Mould Making

[wobblycogs]

Hi everyone, not been around for a while hope everyone is doing well.

Anyway, getting to the point, my dear old mum needs hard plastic inserts in her shoes to correct a problem with her feet. She has one pair of inserts but another pair would be £300+. While you can certainly live with just one pair I agree that it's a pain in the neck to have to keep moving them between shoes and it means you can't glue them in place so they slip around.

Being the kind son I am I've offered to try and make a couple of copies for her. I've done a bit of research and I think the way a business would do it would be to make a two part rtv silicone mould. The mould would be good for making 10's if not 100's of pairs of inserts. The downside is that rtv silicone isn't cheap, I think I'd need a couple of kilos which would set me back the better part of £50 and then there's the resin and other bits and bobs on top of that.

Bearing in mind that I only need to take a few castings (half a dozen max) I was wondering about making a two part plaster or perhaps concrete mould. Since the plaster / concrete mould will be porous I'm guessing I'll need to do some pore filling before casting, for that I was thinking multiple coats of dilute varnish then a liberal coating in some release agent (possibly PTFE spray). My initial casts will probably be in polyurethane as it seems to have all the right properties.

Anyone here tried anything like this, got any pointers?

Cheers

 

[Jacob]

I think you can get them on the NHS. Ask your doctor

 

[marcros]

have you got a pic or image of the finished article? i did a candlemaking course where they made moulds from silicone rubber. although expensive, i wouldnt have thought it would be that bad. however, something like plaster of paris should work, and may be easier to use- they were casting lead in it on "this old house" the other day. The metal set so quickly it didnt even need securing with a few brads. In seriousness, they let the plaster almost go off, then pushed in the item they were copying. i would think that this would be exactly what you are trying to replicate, even if you did have to do it in 2 halves and join them together with epoxy.

or get the carving chisels out...

 

[carlb40]

Have a read up on glassfibre moulding. You will see they use pva - multiple coats and then a wax for none stick/ release agent. Not sure how silicone might affect things down the line.
I think plaster of paris will be a good option, pretty sure that is often used for moulds.

 

[wobblycogs]

I'll see if I can get a picture later. To be honest I've not actually calculated how much silicone I'd need yet, 2kg is a guess, so I might be pleasantly surprised. They are about half the length of your foot though so I think the mould would end up requiring a fair bit of silicone. I'd have liked to do a one piece mould as well but the shape is just too awkward for that. I'll have a read up on glassfibre moulding but I think I'll be having a crack with plaster this weekend...

As for getting them on the NHS, yes, you can get a single pair (that are next to useless apparently). If you want more than one pair it's time to get your cheque book out.

 

[marcros]

personally, i would have a go with the plaster first. make a simple wooden box, or use an icecream container. wrap item in clingfilm. plaster is cheap enough that you could try to make 2 halves of a mould and make it into a single piece one, although that would make it a single use mould. at least in 2 halves, you can adjust the size/thickness.

a bag of fine casting plaster (25kg) might work out cheaper than a couple of bags of the plaster. i wouldn't even bother with the silicone rubber.

 

[wobblycogs]

I've just measured up for the mould and leaving the recommended 20mm all around the piece would mean I'd need a little over 3kg of silicone which is just not going to happen. I'm pretty sure I could get away with a lot less than 20mm but I definitely agree having a crack with plaster first is the order of the day. Thanks for the heads up regarding fine casting plaster, I was going to try with regular skim coat initially as I've got some laying around but I'll grab a bag of that instead.

 

[DrPhill]

When I were a lad.......

Some neighbours kids used my hand as a pattern for casting. They built a wall of plasticine around it and my fingers** (placed on a table), coated the area with vaseline, and poured fairly thick plaster-of-paris mix over the top. When this had set they turned my hand over, removed plasticine, coated with vaseline, and repeated. When the plaster had set they craked the two parts open and cast a resin hand. It was accurate enough to be spooky without the paint.

It is simple enough to give it a spin, surely>

** they made dimples in the plasticine as locators.

 

[wobblycogs]

Progress so far

The box is made from facia board and mdf. I'll mount the piece in plasticine tomorrow and then pour in plaster.

 

[JakeS]

I'm pretty sure I could get away with a lot less than 20mm but I definitely agree having a crack with plaster first is the order of the day.

Having done a lot of RTV silicone and a fair bit of plaster mould-making and casting:

- In my experience you need the walls to be about 10% of the width of the mould for most silicone casting. 20mm is totally over-the-top excessive for small parts, and as you've noticed, silicone is often uneconomical for big parts anyway. It's mostly used because it holds very fine detail, but it looks like you don't need fine detail at all for this piece.

- Silicone is very flexible. Which means that it's great for moulds with a degree of undercutting, because you can bend the mould to get the piece out... but it can be bad for wider, flatter pieces (like this looks like) because the mould can sag and the cast can end up thicker or thinner than you wanted. This is one reason for the 20mm figure, because obviously thinner walls sag more. You can reinforce it to a degree, though, but you have to be careful about how you do it. One method is to do a first pour that covers the top of your master by a few mm, then lay on reinforcing bars (strips of styrene have worked for me in the past) and then a second pour of another 5mm over the top of those can work, and can save you a lot of silicone. You just have to be careful not to flex the mould too much after you've done this, or the reinforcements may tear out of it.


- If you're not going to paint the cast pieces, vaseline is probably the best mould release agent you could ask for. Make sure you make those locating keys DrPhill mentioned; if you're using plasticene you can just stab the blunt end of a biro into the bed on one side to leave dents for this. Just as important, though - whatever you pour the mould with, do a small amount first and stab all these key holes with a cocktail stick or something to get any trapped air out, or you'll find that your locating lugs may be only half a mm tall and wear off easily!

- Another advantage of silicone is that it's very easy to cut cleanly with a sharp knife, so it's easy to cut pour holes and air channels into it after the silicone has cured. If you're going to do a two-part mould in plaster, think about the mould layout beforehand, make sure air can escape from all parts of the mould and you have plenty of space to pour. In a brittle mould medium like plaster I'd advise rolling a sausage of plasticene and laying it over the top of your flat plasticene bed to create an inlet channel - you'll then need to leave that plasticene in place for the second pour, so that when you take the mould halves apart there's already a hole to pour the resin through.

- Bear in mind that you may well get the mix of the resin wrong or something - it's pretty easy to do with some resins - so make sure you're careful enough with the mould that you can make more copies than you expect to need, just in case! If you're casting in polyurethane resins in particular (one of the easier and safer options) it's also really worth keeping the moulds. Polyurethane gets brittle over time, particularly if exposed to UV, so you may find that in a year's time you need to make replacements. (I don't know about other plastics because I've not used them so much, if at all, but I wouldn't be surprised if this wasn't uncommon.)

- If you don't find that plaster works for you, you could also consider plaster-backed latex. Simply paint the latex on to the master, the bed of the mould and up the outside walls to a thickness of a couple of mm, then pour plaster in behind it to bulk out the mould and give it rigidity. You can do this with silicone as well, but latex is much cheaper and it's easier to get it to stick to the vertical walls of the mould case.

- If you give up and use silicone in the end, then one of the really nice properties of RTV silicone is that it sticks to RTV silicone really well. So once you're finished with a mould, you can cut it up into small chunks and use it to bulk out another silicone mould (or pass it on to someone else who does more mould-making than you for the same reason). The inverse is also true - if you know someone who has an old unwanted silicone mould already, you can cut it into chunks and dramatically reduce your own silicone costs. (I've never tried this with - say - old silicone cake decoration moulds, but I wouldn't be surprised if that worked too.)

 

[wobblycogs]

Thanks Jake, I think you've answered every question I'd thought of in one go. I was a bit concerned about sag in a silicone mould as one part of the shoe insert is quite thin (2 to 3mm I'd guess from the way light passes through it). I did a little trial run yesterday with vaseline and it worked well, these items won't be painted so there won't be a issue with trying to get it off the finished piece. Can't wait to get some plasticine now and give it a go.

 

[andersonec]

Liquid latex rubber,,,,, for mould making.

Andy

 

[Steve Maskery]

Just a word of warning about casting hand moulds. It's not very long ago that a young girl at school had her fingers burned off in a casting exercise. The mould got so hot and set so quickly that she lost her fingers. I was a very nasty event. I'm not trying to be a killjoy, just raising the point.

http://www.telegraph.co.uk/news/uknews/ ... esson.html

@DrPhil
When I were a lad, things were ever so bad
But not quite as bad as when me dad were a lad
And when me dad were a lad it were nearly as bad
As when me dad's dad were a lad.

 

[toolsntat]

Just a thought :idea:

http://www.tomps.com/shop/polymorph-the ... p-206.html

Andy

 

[DrPhill]

Just a word of warning about casting hand moulds. It's not very long ago that a young girl at school had her fingers burned off in a casting exercise. The mould got so hot and set so quickly that she lost her fingers. I was a very nasty event. I'm not trying to be a killjoy, just raising the point.

http://www.telegraph.co.uk/news/uknews/ ... esson.html

Wow, I would never have thought that could happen. When my hand was encased the plaster got warm, but it was only an inch or two thick, and one side of the hand only. Just goes to show that hazards can hide in the most innocent looking places.

@DrPhil
When I were a lad, things were ever so bad
But not quite as bad as when me dad were a lad
And when me dad were a lad it were nearly as bad
As when me dad's dad were a lad.

Nice one, not heard that before. But why does my mind star doing a regional accent when I read it?

 

[Steve Maskery]

It's the chorus of a song. I go to a singaround every Wednesday and someone did that one. It's been an earworm ever since.
S

 

[wobblycogs]

Wow, that story is horrific, I feel very sorry for the girl. I knew plaster / cement etc got hot while setting but I'm surprised it got that hot.

I performed a little experiment yesterday (no laughing when you hear the details). The wife couldn't find any plasticine or clay when she went shopping so I made up some salt dough and half embedded a crayon in it as a test piece. I then poured regular skim coat plaster over it and left it over night. I cracked it open this morning to find the salt dough had gone a bit weird but it had lasted long enough to give a pretty much perfect top half of a two part mould. I wouldn't recommend it as a way to make moulds but I'm surprised how well it worked. The plaster has taken a perfect copy of the crayon and even has an imprint of the salt dough. I made up a slightly wetter than normal skim coat plaster which made getting bubbles out easy with a little agitation. My next attempt will be with plasticine or clay just as soon as I find somewhere around here that sells some.

 

[Eric The Viking]

I got caught out once.

We've restored a few marble fireplaces down the years. They're held together with plaster of Paris (traditionally), and to attach them to the wall you basically pour plaster down the back of the columns then put the shelf on (there are usually hooks at the top of each column that also engage with the wall). Anyway I thought I'd use up a half bag of casting plaster from earlier in the job, and started mixing it by hand in a bucket (as you do, with casting plaster). It went off VERY fast - about 30secs, and got really hot in the process. I just about realised what was happening and got my hands out of the bucket - still had to break off some inch-thick "gloves" though.

Not comfy, and although I'm surprised she was hurt that badly, I can see how. That bucket was steaming by the time it set.