Cake dummies are typically pieces of some type of Styrofoam™, plastic or other reusable material that are shaped like baked cakes. There are three basic uses for different types of cake dummies: as displays, as demo models, or as learning tools. Displays and demo models are often used professionally to show customers a design for them to approve, all without the expense of baking a cake. Dummy cakes are also good cake models for someone learning cake decorating techniques. How you plan to use the cake dummy will help you choose which one might be best for you.
Bakeries often use cake dummies to have items on display without the need to bake actual cakes. Display cakes often sit out for weeks or months with no refrigeration; a real cake would eventually mold and break down. Cake dummies are typically made of materials designed to resist mold and rot so that the finished cake displays can be used indefinitely, saving a bakery time and money.
A cake dummy can also mean a lot of savings for a bakery that needs to show a design for approval. This is becoming common in bakeries that sell expensive designs, such as those common in wedding cakes. Many brides prefer to see the design they have chosen on an actual cake rather than just in a picture before they decide it is the right one for their wedding. A cake dummy allows the cake designers to decorate a fake cake that should look just like the finished product instead of creating two complete wedding cakes for one order.
People who decorate cakes typically need a lot of practice to become good at it. Baking a new cake for every practice session could quickly become expensive and wasteful. New techniques can cost much less to practice on a cake dummy that can be cleaned and used over and over.
A cake dummy made from a material that is easy to clean, such as plastic and some hard foams, can make it easy to use it many times. Foam display cakes may be best for a more realistic appearance to show off a baker's skills or to present a design for customer approval. Plastics and other materials might not accept a layer of frosting in quite the same way as foam, but may be better for simply practicing a design. Almost any type of material can be made to work in most situations.
Artificial cakes that will be on display for a long period of time should probably also be frosted with artificial frosting, which can typically be purchased in large tubs and colored in the same way as real icing. A pretend cake may not mold, but mold could eventually grow on real frosting. Dummy cakes can usually be purchased in almost any size or shape you need, from large sheet cake sizes to cupcakes. If you only have an occasional need for a dummy cake, you might try turning your cake pan over and practicing on the bottom as if it were a finished cake to see if that meets your needs.