Soap molds determine your soap's finished size and shape. A wide variety of soap molds are available from companies that sell soap-making supplies, but they can be expensive. Look around your home for household items that can be used as soap molds. When you are considering what you can use for a mold, think about how you will cut the soap and what the finished size and shape will be. Also think about how you will get the soap out of the mold. Inflexible molds need to be lined with plastic wrap so you can pull out the soap.
Wash and dry the PVC pipe. Put several layers of plastic wrap over one end and hold them in place with rubber bands. Fill the pipe with melted soap, place the pipe upright in a warm place and allow the soap to set for 24 hours. To remove the soap, run hot water over the outside of the pipe, remove the plastic wrap and push the soap out of the pipe. Use a knife to slice the soap into bars.
Line a square or rectangular baking dish with plastic wrap, laying one piece lengthwise and another one over it crosswise. Pour in the soap, and allow it to harden. Once the soap has hardened, about 24 hours, pull on the plastic wrap to remove the soap from the dish. Cut the soap into rectangles and allow them to continue to cure.
Wash and dry quart-sized milk cartons. Cut off the top so you have a square opening. Fill the cartons with soap and allow it to dry overnight. Fill paper cups with soap and allow it to dry overnight. To remove the soap, tear the carton or paper cup away from the soap. Slice the soap with a knife and allow it to cure.
Choose boxes with sturdy sides. Line each box with two layers of plastic wrap long enough to hang over the sides. Pour soap into the box to the desired level. Let the soap harden overnight and then pull on the plastic wrap to get the soap out of the box. Cut the soap into rectangles and allow it to continue curing.
Flexible silicone baking dishes work well as soap molds because it is easy to turn the soap out of the dishes once it has hardened. Make a wooden box out of 1/4-inch plywood, approximately 8 inches wide and 3 inches deep. Make it as long as you like. Line it with plastic wrap before pouring in the soap. When you remove the soap from the mold, cut it in half lengthwise and then cut each half into bars of soap.
If you just can't get the soap out of the mold, put it in the freezer for a couple of hours. Soap is usually easier to remove from a mold when it is very cold. If you use metal objects such as baking dishes as molds, be sure to line them very well with plastic wrap, as the lye in the soap can react with the metal and ruin your soap, and your baking pan, too.