Casting Resins, Water Clear & Fast cast Polyurethane Resin, Clear-cast Resin, Guidance for Casting in Fastcast, Clearcast, Water Clear Resin, Polyester....

MARAGON - ARTS & CRAFTS

Casting, Moulding & Modelling - Materials & Kits

Casting Resin Water-clear/Clearcast Resin Fastcast Resin Glass-clear PU Resin

FOUR SORTS OF RESIN

CASTNG RESIN

The Material
The casting resin is a general-purpose hand-laminating polyester resin with an extended gel time and is deal for making small to medium-size unreinforced models. It has an intrinsic release agent for improved application on non-gel coated surfaces. It has a low styrene content which helps to reduce the heat build-up during curing. Although giving perfect results for all types of work, the choice between this polyester resin and fastcast polyurethane resin (see below) is usually dictated by cost - fastcast is easier to use and produces finished castings within 20 minutes, but is generally more expensive.

Mould types
For latex moulds, a PVA release agent is advisable washed or brushed over the mould surface (not to be confused with PVA adhesive), but no release agent is required with silicone moulds. Plaster moulds can also be used for resin casting if the original pattern is smooth and uncomplicated, but a thorough coating of wax or vaseline is required right across the mould surface, and slight warming of the mould may be necessary to allow separation of the cast.

Fillers & Pigments
Fillers may be used for decorative effect, to improve fracture resistance, to reduce shrinkage and also to help minimise heat build-up during the curing stage on larger castings. For proper consistency and full dispersal, always add the catalyst before adding fillers and pigments. The weight of stone fillers such as marble powder should generally not exceed 60% by weight of the overall casting. (It is usually more convenient to use fastcast polyurethane resin (see below) for cold-cast metal work, but follow the the same application notes if using polyester resin). The resin can safely be used without fillers in smaller castings, but on larger castings excessive shrinkage may occur - with possible cracking - and the heat of the curing reaction may damage the mould itself. Pigments can be added up to 10% of the weight of resin used in the casting.

General
Unless you have had previous experience of the work-in-hand, constantly check progress throughout the curing period for excessive heat build- up. A heat sink or water-bath arrangement can usually be devised for most types of work and is always a wise precaution in warm or unpredictable conditions or with large castings. For consistent results, however, start with the resin at around 20ºC - below this the greater viscosity will trap bubbles and make mixing a less efficient process. In some cases the scale of the project may dictate that the casting is built up in stages to avoid excessive heat build-up, in which case the subsequent layer can be added when the casting has cooled but while the surface is stll tacky. Because of shrinkage, open surfaces will set with a rippled effect - this can be avoided by placing mylar on the unset resin (taking care to exclude air) which can be peeled away later.

As a rule, the cast can be demoulded at 24 hours, provided the open surface is completely tack- free; any release agent used with latex moulds can be rinsed off with warm water and a mild detergent. The cast should be left for a few days after this before carrying out any further work on it - preferably in a warm well-ventilated situation, but out of direct sunlight.

WATER-CLEAR/CLEAR-CAST RESIN

The Material
This is also a polyester resin and is used to make paperweights, preserve medical and biological specimens, etc., and is widely and successfully used in the making of jewellery. Water-clear resin can also be used to make ponds and streams or various water effects in landscapes.

Moulds
Moulds specifically designed for clear casting are available, but you can also use a silcone mould or make a rigid mould using polythene, perspex, and most plastic-coated materials. Moulds can also be constructed from wood, glass and melinex covered card. Polystyrene containers cannot be used and latex moulds should be avoided as they, too, inhibit the curing process. Rigid moulds should be very slightly wider at the top for easy removal. Because of the residual moisture, alginate moulds leave a soft, poorly-cured skin on clear resin - but this will harden off if the casting is kept in a very warm, dust-free situation for a week-or-so.

Preparing The Specimen/Subject
Different treatment is required according to the material. Flowers, leaves, etc: these specimens must be thoroughly dried first. This can be achieved by placing in a container and filling with either silica gel crystals or dry fine silver sand, then leaving to dry for several days in a warm area. After the drying out they should be sealed with polyurethane varnish or hair lacquer, bearing in mind each different plant may take longer to dry than others, the water content taken from a flower may cause it to fade also. It is often a good idea to dip the specimen in uncatalysed resin and leave overnight to drain off. Insects: you must remove all the natural oils present by dipping into a grease solvent such as acetone. Leave the specimen to dry completely and dip in uncatalysed resin, then leave to drain. Paper items: seal with a thin coat of polyurethane varnish or hair lacquer. Medals, coins, stones and shells: wash thoroughly to remove dirt and grease then also dip in uncatalysed resin, then leave overnight to drain.

Working with the resin
The normal working proportion of catalyst is 1% by weight (i.e. 10ml catalyst to 1kg resin) but this can be increased to 2% for very small quantities of resin, or in low working temperatures (a level teaspoon holds 3ml). It is important not to use too much catalyst as this may result in the casting becoming cracked or discoloured, with excessive shrinkage inside the mould. Large masses of resin can generate considerable heat, so larger castings should be done in more than one layer; because metal and solid objects tend to expand or contract at a different rate from the resin, casting in layers will reduce the tendency for the resin to crack or craze. For consistent results, however, start with the resin at around 20ºC - below this the greater viscosity may trap bubbles, etc.
Casting in a single mass: In a small casting the specimen can be covered in one pouring. Light specimens will tend to float and need to be glued or attached to the base. Stir the required amount of catalyst into the resin and allow all the air bubbles to completely clear then slowly pour the resin into the mould around the base without disturbing the specimen. Depending on the object, the casting can usually be formed upside down, with the specimen being suspended on thin gut or filament, etc, from the open end of the mould - i.e the base of the finished casting.
Casting in layers: For a casting in which the specimen has to be positioned centrally, etc., a preliminary, base layer will be needed; similarly, two or more layers may be needed to avoided excessive heat build-up in large castings. Pour the resin into the mould to form a base layer, cover the mould to protect from dust. At about 20 minutes at 20ºC the resin will have reached gel consistency. (Avoid leaving it much longer, as the curing shrinkage will cause the resin to detach itself from the mould, leaving a space into which the subsequent pouring can seep). Place the specimen (upside-down if working from top-to-bottom) on the base layer; light specimens will tend to float and can be glued down using supaglue, or can be semi-submerged in a thin, interim layer of resin; pre-soaking the specimen in resin will usually help to exclude air and bubbles. Repeat the procedure for each layer.

General
When you have finished casting wait until the resin, or final layer of resin has gelled (started to set), then cover with a layer of cellophane to exclude the air, otherwise it may set with a tacky surface. Because of shrinkage, open surfaces will set with a rippled effect - this can be avoided by placing mylar on the unset resin (taking care to exclude air) which can be peeled away later.

When the casting is hardened (allow at least 24 hours), you can remove it from the mould. The cast should be left for a few days after this before carrying out any further work on it - preferably in a warm well-ventilated situation, but out of direct sunlight.

FAST-SETTING ('FASTCAST') RESIN

The Material
This is a two-part polyurethane resin with a 4 to 5 minute pot-life (working time). It has an intrinsic release agent for improved application on non-gel coated surfaces and is normally used in conjunction with silicone moulds - which are much more resistant to the effects of heat than latex and other flexible mould materials. Its main purpose is in producing small-scale prototypes and models, or sets of models, and its very short curing period/mould separation time also makes it ideal for cold-cast work. It produces a stronger, more resilient casting than other resins. Utensils can be cleaned in warm water and washing-up liquid while the resin is still fluid.

Working with Fastcast Resin
The component liquids are combined in the ratio of 1:1. At normal room temperature, mixing should be completed within one minute and pouring should be completed within 3 - 4 minutes of bringing the component liquids together. Typically, the resin will have set hard and be tack-free in a little over 20 minutes. However, even small amounts of the mixed resin will generate considerable heat within a few minutes of mixing, therefore unfilled Fastcast resin should only be used on small sections - typically up to 35mm average section. The scale of the project may dictate whether the casting is built up in stages, and whether stone or other fillers are used to avoid excessive heat build-up. Fastcast cures to a mellow, ivory colour, but pigments can be added up to 10% of the weight of resin used in the casting.

Fastcast resin is particularly suited to ‘cold-cast’ metal work; to save on the amount of metal powder required, normal practice is to bathe just the inside of the mould with the filled resin so that, in effect, there is only a veneer containing the metal powder (pre-dusting the mould with metal powder tends to give variable results and uses more material).

Metal Powder Powder:Resin
by Weight

Aluminium 1:1

Brass 4:1

Copper 4.5:1

Iron 7:1

Bronze 8:1

Table showing the actual weight of metal powder required for
cold cast work assuming a typical volume ratio of 1:1

It is important to stipple this initial coating well into the detail of the mould - and to stir the resin in the mixing pot continuously to prevent the metal powder sinking; likewise, until it has started to gel, continuously work the resin in the mould around to prevent the powder slumping to the bottom. This outer layer will set within a few minutes and the core can then be filled with plain resin or resin filled with marble powder, etc. For an antique bronze effect, graphite can also be added in with the metal powder at a rate of 5 - 10g per 100g of resin, but a similar effect can also be achieved by staining or even using shoe polish on the finished piece if it is staying indoors.

General
Unless you have had previous experience of the work-in-hand, constantly check progress throughout the curing period for excessive heat build-up. A heat sink or water-bath arrangement can usually be devised for most types of work and is always a wise precaution in warm or unpredictable conditions - preparations should be made well in advance of casting.

After de-moulding, any release agent used with latex moulds can be rinsed off with warm water and a mild detergent. Cold-cast bronze, etc., can then be lightly rubbed over with iron wool or a pan-scourer, etc., to produce a polished or burnished metal effect and a clear lacquer spray can then be applied to preserve the finish, but this work is usually best left for a few days after casting.

GLASS-CLEAR PU RESIN

The Material
This is a slightly different polyurethane resin. It is used to make paperweights, preserve medical and biological specimens, and is widely and successfully used in the making of costume jewellery. Glass-clear Casting Resin can also be used to make ponds and streams or various water effects in model landscapes, etc; It cures quickly and is harder and more durable than polyester resin, but the uses and techniques are otherwise very similar. The resin consists of 2 components, Part A and Part B, neither of which should be regarded as a catalyst. The two components should be mixed 1:1 by weight. Small plastic cups (the type used in coffee machines, etc.) make excellent mixing vessels, as the corrugations on the side make perfect calibration lines! Working time is a little under 30 minutes, but curing may take up to a day.

Moulds
Moulds specifically designed for clear casting are available, but you can also construct a mould using silicone, polythene, perspex, and most plastic-coated materials. Moulds can also be constructed from wood, glass and melinex-covered card. Polystyrene containers should be avoided. Rigid moulds should be made very slightly wider at the top for easy removal. Because of the residual moisture, alginate moulds leave a soft, poorly-cured skin on clear resin - but this will harden off if the casting is kept in a very warm, dust-free situation for a week-or-so.

Preparing The Specimen/Subject
Different treatment is required according to the material. Flowers, leaves, etc: these specimens must be thoroughly dried first. This can be achieved by placing in a container and filling with either silica gel crystals or dry fine silver sand, then leaving to dry for several days in a warm area. After the drying out they should be sealed with polyurethane varnish or hair lacquer, bearing in mind each different plant may take longer to dry than others, the water content taken from a flower may cause it to fade also; you must remove all the natural oils present by dipping into a grease solvent such as acetone (note that acetone is highly flammable). Leave the specimen to dry completely and dip in uncatalysed resin, then leave to drain. Paper items: seal with a thin coat of polyurethane varnish or hair lacquer. Medals, coins, stones and shells: wash thoroughly to remove dirt and grease then also dip in uncatalysed resin, then leave overnight to drain.

Working with the resin
This resin can generate considerable heat during curing, so larger castings should be done in more than one layer; because metal and solid objects tend to expand or contract at a different rate from the resin, casting in layers will reduce the tendency for the resin to crack or craze. For consistent results, however, start with the resin at around 20ºC - below this the greater viscosity may trap bubbles, etc. Note that very small amounts of resin (e.g. under 10ml) are often much more diificult to mix efficiently and may take dramatically longer to cure. N.B. The Part B component may crystallize/cloud slightly when cold* - simply bring it up to room temperature slowly before use . *It is good practice to measure out part B and inspect it thorougly before adding part A.
Casting in a single mass: In a small casting the specimen can be covered in one pouring. Light specimens will tend to float and need to be glued or attached to the base. Mix the two components thoroughly and allow all the air bubbles to completely clear then slowly pour the resin into the mould around the base without disturbing the specimen. Depending on the object, the casting can usually be formed upside down, with the specimen being suspended on thin gut or filament, etc, from the open end of the mould - i.e the base of the finished casting.
Casting in layers: For a casting in which the specimen has to be positioned centrally, etc., a preliminary, base layer will be needed; similarly, two or more layers may be needed to avoided excessive heat build-up in large castings. Pour the resin into the mould to form a base layer, cover the mould to protect from dust. At about 20 minutes at 20ºC the resin will have reached gel consistency. (Avoid leaving it much longer, as the curing shrinkage will cause the resin to detach itself from the mould, leaving a space into which the subsequent pouring can seep). Place the specimen (upside-down if working from top-to-bottom) on the base layer; light specimens will tend to float and can be glued down using supaglue, or can be semi-submerged in a thin, interim layer of resin; pre-soaking the specimen in resin will usually help to exclude air and bubbles. Repeat the procedure for each layer.

General
When you have finished casting wait until the resin, or final layer of resin has gelled, then cover with a layer of cellophane to exclude the air, otherwise it may set with a tacky or contaminated surface. When the casting is hardened (allow at least 2 hours), you can remove it from the mould. The cast should be left for a few days after this before carrying out any further work on it - preferably in a warm well-ventilated situation, but out of direct sunlight. Because of shrinkage, open surfaces will set with a rippled effect - this can be avoided by placing mylar on the unset resin (taking care to exclude air) which can be peeled away later.
Finishing: Surface tack or grease etc. can be removed with washing-up liquid, followed by a rinse in warm water. This should not be done until the resin is fully cured (about seven days). If necessary, the casting can be then be shaped further with a hacksaw, file or sander followed by wet-and-dry paper and polishing if required.

Full instructions and guidance are included with all materials