What's in a Name? Part 2

16 May, 2007 By: Jeffrey Rowe

RP vs. RM: Do processes define production terms, and when does prototyping become production?

In last week's "What's in a Name?" I discussed the industry's dilemma regarding what to call rapid prototyping and manufacturing (RP&M). Can we find a simple term for this technology that we can universally agree on and understand?

In that article, I overlooked another term that we should include in the mix: SFF (solid freeform fabrication). A technique for manufacturing solid objects, SFF is sometimes used as an umbrella term to describe RP (rapid prototyping), RM (rapid manufacturing), layered manufacturing and additive fabrication. Note that in every case, rapid is a relative term, and depends on the machine type and model size.

This week I’ll pose another daunting question: What’s the difference between RP and RM? I’ll bet if you asked 100 people, you would get 100 different answers. This becomes an issue of prototyping vs. production, although several factors are involved in attempting to answer the question. Some RP&M machine manufacturers, observing the increasing adoption of RM, are attempting to expand from traditional RP to RM by developing machines with greater throughput, as well as new materials that have physical characteristics that more closely resemble those of traditional/conventional manufacturing processes.

RP: The Old Standby
RP is generally regarded as the construction of physical objects using any one of several solid freeform fabrication techniques. RP takes digital designs from CAD software, transforms them into cross-sections, then recreates each cross-section in physical space. This additive process continues layer-by-layer until the model is finished. Ideally, it is a WYSIWYG process -- that is, the digital model and the physical model correspond to each other.

In additive fabrication (a universal term that could be used to describe both RP and RM), the machine reads data from a CAD drawing and lays down successive layers of liquid or powdered material, and in this way builds up the model from a series of cross-sections. These layers, which correspond to the virtual cross-section in the CAD model, are glued together or fused automatically (often with a laser) to create the final shape. The primary advantage to additive construction is its ability to physically create almost any 3D geometry.

In RP, production usually is limited to relatively low volume, and parts produced usually are used only for development purposes.

Some additive fabrication techniques in both rapid prototyping and rapid manufacturing use multiple materials to construct parts. In some cases, the material used for the actual part has a high melting point for the finished product, while the material used for its support structure has a low melting point or is water soluble. After the model is completed, it is heated or immersed in water, causing the support material to dissolve away and leaving a functional plastic prototype.

Although traditional injection molding is still less expensive for manufacturing larger volumes of most plastic products, RP is being used today to produce small quantities of finished goods in a single step minus the expensive tooling cost. This is where the distinction between RP and RM begins to blur, but the generally accepted distinction is made with regard to production volume and function of the part produced (for development purposes or end use).

RM: The Hip, New Cousin
Like RP, RM is a technique for manufacturing solid objects. However, RM is often performed in a parallel batch production mode that can provide an advantage in speed and cost overhead compared with alternative manufacturing techniques such as die casting. As defined, RM involves the series production of products or using the created part in a production environment as a finished part.

People are often surprised that RM can be used for producing relatively large parts from metals, ceramics and polymers. RM is becoming better known for several industrial applications in the military and aerospace industries. On the opposite end of the scale, RM is being applied to small products and micro technologies in the medical, diagnostics and sensors industries.

RM can be especially well suited to producing short runs of parts, spare parts and complex parts that are difficult to manufacture by conventional methods, and for bridge parts, which are produced while tooling, such as injection molds, is being built for high-volume production.

RM Has Its Limits
As good as RM may sound, it’s not without its drawbacks and limitations. For example, RM requires a modified approach to part design, one that takes into consideration the RM process. Material selection and build orientation are paramount considerations -- much more so than for conventionally manufactured parts. You must keep in mind that fewer materials are available for RM than for conventional manufacturing. And finally, because of all these considerations, part design can actually take longer than the actual manufacturing. A lot of careful decisions must be made to arrive at the best process to produce the parts -- in other words, you have to design to the process.

Probably the biggest issue holding back RM is that it suffers from a general lack of awareness by many potential end users who don’t understand its possibilities as a comprehensive manufacturing process. However, RM is finding favor and being used as an intermediate step in some conventional manufacturing processes.

Yes, there are design constraints and material limitations associated with RM, but if RM vendors can improve the materials available for the process, as well as improve speed and capacity of machines, it’s likely that it will finally move to the manufacturing floor alongside other conventional manufacturing methods.

While RP and RM do share some similarities, they are different in terms of the volume and ultimate use of the parts produced with the respective processes. They are both, however, about doing things differently in a more efficient manner throughout a product’s development and production cycles.