The beauty of the MK 17, out of sight out of mind.
SPEC Boots for MK10 and MK15 primary regs
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OK, so I report on the further experiments I made with my aluminium mold.
I made a second silicon SPEC Boot, which came out identical to the first one, perhaps with less eccentricity.
then I made a third one, adding some black acrylic dye to the mixture. Here you see the result...
First the silicon rubber yet inside the mold, after cutting away the excess:
Significant air inclusions are evident. They were not present in the previous orange samples. Adding the black dye past made the mixture more solid, it does not flow so easily, so air remains trapped in it. Probably I did employ too much of this black dye
Than you see the part after removing it from the mold, tilted upside down:
As you see there are not air inclusion in the part which remained inside the mold, so this boot seems to be usable.
However, after fitting it on the MK10, here is the result:
If you look carefully, you see that now the boot is slightly shorter and a minimum gap remains between the silicon rubber and the brass. This means that the rubber had some shrinking while solidifying, which did not happen to the orange parts. Again, probably, this is the effect of adding too much black paste, which loses volume while drying.
However the boot seems capable of keeping the silicon grease inside, without leaking. So I packed the MK10 with silicon grease and mounted the rubber sleeve covering the holes, let's see if after a few days there is some leakage or not...
In the meanwhile this morning I injected the mold with a new mixture, made using much less black dye: tomorrow I will show the results.
I made a second silicon SPEC Boot, which came out identical to the first one, perhaps with less eccentricity.
then I made a third one, adding some black acrylic dye to the mixture. Here you see the result...
First the silicon rubber yet inside the mold, after cutting away the excess:
Significant air inclusions are evident. They were not present in the previous orange samples. Adding the black dye past made the mixture more solid, it does not flow so easily, so air remains trapped in it. Probably I did employ too much of this black dye
Than you see the part after removing it from the mold, tilted upside down:
As you see there are not air inclusion in the part which remained inside the mold, so this boot seems to be usable.
However, after fitting it on the MK10, here is the result:
If you look carefully, you see that now the boot is slightly shorter and a minimum gap remains between the silicon rubber and the brass. This means that the rubber had some shrinking while solidifying, which did not happen to the orange parts. Again, probably, this is the effect of adding too much black paste, which loses volume while drying.
However the boot seems capable of keeping the silicon grease inside, without leaking. So I packed the MK10 with silicon grease and mounted the rubber sleeve covering the holes, let's see if after a few days there is some leakage or not...
In the meanwhile this morning I injected the mold with a new mixture, made using much less black dye: tomorrow I will show the results.
I love this project.
drk5036
Contributor
@Angelo Farina, just to make sure everything is good, why not try placing a piece of your silicone rubber in the silicone grease you are using to fill the regulator? Basically you want to make sure it doesn’t undergo any change in volume due to swelling.
My PhD is in polymer science, and my best friends research group focuses on the mechanical of soft silicone rubbers. It’s well known that commercial platinum-based silicone kits are made up by a large percentage of “free” silicone chains (not connected chemically to the network). In my friends research, they use octane or similar low molar mass molecules to remove these free chains and compare mechanical response. Likely the molar mass of whatever silicone grease you are using is high enough that it won’t be capable of swelling the rubber, but if it can, I think this is something you’d like to know about ahead of time. And it’s a simple experiment to check. If it does swell, we have some options about how to overcome this problem...
My PhD is in polymer science, and my best friends research group focuses on the mechanical of soft silicone rubbers. It’s well known that commercial platinum-based silicone kits are made up by a large percentage of “free” silicone chains (not connected chemically to the network). In my friends research, they use octane or similar low molar mass molecules to remove these free chains and compare mechanical response. Likely the molar mass of whatever silicone grease you are using is high enough that it won’t be capable of swelling the rubber, but if it can, I think this is something you’d like to know about ahead of time. And it’s a simple experiment to check. If it does swell, we have some options about how to overcome this problem...
Well, the black boot is actually filled by silicon grease just now. In few days we will see if there is any interaction between the two materials...
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@raftingtigger totally convinced me of the need to de-gas certain polymers before curing. It is just a fact of life...
I did the bolding in the above quote.
MOST interested!!! Before or after curing??? Proprietary coating? I've been in research too, not asking anyone to give away unpublished work.
References would be most appreciated, though.
For now everything is fine, no silicon leaked out the and the boot maintained shape and hardness.
I have currently built 6 boots, 2 orange and 4 black.
However, I am not fully satisfied of the results, for 3 reasons:
1) The final product is too soft, this silicon rubber has and hardness which is just 30 Shore A.
2) Adding the black acrylic dye makes the stuff a bit too viscous, so it does not penetrate perfectly in the mold, and I have got some imperfect samples, with some part missing.
3) Adding the dye also causes air bubbles to remain trapped in the mixture, hence there are some small surface imperfections.
So I ordered an alternative product, which is a polyurethane black rubber specifically manufactured for molds of model tyres:
sintagom-m-nero-per--modellismo-resina-per-gomme
The finished product has an hardness of 50 shore A, there is no risk of interaction with the silicon grease, and it is already full black.
I hope to receive this liquid rubber soon (Covid-19 permitting) and I will report here how it works.
@couv already asked for a number of these boots. And of course some are for @raftingtigger, who created the 3D model from which I started the design of my mold (changing some measures, indeed).
Anyone else wanting some samples, either by silicon or by PU rubber?
Hey @Angelo Farina , when you make some for @couv, toss one more in for me to try, if you'd be so kind. He can mail it to me.
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