In the last part I wrote about laser engraving, which is a great step towards mass producing custom made insignia for your vehicles or troops. In this part I’ll show you how.
The aim is to create the symbols in various sizes so that I could directly glue them on a chassis or on a SM backpack. I hope they turn out good.
The method to duplicate the simple quasi two dimensional things is the most basic type of resin casting technique called open mould casting.
The first thing to do is to cut the rubber positives from the sheet. For bigger ones, you just cut the outlines with an X-acto knife or with something very sharp. Small and tiny ones are more tricky: the way to go is to cut the upper half off the rubber core. For this I use a plain old style razor blade. You don’t want to cut under the bottom of the symbol (given that you have a design that is not one solid piece, like a cross or something).
Usually the design is too deep for more delicate symbols, thus I cut them shallow, usually around 0,5mm. You have to cut cautiously, parallel with the bottom for best results. I found that this is done easiest by cutting the symbol from the mat and fixing it to some rigid surface like 2mm plasticard.
When all positives are cut they are glued carefully on a piece of plasticard. For open mould casting we need a perfectly smooth and level surface for the mold.
When you finish gluing you can start calling it a master.
Place the master face up on a big enough flat LEGO (actually I use an old cheap alternative, just I love LEGO too much still to abuse it like this) piece and surround it with blocks. You don’t have to build too high, this technique does not require deep moulds. Usually it makes sense to cut the plasticard piece of the master to fit exactly between the block wall, though it is not essential.
Now we pour latex rubber on top. Be careful, some stamp rubber materials stick to latex rubber and unless you use some surface treatment like vaseline they may end up inseparable. Worth checking in order to avoid nervous breakdown. I did, and fortunately my stamp rubber did not stick to latex at all.
Latex rubber comes in different varieties. I use two types from T-Sylox (a local brand, but they deserve to be mentioned here) RP is a super durable variety that is impossible to break, ideal for one part casting or deeper undercuts, while OXAM C1 is a much better flowing and much less durable material. Of course their characteristics are reflected in their price, RP costs about €30 a litre, while C1 only 10.
Tonight I chose C1 as this type of mould does not require much structural durability, and the lower viscosity is a benefit. The latex comes with it's own hardener oil, from which you need 5-7 mass percentage, so acquiring a digital scale is advisable. (and the use of gloves as well since the catalyst is harmful)
After you thoroughly mixed the latex base with the catalyst take a larger synthetic brush – ironically the cheaper the better – and using a little rubber mix paint it into the small recesses on the master. This is important, because the latex is quite viscous, and you want to avoid the forming of small bubbles, at least on the direct surface of the master. Of course if you have access to a vacuum chamber, it is worth trying for best results – unfortunately I don’t have the space for one in our flat (and my wife would not tolerate it anyway. there are limits...)
It is said that pouring the rubber from the height of 50-60 cm should eliminate most of the bubbles, but I haven’t tried it out yet. I usually shake the mould and tap and knock it from under vigorously. Being a persistent knocker I almost always have good bubble-free results.
When it is poured, leave the latex to cure overnight. In the morning you you can remove the master with ease and marvel at your perfect open mould.
Resin casting actually is quite easy.
Nowadays I use Sika Biresin G-26. It is quite good, but not perfect, I think when it runs out I try out something else, most likely Rencast. G26 sells for about €40 for a kilogram of resin and hardener each.
First pour the amount you want to use into cups. Usually I use 3 plastic cups: one for the resin, one for the hardener and a third for excess mixed resin and other junk. Biresin 26 is a 50-50% mass percent resin - which means that for each gram of resin you need a gram of hardener - but since the hardener is just a little bit lighter, if you use a little bit more volume of hardener than resin you cannot go wrong.
One important thing: always wear gloves when dealing with urethane resin; though cured resin is harmless, the raw resin is irritant and the hardener is harmful, so you don’t want them on your skin.
I use a 20 ml syringe for casting: first I suck up the resin, wipe the tube and suck up the hardener. I let air into the syringe for that I can shake the components together with the air bubble, and also because if I don’t, some hardener will remain in the tube unmixed with resin, so injected it will not cure properly. Using a syringe instead of pouring allows you greater control.
One very important part which is often overlooked is that you have to powder the mould thoroughly with talcum. Pure talc - hydrated magnesium silicate (you can buy it in your local pharmacy)– works best and but any talc based commercial product will do. Do not use corn based products.
Talcum dramatically reduces surface tension within your mould, and helps you to avoid air bubbles.
Take a big brush and brush into the mould a generous amount of talc, then remove most of it, and blow away the rest. Only a very fine layer will remain, that enhances your casting results greatly.
I fill up the open moulds with the desired amount of resin, if I want a thinner emblem I just inject a bit less.
For open moulds most de-airing techniques don’t work, so the best way is to use a toothpick and scratch the surface of the mould gently so that small air bubbles that may form in recesses would leave the mould surface.
The cast resin cures in 6-7 minutes, and can be removed from the mould in about 20-30 minutes. The product stays somewhat flexible for an hour or two.
I’m pretty satisfied with the results, all of the insignia is good for something, some of them could be primed and painted right away.
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