Create a plaster mold of your desired object, for example a piggy bank. Cover the bank with releasing agent with a brush. Suspend the bank inside a cardboard box or plastic container so the plaster can reach all around the bank . Make sure this box is deep enough and wide enough to form a final plaster cast with a thick backing that will not break in two.
Pour plaster of Paris or Hydrocal into the container so that wet plaster covers the bottom half of the bank . Allow this casting to harden. Remove the bank carefully. Remove the plaster casting from the box or container. Get a new casting box and repeat the process, casting the top half of the bank .
Test-fit the two halves of the cast together. If necessary, shave down the hardened plaster so the two halves fit together neatly. Be careful not to break the molds. Avoid any large gaps between the two pieces. Remove any dust from the molds with a brush or air can and allow them to dry.
Coat both sides of the mold in releasing agent, according to the releasing agent instructions.
Gather all of your foam mixing materials and equipment within easy reach. Cold foam casting is a process that begins instantly when the two foam chemicals are mixed. You will have only a few seconds to pour the foam, fit the mold together and weigh it down.
Measure the two cold foam chemicals according to the instructions and using an accurate gram scale. Do not estimate measurements. Any excess amount of either chemical will ruin the foam.
Pour the two chemicals together into a glass bowl. Begin mixing immediately. Use an electric mixer for quickest mixing. Mix the foam for just a few seconds. It will instantly begin to foam and increase in size.
Transfer the foam batter into the bottom side of the mold, very quickly. Lower the top of the mold into position. Seconds count: move fast. Place a weight on the top mold. Use between 20 and 40 pounds of weight that cannot shift. Do not interfere with the molds or touch the foam now.
Wait 10 to 15 minutes. If all went well, you will be able to pry the top mold off without tearing the casting inside. Remove the bottom mold as well. The foam should be stable, not sticky, and perhaps only slightly warm to the touch.
Decorate or cover the foam rubber casting to suit. The rubber will accept dyes if you want to stain it. Painting it will provide only limited satisfaction, as paint will tend to crack and peel off as the object flexes. Upholstering the object is another option.