How to Make a Mold Negative for an Injection Molding Machine

This is a long term project that I've designed and prototyped from scratch, and for our schools Precious Plastic Club, I decided to make another product for our club to showcase as a way for our club to produce our own custom recycled plastic product utilizing our injection molding machine. This product that have designed and made is a Precious Plastic (fridge) Magnet Mold intended to be used to create copies of the mold positive design out of recycled plastic material via the injection molding machine

see list in image above, plus:

1. STL and resin 3D printer
2. Several ounces of isopropyl alcohol
3. A resin curing machine
4. Bandsaw
5. CNC Mill

These are my brainstorming sketches of the product I envisioned, as well as the inspiration which is the Precious Plastic logo

3D model your design: CAD file here

This will include three primary steps if you follow my exact process:

1. The Mold Positive - or rather known as the final product. This in itself took me a very long time with a lot of adjusting along the way, I started with very simple geometry for the baseline shape, but the design of the plastic bag flag requires using organic shapes, which for me involved using the form feature on Fusion 360.
2. The Mold Negative - This is what the melted plastic will be inject into in order to replicate the geometry of the mold positive.
3. Make a solid body to start off of, it has to be bigger than your mold positive
4. Use the combine tool on both the solid body you just made and the mold positive
5. Use the split body feature, with the mold positive being the tool body, to essentially leave the imprint of the mold positive onto the solid body
6. Cut the mold positive into two so the overhanging features of the product do not make it get stuck in the mold
7. Add extrusions and slots for the two halves of the mold negative to interlock, do not forget to include offsets for this step.
8. The Casing - This is the metal surrounding the mold negative insert, clamped around it with a channel for the plastic to be injected into the mold from.
9. One half will house the mold negative, make sure it also has an offset to the outer dimensions of the mold negative so it will be easy to remove.
10. The other half, or as I call it, the "flat", will essentially just be a flat piece held against it all to keep everything together and give the product a flat backing

This design went through many iterations in my process, with the final being displayed above. I 3D printed each prototype on a STL printer by directly using my CAD model by saving the desired bodies (of the mold negative) as mesh in 3MF and slicing that download in a 3D printing software, etc.

The silicon will provide information regarding whether your mold design is functional, basically a prototype for injecting the melted plastic.

As you can see in the images above, the silicon, or "liquid rubber", I used was from Diamond Driven. I just used what was on hand at the shop, however I imagine an kind of air-drying silicon would work just the same. The instructions should be provided with the silicon you use.

After mixing equal parts of liquid A and B, I poured it into the 3D printed mold negative, let it cure, and would make sure it was removable. This step is especially important as the real deal will not be as flexible and forgiving as the silicon, you have to make sure it would be removable if it were completely hard.

This is the final print of this project, the high temperature resin will in be able to hold up to the hot, melted plastic from the injection molding machine, while the print itself should be finely detailed from the resin printer.

The printer I used was the Formlabs Form 2 printer, with the slicing software being Preform.

1. With your final mold negative CAD model, save the bodies as mesh as 3MF, and input it into Preform.
2. Auto Generated supports and orientation work just fine for this print, I personally separated them just a little bit more on the x and y axis.

Once printed you will need to cure the resin mold negative:

1. First you will need to submerge it in 90% isopropyl alcohol for around 10 minutes
2. Last you will need to seal it all with a curing machine that uses UV and heat, one cycle at 90 degrees celsius will work just fine

Then just remove the supports and sand as needed

To prepare the CAM for the CNC Mill, go to the manufacturing workspace on Fusion 360

Create a setup according to the CNC Mill you have at hand as well as your stock parameters.

The first operation will include:

1. Drilling
2. Facing
3. 2D Adaptive Contour
4. 2D Contours
5. 2d Chamfer

The second operation will be from the bottom, so you will need to flip the stock in the vice in real life here

1. Facing
2. 2D Contours
3. 2D Chamfer

Lastly save the operation to their own NC Programs

According to your CAM preparation, prepare your aluminum stock; I used approximately 9x4.3x1 so both halves of the casing can fit on it plus some extra tolerance just to be safe.

Deburr the cut edges as needed.

Machining Stock:

1. Set up stock in vice
2. Probe your stock and tools
3. Run your first operation
4. Adjust Stock
5. Repeat the first three steps for the bottom side

Now you have all of your components!!

Place the mold negative into your aluminum outer casing, and tighten it all together with screws in the outer drilled divots. Now you are ready to inject👍