DFM Night Light

October 2023

Design for Manufacturing of a custom night light

This project was designed around the following scenario:

We have a customer who wants to make nightlights in batches of 10-100 per month. We have the capability to vacuum-form product shells using CNC molds or cast urethane parts in a silicone mold. Then, the product is to be batch-assembled with the help of mechanical and wiring fixtures. Our client has already designed and manufactured a printed circuit board (PCB) for the nightlight, and they're hiring us to design and manufacture its enclosure, from which a stream of derivative future products could be efficiently developed and produced.

The learning objectives for this project were:

To learn about integrating electrical components in a mechanical enclosure
To design this enclosure to enhance the product's brand and using the surface tools in SolidWorks
To learn about casting processes by creating and using a silicone mold
To gain vacuum-forming experience

My project partner and I decided that we wanted to make a nightlight based on a plastic pushpin like the one shown in this photo. We were interested to see how we could recreate the mold artifacts on most pushpins, such as the seam around the edges, through an entirely different process--thermoforming. We also thought that the many colors of pushpins paired well with the types of plastic available to us, which gave us many options to create the best light diffusion possible for the night light.

We started out by doing some quick notebook sketches (like the one shown here) to get a sense of what we wanted our design to look like--whether we wanted the PCB to be in the base of the nightlight or in the pushpin itself, how would make sure that the battery and buttons were accessible, where the LEDs would go, etc. This was also the stage at which we figured out a rough idea of most of our major dimensions, and discussed what parts would be made out of what materials through which processes. We decided that we wanted the nightlight to look as if it had been pushed into a piece of corkboard. The corkboard would sit on top of a cast urethane base, with a panel in the bottom that concealed the buttons. We also decided that the top of the thumb tack would be thermoformed out of PETG or HIPS plastic.

Once we had our overall design laid out on paper, we began creating CAD models of our design in SolidWorks. My project partner taught me how to use configurations so that we could model both the vacuum-forming buck and the finished vacuum-formed part in the same SolidWorks part. This proved very helpful later when we needed to scale the entire pin-head down to a smaller size.

My project partner modeled the initial part, and I added the thermoforming features--draft angles, air holes, and a brim around the outside of the buck. I used several SolidWorks features that were new to me to create these features. I used a combination of the mold and surface tools to create the brim, and I used the pattern fill tool to create some of the holes. I also gained practice using some SolidWorks tools that I had used before but wasn't very familiar with, such as body split, combine, and delete.

When the CAD was complete, I 3D printed the buck out of ABS. Then, I used the vacuum former to pull our pushpin in a sheet of PETG (for the clear pushpin) and HIPS (for the read and blue pushpins). The corners of the pushpin tended to create some webbing, but with careful timing we were able to minimize that. The photo on the left is one of the first forms that we pulled. We let the plastic heat up for far too long, causing it to stretch too much and leading to webbing. The one on the right is a more successful thermoformed part. There's still a little bit of webbing at the corners because of the geometry of the part, but not so much that it caused us problems during assembly.

Here's a video of us thermoforming another part. Unlike the two in the previous photos, which were made out of clear PETG sheets, this one is made out of HIPS.

For the base of the night light, we designed and 3D-printed two parts to be the positives for our silicone molds. I took the lead on the base, shown in this photo, which held the PCB in place and supported the aluminum pipe.

My project partner designed the PCB cover. Originally, it was to be secured with bolts and heat-set inserts, but after casting, a press-fit was enough to hold it in place. However, through holes for the bolts are still in the cad and shown in this rendering. Instead of holes to allow access to the power and mode buttons on the PCB, the cover had tabs that transmitted force from the user into the buttons. To replace the battery, the user simply had to remove the cover.

We molded the PCB cover first so that we could test our process with our smallest part before moving on to the larger nightlight base. It was a good thing that we did this, as we had to troubleshoot several issues with our process. Our first cast part, shown in this photo, had a poor surface finish, lots of air bubbles, and completely lacked the through holes it was supposed to have for the bolts to secure it to the base.

However, by printing the mold form more carefully, using a vacuum chamber to reduce the air bubbles, and doing a 1-part mold instead of a 2-part mold, we were able to cast several more PCB covers at a significantly higher quality.

Based on what we learned from the PCB cover, we made some changes to the CAD for the nightlight base. Most significantly, we adjusted the design so that we could do a 1-part mold. We also added draft angles to the smaller holes and fillets to whatever edges we could to make separating the cast part from the mold easier. We had just enough silicone left to make our mold. In this video, my project partner is pouring the urethane into the one-part silicone mold. We used a popsicle stick to pop some of the surface bubbles just after the end of this video.

Once enough of our parts were cast and thermoformed for us to make two nightlights, we began assembly. The first step was to solder the LEDs onto the PCB. We used wire leads to attach them so that the LEDs could sit inside of the aluminum tube at the center of the pushpin while the PCB was contained in the base. We also sanded down our urethane parts on a belt sander to remove any artifacts left over from casting, and attached the cork to the base.

The next step was for us to cut out the thermoformed parts and attatch the two halves. We also sandblasted the interior of the clear sections to create a frosted effect. When we were ready to attach the halves, we used clips to hold the edges together while superglue dried, as shown in this photo. Once they were attatched, we used a spindle sander to remove excess plastic from the seam.

The final step was to put all the pieces together and turn it on! Here's a photo of both nightlights. You can see the seam that inspired us running down the center of the nightlights.

Engineering Drawings

Because our parts were made out of plastic and we weren't sure how well the molding and thermoforming would go, we designed them not to require tight tolerances to work in the assembly. But we did still have some critical dimensions, which are documented in these drawings.

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Bill of Materials & Cost Analysis

Here's a bill of materials that tracks the approximate cost of the two nightlights we made and the estimated cost for making 100 nightlights. The cost of labor is also calculated, and is based on an estimate for how long it would take a skilled person with access to the correct equipment to do the manufacturing and assembly.