In what situations does it make sense to use 3D printing technology? Here are a few examples of how people are successfully using 3D printing:
Inventors use 3D printers to rapidly create prototypes for testing. When you can print an idea and test it a few hours later, your design cycle improves, which helps to get the product to market faster.
Inventors may also sell 3D prints as finished products before mass-producing. The Glif is a great case study of a design team that 3D printed their first batch of iPhone-to-tripod adapters. (http://www.therussiansusedapencil.com/post/2794775825/idea-to-market-in-5-months-making-the-glif).
Designers can use a 3D print service to make and sell their designs online. This works well because you can focus on making more designs, and let somebody else take care of producing and distributing the finished products. Bathsheba Grossman is a great case study of an artist using a print service for manufacturing and selling her designs online (http://www.bathsheba.com/).
Hard-to-find replacement parts can be made with 3D printing. Some examples are old architectural hardware, antique car parts, and obsolete equipment. This case study explains how a landlord saved hundreds of dollars by printing window parts that are not manufactured any longer (http://www.denali3ddesign.com/landlord-3d-printing-replacement-window-parts/).
When modeling for 3D printing, there are two ways to approach the design—either you'll be prototyping before traditional manufacturing or designing products specifically for 3D printing and ready-to-use right out of the printer.
If you design for prototyping before traditional manufacturing such as injection molding, then you'll want to design as per the injection modeling guidelines and find a 3D printing technology to fit the specifications of your model. Do not change the design to accommodate a specific 3D printing process, but rather look for a 3D printing process that will meet your prototyping needs.
If you are designing specifically for 3D printing, then you must understand and design around the specific requirements for the material of your choice. For example, in SLS printing, no details smaller than 0.2 mm will show on your model, but for 3D printed ceramic, details must be larger than 2 mm. This is important to know, because if you're printing the same model in different materials, you may need to tweak the design for each material.
If using a 3D print service, look for guidelines on their website that describe how to design for each material. Alternatively, if operating a printer yourself, find the manufacturer's guidelines for model specifications.