Design for Manufacturing


Design for Manufacturing, or DFM, is a course in which students learn about and apply design for manufacturing and assembly techniques to develop and optimize products for different manufacturing processes and production volumes. As a product designer, I want to be able to create designs that are realistic to manufacture, and in DFM, I began to learn about the factors and constraints to consider when designing for different manufacturing processes.

The course was divided into 3 short team projects exploring different manufacturing methods and increasing in production volume over time. The first project, a CNC-milled treasure box, introduced CNC milling in a low volume (40 per month) application as we focused on tolerancing critical dimensions and reducing cycle time. The second project, a nightlight based on the existing ‘Olin Nightlight Platform’ made up of a urethane part cast in silicone and a vacuum-formed part, introduced casting and vacuum forming at a higher production volume (400 per month) while focusing on draft angles and easy-release geometry. The final project, an injection-molded toy with some form of assembly, introduced the injection molding process and custom mold design for high volume (1,000-10,000 per month) production. For the final project, we created designs that met tighter constraints to maintain even shrinkage during cooling, simulated plastic flow to choose optimal gate (injection site) locations, and revisited CNC milling to cut custom aluminum molds.

Injection Molding overmolded fidget spinner

  • Black fidget spinner with exposed bearings held between fingers
  • Three ball bearings hovering over plastic part with three holes, one for each bearing to fit into
  • pile of 7 fidget spinners in different colors
  • Two overlapping fidget spinners glowing in the dark
  • Diagram representing ball bearing surrounded by a mold with orange lines showing how the bearings are inset into the mold
  • Two aluminum blocks with the negative for injection-molding carved into them, forming a mold
  • Pliers, 2 sets of bearings, a small pile of bearings, and a large pile of bearings sitting by an aluminum block mold to show that many bearings do not fit into the mold
  • Clear fidget spinner with exposed bearings and large air bubble
  • Air bubble in end of red and blue fidget spinner held up to camera

For our injection-molded toy, my team designed and produced 8 fidget spinners using custom machined molds. Our final spinners do spin, although not as freely as commercially available alternatives. The linearly aligned bearings in our design, as opposed to the standard tripoint, keep the spinner narrow and easily pocketable. 3 large skateboard bearings add weight to the toy and enable multiple modes of play using different pivot points.

Fidget spinners generally have press-fit bearings, but since our injection molding machine lacked the ejector pins necessary to remove a part with small draft angles, we chose to overmold the bearings in place. Preventing plastic from flowing into the bearings and jamming required tight tolerances on the fit between the bearing and mold, which we achieved by insetting the bearings into the mold and sorting through our low-cost bearings to find the few that fit.

Fidget spinners are typically cheaply made with visible surface defects, so minimizing shrinkage and sinks was not critical for our design. Our final parts have visible sink marks, and we are okay with that. However, making sure that the ‘rings’ of plastic filled and welded properly to surround the bearings was critical. I conducted a number of injection-molding simulations for our team to test different gate placements during the mold design process. We found that gates on either end of the part led to air traps on the top and bottom surfaces of the mold, which was concerning, but we moved forward with that design due to challenges with getting accurate simulation results for diagonally opposed gate placements.

Our team chose to treat the injection molding project as a learning experience rather than focusing on producing a product. Having experienced multiple DFM projects previously, we understood that projects tended to increase in complexity as we worked through them, so we tried to reduce the scope of the project from the start. I found revisiting the use CNC mills after the first project, this time with a shallower design, helpful for building confidence with a process I struggled with initially. I enjoyed troubleshooting the molding of our part, from which I gained experience with the process and now have a better understanding of how plastic flows into different types of mold geometry.

Casting & Vacuum Forming Snom nightlight

  • Bug-shaped nightlight turned on sitting on a cork surface
  • Exploded view of bug-shaped nightlight showing how the top/bottom halves and circuit board fit together
  • Two iterations of 3d printed mold positives for silicone casting. One with a narrow groove with silicone stuck in it, and the other with a wider groove that releases easily
  • Plastic base of bug-shaped nightlight with resin in the process of curing to fill air bubbles
  • Freshly vacuum-formed part still in the machine with buck (mold) visible through the material
  • Rendering of bug-shaped nightlight glowing against a warm glowing background

For our urethane cast and vacuum formed nightlight project, my team developed a Snom-inspired (Pokémon character) nightlight based on the Olin nightlight platform, with a custom coin-cell powered PCB controlling 6 warm-white LEDs enclosed in a urethane cast base and vacuum-formed, light-diffusing, top shell. The enclosure is glued together with epoxy, although a redesign for proper fits/fixtures and finishing would help with repairability and surface finish. Our lamp lights up, although not enough to provide usable illumination in the dark, and the top shell’s combination of light ridges and sand-blasted texture create a shifting depth effect as you look around the lamp.

The base of the nightlight houses the PCB and serves as the ‘body’ and mouth of Snom. A large hole in the base allows for access to the buttons and battery holder on the underside of the PCB. The current design has 8 leg-like spikes sticking out of its sides to help incorporate more spikes into the design, which would otherwise be limited by vacuum-forming draft angles. The base extends out towards the front to form the bottom half of Snom’s face, with two hemispheres, making up up the character’s large cheeks/mouth, protruding upward in front of the top shell.

The top shell of the nightlight slots into the base, hides the electronics from view, diffuses light, and forms Snom’s head and back. We explored multiple shell designs using SolidWork’s ‘freeform’ tool to pull softened spikes out from a cylindrical base. Our initial design for the shell did not include Snom’s head, but in redesigning the base for ease of fabrication and assembly, we chose to separate the face into 2 halves, with the top shell forming the top half of the face.

Through this project, we learned the basics of silicone casting, vacuum forming, and sandblasting. We iterated on designs to improve the manufacturability of our parts and salvaged imperfect parts by manually filling holes and finishing to remove defects. I found it helpful to learn from experience and mistakes throughout the project. Because we started testing our designs and fabrication methods on the earlier side, we were able to make adjustments and understand what factored into vacuum forming and casting failures. I had no experience with vacuum forming or silicone casting but was interested in exploring the processes, and now I feel more comfortable with the idea of doing personal projects involving them. I was also a little intimidated by draft angles and how that would affect my designs, but incorporating them into a design ended up being simpler than expected.

  • Render of a nightlight with round base and slightly spiked top. Glowing orb of light inside the light.

In the future, I may explore creating a non-character version of the nightlight as an industrial design portfolio piece. I have a personal interest in lighting and would like to continue working with the vacuum forming process to produce complex, but still organic, forms for light diffusion. Were I to continue the project, I would explore using multiple diffusion layers for a 3D effect and potentially shift it from a nightlight to an accent lamp by scaling up the size and increasing brightness to provide usable illumination.

CNC Milling aluminum treasure box

  • Machined aluminum box with pill-shaped internal pocket. Box open with lid resting at angle over base.
  • Angled view of closed machined aluminum box
  • Machined aluminum box with pill-shaped internal pocket. Base and lid standing vertically side by side with internal pocket facing the camera
  • Sketch of initial concept for round-edged aluminum box with interior hollowed out except for cylinders of material for mounting holes
  • Closeup view of side of milled aluminum box showing textured surface due to jitter during fabrication process. Tool placed next to box to show deep cut depth
  • Closeup view of interior of milled aluminum box showing textured surface due to jitter during fabrication process
  • Render of rounded-edge square box with pill-shaped internal pocket made of polished aluminum

For the CNC-milled treasure box project, we developed a minimal box milled from two solid blocks of aluminum for a clean design, with rounded corners, crisp edges, and a obround (pill-shaped) internal pocket extending into both the base and lid. When closed, the box forms a solid, reflective, block, concealing items stored inside. Once opened, the base and lid can be separated and used individually as open storage.

Our box is made up of two main components, the base and lid, secured by a pair of diagonally opposed dowel and diamond pins. The base and lid of the box are mirror images of each other apart from the hole sizes for pins. While not critical, the outer edges of the base and lid are intended to align when closed. An obround pocket on each component forms the usable internal space within the box. The final photos show box components with pedestals still present, though they could be milled off to produce the final parts.

Our design was simple, so we spent most of our time making small changes and working on CAM to reduce machining time. Coming into this project, I had very little experience with machining, just trainings and a simple stamp demo. We struggled with tool deflection leading to jitter on the vertical surfaces of the box, so I learned to design for shallower cuts in the future. While machining, I had a chance to learn a lot about what can go wrong and what adjustments can be made to address issues as they come up. I spent a lot of time standing by the mills listening to the sounds being made, and by hearing how it sounded when settings were okay versus when the machined needed to be stopped or adjusted, I got a better understanding of what to look out for.