TL;DR
The mold cavities are the easy part -- hot soapy water and a bottle brush finish them in a minute. The lid is where popsicle molds actually fail, because sugar syrup seeps into the stick slots and the silicone seal groove and dries into a sticky varnish. Take the lid apart every wash, and dry every piece fully before storing.
Where the residue actually hides
A popsicle mold looks like a simple set of cups, but the lid is a small assembly with several places for syrup to collect.
The stick slots. Every slot has a narrow gap around the stick, and juice wicks up into it as the pop freezes and expands.
The seal groove. Most lids have a silicone or rubber ring seated in a channel. Syrup runs under the ring and stays there.
The underside ribs. Molded plastic lids have reinforcing ribs on the underside that form little pockets.
The rim of each cavity. Overfilled molds push liquid over the rim, and it runs down the outside.
Sugar syrup dries into a tacky film rather than flaking away. Left long enough it goes hard and glossy, and it becomes a place where residue accumulates and smells develop. Recessed grooves and seals are a recurring problem across kitchen gear, as covered in the guide to cleaning kitchen tools without damaging them.
How do you clean popsicle molds?
Take the lid off and remove the silicone seal, then wash every part in hot water with dish soap, using a bottle brush for the cavities and a small brush for the stick slots and seal groove. Rinse with hot water, air dry all pieces separately, and reassemble only once everything is completely dry.
The step by step
1. Rinse immediately after unmolding. Sugar dissolves instantly in warm water while it is still wet. Left overnight, it becomes a job.
2. Take the lid apart. Pull out the silicone seal or gasket if there is one. This is the step everybody skips and it is the one that matters.
3. Wash the cavities with a bottle brush. Hot water and dish soap. A narrow brush reaches the bottom of a deep cavity where a sponge stops halfway.
4. Brush the stick slots. A small nylon brush, a straw brush, or an old toothbrush. Work from both sides of the slot.
5. Scrub the seal groove. Run the brush all the way around the channel the seal sits in. Wash both faces of the seal itself.
6. Rinse hot and thoroughly. Sugar residue plus soap residue is the reason a mold still feels tacky after washing.
7. Dry every piece separately, fully. Reassembling a damp lid traps water against the seal, which is where stale smells and dark spots begin.
Cleaning silicone molds
Flexible silicone molds are easier to unmold and slightly harder to clean, because silicone holds a fine greasy film that plain water does not remove.
Wash them in water as hot as your hands can comfortably stand with a degreasing dish soap, and turn each cavity partly inside out so you can see and reach the bottom.
If a mold feels faintly slick after washing, that is the film rather than soap. A baking soda paste scrubbed in and rinsed with hot water clears it.
Silicone also absorbs strong smells. Air the mold uncovered for a day after washing if it has held anything intensely flavored. That is the same behavior described in how to clean ice cube trays.
Are popsicle molds dishwasher safe?
Many are, on the top rack, and dishwasher temperatures handle sugar residue well.
Three cautions apply. Thin or soft plastic molds warp in dishwasher heat, and a warped mold leaks. Silicone seals should be removed and washed separately or they trap water in the groove. And deep narrow cavities may not get full spray coverage, so check them after the cycle.
Check the underside for a dishwasher safe mark. Anything unmarked is safer by hand.
Fixing a sticky or smelly mold
If a mold still feels sticky after a proper hot wash, there is dried syrup somewhere you have not reached -- usually the seal groove or a stick slot.
Soak the lid assembly, seal removed, in hot soapy water for fifteen minutes. Hot water dissolves hardened sugar without any scrubbing at all.
For odor, a baking soda paste or a soak in one part white vinegar to three parts water for thirty minutes both work. Use one or the other, not both together, and rinse well with hot water afterward.
If a silicone seal has gone permanently discolored, hard, or cracked, replace it. A perished seal no longer seals, and the groove behind it becomes a trap. The same principle applies to storage jars, covered in how to clean jar lids and rubber gaskets.
Avoid bleach here entirely. It offers nothing against sugar, it can discolor silicone, and it must never be combined with ammonia, with vinegar or any other acid, or with rubbing alcohol, because each of those combinations releases a toxic gas.
Reusable sticks and handles
Plastic reusable sticks are usually part of the lid assembly and often have a drip catcher shaped like a small cup. That cup holds syrup and needs a brush.
Wooden sticks are single use. They absorb liquid, splinter, and cannot be properly cleaned, so they go in the bin after one pop.
Silicone or plastic sleeve handles should be pulled off and washed inside and out. Reusable drinking gear behaves the same way, which is covered in how to clean reusable straws and their brushes.
Cleaning a mold well reduces residue and odor carried into the next batch. It is not a food safety program on its own.
Source: USDA Food Safety and Inspection Service guidance on cleaning food contact surfaces
FAQ
How do you clean sticky popsicle mold lids?
Take the lid apart and remove the silicone seal, then wash every piece in hot water with dish soap, brushing the stick slots and the seal groove with a small nylon brush. Rinse with hot water. If it is still tacky, soak the lid in hot soapy water for fifteen minutes.
Can popsicle molds go in the dishwasher?
Many can, on the top rack, but check the underside for a dishwasher safe mark first. Remove silicone seals and wash them separately so water is not trapped in the groove, and inspect deep cavities afterward, since spray does not always reach the bottom.
How do you get the smell out of a silicone popsicle mold?
Wash it first in very hot water with a degreasing dish soap, since the odor usually rides on a thin film. If it persists, use either a baking soda paste for thirty minutes or a soak in one part white vinegar to three parts water, never both, then rinse hot and air the mold for a day.
Why do my popsicle molds leak?
Usually a seal problem -- syrup dried in the groove holding the seal proud, a seal that has hardened or cracked, or a lid warped by dishwasher heat. Clean the groove and both faces of the seal, and replace the seal if it is stiff, split, or permanently discolored.
Can you reuse wooden popsicle sticks?
No. Wood absorbs liquid, softens, and splinters, and there is no way to clean the inside of the fibers. Discard wooden sticks after one use. Reusable plastic or silicone sticks are designed to be washed, including the small drip cup many of them have.
How soon should you wash popsicle molds after use?
Right after unmolding, while the syrup is still wet, because sugar dissolves in warm water almost instantly at that stage. Left overnight it dries into a tacky varnish that then needs a fifteen minute hot soak and brushing to remove.
Molds take a few minutes. The kitchen and freezer around them take longer. If you would rather hand that off, get a quote or read more about house cleaning services.