How are Resins Produced?
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.
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.)
Rubber molds tend to swell a little with each casting, therefore usually it is only safe to do 2 to 3 pourings per day, 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 copies need 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.
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 resins react with the rubber and cause gradual degradation. RTV rubber itself can also dry out and become useless over time, often within a year, even if it is not being used for casting.
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