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Whenever I get tasked with unloading the dishwasher, I'm always amazed at the amount of water that's still stuck (always in droplets) to our plastic kitchen utensils and storage containers.

We have a dishwasher with three racks, but it doesn't seem to matter which rack you use, plastic spatula's in the top cutlery rack, plastic containers or cups in the middle rack or our plastic cutting boards or plates in the bottom rack, all are equally wet when unloading the dishwasher, while the regular cutlery, glasses, metal pans and ceramic plates are all perfectly dry.

Why do the plastic kitchen utensils never dry properly in the dishwasher? And is there anything that can be done so the plastic does dry in the dishwasher? (besides using a towel or just waiting three days before unloading)

  • 4
    I recently got a new dishwasher which – to my immense joy and surprise – actually gets plastic items almost completely dry. So the obvious follow-up question then becomes: why does plastic dry in newer dishwashers? – Janus Bahs Jacquet Dec 31 '18 at 13:54
  • @JanusBahsJacquet go for it...! – Stephie Dec 31 '18 at 19:52
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Causes

According to this article the problem seems mainly two fold, conductivity and thermal inertia (among other factors).

During washing temperatures get relatively hot (depending on the particular program chosen) to promote sterilization and help with cleaning.

  • Conductivity: Different utensils are made of different materials which will absorb this heat at different rates. Plastic has relatively low conductivity compared to say a metal pan or stainless steel object, thus both gaining heat slower and transferring that energy to water less efficiently leading to less evaporation.

  • Thermal Inertia: Plastic objects are generally thinner and lighter, plastic is generally also less dense than other common kitchen materials, leading to retaining less heat, and conserving less energy, thus remaining warm for shorter periods, again promoting less evaporation.

There may also be other factors at work; such as surface properties of plastics like roughness and porosity that make it hydrophobic, which may cause water droplets stick more to its surface, or evaporate slower.

Possible Solutions

Briefly open the door

I have recently developed a practice that I feel helps getting most items (even plastics) almost dry.

After the cycles finishes, (the sooner the better so that the least amount of heat is lost), immediately turn off the dish washer and open the door.

Leave it open for a few brief seconds, long enough to let most vapor escape, but short enough that the minimum amount of heat is lost. After that close the door again and let it sit for a while, no need to shut it tight, leaving it ajar will suffice. Ten to twenty minutes is probably more than enough if you are in a hurry.

This will ensure a lot of the humidity will leave the compartment while still remaining warm, promoting quicker evaporation, ensuring that most items will be either almost dry or have a minimum amount of water when you return to unload the dishwasher.

Shake Plastic Items

One other optional thing you may do in addition to the previous procedure, (if you can waste the time and have the energy) is, after opening the door while waiting for the steam to escape, individually grab any plastic objects and one by one give them a vigorous shake to loosen any droplets on its surface.

If your ratio of plastics to other materials is anything like mine those should be a minority so it shouldn't take too long. One or two shakes per item is generally enough, just let them splash into the sink, or even back into the dishwasher to get most of the surface water off. Then just put them back in and close the door again and let them dry along with other items, the remaining heat should take care of most residual humidity.

As mentioned in the comments, if you have no delicate items that might break, you can also just give the whole rack a shake instead. Items with intricate designs or crevices that pool water may still benefit from a good individual shake though.

Use Rinse Aid

In case you aren't already for some reason, using rinse aid seems to helps considerably achieving drier items after the cycle.

Backstory: After purchasing a new "fancy" zeolitihc dish washer we were told it needed no rinse aid since local water quality is satisfactory; so we didn't for a while, and were pleased with the results. In time however we found, that despite all the bells and whistles the new dishwasher seemed to achieved poorer results compared with the previous one.

So for a change we decided to add some rinse aid once and it made a very considerable difference in the wetness of all item types after a cycle, not only plastics but also, metals, glass and ceramics (in addition to considerably reducing cycle duration by 1/3 of the time, thus also saving energy).

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    Some newer dishwashers pop the door open at the end of the cycle and leave the heater running for a bit. Their R&D, our gain. I'm not really adding anything to this answer other than it's a valid approach that we can take advantage of. – user3190797 Dec 27 '18 at 20:50
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    I grab the entire rack and give it a shake instead of fussing with shaking individual items. Only works if nothing will break when you do this! – elliot svensson Dec 27 '18 at 22:03
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    @elliotsvensson True that also works in most cases, though the individual shake is probably more effective, especially if the items have intricate designs with crevices or shapes that accumulate water – Duarte Farrajota Ramos Dec 27 '18 at 23:55
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    Tussle the Tupperware, shake both drawers and then pull them all the way out to finish drying. – Mazura Dec 28 '18 at 0:16
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    Don't open the door all the way. Open it a little, shake the top rack vigorously, then leave the top rack pulled out a little so the door is ajar. This will let steam pour out while not cooling the inside as fast as having the door fully open. The darn good shake of the top rack also helps if there are hollows on the bottoms of mugs that have water in them. – Kate Gregory Dec 28 '18 at 1:25
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In addition to the lower heat capacity (see other answer), a main reason, quite counterintuitive, why plastics don't dry well is that they're hydrophobic. That's right: they keep water sticking to them because they're water repellent (but not completely water repellent).

The reason for this strange behaviour is that any small amount of water on the surface of plastics immediately contracts to a compact droplet. This minimises contact with the plastic, but also with air, which is the problem: for efficient evaporation, you need a large air-to-liquid surface. You do get such a large surface on glass, ceramic and metal, because these are (at least when freshly cleaned) hydrophilic, so the water stretches out to a thin film coating a whole lot of the surface.

Not so with plastics. Only with some shaking will the droplets start running down the surface, and may combine with other drops and then drip off. With super-hydrophobic materials you'd be ok again because even tiny drops would immediately pour down, but most plastics are exactly at the sour spot: too hydrophobic for evaporation, but still not hydrophobic enough for a lotus effect.

Melamine resins tend to be among the better plastics in this regard, as they're still pretty hydrophilic. Still, they tend to dry only incompletely, probably because of the low heat capacity.

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    Hydrophobic surfaces also explain why non stick pan's can have beads of water on the inside while the outside is dry at the end of the cycle. – Chris H Dec 27 '18 at 22:58
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    @ChrisH, on the other hand, the hydrophobic coating of non-stick pans also means you can dry them with a quick shake. Plastics aren't hydrophobic enough for that to work. – Mark Dec 28 '18 at 0:03
  • This answer could be improved a lot if you actually explained the thermal issues as well. As it stands it seems very incomplete though very interesting. If this answer had both parts it would be a very complete and excellent answer. – Rubiksmoose Dec 30 '18 at 21:55
  • @Rubiksmoose yeah, I actually intended this answer to be more of a supplement to Duarte Farrajota Ramos'. Didn't expect it to outperform that in the votes... – leftaroundabout Dec 30 '18 at 22:00
  • I'd give you a +1, but didn't to try to get Duarte's post back on top as requested. – Joe Dec 31 '18 at 1:18
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There are several variables that go into this so I may not touch one the one(s) you are facing but I will try.

Some options to help in no particular order:

  • Use a rinse aid. (This would be my first suggestion) Rinse aids are designed to coat dishes and then repel water. It makes drying a snap. The lack of splotching is secondary to me.
  • Use the heat dry setting on your dishwasher. (If it has it and it works)
  • Unload the bottom first can help prevent the water on the upper racks spilling onto the lower dishes.
  • Load dishes carefully. Make sure when loading the dishwasher you aren't placing anything in such a way that it pools water. Also packing things tight so they don't shift too much and then pool water.

As to why. I can't answer that very thoroughly but many dishes have an enamel on them that is very smooth and dries quickly. I suspect your plastic dishware aren't as smooth. Especially as time progresses. I know some of my oldest plastics are very rough and don't dry so well. I usually put them on a drying rack after they come out of the dishwasher.

  • Took the liberty to fix, I think you meant upper racks. Feel free to roll back if not – Duarte Farrajota Ramos Dec 28 '18 at 0:34
  • Your suggestion to use a rinse-aid directly contradicts leftaroundabout's answer, where he suggests that repelling water is the problem. – Martin Bonner supports Monica Dec 28 '18 at 11:37
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    Martin, actually a rinse-aid might help the water on a plastic in the dishwasher not stick to itself so much (inside the droplets that form), helping it spread out and dry up. thewirecutter.com/blog/dishwasher-rinse-aid-cleaner-drier – mlibby Dec 28 '18 at 23:03
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    I can confirm rinse aid really helps. We just got a new dishwasher recently and noticed this issue (and not just on plastics, everything was coming out wet). Having never bothered with rinse aid before we decided to give it a go and now there is hardly any water left on items. Plastic does still gather some droplets but nowhere near as bad. – adaliabooks Dec 29 '18 at 18:04
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    Another confirmation on rinse aids: I got plagued by wet plastic for a long time, but just changed the rinse-aid recently, with a great effect: If it doesn't work for you (yet), try a different one. It's not perfect, but a lot better. I won't name the brands here, as they'll likely be very different in different areas of the world. Note: I'm using basic detergent tabs and liquid extra-rinse-aid that the machine adds late in the cycle, rather than the 42-features-in-one-tab cleaners that apparently include little gnomes scrubbing and drying your dishes. – Olaf Kock Dec 30 '18 at 10:22
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To answer you question, we have to take it from a scientific point of view. Its been a while since I did A level chemistry but i think i know enough to give you a simple enough answer. The atoms of hydrogen and oxygen in water, as well as hydrogen and carbon (and other elements which will form the plastic, while hydrogen and carbon are the main ones) give out intermolecular forces- van der waal/dipole dipole forces, hydrogen bond (still a force, but a strong one of that) and temporary dipole forces. They occur due to the attraction between atoms, which will all have different arrangements of their electron shells, number of protons and number of neutrons. Hence different atoms have different levels of attraction. Secondly, static also builds up on plastics when put under energy, thermal in this case, so the water is attracting to the plastic in clumps. You can test this with a balloon rubbed over the carpet, then run under the tap. some water will remain.

To conclude, The plastic and water are just attracting to each other, by intermolecular forces as well as static forces. Think of it like the static collects at several particular points, then the intermolecular forces also help to keep it together.

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