Ergonomics and Rapid Prototyping

What is rapid prototyping or RP? It is really a group of different technologies that can used to quickly construct physical models from Computer Aided Design (CAD) data. Different technologies have their own strengths and weaknesses. Examples of rapid prototyping technologies include stereolithography (SLA), 3-D printing (3DP), fused deposition modeling (FDM), computer numeric controlled processes (CNC), and selective laser sintering (SLS). More technologies are being developed every day. Rapid prototyping is becoming more accessible and affordable than ever before. You can learn more about the various RP technologies below (RP Technology Overview).

 

A prototype is just the first model of something. Just like any model, it only has to be close enough in detail to the object being studied to be useful. Most ergonomists already use prototypes to evaluate designs. We might use a hole cut in foam board to simulate hand access into a service panel design. Or we might mockup a load station by stacking cardboard boxes to the right height and then mark the reaches. While it is easy to lose perspective looking at CAD, a prototype lets us interact with the design. Some designs are too complex to mockup using common materials. For example, a prototype of a new electrical connector design would be hard to make without RP.

 

 

One of the manufacturing plants I work with was having trouble installing a plastic trim piece on their product. It wasn’t fitting correctly and required too much force to install. I analyzed the ergonomics of the task and provided the appropriate force guidelines. I discussed the proposed changes with the design engineer, and he asked me if I thought there was any point in looking at a rapid prototype model of the new part. I told him that I couldn’t buy off on installation forces for a part that was not production intent, but I would like to have a look. Usually, I would review CAD drawings and wait until a first part was available to take measurements. I knew the production part would be a tough plastic and rubber molding, but it wouldn’t be available for months. The prototype was available now. The prototype he showed me was from a white, brittle plastic. We looked at it. At first I was skeptical, but it really helped us see the key changes in the part we were talking about. We even installed this prototype part and saw how the changes affected part fit. Part fit was perfect. Although we couldn’t make accurate force measurements because the materials were so different, we could tell that the design changes were in the right direction. Looking at the prototype was so much better than just looking at a drawing.

 

"How long have we been rapid prototyping parts like this?" I had to ask. The engineer said we have been doing it for years. He explained that designers and engineers had been using rapid prototypes to help develop designs more quickly at a reduced cost. The whole lesson taught me to ask the simple question, "Is a rapid prototype available?" I became very interested in rapid prototyping. Rapid prototypes can be another valuable tool in the ergonomist’s toolbox.

I asked the engineer how the rapid prototype was made. He invited me to visit the lab that made it. Using math data on a desktop computer and a small fused deposition modeling (FDM) machine, the lab technician showed me how they "printed" out a part using plastic. I asked, "How much does a machine like this cost?" I was surprised it was around $10k. The technician told me that prices for FDM machines have come down. He also explained that with just the math data (CAD file), there are many online companies that will make a rapid prototype and send it overnight. I became very interested in Rapid Prototyping and I took a few classes at my local college. I learned that many advances have been made in this field since I was an engineering student.

 

 

One of the first technologies used in RP was stereolithography (SLA). SLA first appeared in a usable form around 1986. In this technology, a UV laser scans the surface of a vat of UV sensitive photopolymer liquid to solidify it and create a layer. An elevator platform inside the vat moves this layer down a tiny amount and the laser again scans the surface of the liquid. The process is repeated until the object is formed. Naturally, the size of the object that can be made using SLA is limited to the size of the vat. Larger parts can be made in pieces and glued together later. A drawback of SLA technology is that there can be no unconnected internal parts to the model.

 

An exciting new technology is 3-D printing (3DP). It is similar to SLA, but instead of a liquid and a laser, it uses powder and glue. Successive layers of powder are built up inside a special machine. A computer controls a nozzle to spray a binding agent into certain areas the powder between layers of powder. At the end of the process, the extra loose powder is vacuumed away leaving the 3-D model. The powder that was not used can be recycled. Like SLA, the size of the model is limited by the size of the 3DP machine. Because the loose powder supports the model, unconnected internal parts can be modeled. Inks can be added to the binding agent so some 3DP machines can actually print their models in color.

 

Another new technology is fused deposition modeling. Imagine a printer that prints in melted plastic instead of ink. An FDM machine builds up layers of plastic to form a 3D model. Some FDM machines also print in water soluble substrate. By alternating the water soluble substrate with plastic, voids can be created in the model. After the machine is finished, the whole part is dunked in hot water and the substrate dissolves. This allows unconnected, internal parts to be in the model. A complete assembly with moving parts can be created. 

 

Probably the oldest technology for Rapid Prototyping is CNC machining. Even though it is an older technology, it is still the best for some applications. A computer controlled spindle is used to precisely cut away areas of a block of material. CNC can also be used with water jet, laser, and plasma. It is not unusual to see CNC machines with tables that measure 4’ by 8’ and larger, so some very large prototypes can be made with CNC. Although once very expensive, the software and even the machines themselves have become much more affordable in recent years. CNC technology can be used to make prototypes out of plastic, wood, or metal.

 

The latest technology in RP is selective laser sintering (SLS). It is like the technology used in 3D printing except a high powered laser scans the surface of layers of powder. It is used to fuse powdered material together. One of exciting things about SLS is that it can be used to make working metal parts. Very complicated designs can be created with this technology. Today, SLS machines capable of making metal parts are very expensive. Someday, instead of a warehouse full of spare parts there might just be a large SLS machine in the corner ready to make a replacement for any part that breaks. Like 3DP, SLS is also very efficient because the powder that isn’t used can be recycled.

 

 

It is a great time to be an Ergonomist. When I first started my career in Ergonomics 15 years ago, some problems were caught too late and thus were just too big to fix. Naturally, when Ergonomics is applied earlier it is much more effective. Rapid prototyping helps us to apply Ergonomics earlier in the design process. While looking at drawings of designs is useful; with RP an Ergonomist can actually hold the designs in his or her hand. We can use RP models to measure dimensions and clearances and see how well a part fits together with other parts. We can even measure some forces so long as we are careful to understand how material properties may be different between RP materials and production parts. The best part about RP models is that they help us make changes before a design is final. I’ve heard it said in Rapid Prototyping that a picture may be worth a thousand words, but the part tells the whole story.