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DIY Casting

DocRef : JC1a

Casting at Home in Resin and Whitemetal.

Note : Specific applications of these techniques can be found useful in the batch construction of rolling stock.

See also Wagon Making

If the builder requires batches of identical parts, one of the quickest and least tedious methods is to produce a 'pattern', pour a mould and cast as many items as required. There are cold casting and hot casting techniques which employ a variety of mould-making materials,casting compounds and metal alloys.

Cold castings are commonly made using polyurethane or polyester resins but can be of epoxy resin, (e.g. Araldite), or car body filler. In fact this can be anything which is a liquid or a paste and will cure into a suitably strong material for the intended application.

Hot castings usually involve metal alloys. These are low melting-point alloys based around Lead and Tin, (these are known generically as 'whitemetal'), and are commonly used for 'home casting'. But other, higher melting point metals, such as brass, are frequently used in commercial processes.

For the purposes of this guide we shall be looking at 'home casting' employing resins and 'whitemetal'. The materials used are potentially hazardous and see the Appendix to this article regarding the safety precautions to be observed.

Methods.

The starting point for any casting is a 'pattern' from which to produce the mould. This can be made of any material which can stand up to the temperatures at which the mould making material is poured.

For resin casting, a soft, flexible, silicone rubber is commonly employed for the mould and this is poured cold. It is possible to use hot poured vinyl based mould material which has the advantage that it can be remelted and reused. For hot casting of white metals, a harder, less flexible, Room Temperature Vulcanizing, ('RTV'), rubber is the usual choice.

For details of various available types of mould-making materials see the Appendix to this article.

Patterns.

For cold casting these are easily made of styrene ('Plastikard') sheet and profiles or can be of soldered metal or even timber. For hot casting, using RTV, the same materials can be used as for cold casting. If using a hot poured mould, then the materials chosen must be able to resist the casting temperature. If reproducing parts from a white metal pattern, bear in mind that the mould making material may be sufficiently hot to melt the white metal alloy.

When making the patterns, bear in mind that the mould will need to be easily removed, and that any 'undercuts' in the design will need to be within reasonable limits, taking account of the flexibility of the chosen mould material. If the undercuts are too deep, either the mould will tear when removing from the pattern, or the castings will break when being released from the mould.If the pattern has a smooth, impermeable finish then I have found a 'mould release agent' to be unnecessary, but for other types of surface, a coat of sealant, (e.g. varnish), and a spray of silicone agent is advisable.

Moulds.

If the item to be produced is required to have only one side bearing detail, examples of which might be the sides of an enclosed object such as a building or a covered wagon, then a simple flat mould with an open top can be made. The pattern needs to be fixed to a dead level baseplate and to have sides built up around it to contain the silicone and deep enough to provide at least 3mm depth of silicone above the highest detail of the pattern. I normally make the baseplate of thick plastic card and the perimeter wall of the same material. The pattern needs to be on a dead level surface when pouring the silicone and the mould needs to be on a dead level surface during the resin casting process.

If the object is to carry detail on all aspects, then the mould making is more complicated and involves pouring in two parts, with registration spigots and holes to ensure the two halves engage accurately.

An easy method of producing the two part mould is to impress the pattern, to half its depth, into modelling clay (Plasticine). Next insert spigots into the clay (e.g. short stubs of plastic rod) and then pour the first part of the mould. When the mould has cured, remove the clay and spigots (leaving the pattern in the half-mould), treat the cured surface of the half-mould with a release agent, (specialist silicone based sprays are available or simply brush on Vaseline/ petroleum jelly), and pour the second half of the mould. You should now be able to separate the two halves of the mould and remove the pattern without too much difficulty. As with the single facet pattern, the cautions on positioning "undercuts" must be observed and the clay with embedded pattern needs to be enclosed in a built-up container during the first stage and, in the second stage, the half-mould with embedded pattern needs to be similarly contained.

Enclosed two-part moulds in the D.I.Y home environment are really only suitable for hot casting of white metals, since without a vacuum chamber to 'de-gas' resin, accumulations of air bubbles will almost certainly spoil any resin castings.

Enclosed two-part moulds in the D.I.Y home environment are really only suitable for hot casting of white metals, since without a vacuum chamber to 'de-gas' resin, accumulations of air bubbles will almost certainly spoil any resin castings.

Enclosed two-part moulds need to have channels cut both to pour in the molten metal, to release the displaced air from the bottom of the mould, and any side 'branches'. These venting channels must exit at the top of the mould, otherwise the molten metal will run out before solidifying. When mixing the mould rubber, try not to introduce too much air. When pouring the mould, work around the edges and allow the mixture to creep over the pattern surface, it is less likely that air will be trapped this way. When you have poured the mould, sharply tap the mould on the bench several times to release any trapped air, (you will see bubbles rise to the surface).


Casting Materials.

Resin.

These are obtainable in various types or grades according to application, (see Appendix to this article), but there are general points to be observed when handling, mixing and pouring. Temperature is a critical aspect in that the 'pot life' (i.e. the time period during which the mixed resin remains usable), varies from as little as a couple of minutes up to 30 minutes or more, this is governed by the type of resin chosen. The warmer the materials and ambient temperature, the faster the resin will cure.

Ideally, for the purposes of efficient model-making, a pot life of 4 minutes and a cure time of 30 to 45 minutes is a good combination. The biggest enemy of DIY casting is the air bubble! Resin curing is an exothermic process and tiny bubbles are produced which can ruin a casting. The source of these bubbles can be from:

  • Air trapped in the mixing process.
  • Air trapped in details in the mould. (Of which rivets and bolt head detail are prime examples).
  • Moisture present in the mould.
  • Moisture present in the resin. (If it has not been stored properly they absorb atmospheric moisture).

To reduce the risk of bubbles, and it is very difficult to eliminate them all without 'de-gassing' equipment, use the following procedures:

  • Open resin and hardener container lids as briefly as possible and replace tightly.
  • Ensure moulds are dry.
  • Warm ingredients to 18/20C.
  • Avoid mixing air into resin / stir gently.
  • Lightly brush French chalk (talc) over the dry moulds, this reduces surface tension, (do not use a mould release agent).
  • Use a tool with a rounded head, (e.g. a brass rod), and run over the rivet heads or other mould detail immediately after pouring the resin.
  • For a few seconds vibrate the surface on which the mould is standing to release any trapped air.

Example of stages in producing a polyurethane wagon side (in this case for the "Flexikit" LMS D1839 5-plank)

Pattern

Silicone Rubber Mould

Casting

White metal.

This is a generic term covering a whole range of low-melt alloys with differing characteristics of hardness, malleability and melting points. See the Appendix for information, but in general terms, the higher the Lead content the more malleable, (soft), the alloy will be. The characteristics and recommendations for various applications will be found on the suppliers' websites and in their printed catalogues. Temperature is again critical.

  • Too low and the casting will not be properly formed.
  • Too hot and the mould may be damaged and potentially harmful gases and vapours, (e.g. Lead or Cadmium), will be given off.
  • A temperature controlled electric melting pot can be purchased for as little as £60 with more expensive and larger models available, (see Appendix).

When an RTV mould has been left to cure for around 72 hours it will be ready for use. There are lengthy 'tempering' processes involving baking in the oven which will prolong the life of a hot casting mould but I have never found this to be necessary. To produce a good casting, the mould needs to be heated. You can place this in an oven but I usually just pour two or three castings which are discarded and this is sufficient to heat the mould sufficiently for the 'production' run. Dust the mould lightly with French chalk to help the metal flow into the mould and clamp firmly in the vice. When the molten metal has been poured into the mould, a light 'tap' or two on the side will help release any air and help the metal settle into the detail of the mould.

Example of pattern for a Midland Johnson smokebox door (of plastic card and brass rod) together with mould of RTV-101 rubber (note the venting channels in the mould and the large filler hole which produces a "plug", the weight of which, helps force the molten metal into all the nooks and crannies of the mould.

The final product cast from "Tiranti No.3 Low Melt Alloy"

If you want to experiment with low melt alloy casting, it needn't be expensive : You don't need to invest straight away in an electric melting pot. A simple pressed steel ladle plus a butane blowtorch will work perfectly well. You can later graduate to a melting pot and cast iron ladle if you wish.

Below : All the equipment you need to get started.

An electric melting pot and heavy-duty iron ladle can be added later, if you plan on doing a lot of casting.


APPENDIX.

Safety.

The chemical compounds required to make moulds and cast in resin are hazardous materials and should be handled with care. They can give off odourless fumes which may cause respiratory distress, while contact with the skin may cause irritation, inflammation and other skin problems. Always read the safety instructions carefully and where necessary adopt precautions which may include the wearing of disposable gloves, eye protection and the use of an active carbon filter mask or respirator.

'White metal' alloys contain metals which are potentially harmful, particularly by ingestion or inhalation as a hot vapour or dust. According to the composition of the alloy being used, ingredients may include Antimony, Bismuth, Cadmium, Lead and Tin.

'Dross' (i.e. the deposit which accumulates through oxidation on the surface of molten metal in the pot or ladle) should be disposed of carefully and inhalation of the dust must be avoided. Needless to say, even at the lowest melting points the molten metal will cause severe burns if it should come into contact with skin. Always read the safety sheet provided and sensible precautions include heavy-duty work gloves, eye protection and protection against the inhalation of dust and fumes.

When disposing of waste material observe the specified precautions as to hazardous waste.

Specialist Equipment and Supplies.

Melting Pots for alloys and ladles for pouring molten metal are available from:

They supply two low cost thermostatic examples priced between £60 and £100 up to models costing several hundreds of pounds.

Mould-making rubber, resins and whitemetals are obtainable from:

The specifications and recommended applications will be found on their websites.

From experience, my personal preferences are :

Mould making compounds.

  • For cold casting moulds silicone rubber type T28 (Tiranti) or TOMPS Value Range RTV Silicone
  • For hot casting moulds type RTV-101 (Tiranti)
  • Polyurethane Resins : Biresin G26 (Tiranti) or Axson F19/F119 (from TOMPS)

White metals.

Select type required according to properties required for project. See Alec Tiranti for full list. I generally keep a large supply of Tiranti No.3 and No.4 Low Melt to hand and use 'Wood's Metal' for soldering white metal components. More recently (2018) I have obtained a supply of "lead free" pewter. This eliminates some of the health hazards associated with the lead-based materials but requires a higher melting temperature.

Safety equipment and sundry items; measuring beakers, mixing sticks, fillers, French chalk, mould release agents, etc. are available from the above-mentioned suppliers.

Digital Scales.

The majority of components are mixed by weight and a set of kitchen type scales will be required.


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Page last modified on February 28, 2018, at 10:52 AM