Additive Manufacturing

A Complete Guide to Rapid Prototyping

Rapid prototyping is used in everything from software development to parts design. It attenuates the design process and reduces the potential for costly mistakes during full production.

What Is Rapid Prototyping?

In the simplest of terms, rapid prototyping is the speedy creation of models that are visually and/or functionally evaluated during the product development process. Rapid prototyping may be repeated numerous times as a design is perfected.

Although the terms “3D printing” and “rapid prototyping” are occasionally used interchangeably, the former is the process and the latter the end result.

A variety of different additive manufacturing (AM) techniques are used in rapid prototyping.

In the mid-1980s, stereolithography (SLA) became one of the very first 3D-printing technologies used for rapid prototyping. Other AM prototyping processes have followed, including fused deposition modeling (FDM), selective laser sintering (SLS), laminated object manufacturing (LOM), inkjet printing and solid ground curing (SGC).

All of these techniques utilize the three-dimensional visualization of an object via computer-aided design (CAD) data. To adapt this data for use in 3D printing, the digital object is “sliced” into thin cross-sections. The resulting information guides a 3D printer to create a physical object one ultrathin layer at a time. The first layer is deposited directly onto a print bed, and each additional layer is deposited onto the previous one. When the printing is complete, any support structures are removed. The scale model is then cleaned and finished in post-processing.

In many ways, additive manufacturing is the perfect process for rapid prototyping. It facilitates the quick and efficient distillation of user experiences into a mockup, making it possible to quickly produce and evaluate a 3D-printed prototype. Ideas garnered from repeated feedback cycles yield added refinements that are quickly incorporated into subsequent iterations.

The rapid prototyping process may be repeated many times until the component meets a variety of demands, including cost-effectiveness, compliance requirements and user needs. With 3D printing, prototyping cycles that were once measured in weeks are now frequently measured in days or even hours.

What Is Rapid Prototyping Used For?

Again, rapid prototyping is used in the development of both physical and nonphysical products. For example, in software development, Rapid Application Development (RAD) uses rapid prototyping as an alternative to the traditional, sequential “waterfall” method of software development.

In the development of physical objects, prototypes are commonly used to aid visualization and to solicit reactions from different audiences. However, some prototypes are also functional to varying degrees. Concept models created via rapid prototyping may accelerate early stages of product development.

Some prototypes are faithful representations of the intended end-use product, while others vary in dimensional stability and accuracy. For example, a prototype’s aerodynamics can be evaluated in wind tunnel testing even if all the internal parts are not present. Test prototypes allow for cost-effective testing that determines viability and reveals a need for improvements. Rapid prototyping is often used to produce male models required for investment cast tooling and silicone rubber molds.

In some cases, a prototype part may be fully functional and virtually indistinguishable from the proposed production part. In other instances, it may lack the strength or dimensional accuracy of final production parts. Thanks to the inherent nature of the additive manufacturing process, 3D-printed prototypes can include intricate structures, including parts nestled inside of other parts.

Why Use Rapid Prototyping?

Even in the pre-digital era, prototyping offered many benefits, although it was primarily used as a cost-effective way to communicate ideas visually. When coupled with 3D printing, rapid prototyping encourages the use of multiple iterations of an item during its development. This often promotes the development of a final product with desirable design refinements. Such a product may more perfectly meet user expectations, lifespan goals and budgetary requirements.

Rapid prototyping promotes timely input from important areas of a business, including engineering, marketing and purchasing. The proposed product is analyzed from radically different perspectives at an early stage, increasing the likelihood that improvements will be made at a cost-effective stage in the process. During the rapid prototyping process, it is possible to quickly and inexpensively test benefits and eliminate redundant features. Ultimately, AM rapid prototyping dramatically reduces scrap and requires less development time.

Rapid Prototyping Techniques

The primary, commercially available AM rapid prototyping processes are stereolithography, selective laser sintering, solid ground curling, inkjet printing, laminated object manufacturing and fused deposition modeling.

Stereolithography (SLA)

The stereolithography (SLA) process is a very accurate, cost-effective AM process that employs an ultraviolet (UV) laser to cure and solidify ultrathin layers of photopolymers such as acrylonitrile butadiene styrene (ABS) and polycarbonate (PC). The use of available clear resins allows for lens-like prototypes. When desired, clear SLA rapid prototypes can be painted or plated. Layer resolutions of 50-100 microns are common, and high-res prototypes with layer thicknesses of 20-40 microns are possible.

SLA prototypes up to about 2 cubic feet in size are possible, although sections can be bonded together with the same component photopolymer resin to produce even larger prototypes. Overcuring can cause warping, and raw SLA prototypes may be somewhat brittle. Additional curing during post-processing may be employed to further stabilize the object.

Selective Laser Sintering (SLS)

Selective laser sintering was another one of the early AM processes. Like SLA, the process was first patented in the 1980s. It uses thermoplastics like nylon, polycarbonate and glass-filled nylon as build materials. Since these plastics can approximate the performance of engineered materials, SLS can be used to produce partially or fully functional prototypes.

Since SLS yields prototypes with a somewhat powdery, porous surface, it is common to apply a sealant. The sealant also strengthens the prototype to a degree. Objects printed via selective laser sintering are somewhat less vulnerable to residual stresses than those produced by stereolithography. SLS prototypes can also be easily machined in post-processing.

Laminated Object Manufacturing (LOM)

Although not as widely used for rapid prototyping as SLA and SLS, laminated object manufacturing (LOM) is still popular when visualization, rather than functionality, is the primary goal. The process uses thin layers of paper, thermoplastics or metal powder bound together with adhesives (paper, plastic) or sintering (metal). One advantage of LOM is that it allows for the fabrication of larger prototypes than a number of other AM processes do. CAD data converted to an .STL file directs the movement of a laser that cuts each layer or cross-section to size.

During the process, waste areas are cross-hatched to facilitate removal during post-processing. Those areas act as natural supports for overhangs and other delicate parts of the prototype. Post-processing often includes sealing, particularly when layers of paper form the printed object. Sealing also inhibits moisture intrusion and subsequent warping.

Fused Deposition Modeling (FDM)

Fused deposition modeling uses thermoplastic filaments as build materials. It is another AM process developed in the 1980s. The selected filament is heated to the point that it can be extruded onto the print bed one layer at a time. As it cools, it fuses to the preceding layer. The thermal printhead is directed by an .STL file derived from CAD data that digitally represents the desired object. Faster FDM machines lay down thicker plastic layers that may lack dimensional accuracy, although slower machines that lay down thin, more accurate layers are materializing. Thermoplastics like ABS are amenable to sanding, drilling and machining during post-processing. Prototypes almost 2 cubic feet in size are possible.

Solid Ground Curing (SGC)

Solid ground curing uses a plastic spreader and a wax spreader to fabricate and support the desired prototype. Like SLA, a photosensitive polymer is used to create object layers, and an ultraviolet laser cures them. As each layer is printed, uncured resin is vacuumed for reuse. Wax is used to fill voids and support the object. Each layer of cured resin and wax is milled to ensure vertical accuracy. In post-processing, the wax is removed and the prototype is sanded, drilled and/or milled as desired. Although the SGC process generates somewhat more waste than other AM rapid prototyping methods, it is gaining favor because of its production speeds.

Inkjet printing techniques

3D inkjet printing is very similar to 2D inkjet technologies. Tiny droplets of heated plastic are jetted onto the print bed, and then onto the preceding layer. It is also commonly referred to as “binder jetting” because a binder is applied between the layers. Like established 2D inkjet printing methods, the low-odor process can be managed in a relatively small area within an engineering office or other convenient space.

Rapid prototyping using multi-jet technology produces objects at relatively fast speeds of 1-2 vertical inches per hour. Inkjet printing techniques are good for producing visual rather than functional prototypes. As with SGC, milling between layers improves vertical accuracy.