INJECTION MOLDINGFall 2018 | Computer-Aided Manufacturing Methods course | Northwestern University
An injection molded tiger coin holder project from a design for manufacturing course. Focus on design for manufacturing and mass production, GD&T, polymers, and the polymer processes.
A class of 4th graders came up with ideas that they wanted us to make, and our professor gave us a few of their drawings to loosely base our designs off of. My team combined the ideas of a sloth that grabs coins and a tiger with wings to create a tiger coin bank. This coin bank would consist of two parts, a front with a tiger face and a slot to put coins in and a back container.
Part Design
The first step in our project was to design a part using Siemens NX. Our original idea was to have the whiskers of the tiger on the side, but tooling and tolerancing constraints made us move the whiskers onto its face. The two parts assembled through an interference fit, where the top fits over the bottom.
CAM & Manufacturing
The second step of our project was to design core and cavity molds for each part, generate manufacturing programs and g-code for each, and machine the molds. I learned how to minimize manufacturing time by clearing out large areas with larger tools and using smaller tools for finishing passes and details, as well as how to calculate spindle speed and step over, and use a HAAS CNC. During this step we also added draft to our part so it could be ore easily removed from the mold, chose our parting line, and added runners and gates to our molds.
Production Run
This quarter, our rapid prototyping labs had two new injection molding machines, and while this was very exciting, it meant a lot of learning and trial and error to make them work. My team started by trying to injection mold our bottom piece, and we kept getting what we thought was short shot, where our part would not fill all the way. However upon further inspection, we realized that our part was actually misaligned, causing the molds to touch in one area and not allowing plastic through. After fixing this alignment issue, our part was experiencing a lot of flash, which we remedied by adding a vent to our cavity mold.
For the front piece, we immediately realized we had made a mistake after the completion of our molds. We put a core in the cavity for the mouth, and it did not align with the hole in the core. The hole in the core should have been a raised piece that sealed off against the flat cavity, but we did not have time to redo both mold halves, so we machined down the extrusion in the cavity to create a seal against the core. After this adjustment, we were able to adjust temperatures, pressures, and shot size relatively easily to create the front piece and assemble our tiger coin bank.
