Injection Molding With Hot Glue

Injection molding is simply forcing a melted thermoplastic into a mold of some sort, letting it cool, and then prying the mold apart to get to the finished piece. Hot glue guns are basically handheld thermoplastic extruders, so when [scorch] dug up some old injection molds he had sitting around, it didn’t take long to put two and two together.

Injection molds aren’t something any normal person has sitting around, but a few years ago [scorch] found two books published by Gingery, the same people who have published instructions on how to build a metalshop from scrap. [scorch] created his molds on a small CNC mill – a Sieg X3 – and his initial experiments with injection molded plastic were fairly successful, even if the molds were made from self-cast billets.

After molding a few hot glue LEGO parts with his equipment, [scorch] had a look around the Internet and noticed this was nothing new. One company even sells a hot glue gun-based injection molding kit using polyethylene glue sticks. Their demo video (seen below) seems much more complicated than [scorch]’s efforts, so  we’ll say he came out ahead on this one.

24 thoughts on “Injection Molding With Hot Glue

  1. He needs to add sprues to allow excess material to pass through so you get a full mold filling. Surprised that the books he read on the injection molding did not mention that.

    1. While I cannot think of a practical use for things made from hot glue, but the second link was very interesting! Certainly the first time for to read about actual injection molding at home.
      I thought the same as fartface, mostly because sprues are necessary for casting. For (proper) injection molding, the air can probably get squeezed out via the gaps between the molds. I don’t think this warrants improvements for the (gimmick) hot glue.

      1. From what I can tell, the company selling the kit is actually selling plastic (not adhesive) rods for use in the gun. I know of several practical things that can be molded from plastic. I also know of several companies that have converted machinery from hot melt adhesive to plastic in order to make plastic rods in varying dimensions.

    2. Injection molds do not need risers, the plastic is pressurized and air escapes through the seams. You do need it for molding parts that rely on gravity to fill it like metal casting and resins.

      1. Sorry, but false. Industrial inyection molds needs a way for air to escape. If you depend on the seam for the air scaping, you will get burrs, and this is inacceptable.

        Also, in the last parts to fill, you will have an small volume of highly presurized air and plastic gases, that will detonate, and this shows in the final part also. Another air relieve, and you need simulation or a lot of experience and intuition to place it. (Or some testing before delivery, the usual way.)

        So, air relieve is needed. Usually in the form of 0.05x5mm canals spaced evenly arround the seam.

        (Actual mould designer here. Sorry, I don’t know the correct terminology in english)

        1. This is quite correct, the cavities are vented where the mould halves shut off otherwise the meltfront compresses and ignites the air in front off it. – injection moulding 20 years.

    3. Yes, he needs to cut a couple vents for safe and proper mold flow… More importantly, these molds can pretty obviously be adapted to a stand-alone 3D printer head so that a material other than hot glue could be employed. ‘Why bother???’; improved flow dynamics will create a more physically resilient finished part than a 3D printer would be able to accomplish through composite. In a 3D printed part, the cooling surface of every layer creates a week joint along two axis. While the composite may be of improved resistance to bending along one axis, it will be weak along the third. A similar molded part wouldn’t suffer the same inconsistency. In reality, while serving it’s unique purpose, 3D printing also relies on the worst possible plastic flow design.

      1. I dont think a typical 3d printer head would meet the ‘injection’ requirement. To properly fill a mold you need to push it in with a fair amount of pressure, and in a short enough timespan that it does not start cooling before filled. I believe you can achieve this with the plunger on a glue gun but i doubt the wimpy feed gear on a filament extruder would be up to the task.

        If anyone’s interested in the basics of injection mold design, you can get a pretty informative ‘Injection Mold Part Design for Dummies’ pamphlet from ProtoLabs (I got it for free with a coupon code I found in a back issue of LEDs Magazine).

  2. Years ago I knew someone who built their own small-scale molding machine and it worked very well – similar to the “Gingery” unit mentioned in the post. Using casting epoxy, you can also take molds of things to reproduce since the temperatures are low and the materials are soft.

    Here’s a .pdf of something similar

  3. Gingery Is not a group of people. Gingery is David J Gingery . He died several years ago. But before he did he wrote several books. With his books one could go from literally stone age tools to full mills. As he puts it. 500 years ago when a man wanted to start a machine shop he did not go down to the local store. He built everything from scratch. Here is a link to the series for doing just that. And no I don’t have a tie to this site. I just like his books.

    1. No one had a machine shop 500 years ago. Not even 250 years ago. That kind of technology is more recent than that. The first practical screw cutting lathe wasn’t made until 217 years ago. The Stone Age ended for most of humanity about 12,000 years ago, so that leaves about a 11,800 year gap between the Stone Age and the beginning of the Machine Age. What did humanity do for all of that time you might ask? Mostly we slaughtered each other, or died of famine, or pestilence. None of that required much in the way of machine tooling either.

      Now try to find the story of how this gent got his family name

      Because it is pretty funny.

    2. yes, David J. Gingery did produce a series of pamphlets on making your own machine shop from “scratch”, another man by the name of Vincent R. Gingery(who assume was his son) produced a series of pamphlets of a similar bent, but working with plastics. his pamphlets include a vacuum former, a slip roll machine, an alcohol still(for cars, not consumption), and guess what? the book this article mentioned, injection molding. if you want paper copies of the books, feel free to buy them, i know lindsay books carries them, a quick google search should turn them up, and if you’d like a pdf version, as they’re out of print i won’t feel bad in sharing them, does anyone know a good sharing site? i have them on google drive, but everytime i go to the link google gives me, if it’s more than a month past the link doesn’t work. is there enough interest for me to make a github? i also feel like it’s important to know, that i’m not giving anything away that i didn’t get for free elsewhere, i just gathered it to one place.

  4. I used to run an aluminum die casting machine (injection molding) at a big auto maker and know wayyyyyyyyy more about injection molding (shooting something into a die or mold at high pressure) AND permanent molding (pouring something into a mold and letting gravity / fluid dynamics do the work) than any one person should probably know. The technology for large diecast parts such as engine blocks goes way beyond what this guy is doing, but since these are tiny little plasticy (gluey?) parts, he can get away with letting the die off-gas through the parting lines without the need for “overflows”, which are extra voids in the die that help the part to fill out without causing “poorfill” and “porosity” and the gazillion other defects of injection molding done poorly. In real die casting, the injection pressures involved can be thousands of tons and the speeds at which the media is injected on the order of miliseconds. Imagine taking 1300 degree F molten aluminum and squirting 50 pounds of it into a closed container in under a second at 30,000 tons of force… you DO NOT want to be anywhere near the parting lines of that die!!! If it “flashes out” or molten metal escapes the die at high velocity, you’ll be toast!

  5. Most hackers who frequent this forum were not around when 3D printing started in the 80’s, but this is largely how it began. In the 70’s and through the 80’s new ‘hotmelts’ (with and without adhesive) were coming out. Engineers were using them for rapid prototyping. Injection molding with it was a nice way to soft-tool a prototype part.

    3D printing really started when some clever hackers started attaching the glue gun guts to our cutting plotters… some people were even able to modify pen plotters. They were commonly called Santa Claus machines.

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