There are many ways to produce custom plastic parts for a prototype.  In most cases the processes are used only for prototyping, as the setup-cost is low, but the piece-cost is high (the opposite of most production manufacturing processes).  This article is meant to be used as a guide to what is available to make the plastic parts of your new product.

SLS: 3D-Printing
SLS (Selective Laser Sintering) is a very common process for making plastic prototypes.  The process works much like an laser printer, except that it prints layer after layer to develop a 3D part.  A thin layer of plastic powder is leveled, and a laser fuses the powder in the appropriate spots.  These layers tend to result in a ‘stair-stepping’ effect that you can see in the surface of the final part.  The advantage of SLS is that it uses real plastic, and the parts are nearly as structurally sound as the a production part.  SLS is available in Nylon, ABS, and other blends of plastic.

SLA: 3D-Printing
SLA (Stereolithography) is the original plastic RP process.  Instead of powdered plastic, a vat of UV-curable photopolymer resin is cured in layers with a focused light beam.  a very common process for making plastic prototypes.  The accuracy and surface finish of SLA is better than SLS, but it does not create a true plastic part, and consequently the parts are more fragile.  New, high-performance materials have been introduced in recent years, and the structural properties are getting better.  SLA can also print transparent parts.  However, to get a crystal clear finish, extensive hand finishing is required which makes these parts very expensive.

PolyJet: 3D-Printing
PolyJet is a newer process that works similarly to an inkjet printer.  Instead of the bath of photopolymer resin that SLA uses, PolyJet deposits droplets of the resin and instantly cures them with a focused UV laser.  PolyJet parts have extremely high accuracy, and very smooth surface finish.  New materials are becoming available that rival or surpass the structural properties of SLS.  Many parts can be made for very low cost with PolyJet, as well as the existence of a network of suppliers that offer very rapid quoting and turn-around. Lastly, elastomeric materials (rubber-like) are available with PolyJet, at a range of hardnesses, and can even be printed at the same time as hard plastic!  This means that you can easily print a rubber over molded part.  At LTS, I make use of PolyJet RP very often.

FDM: 3D-Printing
FDM (Fused Deposition Modeling) is the poorest quality plastic RP process available.  However, the equipment to produce it is affordable for many small companies, and can quickly produce ‘form & function’ parts.  It works by a print head laying down a molten bead of ABS plastic, that weaves back and forth and builds up a layer of solid material.  Although it does use true plastic, the poor adhesion between layers and beads results in rather poor structural properties.  Also, the bead size is a limiting factor, so the stair-stepping is very pronounced, as is the lack of precision.

Rubber Mold Casting
This process has been used to make high quality prototype parts long before 3D printing machines existed.  It is an excellent technique for making multiple exact replicas for a fairly low piece-cost, after the initial molds have been created.  You may have seen this process used on shows like “Mythbusters”.  It all starts with a master model, usually carved from wood, plaster, or resin.  The master model can also be a machined or 3D printed part, or made from a modified existing part.  A silicone rubber mold is carefully casted around the model…there are several methods of performing this step.  Then, the model is removed from the mold, and additional copies can be made by casting liquid resin into the mold.  Flexible silicone parts can also be created this way.

Machining
Traditional CNC machining processes can be used to make a plastic part, but certain geometries simply cannot be machined due to thin walls or lack of access.  If the geometry is simple, machining plastic may be the cheapest method to make your parts.

Rapid Manufacturing/Tooling
If you need an actual injection molded part for testing, or low-volume production runs, you still have some low cost options (when compared to conventional tooling).  Rapid tooling is a process that makes injection mold tools out of aluminum, rather then conventional tool steel.  There are several companies offering this service that can turn around this tooling in days, rather than months.  The rapid tooling does have some geometry restrictions, and will only be good for a few thousand parts, but often that is all you need.  Best of all, it produces actual production quality parts.