The Miracle Of Injection Molding: How Does It Work?

Pretty much any household item nowadays has an involved, extremely well-thought-out manufacturing method to it, whether it’s a sheet of paper, an electrical outlet, a can of tuna, or even the house itself. Some of the stories of how these objects came to be are compelling, though, as one of the recent videos from [This Old Tony] shows as he takes a deep dive into a $5 ball valve, and uses it to talk about all of the cool things you can do with injection molding.

Injection molding is the process of casting molten plastic into more useful pieces of plastic. In this case it’s a plumbing valve which might seem simple on the surface, but turns out to be much more involved. These ball valves are extremely reliable but have a very small price tag, meaning that a lot of engineering must have gone into their design. What is unearthed in the video is that injection molding allows parts to be cast into the molds of other parts, and the means by which those parts don’t all melt together, and how seals can be created within the part itself. All of this happens with a minimal number of parts and zero interaction from a human, or from any robot that isn’t the injection mold itself.

The video goes into exceptional detail on these valves specifically but also expounds on various techniques in injection molding. Similar to the recognition the seemingly modest aluminum can deserves, the injection molded ball valve deserves a similar amount of respect. While [This Old Tony] usually focuses on metalworking, he often tackles other interesting topics like this and this video is definitely worth checking out.

18 thoughts on “The Miracle Of Injection Molding: How Does It Work?

    1. That’s a lot of mass to keep warm whilst you pump it in. Not just the extruded material but the mould itself too.
      Possible, yes. Financially viable without the infrastructure to mass produce, not so much.
      Provided the plastic was uv stable, it would last a while though. Easy maintenance, refurbishment, extension, cleaning.
      Insulation may be an issue. Would probably have to be added after since injection moulding isn’t great for making hollow enclosed objects.

      Perhaps you could injection mould a flat pack house. That would be far easier. Maybe filling the sections with expanding thermal foam.

      1. There are methods of gas injection to create hollow injection molded parts. They’re sort of like blow molding except the gas is shot in with the liquid plastic instead of using a molded preform clamped into a heated mold then blowing air or another gas into the preform.

        What’d work for making room sized building components is rotational molding. Easier would be using that to make wall panels. That’s already been done for some playground equipment, scale it up to house size.

  1. It’s true – that’s quite a marvel of modern molding technology!

    One part that he doesn’t mention in the video: not only do the ball and the seals have to be fabricated of chemically incompatible material from PVC (so the housing doesn’t stick to them), those parts (ball, seals) have to not melt when the housing is formed. So they must be made from a material that has a much higher melt temp than PVC (which isn’t all that high, to be fair).

  2. 3D printing will have truly arrived when it is routinely used to make patterns for producing cast zinc alloy injection molds for short run production.

    3D printed mold –> plaster part replica –> cast zinc alloy injection mold


    3D printed part –> plaster mold –> plaster part replica –> cast zinc alloy injection mold

    A plastic injection molding setup does not need to be big or expensive if you’re only going to produce a few hundred units.

  3. Longtime This Old Tony fan here. Beware – he kind of expects you to know a few things already – there’s a ton of dry humor in his vids that might mess up the literal minded…but it has me rolling – it’s really brilliant stuff.

    1. ++ This Old Tony is great. I stumbled on his channel on accident a while ago, and proceeded to binge watch essentially all of his videos. Great sense of humor, really clean editing and he’s got a knack for filming (visual gags, etc). Plus he’s a great machinist who explains things well!

      Can’t say enough about that channel, it’s wonderful.

  4. “So they must be made from a material that has a much higher melt temp than PVC”

    Actually, a cold ball will take heat away from the valve body where it first touches the injected material. The ball can never quite warm to the injection temp. Then the mold can be chilled a bit by water flow or the whole valve popped out and into a cool bath… but this part only helps a part to not deform until cooled. The key is the first part.

  5. Don’t know if it is just his comedic approach to the subject, but the “5” recycling symbol referred to at about 6:20 is in fact the plastic classification number (indicating polypropelene, or PP) and not the number of times it can be re-used.

    The number is a requirement whenever the recycling triangle in printed on something, but the letters “PP” are not a requirement – just a nicety. I believe the numbers range up to 7, with each number representing a particular type of plastic.

  6. Injection molding is definitely fascinating! I actually enjoyed it and got a couple of really interesting facts, so thanks for sharing.

    The only question I have is why is the valve moving by itself from 0:30 to 0:40? lol that was really weird

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