Design for Manufacture

Injection Molding: Pillbox Assessment
March 16, 2021

Hi, I’m Tam Black and today we’re going to be talking about a clever plastic item I use every day, this pill organizer! At the end of this video, you should have a better understanding of how *this part [wiggles box, pills rattle] was made.

From this handheld view, I can show you what I’m talking about.

The very first thing that I notice when I look at this pillbox, is that it’s all one material, 4 compartments in a single piece. There was no glue or fixtures required to assemble this product. Because of this, the case is quite strong and does not visibly flex.

From this material stamp, I can tell what kind of plastic this was made from. It reads 05PP- for plastic type 5- Polypropylene. PP is a great choice for this application, demonstrating bright translucent color, and high shine.

The whole box is also recyclable, so if and when it breaks, this entire piece can be melted down and remade into new parts.

Even though it was molded from a single material, it forms 4 separate compartments that can be opened individually empty one into hands. This flexible fold here is called a living hinge, and it creates a light seal. It’s called a living hinge because it didn’t require any additional assembly or parts to be a moving part. Plastic rigidity depends a lot on part thickness. The thickest walls here are hardly flexible at all-and yet a single finger can close and seal the compartment. ‘that’s satisfying’.

I can tell from this separation line here, that the mold came apart along this plane. The parting line you can see on many products demonstrates where the different mold pieces come together and apart to make hundreds and thousands of copies. You can sometimes tell where the separation line is by touch, it’ll feel a little sharper, and is often along an edge for best results.

Each injection cycle pressure fills molten plastic into the cavity or ‘negative’ of the final unit. Some parts require multiple injection sites in order to fill the entire cavity. Because this item is handheld and relatively small, it only has one.

Here is the injection site, well placed to have the plastic flow from a single spot. In just a few seconds the molten hot plastic flows to fill the entire mold cavity, leaving no room for bubbles or gaps.

To make a single piece of plastic this complex, I know the mold must have contained moving parts. Parts of the mold that move into place every injection cycle are called ‘pulls’, because they ‘pull’ out of place for part ejection. After the part has sufficiently cooled, the mold comes ap art- pops out the finished parts, and forms back up to repeat the sequence.

At the end of the cycle, symbols or text get stamped to the compartments, the products get packaged up, and sold to consumers.

This pillbox is made entirely from recyclable polypropylene which was expertly injection molded to create clarity and shine. Faint parting lines tell the story of moving pulls in the mold to form 4 separate sealing compartments, that open and close with a plastic living hinge.

Now that we’ve taken a closer look, I hope you have an appreciation for how this little guy made it into my hands.

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