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Here's my guess, but I would love to hear another perspective.

I think this is injection molded. This colander has draft angles and the two mold halves use an intricate step-like wall pattern to create the final holes on the wall.

(For extra point, how would you efficiently produce the wall pattern in CAD?)

OXO colander

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  • $\begingroup$ OXO is a gravy cube - when did they start manufacturing? $\endgroup$ – Solar Mike Jul 11 '20 at 16:54
  • $\begingroup$ Bonus really depends on cad application. I mean in creo you would use the dimension pattern with a alternating rule but in something like inventor youd have to do it with 2 patterns $\endgroup$ – joojaa Jul 11 '20 at 22:08
  • $\begingroup$ @SolarMike Perhaps you are just making a joke, but if not, OXO is one of the largest manufacturers of kitchen utensils and other things. Most known for improved ergonomics. They make a lot of pretty clever things. oxo.com $\endgroup$ – Eric S Jul 12 '20 at 19:06
  • $\begingroup$ @EricShain well, check out en.m.wikipedia.org/wiki/Oxo_(food) $\endgroup$ – Solar Mike Jul 12 '20 at 19:13
  • $\begingroup$ @SolarMike Oh, I’ve experienced Oxo bullion cubes. $\endgroup$ – Eric S Jul 13 '20 at 14:17
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This colander was definitely not CNC machined, it was injection molded. Without being able to inspect the part closely, I will guess that the shutoffs (the open spaces between the plastic ribs) were formed by a retractable feature in the mold which presses against the mold core during the injection process, and then is pulled away from the core when the mold is split, so the part will not hang up in the mold.

The other way to do this without a mechanism in the mold as described above is to cleverly shape the halves of the mold in such a way as to form the shutoffs naturally as the two halves of the mold are pressed together. This is a tricky thing to accomplish because it creates an extremely complex parting line between the mold halves which greatly increases the chances of creating flash, where small amounts of plastic squeeze inbetween the shutoff zones and leave thin webs of plastic that close off the shutoffs.

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  • $\begingroup$ 1.to clarify, when you say "retractable feature", are you referring to side actions? if so, since there are open spaces on all 4 sides, would this require 4 side actions? also, wouldn't such a feature dramatically increase price? 2.you raise a great point about increasing flash. i didn't think about that and this "clever" way would increase the parting line significantly. due to cost and complexity, i doubt they will add a post processing step to cut the flashes. so my conclusion is that this is either made using highly optimized injection molding settings or used a different method entirely. $\endgroup$ – Wei Xun He Jul 11 '20 at 17:33
  • $\begingroup$ yes I am referring to slides and the mold would require 4 slides either in the sidewalls or the core. It would not dramatically increase price of the part since the added mold cost is distributed across tens of thousands of molded parts. If I could hold the part in my hands I could trace the parting line(s), find the ejector pins and blades, and locate the gate and then tell you exactly how it was molded! $\endgroup$ – niels nielsen Jul 11 '20 at 19:33
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Looks like pretty routine machining for the steel molds ,probably with a CNC to speed it up. I doubt it is complex enough to need side actions in the molds, only ejector pins to push the the solid plastic part out of the mold. If you want to see clever machining , look inside a sewing machine, some parts do not seem to have a straight, flat surface anywhere . Long ago I worked for an investment foundry that made several parts for Singer , no CNC to help.Parts were molded in plastic then investment cast in steel allowing for thermal shrinkage. Better yet ,look up a Hydril "wedge thread" for oil country tubulars , although pretty difficult to visualize from a picture.

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    $\begingroup$ Clarified that I was talking about the steel mold, I thought that was obvious. $\endgroup$ – blacksmith37 Jul 11 '20 at 18:56
  • $\begingroup$ Many people asking questions on this site are pretty clueless about engineering. $\endgroup$ – Eric S Jul 12 '20 at 19:01

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