This section covers most aspects of resin casting where clarity is not required. The basic requirements for successful casting are extremely simple and providing good sense and sensible working practices are observed there is no reason why you should not be able to produce satisfactory castings. Polyester resins are widely used to produce such items as chess pieces, statuettes, and miniature cottages, museum replicas and for the âpottingâ of electrical components.
General-purpose polyester resins are often used for simple low cost castings. Casting Resins have lower viscosity, which allows higher quantities of fillers to be added. A variety of fillers can be added. Depending on the filler used, the mix may vary from 50:50, filler: resin, up to 70:30 filler: resin by weight. Both resins can be used unfilled for mini castings up to about 100g, castings above this size without filler are likely to crack and damage your mould.
Polyester resins are suitable for casting in a variety of moulds including silicone rubber, latex, polythene, fibreglass, glass, etc.
Fillers & Effects
A variety of effects and finishes are possible using a combination of fillers and colours, we list a selection:
Hard wearing stone effects can be obtained by mixing various grades of sand, gravel, stone dust, etc., into polyester resin. Colour can be adjusted by using opaque colour paste.
A good ivory effect can be obtained by tinting resin with a combination of white and ivory colour pastes. A certain amount of trial and error mixing will be needed to obtain the best results. Scrimshaw (scratched designs on ivory or bone) is possible provided suitable originals and silicone rubber moulds are used. The use of various stains, especially brown, is important in obtaining an authentic ivory look.
A realistic white cast marble effect is difficult to obtain. Resin filled with marble dust and a little white pigment gives reasonable results. The addition of marble dust into water clear casting resin, however, gives excellent results, especially if this is used as a back up layer behind a white tinted gel coat. If flexible moulds are being used without a gel coat, however, extra catalyst will be necessary to effect a rapid tack free cure.
Resin Metal Casting
The use of sintered metals to give metallic finishes is a popular and relatively simple process.
Described is a small chess set casting using casting resin and it is assumed that you have a latex mould ready for use. It is recommend that, because of the smell of the resin; it is advisable not to use this material in a domestic situation. A well ventilated heated garage or outbuilding is ideal. The working temperature should be around 20ÂºC (68oF). No release agent is required. Prepare pieces of card with a central hole to act as supports for suspending the mould when filled. Give the resin a stir but try not to introduce too much air. Pour about 200g of resin into a small container. This quantity of resin may fill several moulds.
Thorough mixing of catalyst into resins and gel coat is very important. Also the correct additions should be observed to maintain good results. Dispensers are advised for accuracy. The table below gives the correct ratios of catalyst to resin and gel coat by weight. 1% is considered a slow mix, 2% is ideal, 3% is a fast mix. Additions outside these bands in not advisable for proper curing, in fact adding more than 4% may result in a failure to cure. The pot life of these mixes is also determined by temperature. The higher the temperature the faster the cure. As a general guide 2% addition at 20ÂºC gives 15-20 minutes pot life. The resin will always cure quicker if left in a mass such as the mixing bucket or in castings.
Catalyst Mixing Ratio
Addition of Catalyst (ml) 50g 100g 250g 500g 1kg 5kg
1% 0.5 1 2.5 5 10 50
2% 1 2 5 10 20 100
3% 2 3 7.5 15 20 150
Preparing Resin for Casting or Laminating
All resins, whether for casting or laminating, require the addition of catalyst (hardener) to initiate the curing process. Use a safety dispenser to add 20ml of catalyst per kilo of resin. Stir thoroughly. The hardening process begins immediately, so only catalyze a working quantity or your mixing containers will soon be full of solidified resin.
If pigments are being used these should be stirred into the resin before adding the catalyst. Add up to 10% of pigment, depending on the depth of colour required. To maintain consistent colour on a large project, it is often a good idea to pigment all the resin and then decant working quantities to be catalyzed as required.
Once catalized the resin gradually cures, taking on a jelly-like consistency in about 10-20 minutes before becoming hard in about 30-40 minutes at room temperature (about 20ÂºC). The curing process generates heat (known as âexothermâ) within the resin. Too much catalyst or large volumes of resin increase this heat, so a thick laminate or a large casting should preferably be built up in stages.
The term resin metal casting refers to a relatively simple method of producing quality metallic castings using pure metal in a fine powder form mixed with polyester resin and catalyst. These are usually produced from a latex or silicone mould.
We will describe the procedure to enable you to produce fine quality castings either for your own pleasure or as a profit making venture and it is assumed that you already have a mould ready to use. No release agent is required when using latex or silicone moulds.
As with all chemicals safety procedures must followed. We recommend that because of the smell of the resin it is advisable not to use this material in a domestic situation.
A heated garage or outbuilding is ideal. The working temperature should be around 20ÂºC (680ÂºF).
THESE MATERIALS ARE UNSUITABLE FOR USE BY CHILDREN UNLESS UNDER SUPERVISION.
The casting is made following two procedures. 1. The outer shell/gel coat layer and 2. The infill. The outer metal shell is a mix of about 3/4 parts of metal to 1 part of resin aiming for a just brushable paste. General-purpose resin and Casting resin, work well but casting resin will accept more metal powder.
By adding up to 20% gel coat you can reduce the amount of metal powder to 2/3 parts to 1 part resin with acceptable results.
The infill resin is poured into the mould when the outer shell has cured. The infill resin can be loaded with low cost filler powder such as talc and microdol to reduce cost and shrinkage. Dried sand can also be added to the infill for added weight.
For larger castings Fillite should be used as part of the filler addition to extend the resin with the benefits of lower shrinkage and less heat generated.
Applying the outer shell
First weigh out the resin, next add catalyst at a rate of 2% and mix thoroughly,
(refer to catalyst chart on next page)
The metal powder is then added at the required amount. All metals intermix, e.g. 4 parts of nickel silver to 1 part of Aluminium gives a pewter effect. catalyst can be added after the metal powder but will prove much more difficult to mix in. The catalysed mix is then applied to the mould surface.
Remember, that after adding Catalyst you have approximately 25 minutes working time before the mix sets.
If you are using a reversible latex mould you may find it easier to turn the mould inside out, paint on the resin/metal mix and then reverse.
If this is not possible the mix should be thinned enough to be able to revolve the mould until the surface is covered. If this method is used it is advisable to add a second coat to make sure the shell is thick enough.
After the outer skin has set (approx. 2 hours) simply add catalyst at 1% by weight to resin and add filler powder as required. This should set in about 30 mins.
A useful tip is to pigment the infill resin to the approximate shade of the finished casting; this will help disguise any thin areas in the metallic shell.
Tiny casts should be done in a one-shot resin/metal mix only. For larger casts the infilling can be in two or more layers; pebbles, metal or stones can be added which will not only save resin but also add weight.
While curing, the resin will get hot, so to preserve the mould it is advisable to remove the casting before the exotherm peaks. (Resin is at its hottest.
See mixing catalyst chart for addition of hardener to resin.
Finishing and surface finishing
Allow the casting to fully cure for at least 72 hours, cut back with wire wool and finish with metal polish. If polishing mops are available cut & polish back with cutting compounds. A coat of wax polish or clear lacquer will prevent tarnishing.
Antiquing can be achieved by painting on a spirit-based stain, leave to dry then abrade and polish; shoe polishes can also be used. As an alternative graphite can be mixed to the resin. 5% - 15% by weight can be added depending on shade.
Castings can be treated chemically to obtain a patina, the method and chemicals used are often the result of experimentation. Patination can be âfixedâ by wax polishing. Further Information on this subject is available on the Internet.
For some sintered metals patination is not possible unless small quantities of other metals are added, e.g. Aluminium will need 10% Lead added and Brass will require the addition of a small quantity of Copper or Lead.