Manufacturing Methods: Part 1 – Injection Molding
One of the strengths of the Quirky community is the broad range of backgrounds that our members bring to the table. That variety does mean that some community members are intimately familiar with what happens “from sketch to store,” and others might be getting a bit lost when the conversations get more technical. So for my next few bi-weekly blog posts, I’ll be trying to explain the basics behind some of the most popular manufacturing methods we use. For those in the know, please excuse some broad generalizations.
Far and away the most common process we use is injection molding – it’s probably the most common manufacturing method for any plastic or rubber. Molten plastic is shot into a steel mold at high pressure, allowed to cool slightly, and then ejected from the mold. The steel mold is called the tool, and tooling costs tend to be pretty high for injection molding ($1K-$80K) because steel is expensive and labor intensive to cut. On the other hand, the part cost (basically the cost of the plastic used) is pretty low, because it’s pretty easy to automate. The most basic mold would have two sides and no moving parts (except opening and closing). For more complex geometries, or to add features like threads, mold makers add more parts to the mold (which, of course, adds more cost).
While the plastic is cooling, it also shrinks, and that’s the basis of most of the limitations involved in injection molding. One example is wall thickness. Because the cooling occurs at different rates depending on how thick the part is, parts need to have more or less even thickness – otherwise the part warps, because the middle of thick sections are still liquid while the thinner sections are trying to shrink. If you want to know more about different design constraints for injection molding, just google it – there are tons of guides out there.
Two of the most powerful things you can do with injection molding are overmolding and insert molding. For overmolding, you mold two different materials in the same mold – that’s how you get all of those objects that have rubber grips permanently attached attached to hard plastic. Overmolding is very popular in toothbrush and sneaker design. Insert molding is where you take something and put it in the mold before the process starts, so that it ends up embedded in the part. For example, Cordies has an insert molded steel piece that adds weight to the ends.
There are lots of books out there on injection molding, and lots of slight variations on the process, but that’s the general gist of how it works.
Addendum from Ben: How does this all work in the real world? Check out our awesome PowerCurl molds!
