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Resin Casting |
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A resin mold is usually made from an original using room-temperature-vulcanizing (RTV) silicone rubber, or with a painted on and reinforced latex 'skin'. Careful attention is paid to setting the seam lines in the best places to allow the copies to come free of the mold after casting, as well as keeping in mind finishing aspects such as seam removal. Because the molds are flexible, resin can be used to cast pieces with high levels of fine details and undercuts.
Mold-Release is a silicone-based lubricating spray that is applied to the inside of a mold before each casting. This helps the final casting to "release" from the mold more easily, reducing the likelihood of the mold ripping or tearing. Since it allows the casting to come out more easily, it helps to reduce "warping" that can occur when the the casting is removed.
Unfortunately, a very thin layer of silicone will end up on the surface of the final casting and it can be detrimental to getting paint to "stick" to the horse later. However, this is easily removed in the "prepping" stage of creating a finished, painted, resin-cast model, by scrubbing with an abrasive cleanser (such as "Bar Keepers Friend").
Simplistically, the two components of the resin are mixed and then poured into the mold and allowed to cure. Curing time can take up to an hour or more, depending upon the resin, and the ambient temperature (resin cures slower at cooler temperatures.) Resin production facilities use an injection system to push the mixed resin into the mold... as resin tends to harden and cure very quickly, and even moreso in large volumes.
While some small and simple items can be cast with a simple "pour in" method, items that are large with complicated multi-piece molds benefit from some mechanical assistance. Resin can also be rather heavy if it is cast solid, which is why many resin model horses are "hollow cast". The central part of the body is hollow, with the body being more of a "shell" of resin (similar to an injection-molded plastic model). This achieved with a machine known as a "roto-caster", that holds the mold and rotates it in all dimensions until the resin sets and cures, coating the inside walls of the mold with resin.
Rubber molds tend to swell a little with each casting, therefore usually it is may only be safe to do 2 to 3 pourings per day (sometimes more in optimal settings), else distortion of the copies will result. A mold needs to be "rested" for a day or two in between casting sessions to offset this, not to mention that it can take a little time for each copy to be carefully removed, to avoid tearing the mold itself. Resin casting is not well-suited to rapid mass-production means in most cases, being rather labor-intensive and time-consuming, compared to injection-molded plastic.
A quality RTV rubber mold that is handled carefully can yield anywhere from 40-80 or so castings before it may need to be replaced (50-60 is average), as the resin reacts with the rubber and cause gradual degradation. RTV rubber itself can also dry out and become useless over time, sometimes within a year, even if it is not being used for casting. Removal of the castings from the mold ultimately results in little pieces (and sometimes larger ones) tearing off of the mold. The lifespan of a resin-casting mold is relatively short, compared to some other reproduction methods.
