Introduction: Transfer Board for Wheelchair Users
As part of the Impact Residency program at Pier 9, it is my goal to mass produce products that make a measurable impact on the world. My company, IntelliWheels, Inc. will then take those products and make them available to wheelchair users all around the world through our traditional distribution channels.
People who use wheelchairs often times use a product called a transfer board in order to help them transfer in and out of their wheelchair. Transfer boards are simple devices usually made from wood, and they cost about $50-$80 for a good one. Unfortunately, transfer boards are normally not covered by insurance, and $50 can be a big investment for someone on a fixed income. Therefore, we decided to see if we could make one lighter weight and lower cost by using a plastic injection molding process. We expect to be able to manufacture a $20 injection molded transfer board that will surpass the affordability barrier for users around the world.
In this instructable, I'll show you a little bit about our process going from concept to product.
Source of the gif: https://www.youtube.com/watch?v=CtrmDpsY9BY
Step 1: Gathering Feedback
After we have the concept, the first step is to get out and ask people what they like about the design of current transfer boards. We wanted to talk to wheelchair users, therapists, and families of people who use wheelchairs. After as many interviews as possible, we were able to identify a few areas where we could really make a difference.
While a nicely finished wooden transfer board will usually look nicer to most users, we can successfully create a plastic transfer board that is lighter, easier to hold, and significantly lower cost.
Step 2: Choosing a Design
After the interviews, it was time to create concept models for people to provide feedback on. After much discussion and considering manufacturing concerns, user aesthetic preferences, and survey data, we settled on a single design concept. The chosen design has rounded corners, handles on the corners, and big thick ribs in order to stand up to a 250 lb person sitting in the middle of it. It should be very smooth on the top, the actual sliding surface. People also had a slight aversion to the standard medical white aesthetic of traditional plastic transfer boards. Therefore, we decided to go with a gray or blue color for the finished product. Luckily, the color of injection molded parts is very easy to change later in the manufacturing process.
Step 3: Simulate
This product is planned to be injection molded out of High Density Polyethylene (HDPE) for strength and cost concerns. In order to learn about the design before investing lots of time and money into making a prototype, we needed to do two simulations:
FEA Analysis of 250 lb person sitting on it:
Using Autodesk Fusion 360's built in analysis tools, I ran a simulation seeing how the product would deform under weight. Based on these results, I had to go back and modify the design a few times before I was confident that it would be strong enough.
To simulate this and make certain that we had an injection moldable design, I ran the part model through Autodesk Moldflow Advisor. You can see the way the plastic fills in from the center of the part.
A few interesting things learned at this step:
- Material will be injected at 246 degC (475degF)
- Material will be injected at 14.12 MPa (2000 PSI)
- The two halves of the mold will need to be held together with 500 tons of force
- We will be able to mold 78 parts every hour
Step 4: 3D Print the Prototype
Off to the 3D printer it goes! We used a Fortus FDM printer, and we had to print it in two sections and then glue it together.
This process allows us to see a real version of the product before you go to the tooling step. Unfortunately, this prototype isn't strong enough to do real stress testing but it is big enough to get a good feel for the look of the product.
Step 5: Design the Mold
Designing an injection mold is a very specialized task that is usually outsourced to another company. But, since we're a bit insane here and we think it's fun to do things ourselves, we decided to design it and build it in-house. We used Autodesk Inventor to do the mold design. Stock parts can from DME, a mold supply company in Michigan.
The real mold will be made from aluminum and steel, but the model is color coded in order to see all the different parts.
When finished the mold will weight 1,500 lbs and be the size of a small coffee table.
Step 6: Machine the Mold
For this step, you need a high precision vertical milling machine. Luckily, Pier 9 has a Haas VF2 at the ready which is perfect for holding the tight tolerances required for injection molding. Polyethylene will flash between gaps less than 0.002 inches in width, and the Haas vertical milling machine can guarantee +/- 0.0005 inches when you use it right.
The mold is made up of 11 plates and each plate takes anywhere from 2 to 24 hours to machine.
Step 7: Finally, It's Time for Mass Production
Once the mold is complete, we can finally mold the parts. Of course, the mold is only half finished right now, so we can't make full parts yet, but we will be able to in January.
We work with a factory called PDI based in central Illinois which has 10 different molding machines and makes parts for all different companies. IntelliWheels, my company, will take over the distribution challenge. IntelliWheels has a good distribution network in 20 countries across the globe.