TUTORIAL: Making Moulds and Casts

[Petitioner's City]

Lots to think about, thank you all for responding. Hopefully Lego and silicone will arrive soon so I can practice.

I prefer to make a mould that sits in a removable wall. So if you make a wall around the mould to form the mould in say lego and the slice the sides just stick it back in lego when you cast. You then don't get any issues of warpage that tends to happen if you try another method to keep the cut sides together.

I'm a little bit confused by this but might understand soon

 

[Tiny]

I like making them on the lean side but always cast with lego walls in place. Get more moulds out of any given amount of rubber that way.

Yeah, I used to go this way but I ran out of Lego casting 20-30 moulds at a time.

 

[=][=SW1=][=]

Yeah, I used to go this way but I ran out of Lego casting 20-30 moulds at a time.

Lol ... I've got a huge box of Lego reclaimed from the kids.

 

[=][=SW1=][=]

Lots to think about, thank you all for responding. Hopefully Lego and silicone will arrive soon so I can practice.



I'm a little bit confused by this but might understand soon

I'll get the moulds out with the Lego and take some pics

 

[Punktaku]

I’m pretty sure there’s a whole mess of confessed blasphemy in this thread now. The LEGO Inquisition has been notified.

 

[Tiktaalik]

@cardyfreak this is a very informative thread and I want to start buying the supplies and start making two-part moulds. I noticed that you adjust the mix ratio with your silicone to get the desired hardness. The different silicones I am looking at all have a one to one mix ratio and a shore hardness value. What shore hardness would you recommend for making molds of plastic components? can one do much adjusting with a mix ratio of one to one? I would imagine that that could hinder the vulcanization. Also, Do you have any tips for (re) casting an entire sprue?

 

[cardyfreak]

@cardyfreak this is a very informative thread and I want to start buying the supplies and start making two-part moulds. I noticed that you adjust the mix ratio with your silicone to get the desired hardness. The different silicones I am looking at all have a one to one mix ratio and a shore hardness value. What shore hardness would you recommend for making molds of plastic components? can one do much adjusting with a mix ratio of one to one? I would imagine that that could hinder the vulcanization. Also, Do you have any tips for (re) casting an entire sprue?

I can’t remember the shore hardness of the moulds I made, it’s been a while since I last made one! But I’d say the stiffer the mould the better so just try a mould and see how it comes out. It’ll always set up completely so I wouldn’t worry about the vulcanisation process, the main concern for things this size is working time before the rubber gets too stiff to pour. You have a good five or ten minutes working time usually so it shouldn’t be an issue, but if it comes out a little too soft then add more catalyst, and vice versa.
Casting whole sprues can be tricky, more for the time it takes to get the resin around the whole mould. Most sprues are injection moulded plastic and aren’t great candidates for gravity poured resin. I tried cutting vents into sprue moulds and injecting resin with syringes but it often turned out disastrously. The pressure of the resin being injected would force the two faces of the mould apart and ruin the whole cast no matter how many rubber bands I’d add (stiff moulds are good for this reason), and air bubbles were a constant issue. I toyed with the idea of using pressure pots, but all of my moulds had been made at normal pressure and the mixture wasn’t degassed, so using them in a pressure pit wouldn’t be feasible as any air bubbles in the mould would make the casts look like they had little bumps all over, so I never perused it further and haven’t really made any moulds since.

 

[Sundown Dreamer]

Some manufacturers give recommendations as to which of their silicones is best suited for which purpose. I would best follow them.

I recently used a rather soft silicone with a Shore A hardness of 18-20 after 24 hours and a breaking load of 3.1 ± 0.2 N/mm² for tabletop and bulkheads, for example. This is soft enough to demould fine details in model building and tabletop hobby and has worked very well for me.

Medium-hard silicone grades are usually recommended for large and larger moulds. For example, where I bought mine, Shore A 35 ± 2 is listed as medium-hard and suitable for large molds.

The hardness of the silicone has a great influence on the flexibility of the mold and on whether and how easily a model can be demolded afterwards.

 

[Tiktaalik]

thanks for the responses and sharing your experience. The silicone I am looking at has a shore hardness of 13, I think pliable is good, I just don’t want the mould to contort if I use rubber bands to press the sides together.

 

[Sundown Dreamer]

I create my casts in a Lego brick mold on MDF board, which I additional seal with painter's tape or in English perhaps masking tape. Before I also used foam cardboard for the frame but foamboard naturally gets soggy! But both makes the shape smooth. In order for the mold to be evenly filled and thick, I pay attention to a horizontal surface when drying. If necessary, use a small spirit level.

If, contrary to expectations, your mold is too soft and you have problems with rubber bands, you can also protect the mold top and bottom with small MDF boards or something similar and only then pull the rubber bands over it. What could also work well in this case for pressing are small model construction screw clamps.
I wish you every success with your project.

 

[Hobo86]

I'm currently testing two-part moulds with lego/bluestuff/greenput.

so far I had 2-3 bits (in a dozen) with a markedly higher reproduction quality (2017 enforcer weapons)
on the other bits the deeper grooves end up less deep and sharper edges end up slightly rounded

I'm not sure how I can reproduce the more detailed results in the other pieces but might somebody recognize the above?

maybe pressing the putty too softly or using a too dry putty mix might cause this?
maybe 'wiggling' the bit in the cooling bluestuff could cause detail loss?

 

[Stoof]

The outer edges being blurred is probably due to not enough pressure being applied as it's cooling/not hot enough bluestuff.

Not fully filling deeper grooves is common - air gets trapped in the recess and blurs detail on the indent mould.

 

[Hobo86]

The outer edges being blurred is probably due to not enough pressure being applied as it's cooling/not hot enough bluestuff.

Not fully filling deeper grooves is common - air gets trapped in the recess and blurs detail on the indent mould.

ok... I'll see how I can push in the bits in a way the trapped air might get out

and I'll see if I can heat my water a bit over the boiling point (I have a food thermometer)
I looked up the temperature to avoid damaging the plastics

 

[MusingWarboss]

ok... I'll see how I can push in the bits in a way the trapped air might get out

and I'll see if I can heat my water a bit over the boiling point (I have a food thermometer)
I looked up the temperature to avoid damaging the plastics

Yes, I agree with Stoof that trapped air is a problem there. Is there any way you could add a vent to those areas? That’s what they usually do on both resin and metal spincast moulds.

It could be as simple as adding in a small straw or tube (like the ones from a used spray bottle). You can always trim off any putty that goes in that direction.

 

[Hobo86]

there's a lot of small details / airpockets in those areas, they're too small to create vents
(for a rifle shaped part) if i start by pushing on side in (rifle but) first and then gently tip the other side in (barrel) the air might have a chance to get out.

I am going a bit perfectionist into this... most people would be quite happy with what I got so far.

 

[MusingWarboss]

there's a lot of small details / airpockets in those areas, they're too small to create vents
(for a rifle shaped part) if i start by pushing on side in (rifle but) first and then gently tip the other side in (barrel) the air might have a chance to get out.

I am going a bit perfectionist into this... most people would be quite happy with what I got so far.

That small eh?

Could it be possible to push a pin or needle into the mould near there to create a hole that might act as a vent?

 

[Hobo86]

That small eh?

Could it be possible to push a pin or needle into the mould near there to create a hole that might act as a vent?

we're talking about the small ribs in enforcer boltguns...
de-airing 2-3 of those while it cures... does feel problematic with home equipment

i will wash them before as they could also be a bit greasy atm,
I'm recasting these for the 4th time in a bit... I got quite a few suggestions here and on reddit.

 

[MusingWarboss]

Are you pushing greenstuff/milliput/mix into the bluestuff moulds?

Have you considered using something like UV resin?

 

[Mad Robot]

I have no practical experience with bluestuff, but if y'all have questions about silicone molds or resin casting, I'll share whatever knowledge I can..

 

[Hobo86]

Are you pushing greenstuff/milliput/mix into the bluestuff moulds?

Have you considered using something like UV resin?

I'm using 3/4 milliput 1/4 greenstuff (but considering adding a drop of water and putting a bit more effort in pushing it in better)
I did an experiment with spruegoo... I suppose it can work but it would take some effort/multiple thin coats to avoid deforming

I thought about uv resin but for the time being I don't anticipate buying it
I'm trying to get a qualitative result: testing 3D scanning & printing / bluestuff molds

@Mad Robot, if I see artifacts on the next er... copy I'll upload a few pictures to get your feedback