Introduction: Hot-glue Key-Chain Designs

About: I am an engineering educator and mother of two little makers who loves developing new projects - both personally and for my children and students to work through.

Manufacturing processes aren't the most glamorous and exciting topics for most high school students, but this lesson attempts to intrigue students by having them create their own injection molding system that can produce individually designed key-chains.

I use this lesson in my introductory high school engineering/manufacturing class (grades 9-12) after we have learned how to use CAD software to make 3D models. I introduce manufacturing methods (specifically highlighting injection modeling) within this project and have students apply their newfound CAD skills to help them better understand that process. This project begins by having students design and model an original key-chain shape. Next, they they turn those models into 3D printed molds that they can use to create personal injection molding machines. Finally, they can inject those molds to create their own personalized key-chains.

This project can be used to reinforce the following key concepts within engineering design:

  • design concepts and technical drawings
  • 3D modeling and/or 3D printing
  • manufacturing processes



  • Paper
  • Computer with 3D modeling software*

To create the mold:

  • 3D printer and filament
  • Masking tape
  • Cooking spray
  • Drill and drill bit (drill bit size should be larger than the tip of a hot glue gun)
  • Hot glue gun and glue sticks
  • Optional: Wire, Utility knife or small file

Step 1: Create a 3D Model of Your Design

Once you come up with an idea for your design, it is time to model the project. Any 3D modeling software (that allows you to output a .stl file) should work for this project. I limited each half of my mold to 2" x 2" by 0.5" or less. (This helps for both 3D printing time and glue drying.)

To create the model: First, create a plane that will serve as the surface where the two molds will come together. Determine where this plane will be in your design (sketch) so you can model it accurately. Next, build two part bodies: one should be a model of the front half of the shape (see the gray shape in the above images) and the other should be a model of the back half of the shape (see orange shape in above images). The two part bodies should combine to make the total shape (with no overlap) when visible together.

Step 2: Create the 3D Models of the Molds

Once your shapes are complete, the last step before preparing them to be printed is to "remove" that shape from a box, or mold (as shown above).

Begin by creating a new part body for the box that will become the mold of the front of the part as shown in the yellow images above. The box sketch should be created on the plane you originally set as the middle of your key-chain and be slightly larger than the footprint (Given the project limitations discussed previously, I recommend starting with a 2" x 2" box - but size up slightly if needed based on the dimensions of the model) . Extrude that box sketch slightly past the thickest portion of the key-chain (approximately 0.5", slightly more if needed). Then, remove the key-chain shape part body from the box part body to create the open mold shape. Repeat this for the other side of the model. When done, you should have two mold shapes that show opposing sides of the key-chain. I have attached the 3D model (created in Catia) of the example elephant and the related molds for reference.

Step 3: 3D Print the Molds

Once your mold designs are complete, save each side as an .stl file to 3D print. (I have attached the .stl files for the example elephant molds below.)

Just like you can use any applicable 3D modeling software for this project, any 3D printer and related software should work just fine. My molds were printed on an older Flashforge Creator Pro and turned out nicely.

Step 4: Creating the Injection Molding System

Now comes the fun part: turning your 3D printed designs into an injection molding system.

The first step to prepare your mold is to determine where you will have your gate, which is the location that the liquid material will enter your mold. I always recommend using an area on the back of their part that has the least detail possible (see the elephant's back in the images above).

Drill through the mold completely and remove any debris, etc. from the mold.

Next, wash your molds next to ensure the key-chain you produce is clean.

Before closing up the molds, spray them lightly with cooking spray (canola oil, etc.) to help avoid having your key-chain stuck in your mold when cool.

Line up the molds (ensuring the molds will come together to make the correct internal shape) and tape them tightly closed with masking tape. (This simulates the high pressures that are applied to the injection molding machines during the actual manufacturing process.)

Step 5: Mold the Key-chain

Pre-heat a hot glue gun and then begin injecting the glue into the mold through the gate you made. Take care not to touch the tip of the glue gun to the mold or it will melt the outside of the mold. Insert glue until it starts to bubble up, then wait until it settles into the mold. Continue to slowly add glue until it does not settle below the outside surface of the mold anymore.

Let the mold set for at least 10 minutes to fully harden and then remove the tape to open the mold and lift the completed part out. You may need to use a toothpick (or similar object) to pry out the shape if it is a particularly intricate design.

If any flash (extra material) exists where the molds came together, cut or file it carefully away.

(Note: I used a standard glue stick for this project to show the project with materials many people have lying around, but a colored glue-stick would really make certain designs pop.)

Step 6: Turn Your Molded Design Into a Keychain

You can now turn your molded part using a short (6") piece of wire.

Cut the wire to length and strip both ends. Drill a small hole through your glue-molded part and thread the wire through it. Twist the wire ends together to complete the key-chain!

Step 7: Project Extensions (optional)

I have included the following references if you are using this project in the classroom:

  • If you would like students to create technical drawings of their ideas before modeling and have not already taught this content, HERE is a great reference made by Normal Universe you could use as an embedded mini lesson on technical drawings.
  • HERE is a nice, short video to explain injection molding from Tronicarts. A video is worth a million words in this case. It does a great job stepping through the process.

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