Designing a chapter Part 4. - Resin cast custom shoulderpads

Last time I showed you how to make simple open face moulds and how to cast custom made insignia with them. Now it is time something a bit more challenging. Shoulder pads and one-piece moulds.

Reading the previous post is of course by no means necessary but there are a lot of details explained there that I will not repeat.

I have to confess I’m not very good at drawing. Somehow I lack the ability to draw too well, and even if I sometimes am able to produce a neatly painted mini, I cannot dream of producing enough freehand shoulder pads in constant quality and quantity. Perhaps decals would do, but I want the key symbols white, and  unfortunately I have no access to an ALPS printer, the only thing I know to print white (offset would be overkill). So, given my discovery of laser engraving I opted to go for perhaps a more subtle solution: custom embossed shoulder pads.

The first step is the same as before, carefully cut the symbols from the rubber core. This proved rather challenging for me as the chapter symbol is in a thin circle, and it required a lot of concentration and a fresh blade to do properly.

Once the symbols are cut, I superglued them on standard issue space marine shoulderpads from the bits box. 

This is pretty straightforward, but not easy at all; the pads have a curved surface and superglue tends to stick to everywhere. Be prepared to lose some of the more delicate symbols. Luckily you can salvage the messed up shoulderpads with the help of a small piece of grade 1000 sanding paper.
As usual there is a trick that may help: I glue only the central areas of the symbols first and place them properly on the shoulderpad. Once the glue sets and the symbol is fixed in place glue the edges with a toothpick and position them. In general it pays not to use the super glue jar on small pits and  pieces. 
Anyway here you see the outcome. I'm pretty satisfied on this point.

Next I cut a thin strip of 1mm pasticard, around 1mm wide, and glue it on it a piece of balsa wood. I use Balsa because it is easy to cut with a hobby knife. This bar will serve as the excess basin for the resin.
On the top of the wooden piece fix a tin strip of plasticard, 1 mm wide ad roughly the same of length as the balsa piece and glue it on the wood, on it's side.

Now glue the shoulderpads on the strip of plasticard at even intervals . Calculate how much and what shape of LEGO basin you will need, and build it on the base. Your mould's wall should not be very thick, because it should enable you to remove the products easily.
Cut a piece of plasticard that fits exactly into the opening between the LEGO blocks, glue the balsa stick into the middle of the plasticard piece and you have just finished your master.

Place the master into the basin, and mix some latex. For one piece moulds you need good quality durable and elastic rubber, so for this purpose I always use the more expensive T-Sylox RP. Paint the surface of the shoulderpads with some latex. Be neat and make sure that every little recess is filled.

When you are certain, that there are no bubbles in the rubber on the surface of the master build up two or three levels of LEGO blocks so that the wall should be about 5mm higher than the top of the pads. After the wall is built, pour in the rest of the latex. It seems that tapping (described in the last post) alone is not good enough for shoulderpads, as the "domes" are perfect traps for larger air bubbles. This can be avoided if you take a toothpick and move it around the inside curve of all of the shouderpads. Further tapping will likely reveal an air bubble or two surfacing.
By the way it is useful to regularly blow the surface of the latex as it explodes the bubbles. Again you have to wait at least 6-8 hours before you can proceed with casting.

Above you see the mould as it comes out from the LEGO basin, only the bottom pasticard is removed. When you remove the master from the mould you'll need to use a hobby knife to cut through a thin layer of rubber between the shoulderpads and the strip of plasticard they are sitting on. It is quite easy, be neat.
Now this is how it looks like after a bit of trimming with pirated manicure scissors:

Prior casting you talc the mould similarly to the open faced ones. The difference of course is that you have to stretch open the mould and brush the inside with the powdering brush. face down tap out the

Similarly to the insignia shown earlier your shoulderpads are best cast by a syringe. Open the mould a bit, push the tube of the syringe into the openings, and inject the resin into them, and fill up the casting basin too. Tap the mould on the side and from under, and repeatedly open and close the mouth of the mold to let the bubbles out. This should give you perfect results.

It is worth leaving the pads a bit more time to cure, it makes them easier to remove and much less prone to deformation when pulled out of the mould. A little more than half an hour should do.

This is what become of it. Some are satisfying, others are not good enough, but it is not a problem, because later I will work on the pads and sand and hand-engrave them until I'm happy with the results, when all these are ready, I make the final moulds.

You can finish the shoulderpads by hand and make a new mould of them, since if you use a good quality latex and de-air it properly the new mould should be practically the same quality. These below are the first finished products, as I want to start painting tactic squads soon. I think they are rather good.

The trial painted them for fun last night to:

I'm not really satisfied with the painting of the chapter symbol, but I know now how I will do it when I start my assembly line painting.
To sum up, one piece molds are perfect for cylindrical or spherical shapes like heads or one piece torsos, as there are no mould lines. Unfortunately complex or very delicate items can not be duplicated this way. Fortunately two piece molds solve this problem, and I will do the best I can to help you to shorten the learning curve.
See also:
Designing a chapter part 3. - Resin casting your insignia

Designing a chapter Part 3. - Resin casting your insignia

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.

In the next part I show you the one piece mould technique when I make and duplicate some custom shoulder pads for the Sentinels.  

See also:
Designing a chapter part 2. - Chapter insignia