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How To Make Crystal Clear Castings

RESIN, CATALYST, COLOR  These three materials form the basis for all clear resin projects.  The resin itself, EP4101, is a clear liquid syrup with a sweet smell.  The catalyst, EP4920, looks like water and is the active ingredient which converts the liquid resin into a hard, clear plastic.  Colors are available in transparent, EP7701, and opaque, EP7702, and even a Glow-in-the-Dark.  Decorations can include materials such as glitter, metal powders, sea shells, baby teeth, coins, fishing flies, etc.


MIXING  Mixing is generally done in paper cups with wooden sticks.  For small projects, visual measurement using graduated paper cups is generally close enough; catalyst is proportioned at a starting rate of 10 drops per ounce of resin.  Mixing must be thorough.  Mix until all streaks are gone...and then some.  As an example, for a 11/2 ounce batch, fill a 3 ounce cup half full and then add 15 drops of catalyst, mix thoroughly and pour.  The rule of thumb, of 10 drops of catalyst per ounce of resin, should be taken as the starting point when at 72 degrees F.  Sometimes conditions require different proportions.  As the projects get bigger and thicker, less catalyst is used.  Also when ambient temperatures get higher, less catalyst is used.


MOLD  Many non-porous containers can be used as molds for the curing resin.  Examples include glass, aluminum, polyethylene, wax, rubber etc.  Depending on the material, some molds can be used as is, whereas others require release agents.  For the beginner, polyethylene molds (sometimes called Tupperware molds) are the best choice because release agents are not required.  With rigid molds, mold release wax  is a good choice because wax can be polished to a high sheen.  Mold surface is an important consideration.  Factors such as smoothness, shine, texture, hardness etc. affect the surface of the resin casting.  The resin conforms to the surface of the mold and copies it.  If you want a shiny casting, you need a shiny  mold.



First, analyze the mold and estimate how much EP4101 resin is needed.  For example, fill the mold to the required level with sand or salt, and then pour it into a mixing cup.  If the sand half fills an 8 ounce cup, you need 4 ounce of resin. 


Second, if your project is thin (1/4” thick or less), measure out 4 ounces of resin; add to that the color you desire; then add 40 drops of catalyst.  Mix thoroughly.  While mixing, repeatedly scrape down the walls of the mixing cup and the sides of the mixing stick to eliminate any unmixed residues. 


Third, pour the mix into the mold and allow it to cure overnight.  The curing resin is sensitive to its environment.  Temperature and humidity will affect the cure.  For best results, cure the resin at a steady room temperature of 72 degrees F and low humidity. 


Fourth,  remove the cured resin from the the mold.  Oftentimes, the casting will fall out of the mold by simply tapping or bouncing the mold against a table top.  If the casting is stubborn, place it in a refrigerator for a few hours; then pour warm water over the mold (not the casting).  In this way you can expand the mold, which allows more room for the casting to fall out.



This resin shrinks during cure.  The bigger the casting, the more noticeable it becomes.  Shrinkage occurs during the “jello” stage.  If the casting pulls away from the mold during this stage, the mold is no longer able to control the surface and the surface of the casting becomes wrinkled and inferior. Click here to read about polishing these wrinkles away.


EP4101 is an air-inhibited resin which means that it does not want to fully cure when in contact with air, or more specifically oxygen.  The cure is inhibited by air.  If a test tube were filled with catalyzed resin, the resin in contact with the glass would cure rock hard, but the resin on top which was open the air would appear soft.  Eventually this dries out by itself.


EP4101 exotherms during cure.  This means that heat is liberated and the resin gets hot during cure.  If overheating occurs, the resin can scorch which results in an undesirable amber color.  Using too much catalyst accelerates the cure which in turn speeds the temperature rise.  Overheating is undesirable.  Control it by: not overcatalyzing, keep ambient temperatures at 72 degrees F, keep layers thin, etc.


SAFETY  Chemical  should be handled with respect.  Exercise common sense hygiene and ventilation.


CONCLUSIONS  These general instructions are sufficient to get you started on your first projects.  If further information is required, Eager Plastics has a wide selection of books and reprints dealing with this subject or call for advice

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