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Circuit Owner
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Discussion Starter · #1 ·
I have been making urethane tyres for a while now - I guess I have made about 150 using Shore 20 and Shore 50 urethane. The shore 20 works well for bulky tyres like the old F3 rears (Scalextric C125 Porsche and C128 BMW 3.0 CSL) and the shore 50 for thinner tyres that I don't want to stretch so much (new Scaley F430 etc.).

Both urethanes have a 2-3 minute cure time so by the time I mix the two parts together thoroughly without bubbles I have about 90 seconds to pour.

I only mix 40 grammes in total and that is enough for 16 NASCAR tyres but pouring into what are quite narrow gaps in the moulds inevitably leads to 2 or 3 of the 16 tyres having bubbles and being consigned to the tyre wall pile.

My question - is there a way of slowing down the cure time? OR is there a urethane available in the UK with a slower cure time? I use the Polytek PT Flex. I am more than happy to change brands for a slower curing time.

Any advice greatly appreciated.
 

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Phil Kalbfell
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I don,t know what is available over there, but using a syringe to inject the eurethane into the moulds will speed up the "pour" time. The eurethane can easily be removed allowing the syringe to be used again.
 

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Circuit Owner
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Discussion Starter · #3 ·
Hi Phil,

What a great idea!

I have a 50ml syringe lying around in the garage (from my days as an upgrade/test engineer for medical infusion pumps) so I will give that a go.

Thank you
 

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Are those your only choices? That's an awfully short pot-life. Use the syringe on what you have, and go looking for other plastics.
 

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Premium Member
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I use te syringe as well but find 20ml or smaller easier to handle

I get about 15 minutes pour time with the urathane I'm using which is shore a30 but only about the same as you for th a70

Have a look at your supplier as they should sell a selection and state te setting times.

One thing I have noticed is that a hot room sends the urathane off quicker so I change te mix percentage a small amount so instead of 50/50 I use more like 55/45 to extend setting time. His does not seem to effect the end result.
 

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The chemical reaction creates heat, so I'm not surprised to hear that it speeds up in a hot room.

For pouring resins, I've put my resin in my garage "Beer Fridge" to keep it cool. It gives me almost another 2 minutes of pot life on hot days in the shop. I live in a VERY arid location though, and the repeated cooling/warming cycle of the resin going in and out of the fridge could cause moisture to build up in the bottles in a more moist area.

Have you thought about rigging a pressure tank? That way you mix it, don't worry about the bubbles, pour into the mold, and then pressurize - 35psi works here. No bubbles.
 

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Slot Car Racer and Builder
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I use Smooth -On urethane

I use a Shore-40 for most of my tyre work - the UK distributors are
http://www.benam.co.uk/

Reo-Flex or Vyta-Flex I get 15mins work time easily.

cheers
DM
 

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I may be taking this discussion slightly O.T. but years ago I used to make my own tyres. I used a rubber that was simply heated in a pan until it was liquid, then poured it into the mould, which itself was hot. A cold mould cooled the rubber much too quickly.

The rubber was yellow but I was able to make it black with the addition of black powder paint during the heating process.

As the mould was hot I had time to stir the rubber and thus release any trapped air.

The big advantage with this material is that because there is no chemical reaction involved, it is totally re-usable. So when I felt the grip was going away, I simply re-melted and re-poured the stuff to make new tyres.

The only reason I no longer make them is that my wife became fed up with the smell in the kitchen so I started to use Weird Jack Stinson's silicone tyres but now I'm switching over to Ortmanns.
 

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Hello Mr Modifier

Besides altering the chemical process of your pour's (lowering pour temperatures, altering the amount of activator etc etc) you could also improve your mould.

One of the reasons that air gets captured in the mould is that it can only exit the mould via the same gap as you are trying to pour the UT in.

I dont know if you are using a one piece mould or a two piece mould, but cutting an extra hole/ air escape channel in the mould on the opposite side will help air trapped in your mix/mould to escape.

There are some very handy Youtube tutorials on moukld making

An other tip is to minimize the amount of air in your mix.

Try mixing the UT while the mixing cup is placed on what I would discribe as a "shaker plate"

This could be anything from a Hudy tire truer to any kind of box with a electro motor glued/taped to it....as long as it vibrates

The best way to get air out your mix and mould is off course a vacuum chamber, doesnt even have to be a full vacuum as long as you get it below 1 psi it will work and draw air captured in your UT out.

p.s with any type of casting you'll have residu material, the question is do you want that to be in your mould( in a percentage of faulty casts) or outside your mould (the amount that stays in your seringe and seeps out of the air ecsape channel)

Good luck experimenting
 

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Circuit Owner
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Discussion Starter · #11 ·
Thank you everybody for your advice.

Bubbles in the mix are not a problem for me - I seem to have the mixing well sorted.

I use open moulds so trapped air is not a problem with the first 60 seconds of pouring when the mix is very fluid.

The problems come when the mix starts to thicken prior to setting - it becomes harder to pour and doesn't settle into the mould as easily which inevitably ends up with three or four tyres with large trapped pockets of air (much bigger than bubbles - sometimes 1/8 of the tyre is simply missing!). I do try to pull the edges of the mould open to allow the urethane to get in more easily.

I can usually recover one or two badly poured tyres immediately by using a small spatula to push more urethane into the mould before skimming the excess off the top (a bit like spreading butter on toast) and this seems to work better than you would imagine.

A pot life of 5 minutes would be fine as I could pour (or inject) 30-40 grammes of urethane into 16 individual tyre moulds in 2 minutes easily if the mix was runny.

My Shore 20 urethane has a stated pour time of 5 minutes - no way I say - 2 minutes max before it gets too thick for tyre making.

My Shore 50 urethane has a stated pour time of 8 minutes and I must admit I get less problems with that than the shore 20 but it is still only 3 minutes or so.

Wraith - 55/45 sounds interesting - is it 55 part A and 45 part B? My supplier labels the lighter clearer chemical as part A and the opaque or dark chemical (depending on Shore rating) as part B.

Temperature was an issue last night - the house was probably at 23 degrees. The garage is about 10 degrees at the moment - time to relocate my bottles and where I mix and pour I think (at least until I can get urethane with a longer pot life!)

Time to look for another supplier of the softer urethane at least..

Thanks again for the advice everybody - very, very helpful
 

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Bubbles and pot time, the tyre maker's nemesis.

Using a syringe is the best bet, for many reasons.

By drawing alternate A and B in quarter doses you start the mix early. Just be sure to wipe the tip between them jars...

I don't know how viscose your mix is, but the rest of mixing could either be done by shaking the syringe with to much air in it, and/or add a steel ball and shake.
Or use a jar to pump the mix in and out a few times in.

The next step is bubbles. Here is where the syringe really comes into its own!
Hold it tip up and tap it to escape as much air that way as possible.
Press out the rest. Yeah, some mixture will escape with it. But if you shake it "backwards" you get more air up and mixture down.
Seal the tip.
Tip down.
Draw. And by that make your own miniature and very effective vacuum chamber.
Draw, tap, release. Draw, shake, release. Repeat till satisfied. And keep that tip down!

Now you will have bubble free mixture down at the tip and the extruded air at the upper end.
Go ahead and pour, be happy!

Yes, you could do the first vacuum pulls tip up and press out the extruded air to have even less air in the syringe.
And then continue tip down.

This is by far the best variant I've used.

And going back into history... At first I got myself a 2m³ (!) compressor tank, pump and industrial grade vacuum pump. Didn't go very well. And then the compressor tank gave in at full capacity. The term "gave in" is extremely apt, but doesn't really convey the event...
Then I made a much (much!) better vacuum chamber from a glass jar with metal lid, a bicycle tyre valve, a small piece of rubber (possibly from the same source as the valve) and a neodym magnet (excessive, I know!). And a rather large syringe.
Drill a hole and place the valve backwards through the lid. Another small hole somewhere else, cover it with the rubber and lock with neo.
Ah yes, a short bit vacuum hose. To connect valve with syringe.
Connect, draw, disconnect and empty. You will very quickly build a remarkably hard vacuum.


No, a 50ml, or maybe even a 100ml, syringe used as above will help you tremendously. I hope. It did me.

Good luck and happy casting!
 

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QUOTE (Mr Modifier @ 1 Mar 2012, 10:23) <{POST_SNAPBACK}>...The problems come when the mix starts to thicken prior to setting - it becomes harder to pour and doesn't settle into the mould as easily which inevitably ends up with three or four tyres with large trapped pockets of air (much bigger than bubbles - sometimes 1/8 of the tyre is simply missing!). I do try to pull the edges of the mould open to allow the urethane to get in more easily.

Sometimes the easiest solution is overlooked


What you describe as "prior to setting" is actually already the curing process.
This starts the moment the 2 components are mixed.
Also during the curing process the mix generates its "own" heat, so the more you mix, the more internal heat is generated and this off course accelerates the process.
(Noticed how any mix residu in the pot cures faster then the UT in the mold)
So why not try mixing smaller amounts, enough to pour 8-10 tyres. This would ensure the UT remains fluid enough to flow freely.

another example of the KISS principle
 

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WRP World Champ 2015/2016
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My second favourite acronym that one! (R.T.F.M. being my fave).

Very informative thread with some great ideas. Now s soon as the EU introduce the 28 hour day, I may get to do some pouring.

Fancy a batch of Shore 20's for next Weds Rich? Could spice up the evening a bit (especially when I grease Leary's rears).
 

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Circuit Owner
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Discussion Starter · #15 ·
Hello,

Yes that was my plan "C" - mix less. I was just trying to avoid the fiddling about and make this as efficient as possible.

The Smooth-On urethane has a pour time of 30 minutes so I think that will be my next purchase.

I notice also that the info sheets state that once the bottles are opened the cure time of future mixes from those bottles is actually reduced - so I guess I am suffering from buying a 1 litre kit instead of a 500ml kit. False economy it would appear!

For now I will use up the remaining urethane over the next week - mixing in smaller batches in a cool room and using a syringe to pour.

Thanks again for your advice.
 

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Circuit Owner
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Discussion Starter · #16 ·
QUOTE (snurfen @ 1 Mar 2012, 12:01) <{POST_SNAPBACK}>My second favourite acronym that one! (R.T.F.M. being my fave).

Very informative thread with some great ideas. Now s soon as the EU introduce the 28 hour day, I may get to do some pouring.

Fancy a batch of Shore 20's for next Weds Rich? Could spice up the evening a bit (especially when I grease Leary's rears).

Hi Snurfen - I will get pouring!

Actually 50's would be better for the cars we will use on Wednesday because the 20's are too elastic for those thin tyres and will come off the rims unless you glue them. I think I have enough rears in both compounds anyway - I will check and make more if needed.

R
 

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Slot Car Racer and Builder
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I have found I rarely get to use the whole bottle of urethane the clear one tends to go off. I have taken to sealing it in a zip look bag which has helped.

DM
 
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