Secrets of the injection moulder

Here's a fascinating post on the IDSA Materials and Processes blog about the things you can learn about injection moulding from studying the "ejection marks" on the surface of plastic objects:

So I noticed the marks on the lid of my mother-in-law's trash can and thought about what that says about how this part was made and how this might be something an industrial designer would need to understand. What I was looking at was the ejection mark placed on an angled surface. Because this large HDPE (high-density polyethylene) part will be somewhat soft when it is ejected or pushed out of the mold, the molder needed to be able to bear on several points close to the perimeter of the part (because just pushing on the middle would probably permanently distort the warm part).

Further inspection of this part also showed that the gates (injection points for the part) was on the underside, or opposite side of the part, which told me that the part probably rode back with the moving half of the mold and then was ejected after the side action (see the lip used to lift the lid?) retracted.

What's That?: Ejection Mark On Angled Surface (via Beyond the Beyond)


  1. I always have paid attention to this sort of thing, being an engineer, but got much more interested after thinking about designing my own injection-molded parts.

    I consider it a fun mental exercise to study any old plastic thing to see how the mold was made and what sort of side molds etc. were needed. Clever designs can be done without side action, but most quality pieces have them to avoid draft (slope) on the side faces.

    The “Lego factory tour” is one of my favorite amusements when the kids drag me to Legoland in Calif.

    On a somewhat unrelated note, the crop circles in England provided many theories espoused by non-engineers that were easily refuted simply by looking at how the crops were flattened. These people tended to be utterly unfamiliar with the notion of reverse engineering.

  2. And the parting lines, the injection point(s), the stationary side, and the moving side … too!

  3. Generally speaking, an injection molder thinks about 2 things more than anything when looking at a new project: are there any undercuts/reliefs, and how will the part be ejected from the mold. After that it’s just some 3D cad time and lots of patience.

    Big trash cans like this are usually assembled from two primary molds: lid, base.

    The lid might only be a 3 part mold where the lid-lift or handle is a slide out part on a clam-shell style mold. That allows a undercut/relief to eject the part without distorting it as it’s ejected.

    The base of this can is probably little more complex. Here you’re talking about a 5-6 part mold. You might have 2-3 sections on the inside that creates the hollow. It’s also plausible to have a single piece, but you have to take into account suction when ejecting the part. The slug might actually just stick to the core if it can’t release. The outside is a whole other can of worms. At a minimum a 2 part clam shell style mold. You can usually tell from the part lines on the outside of the can where the mold comes together. If there are plastic handles or features on the base of the can, then you can easily keep adding parts to the mold to get the undercuts needed.

    The molds themselves are HUGE. Usually with framing built around them to hold them in place. You don’t really want a thick piece of metal because it would take forever to heat up and cool down. The the molds are steel with thin walls that are braced in certain areas. Curing is usually done in huge ovens, or in some rarer cases with integrated heating thanks to a large press.

    Either way, molding is crazy business. It’s a short list of people that really know what they’re doing. And thanks to a steady decline of engineers in this country, a lot of this stuff is being engineered and hence fabricated elsewhere. Kinda of a bummer really.

  4. As a web designer who works with a lot of clients who are in plastics and manufacturing, I’m amazed at how much I have learned about the processes and industry, to the point where it has become somewhat fascinating to me. Weird, random facts that always end up coming up in conversation, making people ask “How the heck do you know that? I thought you were a web designer?”

  5. what i really want to know is how do i keep the neighborhood squirrels from eating the lid off my trashcan. I just got a replacement lid from the city and the little terrors are already back at it.

  6. @#4 – A few additional items for your description:

    The bottom “bucket” piece on many (if not most) trash bins are roto-molded and not injection. Not sure how this particular product came to be, but something the size of a trashcan is approaching the limits of any single injection molded piece. Ignoring the complicaitons of multiple part core and cavity molds, there simply aren’t many machines strong enough to hold that kind of pressure.

    You mentioned the size of the machine that made these parts. I haven’t got much first-hand field experience at manufacturing factories, but the size of the injection mold is mostly dependent on clamping pressures. More mold parts and more cooling, and you’re looking at some pretty hefty equipment. The machine for the base of that trash bin is mostly likely in the several thousand ton zone, which are gargantuan machines of fabrication.

    Another item of mention is cost. I like how this article is a little introduction into just how much time, engineering and technology goes into the ubiquitous, but also of note is the substantial monetary investment. A simple run of manufacturing of those molds cost several hundred thousand dollars in tooling alone.

    It’s nice to see when the average consumer gets curious in the design process. A sad truth about actual good design is that it inherently goes unnoticed.

    Now let’s complicate things and try to “disect” an injection molded part that runs a collapsible core!

  7. i work in a plant inspecting/ processing and assembling parts like these. I dont pretend to have as much technical know how about all this as you do but after working with these machines for awhile you have to learn a few things about them.

    one thing you should know. these parts are usually quite thin, and some fans are used to cool them as they go from mold to worker. You would be very surprised how fast something even as large as a trashcan lid can cool even without the use of a fan. Typically I am able to easily (they are still rather warm) handle these parts within 20 seconds of them being made, meaning that in as little at 45 seconds it went from a liquid which was injected into a mold to a warm solid.

    also, i realize that this next part does not apply to all things made in this way, most of our larger parts are removed from their molds via large robotic arms fitted with powered suction cups. For smaller parts we use a system where the multiple parts are attached to a central part with a handle of sorts that is removed by a robotic arm. It is then my job to separate the smaller parts from the central connecting piece and remove, if necessary, many remaining traces of the connection between the two.

    So basically, I have no idea what the hell you’re talking about, though I do understand the concept. To my knowledge this is not a necessary practice and is not used in the plants I work in, one of which makes plastic/ rubber parts for cars and is held to the ford standard of quality. Which from my experience is quite high and would not allow for such marks on their pieces.

  8. some kiln insulation, a few stove elements, a vacuum cleaner and some welding, counter weights and springs, anyone can do it!

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