Optimized Parameters: Thermoformed Biohazard

Our injection molded part from blog deliverable 2 did not change, so we’ve chosen to submit the process parameters analysis for our biohazard thermoforming mold, which we had to change significantly to get a final part with the quality we desired.

The critical dimension of our mold is the outer diameter because it has to have a press fit interface with the ID of our injection molded ring.  After our first trial run, we noticed that the outer diameter was too wide, since we had not adequately accounted for the thickness of the sheet of plastic.  It was slightly too large, and did not fit in the ring (it had an OD of 2.535” instead of 2.5”).

In order to fix this problem, we took our mold back to the lathe where we cut off an additional .03 inch of the diameter of our step.

Every part made thereafter had a snug press fit with the yoyo ring.

Now that our part was correctly fitting into our assembly, it was time to optimize the quality of the detail in its biohazard design. Our baseline part didn’t have acceptable detail, so we changed our parameters with the goal of achieving finer detail. From the basic starting parameters we were given by the lab instructors, we decided to change three main variables: heating time, cooling time, and oven temperature. We decided to increase the ovens to 650 degrees. This allowed us to keep our heating time lower while still getting the plastic to the same optimal sag point. Sag tells us that the plastic is malleable enough to physically deform under its own weight, which is an optimal state to try and fit it to a detailed mold. The reason the plastic needs to have sag is because our mold has some very fine details, namely cavities that are thinner than twice the thickness of the plastic sheet. The plastic has to be as malleable as possible to fit into these crevices, and thus provide the best detail. With the oven temperature set at 650F, the heating time was increased to 40 seconds (further optimizing sag). However, upon only increasing the heating time, the part was not fully cooled while being pulled from the mold. This caused the part to become damaged while being removed, and so we had to increase the cooling time to make sure the part would separate from the mold without becoming damaged. We increased the cooling time to 20 seconds to do this.

With these revised parameters, we were finally happy with our end result. We are now ready to run our production with our biohazard mold.