Dutton Bay Tramway - Casting

Polyurethane Casting

by Peter Knife

Casting of Covered Van

The following notes describe the techniques I have adopted for casting rolling stock body parts and detail components. I am far from an expert in this field, however I have found that these techniques give a very satisfactory result with minimal facilities, well within the capabilities of the average modeller. It should also be noted that I have not attempted two-part moulding, so these techniques apply only to casting of ‘flat’ components in open moulds.

As with any casting process, there are three stages in producing the final result - constructing a master, making a mould of the master, then pouring the actual castings.

Materials

For moulds, I use a two-part room temperature vulcanising (RTV) rubber - Dow Corning’s Silastic 3481 base and Silastic 81 curing agent. It can be obtained in 1kg quantities (with 50g curing agent). This quantity will make somewhere between 6 and 20 typical size moulds, depending on size. After opening, it has a shelf life of 1-2 months, so make sure you have a supply of masters ready before opening it for the first time. The moulds have an off-white colour and are quite flexible.

The polyurethane I use is BUF Fastcast 810, also a two-part material. The smallest quantity available is 500g (2 x 250g bottles), and this will make plenty of castings from the above moulds. Again, shelf life after opening is 1-2 months, so unless you plan on producing large numbers of castings, the 500g quantity is about right for this time frame. The castings have a creamy ‘milky bar’ colour.

These two are particularly well matched, and ideal for vehicle sides and ends, as well as small detail parts. And the best part is that they are extremely ‘user friendly’ for amateur use!

Other items needed are:

* respirator (industrial quality, not just paper masks)

* rubber gloves

* disposable plastic drinking cups / glasses

* disposable plastic teaspoons

* toothpicks

* wooden skewers (kitchen variety)

* kitchen / postal scales with top pan, capable of measuring 1 - 100 gram accurately.

* spirit level

Preparing the Masters

The first stage in the process is probably the most important - preparation of a master (or prototype) of the item to be cast. This should be as detailed and accurate as possible, and should be prepared carefully without blemishes such as sanding marks, as these will be faithfully reproduced in the copies. In fact, the 3481 RTV is so accurate that it reproduces the outline produced by MEK glue when it flows on styrene sheet!

Setting and curing

I have been scratch building in styrene for years, and have found that the same construction materials and techniques can be used for making masters for casting. However it is important to remember that the back of the master (e.g. the inside wall of a vehicle side) must be flat, as it will need to be fixed to a flat surface for pouring of the mould. Also, while the 3481 RTV will handle slight undercuts very well, undercuts greater that about 1.5mm will place excessive strain on the mould and could dama ge it.

Preparation for mould making

After the master has been prepared, it needs to be fixed inside a ‘box’ so that the mould can be made. I attach the master with MEK to a base sheet of styrene (040" for small parts or .060" for vehicle sides etc), then build a box around it with styrene strips (.040" x .188" or .040" x .250" depending on the thickness of the master), allowing about 1/2" clearance all around between the master and the box.

The size and depth of the box is not critical - a smaller box uses less material but is weaker, while a heavier size will be stronger and is especially recommended for larger masters even if the master is not very thick.

One word of caution - once the master is attached and boxed, it is difficult to make changes to it, so make sure that it is as finished as you want it to be before committing to this stage!

Making the Moulds

The 3481 base material is inert, but the Silastic 81 Curing Agent is toxic and must be handled with care. A respirator and rubber gloves must be used, along with a well ventilated environment. Dow Corning will supply a Product Safety Sheet on request, and it is strongly recommended that this be observed.

Workbench

It is imperative that the surface on which the mould is to be poured is perfectly level. Use a spirit level to ensure that this is the case, and adjust if necessary.

Mixing the RTV

The RTV requires a mixing ratio of 20g base rubber to 1g curing agent. I have found that a 40g/2g mix is about the largest that can conveniently be handled before the mix starts to cure. For larger moulds, if the original quantity is insufficient, an extra quantity can be mixed and poured straight on top of the original (it will bond correctly).

Carefully scoop the required amount of base rubber into a disposable cup sitting on the scales, using a disposable spoon and taking care to avoid trapping air in the rubber. Then (remembering to use a respirator) pour drops of the curing agent into the cup until the total required weight has been achieved. Gently stir the mix using a small round skewer (not a spoon), again taking care to avoid trapping bubbles. Vigorous stirring or use of a larger stirrer will only trap air.

The RTV will be workable for between 10 and 15 minutes after mixing.

Pouring the mould

Using the skewer, run a thin layer of RTV into the mould. Start by running a thin line of RTV around all edges, then run a toothpick along all edges and into all crevices to release any trapped air. Then fill in the flat surfaces, working out towards the edge of the box. Again, take care not to trap any air by moving too fast, as ‘folds’ in the material being poured will invariably cause bubbles.

After the master has been covered completely with a thin layer of RTV, pause for 30 seconds or so to allow any remaining air to bubble out. Then carefully pour until the RTV is exactly flush with the top edge of the walls of the box (no negative or positive meniscus).

Setting and curing

Once poured, the box and mould must be left on a perfectly flat surface for 24 hours. After that, the mould should be removed. Run a razor blade around the inside of the four walls of the box to release the rubber, then gently lift it out of the box. Invert the mould and place it (detail side up) on a flat glass surface to cure. Casting can commence 7 days after pouring of the mould.

Casting

Handling precautions

While the polyurethane components are not toxic, they can be an irritant and it may be appropriate to use the respirator. Good ventilation is required.

Workbench

As for pouring the moulds, it is imperative that the working surface is exactly level.

Mixing the polyurethane

The two parts of the Fastcast mix require a 1:1 ratio. I have found that quantities of between 3g+3g and 5g+5g are appropriate, depending on the size of the mould being poured.

Pour the desired quantity of Part A into a plastic cup on the scales, then pour the same quantity of Part B into the cup (i.e. until the total weight has been achieved). Mix them by gently ‘swishing’ the cup for 15 to 20 seconds.

If you don't have scales suitable for measuring such small quantities then perfectly acceptable results can be obtained by using equal quantities by volume.

The working time of the mix varies considerably depending on the room temperature. As a guide, I have found the working time to be around 1 min at 28C, 2 min at 22C and 4 min at 16C.

Pouring the castings

The mix (which has a consistency similar to warm honey) should be poured gently into the mould. Run a toothpick into all crevices and along all edges to release any bubbles. Then ‘top up’ the casting until it is exactly level with the top edge of the mould (no negative or positive meniscus).

Setting and curing

The casting must be left undisturbed for the setting time appropriate to the room temperature. These times are around:

20 min at 28C

30 min at 22C

45 min at 18C

60 min at 16C

If the casting has an undercut, then these times should be extended by 50%.

Gently twist the mould to remove the casting (which will still be pliable, so it should be handled with care). Invert the casting and place it on its back on a glass surface to cure. The casting is ready for use after 7 days.

General Hints

* Although the RTV has a 10-15 min working time after mixing, it is not advisable to pour more than one mould at a time. This is because of the time it takes to pour and de-air the mould properly. For larger moulds, it is better to mix two smaller batches rather than attempting to do it all with one mix.

* Castings can be poured rapidly, but the working time of the polyurethane is very short and it is not advisable to do more than one ‘major’ casting per mix. However, it is handy to have a couple of small moulds for detail parts (brake cylinders etc) on hand so that any excess mix can be used and not wasted.

* The resultant polyurethane castings are quite strong and not at all brittle, but for added strength, small pieces of wire can be embedded in the castings. Prepare the wire before mixing, then when the mould is about two-thirds full, drop the wire into the casting. Then pour the remainder.

I have successfully made bogies using this technique, with cast sideframes and stretcher and with brass bearings set into holes drilled in the castings.

* It is often convenient to put more than one master in a single mould ‘box’. For example, a wagon side and end would normally be cast simultaneously and in equal numbers. This saves on RTV material and results in a larger, more stable mould. The castings can of course be poured simultaneously or individually, as required.

Assembly and Painting

Image of End Platform Goods Brake

The resultant castings (when cured) are surprisingly strong and easy to work with. Flash is usually very minimal, and can be trimmed easily with a razor blade or a fine file. The castings can be cut, filed or drilled with very little danger of damage unless the parts are very fine.

I use CA glue (‘superglue’) gel when attaching castings to themselves or to other styrene parts. Setting of the glue is slightly delayed when attaching two castings, but is not delayed when gluing a casting to styrene.

After assembly, the castings must be sprayed with a primer before painting. I have found Floquil Primer (grey) to be ideal. If the castings are not primed, the final colour will not coat evenly and will always have a ‘blotchy’ finish.

For information about our suppliers of this material, go to the Supplier page.

Return to the DBT Home Page

Last Modified October 20, 1997 - link to Supplier page added December 5, 2002