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post #1 of 8 (permalink) Old 08-12-14, 01:42 PM Thread Starter
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Default How to resin cast multiple bases

This tutorial has been created as I've been asked a few times at my LFGS about how to do resin castings of your own bases/creations and I suspect that it is very likely that there are members here on the forum with this same question. Obviously this method allows you to replicate multiple units of your creation and this is especially useful for bases if you want multiple units of them.

So... on this post I'll present the method of how I make multiple copies of my rocky bases creations. The items you'll require are:

1. Base(s) to be replicated
2. Lego blocks
3. Small piece of 3mm thick MDF board (or suitable equivalent)
4. Some tacky putty (i.e. Bluetac)
5. Mini digital scale, paper cups and stirrer
6. Condensation Cure Silicone Rubber & Catalyst
7. Vacuum Chamber and vacuum pump (optional items but handy if available)
8. Talcum powder
9. 2-part Fast Curing Polyurethane Casting Resin
10. Scissors


To start off, create your base to suit your requirements. In this tutorial, I created a simple 60mm rocky base out of stones/slates and sand specifically for one of my Space Wolves Thunderwolves Cavalry model which can also be used for other models as well. Once this is completed, the most important thing to remember is to allow the glue on the base to properly dry and cure. It is preferable for this to have AT LEAST 24hrs to cure before you start, otherwise you may find that the base you created falling into bits during the de-moulding process.




When the base is all cured, created the mould container using a small 3mm MDF baseboard and appropriately sized Lego blocks. You will need to allow room all around the base to get a good solid mould. The rocky base is then simply secured in place in the centre of the mould container with bluetac.




Now the fun begins... I use the the Silicone Mould Resin Casting Starter Kit from a company called easycomposites here in the UK. This kit costs approximately 65 (including delivery) and gives you the CS2 Condensation Cure Silicone Rubber and Fast Cast Polyurethane Casting Resin plus the handling accessories. Depending on where you are, there will be equivalent silicone mould resins casting kits that work in a similar manner. It goes without saying that you should read the silicone and resin mixing instructions carefully before you start using it.




To work out how much silicone is require to create the mould, measure the volume of the mould container allowing for a minimum of 5mm above the highest point of the item you are casting. In my case, the required volume was 8cm (L) x 8cm(W) x 1.5cm(H). The 1.5cm height was calculated allowing for the extra 5mm above the highest peak of the slates. This works out to a volume of 96 cm3. Considering the density of the silicone rubber/catalyst mix at 1.1 g/cm3 as stated in the supplied datasheets, the weight of the silicone required is calculated to be 105.6g.

With the required Silicone to Catalyst ratio at 100:5, this gives the individual compound weights as:

Silicone - 100/105 x 105.6g = 100.6g
Catalyst - 5/105 x 105.6g = 5g

Having worked out the required weights of the two parts, use the mini digital scale to pour out the correct quantities of the Silicone Rubber and Catalyst into a paper cup. Note that for this particular product, the mixture has a pot-life of approximately 2 hours at 25 degrees C before it starts to set and a 24-30 hours de-moulding time, giving plenty of time to work with it.

After pouring out the required quantities, mix the two compounds thoroughly with the stirrer. This stirring process will by default introduce air bubbles into the mixture (even if you use a slow motion mixing procedure) but there is nothing you really can do about it at this stage.




To deal with the air bubbles in the Silicone Rubber mix, you can do two things.

The first, if you don't have access to a vacuum pump and chamber is to use the long pour method when pouring the silicone rubber into the mould. To do this, start pouring the silicone rubber at one empty corner of the mould container in a small and thin stream and gradually raise the pouring cup to approximately 1 metre above the mould container whilst maintaining the constant thin stream of silicone rubber pour. What this does is to "stretch out" the pouring stream of the silicone rubber and allow the trapped air bubbles to escape during the pour. This is by no means a full solution to the trapped air issue but will reduce the amount of trapped air in the mix by some amount.

The second method is to use a vacuum pump and a vacuum chamber to degas the silicone rubber before pouring. The degassing kit can be purchased online (eBay, Amazon, etc.) or from an appropriate resin casting supplier, costing range from 100 ~ 1000 depending on the capacity and pump requirements. Note that the most basic kit will suffice for this if you are simply doing it at home and not for commercial purposes! The picture below shows the set-up I've got in my mini-garage/workshop at home (ignore the mess!!!).




What you need to do is to place the silicone rubber mix after stirring into the vacuum chamber, seal off the evacuation valve and start the vacuum pump. What this does is to expand the air bubbles when a vacuum is drawn in the chamber and that forces the expanded air bubbles to surface and get extracted from the mixture. Note that when doing this process, make sure that you only EVER fill the paper cup no more that half it capacity as the degassing process causes the mixture to rise during the air extraction, the effect is akin to water boiling with the bubbles surfacing to the top. The picture below shows this process at it takes place at approximately -0.88 bar vacuum.






Once the mixture is properly degassed (approximately 15 minutes in the vacuum chamber), you can proceed to pouring the silicone rubber mix into the mould container. I would still recommend using the previously described long pour method for the pour, albeit without having to raise the pouring cup that high. The idea is to pour the silicone rubber in a thin stream to stop too much introduction of air into the pour. Always start the pour at an empty corner of the mould container and DO NOT move from that position. Let the silicone rubber flow naturally around the component slowly as this will naturally "push" into the gaps of the component as it slow envelopes around it. I've tried as best to show this method of pouring in the picture sequence below.




If you have the vacuum chamber, you can do a second-stage degassing process to make sure that all the trapped air is evacuated from the mould. Note that the large bubbles you see in the pictures below are not from the nooks and crannies for the rocky base but from the very large cavity on the underside of the black plastic base!




After this is done, take the mound container with the silicone rubber mixture and place it is a flat level area for it to cure for the next 24 hours (depending on the temperature, this might be quicker or may even take longer!). In my case, I find that it actually cures sufficiently for de-moulding after approximately 12 hours (i.e. over-night). After you are sure that the silicone rubber is properly cured, you can start to remove it from the mould container (that's where I like using Lego to build the container) and extract the original base. Note that it is normal for some bits of loose sand and sometimes rock flakes to stick to the silicone mould. These can be easily removed by hand (digging with the fingernails in reality ) or with a small toothbrush in the clean-up process.








As part of the mould cleaning up process, it is important to prepare the 4 edges of the mould base by cutting off the raised edges. This will allow the mould to sit properly flat on the tabletop during the resin casting stage.




Once the mould is properly cleaned and probably washed to get rid of all the loose sand, etc., I always give the mould a dusting with the talcum powder before starting the resin pour process. This allows the mould to be properly dried and cleaned prior to the pour. This is also recommended for subsequent casts especially if you are taking the mould from storage as you'll find that the silicone mould somewhat oily with a very thin layer of silicone oil that the mould releases over time. The powdering stage helps clean off this layer.




In a similar manner as with the Silicone Rubber/Catalyst mixture, measure and pour out appropriate quantities of the 2-part resin mix.

Tip* - Place the empty rubber mould on the digital scale and "zero" the scale by pressing the TARE button. Next fill the mould cavities with water until it is full. The weight reading will give you the weight of the water used to fill the mould. Multiply this value by the density of the resin compound and you'll have the minimum required weight of the resin needed to fill the mould... easy peasy!!!

The Fast Cast Polyurethane Resin requires equal proportions of the Part A and Part B compounds for the mix. Unlike the Silicone Rubber mix case where there was a pot life of around 2 hours, the Fast Cast Resin has a very short pot life of only 3 minutes! So make sure that everything is prepared in advance before you mix the two resin parts together as you literally have to pour it immediately after you finish mixing! It is important that the mould is place in on a flat surface and where possible check to make sure that the top surface of the mould is a level as possible as the resin will naturally find its levelling position and if your mould top surface is skewed, you will end up with a cast base which is higher on one side. The resin mixture is fairly viscous so it basically pours like water for the first minute or so. After that, it starts to solidify and starts to behave like thick honey and will ruin the cast.




Once the resin is poured, leave it alone for approximately 20~40 minutes. The resin will cure and can be de-mould after this period. Note that the curing of the resin behaves slightly differently as it cures faster in the thicker region compared to a thinner region. It should also be noted that the resin heats up by itself during the curing process so don't be alarmed if you touch the sides of the mould and find it warm to the touch! Once the resin is cured, you will notice a change in its colour to a lighter shade and it is solid when you tap on it. At this point, the resin cast can be de-moulded by simply pealing off the resin cast from the mould in a similar fashion as you did with the original based when you first made the mould.




...and that is it!!! You've now made a resin cast of your original base creation! The following picture show the final product of this tutorial, a close-up of the resin cast showing the level of detail achievable and in the final picture, how I'm using this particular resin cast for my Thunderwolf Cavalry.








Hope you find this useful and even if you don't plan on doing this, you'll know how the sculpted resin bases you buy online or from your LFGS are made!


*Update* - ...and here's the resin base and Thunderwolf all painted up!!!


Don't leave us engineers with too much free time on our hands...
we WILL tinker around and build something!!!


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Last edited by Wookiepelt; 08-21-14 at 07:52 PM. Reason: Typo corrections and updated pic of the painted base!
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post #2 of 8 (permalink) Old 08-15-14, 09:45 AM
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post #3 of 8 (permalink) Old 08-15-14, 10:45 AM
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fantastic tutorial thankyou, i was just wondering if you can tell us a little bit more about the vacumm chamber, where to get one, what to look for, can you make one? also i was wondering what would be the results if you skipped the degassing?



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post #4 of 8 (permalink) Old 08-15-14, 12:22 PM Thread Starter
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Quote:
Originally Posted by Cypher871 View Post
Top marks for this great tutorial mate. +rep
You're welcome!

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Originally Posted by bitsandkits View Post
fantastic tutorial thankyou, i was just wondering if you can tell us a little bit more about the vacumm chamber, where to get one, what to look for, can you make one? also i was wondering what would be the results if you skipped the degassing?
The vacuum chamber needs to be fairly strong. I know of some fellas who use a strong heavy steel pot at the chamber and that will work too. Just make sure you don't use the tin sheet metal pots as that will get crushed under the negative pressure when you draw a vacuum. The top lid is normally a fairly thick piece of perspex with a rubber seal stuck under it and a tapped hole to receive the double valve & gauge arrangement on top.

This eBay link shows a similar vacuum chamber to the one I'm using.

If the mould making process is done without degassing, it not really the end of the world but you will find that a lot of air bubbles surfaces in the pouring/curing process. The main issue this is that with the trapped air bubbles after curing, the mould might tear easier during the component de-moulding manipulation. It will also cause problems if you are casting a complicated component as you may find that the air bubbles get trapped in the crevices of the component resulting in the silicone mould not capturing all the finer details.

Have a look at this video on Youtube from Easy Composites to see the effects of degassing.


Don't leave us engineers with too much free time on our hands...
we WILL tinker around and build something!!!


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post #5 of 8 (permalink) Old 08-15-14, 12:47 PM
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Excellent tute

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post #6 of 8 (permalink) Old 11-16-14, 11:42 PM
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well done!
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post #7 of 8 (permalink) Old 11-17-14, 11:54 AM Thread Starter
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well done!
Cheers, fotoshark!

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post #8 of 8 (permalink) Old 02-26-15, 03:41 PM Thread Starter
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Okay, here's a quick follow-up to my tute on resin casting, specifically on the topic of the vacuum chamber. I've had quite a few queries about where to get the vacuum chamber and many have been "shocked" when they realise that basic small volume ones (2 llitres) cost around 70 and larger ones were well over 100. Recently I had to upgrade my own chamber as I needed contain a larger mould so I decided to document it to a certain extent and log it here for future reference.

A search around yielded prices upwards of 150 so I decided to make one myself instead using the same Poly(methyl methacrylate) or PMMA material, a.k.a plexiglass or perspex. Total material cost doing it myself was approximately 90 (instead of a similar unit available online for 165).

The first thing required is to do some basic calculations to workout the minimum material thickness allowable to withstand the 1 bar vacuum. The design I was after consisted of a 240mm diameter cylinder and an appropriate lid on top. The options I had was a 3mm or 4mm thick, 250mm long cylinder and a lid thickness of 8mm or 12mm. The following were the thin-walled cylinder and circular plate critical pressure equations I searched out and were used to determine thickness suitability (taking the Young's Modulus as 3.1 GPa and the Poisson's ratio as 3.5).



Having done the calculations, the results showed that a 250mm diameter, 3mm thick cylinder would be suitable down to a vacuum pressure of 1.31 bar and a 8mm thick lid would sustain a vacuum pressure of 50.65 bar. This is therefore sufficient for my purpose as the vacuum pump only pulls a vacuum down to 1 bar. These were then bought off eBay together with a 8mm pneumatic hose and associated connectors, a replacement vacuum gauge (my old one broke ) couple of sheets of 5mm adhesive backed neoprene for sealing and a 8" baking cooling rack. So on with the build!

First thing I did was to mark out the location of the various holes (10mm & 6.5mm) on the 3mm thick aluminium base plate.




These were then drilled in steps using 5mm, 6.5mm, 8mm and 10mm bits...




...and threading the 6.5mm holes using a M8 tap.



Then it was simply the case of screwing on the M8 levelling legs, hooking up the pneumatic hose and connectors, attaching the old vacuum gauge in place and connecting the vacuum pump to the whole setup:






Completed setup with both my small vacuum chamber pot and the new larger chamber.






So... vacuum chamber upgrade done with a 45% saving! I'm a happy chappy!

Don't leave us engineers with too much free time on our hands...
we WILL tinker around and build something!!!


Wookie's Wolfies - A Space Wolves Project Log

Last edited by Wookiepelt; 02-26-15 at 04:14 PM.
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