The difference between a design patent and a utility patent is that a design patent protects the ornamental design, appearance, or shape of the invention. This patent is appropriate when the basic product already exists in the marketplace and is not being improved upon in function but only in style or appearance.
Design patents are far easier to obtain than utility patents. Usually, the easier something is to obtain the less rights that are conveyed. For example, you can obtain a copyright for $30, if you prepare the application and file it yourself. You get tremendously long protection in terms of the number of years but the rights a copyright provides are exceptionally weak.
A protectable design consists of the visual ornamental characteristics embodied in, or applied to, an article of manufacture. For example, distinguish an ordinary steak knife from a butcher’s knife. In any knife there will typically be a handle and cutting blade. A design patent will protect the appearance but not protect the mechanical structure. In this regard it is possible for many different knives to receive design protection even though the basic handle and blade configuration is well known. The question is if this design can be patented and whether the presentation or appearance of the functional item is unique or not. A design patent protects the way something looks but not the way it functions.
When possible, you as an inventor would want to protect the function of the invention because by obtaining a utility patent, you will be able to prevent others from making, using, selling or importing into the United States any product that is functionally covered by the claims in the issued patent regardless of whether the device looks anything like what you are making or anything at all like the drawings in your patent application. This is the reason utility patents are stronger, broader and more desirable than design patents. But, if you cannot get a utility patent because the invention is not unique enough and looks different, then you could consider a design patent.
Notwithstanding all the reasons to consider seeking a design patent, it is absolutely essential to understand what you are getting when you seek to obtain a design patent and to understand the limitations of design patents. Design patents do NOT protect an idea or an invention, but rather they only protect the ornamental design of exactly what is pictured.
Always consider seeking the advice of an attorney to find out what type of patent is best for your invention and they can guide you along the way.