What’s That?: Ejection Mark On Angled Surface


I thought it would be useful to start a series I like to call, “What’s That?” where I point out little details on parts and products I see that help designers understand better how they are made and how that might affect their designs… This is not unlike the kinds of stuff I point out to my wife, who looks at me with the expression of “Seriously, do you really think I care about this kind of stuff?”… But hopefully you guys will…

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.


Here’s a close-up of the ejection mark. The pin that made that mark has little stair steps cut into the end so that the pushing force against the plastic is normal (perpendicular) to the plastic (at those stepped surfaces). Otherwise the pin would be pressing on the angular surface which might work as well…


So what does that mean to you as a designer? Well, depending on the part design, the molder might have to devise a way to eject your part out of the mold without leaving these marks which don’t look all that great. For a trash can, it’s no big deal, but what if this was a cover for a piece of medical equipment? Stipulating where the molder is and is not allowed to place ejection is a decision that you should be a part of because it does have an impact on your design and the product’s aesthetics. Even if they don’t need to eject against an angled surface and have to use the stair-step technique shown, the ejection pins will always leave some sort of mark (see the parting line?) This is because the ejection system is one of the ways the mold is vented (how air escapes as the mold fills with molten plastic). So those gaps that allow the air to escape will eventually fill with plastic and leave a parting line on the part.

If you’re actually doing the production drawings, you should explicitly define where ejection can and can’t go (for example identifying surfaces and marking them as “NO EJECTION MARKS PERMITTED ON THIS SURFACE”). Traditionally, the molder will also give you some sort of review document showing where ejection will be located before the mold construction begins. Even if you aren’t the one doing the production drawings, you should make sure you communicate with your engineer about how your parts will be ejected and what impact the ejection marks will have on yourt design.


20 Responses to “What’s That?: Ejection Mark On Angled Surface”

  1. 1 taregrabit July 21, 2009 at 2:06 pm

    🙂 cool stuff. keep it up!

  2. 3 Chris July 21, 2009 at 2:53 pm

    This isn’t something I’ve ever considered before. Thanks so much! Any opportunity to step back and question is more than welcome 🙂

    • 4 Warren Ginn July 21, 2009 at 3:33 pm

      I hope to have many more of these in upcoming posts… It’s something that I think about all the time, and, much to her dismay, mostly share with my wife :-). So I thought this would be the kind of thing other industrial designers would find interesting and informative… Thanks!

  3. 5 Travis Baldwin July 24, 2009 at 1:37 pm

    excellent post and excellent blog Warren… you’re the perfect guy for the job.


  4. 7 matt July 25, 2009 at 11:41 pm

    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 Warren Ginn July 26, 2009 at 12:06 am

      Hey, thanks for the comments… Yeah, it’s actually quote unusual to see ejection marks like this on such a visible area of a product. But then again, it IS a trashcan. I’m sure you’d never see anything like this in automotive parts – particularly body panels…


  5. 9 Janejira Kalsmith July 27, 2009 at 9:22 pm

    Very informative! I also like to study random parts, and try to figure out the manufacturing process behind it.

  6. 11 gebelik July 29, 2009 at 7:27 pm

    i like this site. i added on my bookmark i thanks

  7. 13 öykü August 2, 2009 at 12:03 pm

    Good solution, thanks

  8. 14 Larry Sliker August 10, 2009 at 8:35 am

    This type of article is more interesting than the latest fad concept made with twigs and bottle caps. But then maybe I am old and boring!

    • 15 Warren Ginn August 10, 2009 at 9:28 am

      No, I agree… I need to post more of these types of articles… I’m glad you like it.


      • 16 Warren Ginn October 14, 2009 at 10:20 am

        Here’s an E-mail from doug@scottmodels.com
        URL : http://www.scottmodels.com


        Not sure I have the correct person. I dealt with your namesake at Clark Equipment in Lexington many years ago on an ID/Model project while Director of Marketing @ 3-D Technical Services.

        I am head of business development here @ ScottModels and hope to provide product modeling assistance again.

        If you are the Clark Guy what are you up to these days?

        Doug Fiessinger

  9. 17 Warren Ginn July 21, 2009 at 5:45 pm

    Hey, thanks for the re-post! Glad you liked it…

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