Resin Casting for Beginners - The Sequel
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This article is a follow-up to two earlier and similarly entitled articles on whitemetal casting and arises from the continued development of the Vee
tipper project covered in the second of these. Upon running my all-whitemetal skips I found them to be overweight and somewhat top-heavy
leading to derailments on rough track and when shunting particularly over points. Given that my first attempts at casting for modelling purposes
were 'epoxy resin' (that is Araldite Rapide) and having read a very interesting article on resin casting in RM a few years back (7mm scale windows
and doors) my thoughts turned to this possibility. Could I cast the 'skip' part in resin and the chassis in whitemetal to reduce the overall weight
and give a low centre of gravity? In short this is what I did and this article records what I learned and how I modified my two part mould whitemetal
casting technique for resin. Its worth noting that with resin you can also use single part moulds so long as your subject has one flat side needing
no detail and limited 're-entrant' features. Many of the comments below apply equally to this technique with the obvious exception of subjects like
'bottom filling'.
I found that 'proper' casting resin is available from Alumilite, an American company. The main difference between this and Araldite Rapide is that it
is much much 'runnier' - lower viscosity - and is designed to fill small cavities. It is also (somewhat) cheaper but in most other respects quite
similar. When first mixed it is the same consistency as a light oil and pours very readily, however it sets in a few minutes and depending on the
casting can be removed from the mold after a 15 minutes to an hour. My first attempts at making resin castings were simply to pour resin into the
skip mould I had made for whitemetal casting. This gave encouraging but not perfect results. The main difference in approach I found to be
needed for resin in the area of mould filling, but also in the type of RTV used to make the mold.
I have used the same type of RTV to make moulds for use with resin as for whitemetal - the 'red' type - and if you only plan to make a few castings
for your own use that's fine and probably the lowest cost solution. If you want to make more than say 10 castings you need to consider using one
of the RTVs sold specifically for use with Alumilite. These types of RTV is 'lower temperature' and cannot be used with whitemetal, but are more
flexible when set than the 'red' type I have used for whitemetal. I believe it may also have different chemical propertied reducing the tendency for
resin to stick to the mould.  This means that a mould made from 'Quick Set' , HS2 or HS3 RTV will last much longer and be easier to work with.
The manufacurer's recommended mould release for resin is the same as for whitemetal - talcum powder. I had difficulties with this at first but
after some trial and error discovered that it does work. The important thing is to get a light dusting everywhere and then knock out the excess. I
have also tried using RTV mold release (used to stop the two halves of the mould sticking together during mould making) but found that although
this 'works' as a release agent the RTV gradually absorbs it and swells up causing all sorts of fatal problems. Regarding the need for a release
there are two related problems if you don't use one. Firstly the RTV inhibits the setting of the resin (this is well documented). Secondly (and
because of this) the resin sticks to the RTV and can tear the mould or damage the casting when it is removed. This is particularly a problem for
small fragile castings which in 009 is often the case! More on wall thickness anon.
Mould filling with whitemetal works best with a simple hole in the top. This is because the main limitation to filling is because the molten metal
cools and sets as it comes into contact with the relatively cool walls of the mould. It is very heavy and expels air from the mould cavity because of
this. Resin by comparison is much more genteel with a slower pour rate, a lower mass and a slower set time. The other important difference with
resin is its tendency to trap air bubbles which then become holes in your casting. Taken together and after much and experimentation and
discussions with a fellow caster (thanks Annie) this lead to me adopting a bottom fill approach to resin cast moulds which eliminates air bubbles
and delivers a perfect casting every time. The cross-sectional illustrations show the skip mould fill channel & vent designs for whitemetal and
I find that with a bottom fill mould the bubbles trapped in the resin stay in the fill channel (as they float). The bottom to top filling process of the
mould cavity also seems to prevent the inclusion of any air pockets. Its important to vent all the highest areas of the mould and to avoid obvious
air traps. You might suggest that the mould illustrated would be better set with the diagonal vertical - this would be true if not for the support
brackets moulded onto each side. If the diagonal approach is taken one of these becomes an air trap which would be difficult to vent because of
the detailed and small nature of this part of the mould. Its important to minimise the volume of the feed channel as almost all the resin you will
use will be wasted in this volume for smaller castings. Use some pipe or rod bent to shape as the 'master' for the channel to make sure it has
smooth walls reducing the chance of tearing in this part of the mould when removing the 'sprue'.
One perhaps unexpected benefit of moving from whitemetal to resin is the minimum wall thickness which can be achieved. Because resin is less
viscous and has a lower surface tension than molten whitemetal (this is obvious to anyone who has poured both fluids) and does not set on
contact with the cool mould you can get it to flow into thinner walls over larger areas. Once I had realised this I was faced with the prospect of
re-making my skip master, a fiddly and time-consuming job. Worse still I was faced with making a plasticard master with thinner materials than I
had previously succeeded with, the difficulty being that thin edges in particular tend to 'curl'. The solution to this was to choose a good whitemetal
skip casting and rework the critical areas (mainly the 'rim') with files & scrapers. The re-worked skip was then used as a master for the resin
mould. This resulted in a much finer rim and improved the overall appearance of the final resin castings. I cannot quote figures for limiting wall
thickness with resin but its certainly less than whitemetal by at least a factor of 2, probably at most 0.1mm, and does not suffer from the same
level of large area limitation. I also soldered two thin strips of brass sheet to the skip rim to provide thin and uniform feed and air vent channels
which double as 'snap off' points for the sprue.
Finally an important note about storing and mixing resin. Resin needs to be stored at reasonable room temperatures, if it gets cold crystals form
and it becomes useless. Keep it away from children indoors in a reasonably temperate place (i.e. not by the boiler or in the fridge). You also need
to keep the caps firmly on except when actually pouring resin as Alumilite and many other resins are Hygroscopic (adsorb water and lose their
properties). You need to shake each container before dispensing as if you don't the setting characteristics will be very variable (i.e. too fast & full of
foam or not at all).
Whilst handling the liquid resin I always wear plastic gloves of the type given out 'free' by petrol stations in the UK. These can also be purchased
at some supermarkets and most DIY stores. This is because uncured resin is sticky and difficult to wash off but is also a good idea from the skin
irritation and health and safety point of view. I mix 1ml of each part (typically labelled A & B, you just mix them 1:1 like Araldite) at a time for my skip
mould using two of the small syringes sold for giving children medicine. I obtained a selection from my local pharmacy both 5ml and 10ml for 50p
each. Obviously you need to work out your mould volume before mixing, this can be done by filling with water to the desired level and decanting
into a measuring cup or syringe. Make sure the mould is ready before you mix the two parts. I use small 50ml medicine cups to mix the resin in
available from Alumilite) stirring well using a cocktail stick then pouring immediately. I find that 'bouncing' the mould slightly - lift it a couple of
millimetres then drop it down - helps the resin move through the feed channel and cavity and dislodges any air bubbles which may form. After
perhaps 30 seconds of slow pouring you should be able to see the resin appear in the vent channel showing that the mould has filled. Leave the
mould until the resin in left in the mixing vessel has set hard then dismantle the mould. You will find from experience how long you need to leave
a particular mould, this varies depending on temperature (quicker in warmer conditions) and wall thickness. If you want to 'push' it make sure the
casting is supported after removal from the mold as it may still be flexible. I use half of an old skip mould for this purpose, and can make a
casting in about 20 minutes.
The photos show my resin version skip mould, castings and the completed final version of my Vee tipper with a whitemetal chassis and resin
skip. Please refer to the previous article for a comparison with the all-whitemetal version which also had a less developed chassis.
Mould before and after filling. Filling is via the round hole, the slot is the air vent.
Mould after splitting showing fill channel and air vent connecting with the mould cavity.
Moulding on 'sprue' after removal from mould
Moulding snapped off sprue and waste sprue
Finished views