Fun With Rubber

[Neil]

You may remember in 'distractions' that resin casting was planned as ammunition for my scattergun. Though the chemistry set arrived some time ago it's only recently that I've had the mental space to make a tentative start. It's all very trial and error, I thought a record of successes and catastrophes might be both fun and helpful.

I already have a selection of plasticard masters, for the first couple of moulds I've used one of these, the seat unit of a four wheeled trolley and a ready made 4mm figure in soapy plastic that I want to reproduce in resin to make plastic surgery a viable proposition.

Here's the soapy fellow, face down in a mound of plasticine, ready for the Lego mould box to be plonked round him.



The wires out of amusing places (for us, not him) are for pouring in (red) and venting (copper).

The seat unit is already set into its plasticine base and walled in by Lego.



You may just be able to spot the dimple pressed into the plasticine to register the two parts of the mould.

Time now moves on, unseen by the camera the rubber was poured into both formers and left for 24 hours to set. Then the whole lot was inverted, the plasticine picked out from what is now the top of the former, the exposed rubber coated with the silicone release agent and a second layer of rubber mixed and poured to the top of the former.

Top tip #1, don't use Lego for the former, the rubber finds all the cracks and creeps part way through. Though I haven't had any terminal leakages, the moulds had amusing flubbery bits that I've had to trim away with scissors.



Here's the post snipping debris. The near mould is that for the figure; I've used a fresh blade in a craft knife to enlarge the in gate. The last photo below shows the seat unit pattern and its mould.



I've found both moulds hard to part, I think it will help next time if the first half can be prevented from creeping into the join between plasticine and the sides of the former. I was worried when pouring the rubber as there appeared to be a good number of bubbles present even though I folded rather than stirred the two parts together, but it appears that during the cure the bubbles have managed to migrate harmlessly to the surface as I cant see any obvious flaws in the mould. The seat unit hasn't got in and out gates. I intend to pour to the surface in the bottom half then plonk the top half on and see what happens. You can see that the pattern is as good as new so I can either cut channels into the existing mould, or add sprues to the pattern and do another mould.

 

[Simon Dunkley]

I have used this sort of process for whitemetal casting - initially when I still had my own place - even going as far as casting whitemetal into steel tyres, which are then held in a rim chuck for boring and removal of extraneous metal (you need a good thick disc behind the wheel to make sure the sopkes cast properly).

Top tip #1, don't use Lego for the former, the rubber finds all the cracks and creeps part way through. Though I haven't had any terminal leakages, the moulds had amusing flubbery bits that I've had to trim away with scissors.

This is down to personal preference, but I find lego to be really convenient, as you can easily build a new layer for stage 2 of the mould making. But the flubbery bits are a nuisance!

I've found both moulds hard to part, I think it will help next time if the first half can be prevented from creeping into the join between plasticine and the sides of the former.

Yep. PITA, but it does help with mating the two halves back together when casting.

I was worried when pouring the rubber as there appeared to be a good number of bubbles present even though I folded rather than stirred the two parts together, but it appears that during the cure the bubbles have managed to migrate harmlessly to the surface as I cant see any obvious flaws in the mould.

Ah yes, another domestic tale arises at this point.

I reasoned that a spell of vibration (stop sniggering) would help release the bubbles, so I had the bright idea of placing the mould on top of the washine machine whilst itwas on the spin-dry cycle. I had sense enough to put some newspaper between mould and machine. What I didn't do was stay with it, or alternative tape the base down to stop it "walking". Which it did, of course. Into the wall behind the washing machine. Red rubber all down the wall: scrape most of it off, and leave rest to set, I thought (it was a Saturday, and the missus was away for the weekend), and peel it off tomorrow.
RTV-31 (they have moved onto to something else since then) leaves a reddish stain on the wall, I can tell you. My ears were quite red after the bashing I got on Sunday evening, too...

Since then, I have mounted a Meccano pulley onto a bush, itself mounted on a Meccano motor screwed onto the moulding baseplate, and use that to vibrate the air bubbles out.

If you can gain access to the right kit, then a vacuum chamber is a better idea, as Simon C has proved.

 

[Simon]

You may remember in 'distractions' that resin casting was planned as ammunition for my scattergun. Though the chemistry set arrived some time ago it's only recently that I've had the mental space to make a tentative start. It's all very trial and error, I thought a record of successes and catastrophes might be both fun and helpful.

Excellent!

Two halves of a mould, one poured onto another - I've never done that!

As regards boxes, I've never bothered with the lego but instead build plastikard boxes, much better as you can make them and break them down easily. As you say though, the rubber creeps into the tiniest of gaps.

I look forward to seeing mass production of widgets, at this rate you are sure to soon be awarded the "Western Thunder O gauge Mk 2 2nd class seats contract"

Anon

 

[BrushType4]

Looking good Neil

Don't throw away your rubber mould off cuts as you can use them as filler to pack out the next moulds to reduce the amount of rubber you need.

 

[Neil]

Thanks for all the advice chaps and I wouldn't worry about losing the Mk2 seat contract Simon, I'm a long way off confident and competent.

 

[Simon]

Thanks for all the advice chaps and I wouldn't worry about losing the Mk2 seat contract Simon, I'm a long way off confident and competent.

Curses!

 

[iploffy]

I look forward to seeing mass production of widgets, at this rate you are sure to soon be awarded the "Western Thunder O gauge Mk 2 2nd class seats contract"

Anon

Ha Ha Mr Simon you were awarded the contract for your services to the 'O' gauge fraternity

Curses!

Snigger

 

[lancer1027]

Did someone mention Mk2 2nd class seats

 

[Simon Dunkley]

You expect there to be no sniggering in a thread entitled "Fun with Rubber"..???
I'm surprised it took so long....

To be fair, Jordan, I was at work, and yet still took less than two hours work some innuendo into things...

 

[iploffy]

I decided that for once I was not going to ruin another thread, SO I left it for you Jordan

 

[Neil]

Extensive cat-widdle related diy (long story, you really don't want to know) has kept me away from the playroom till a quarter of an hour ago. I've just poured the first batch of resin into the moulds, now doing the expectant father pacing up and down till it's safe to split them and see what lies within.

 

[Simon]

Extensive cat-widdle related diy (long story, you really don't want to know) has kept me away from the playroom till a quarter of an hour ago. I've just poured the first batch of resin into the moulds, now doing the expectant father pacing up and down till it's safe to split them and see what lies within.

How exciting!!

 

[Neil]

And the result is .....

Well depends on how you look at it, the figure copy was a total failure but the seat for the narrow gauge trolley went better than I could have expected.



The top is pretty much perfect ...



... while the base has an air bubble but in the right place. The locating holes haven't come out full size and true shape but the mould wasn't too hot in this area. Pleased enough to start making moulds for the other two components (underframe and tented roof) of the trolley. I've noticed that despite how gentle or rough I am mixing the rubber it gives off lots of bubbles just after pouring, but the moulds so far have been remarkably free from them once set. I wonder if it may be to do with the setting time of 24hrs; I've noticed that there are other formulations that set faster.

 

[Simon]

Bravo! I'd call that a result, no doubt the first of a long line of "Rushby's Resins" products

Re bubbles in the rubber, that's just what it does I think and is where (as far as I can see at my current state of knowledge) the de-gassing business is most useful.

That said, the bubbles in the mould have only ever been a total pain where the mould design traps them near to a finished surface, with my stuff this happened with my Pomparles bridge girder and the ill starred "Lowfit" under the lower edge of the "steel" chassis section (which is an upper edge in the mould as it is cast upside down).

What happened with the figure then? I've never tried anything like that and am interested to learn of the pitfalls etc.

Anyway, well done, and here's to many more creative casts

Simon

Hopefully off down to the shed in a moment....

 

[Neil]

... What happened with the figure then? I've never tried anything like that and am interested to learn of the pitfalls etc. ....

The simple answer is that there were bits missing when I opened the mould and I'm not sure why though I can hazard a guess or two. The first stab in the dark would be that the in gate was too small though the resin is pleasantly thin and liquid enough to reach nooks and crannies. The second would be that I got the mould wrong. You might be able to work out from the first photo that the mould results in pouring into a cavity on a mound in the bottom half of the mould. Some resin leaked out and this might account for a void where the lower part of the torso should have been, the feet and legs cast fine. May revisit this one later on when I've got a few more sucessful moulds under my belt.

 

[lancer1027]

Could someone point me in the direction of a simple " mould and casting kit". I wish to have a little go at casting in resin , wont be anything major but help on where to get the stuff from. What quantity/size of mould solution and resin solution do i need ?.

Rob

 

[Neil]

Could someone point me in the direction of a simple " mould and casting kit". I wish to have a little go at casting in resin , wont be anything major but help on where to get the stuff from. What quantity/size of mould solution and resin solution do i need ?.

Rob

I got this kit (01 - the top one) from Sylmasta. I'm finding it to be a good starting point all I've had to add are paper cups (Poundland) to mix rubber and resin in, Lego for the mould boxes (contrary to above chuntering I'm giving it a second go) Plasticine and some pipettes (vital to measure out the tiny quantities of the stuff that sets the rubber) that Cookie kindly chucked in my direction last year.

 

[lancer1027]

Hi Neil, thanks very much for the info. I will be giving it a go very soon.

Rob

 

[Neil]

More playtime spent upstairs has produced two more moulds for the trolley, and a couple of hours were spent this afternoon making some more castings. Not all went as well as the first one, but at least I can see what's gone wrong on those that have been partial successes or abject failures. However enough reasonable castings came out for me to think it worth while going for a test assembly to see if the concept would work.





There's a small bubble hole evident on the solebar, that a spot of filler would soon sort out but a larger and more difficult specimen on the end of the canvas roof. I'm happy with the patterns, but have come to the conclusion that for reliable and repeatable production I need to create easier to use moulds. However that will be later as next up will be the tea van which hopefully will benefit from the little experience gained so far.

 

[lancer1027]

Wooo Hoooo , parcel arrived in todays post. Its my resin casting kit ( thanks for the direction on this Neil).

I wont get chance to try it out this weekend, away at grandaughters 2nd birthday bash at Stevenage. But i will take the instructions with me and read up on them..

The plan is to attempt to make some chairs for my Gresley Buffet. As im only having the one gresley buffet and there are only 24 seats i thought i would have a go at this. If however this goes ok i may move onto seats for my other coaches. Time will tell. Any advice at this stage from any of you budding casters it would be most welcome ( Simon, Neil hint hint)

Rob