you have more rubber left to pour.

Lots of full moulds, now they need to set which takes abit longer than resin, if you are gentle then it should be demouldable after about 30mins, its important to leave it after that or you risk changing its shape and an oval wheel is no good.

 

 

[Stabliofarmer]

To demould turn the mould inside out and gently pull the tyre off then leave it face down to set further.

Hopefully these photos will show the quality of the finished item. The front side wall and the tread are perfect just like the originals. the rear side wall is not as good, it is pitted with air bubbles, sometimes they are much larger than others, sometimes theres barley any, what causes it I don't know, possibly degassing the rubber in a vacuum before hand would help. There is then also the two bits of flash the pour and air hole, I cut these with a pair of scissors straight away and then sand it flat once the tyre is fully set a few days later. The fact there is a poor side isn't an issue with these tyres as they aren't directional, therefore only one mould is needed, although I have three of the Wagon tyres and four super singles as I go through that many it speeds up the process. For directional tyres such as tractor tyres I have made a left and right hand mould which I will photograph another time.

[Stabliofarmer]

That is all for now, I will eventually add more to this including mould complex parts such as whole truck cabs and animals

[Barry]

Very interesting, thanks for taking the time to take the photo's and posting it

[Pete]

Enjoyed reading through that James. Thank you for sharing

[dr-abich]

Thank You for that. I 'm going a little bit different way with results i'm going share soon.

 

[Grassman675]

Thanks for that very intresting

[ginjaninja95]

We need the continuation of this haha

[Fang]

Is it better to have the protruding ring round the centre to hold the tyre on or is it not really needed?

[Stabliofarmer]

It depends on the function of the model. Anything likely to be rolled around you'll want a lip on the tyre. If the models just for display you can get away with no lip, it makes it easer to cast as a simpler on part mould can be used. 

[Janval]

Just a quick question, the piece being moulded - how will it look afterwards? Say if I mould an already painted item, will it be ruined? I've ordered some resin now but wanted to ask this before I start moulding stuff I've already finished up.

Hopefully I don't have to make new builds first, as far as I can see the moulding rubber should not ruin anything. Better safe than sorry I guess.

[Stabliofarmer]

As long as the paint is well adhered to the part and plenty of release agent is used you shouldn't have a problem. There is a small risk the silicone bonds and pulls a little paint with it, but that is very unlikely. 

[IH885XLMAN]

All very interesting  I was thinking of doing a totalnresin rim and tyre then painting the rim to my chosen colour and painting the black in for the resin tyre it would be a one piece mould but would I need to treat the said original tyre so it doesn't fuse to the rubber casting  the plan was to  cut the card to the half circumference of the outer tyre glue to the tyre then to the base card so the tyre is standing right to a slight angle then wall it up then pour in the moulding rubber or would u suggest a slight coating of vasstleine on tyres ????? Cheers Nige 

[Stabliofarmer]

2 hours ago, IH885XLMAN said:

All very interesting  I was thinking of doing a totalnresin rim and tyre then painting the rim to my chosen colour and painting the black in for the resin tyre it would be a one piece mould but would I need to treat the said original tyre so it doesn't fuse to the rubber casting  the plan was to  cut the card to the half circumference of the outer tyre glue to the tyre then to the base card so the tyre is standing right to a slight angle then wall it up then pour in the moulding rubber or would u suggest a slight coating of vasstleine on tyres ????? Cheers Nige 

The only material that RTV silicone rubber will fuse to is itself, so I wouldn't worry too much about using a rubber master pattern rather than a plastic/resin one. It is still advisable to apply some for of mould release to a master pattern before surrounding with RTV silicone. A silicone lubricant spray, specialist mould release or very thin layer of Vaseline would be suitable for the job.

[IH885XLMAN]

Thanks ...that a big help James