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So, I meal prep, and I use bowls like this to carry my prepped lunches to work:

Square plastic bowl

From another angle, showing the lip below the lid:

From another angle

The recycling mark on the bottom says they are polypropylene for the bowl and polyethylene for the lid.

I leave the empty bowls in the office dish-washer and then take the clean bowls home whenever the dishwasher's been run. Due to forgetfulness on my and my coworkers' parts, I have occasionally found myself trying to figure out how many of the stack of bowls on the break room side table belong to me.

To rectify this problem, I have this idea for a design based on my initials that I'd like to work on the bowls I use to carry my lunches so I can identify the ones that I brought to the office.

So, is there a dishwasher-proof way to make a mark on polyethylene or polypropylene?

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Both types of plastic, polyethylene and polypropylene are "slippery" and do not take glue or other adhesive attachments well. By extension, paint products are likely to slip off in some manner.

It would be more permanent to create a "damaging" effect to the plastic in an area that is not structurally critical, a section that doesn't provide for sealing or for strength in handling.

It appears that the corners of the lids may qualify in this regard. You could use a soldering iron or wood marking iron to engrave your mark(s) on the surface of the plastic. If you used sufficient caution and did not puncture the surface, you could mark almost anywhere on the container.

The flange around the rim of the bowl (redundant?) is another non-structural location. Even if you chose not to mark initials or names, a bump or divot or similar depression would provide identification unlikely to be duplicated by another owner.

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  • Or a rivet on each side under the lip – Stefan Apr 1 '19 at 23:32
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I've found metallic (plastic foil) stickers to last surprisingly well through a dishwasher (though I normally run mine on a 50°C programme). My daughter has a few on her plastic, probably PE) water bottle and they have survived quite a few washes with only a little fading.

It's hard to find a clear example online but they're easily and cheaply available in real shops, where they're easy to identify.

Even some marker pens last quite well, but it's not obvious which ones until you test them.

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