Double-wall plastic tumblers: checking the ultrasonic weld, and stopping water and fog between the walls
A double-wall tumbler is two cups joined at the rim. When the joint is good, nobody notices it. When it is not, water gets between the walls, where it can be seen and cannot be cleaned.
How it is built
The inner and outer walls are moulded separately and joined at the rim, usually by ultrasonic welding: the two parts are pressed together while high-frequency vibration melts a small ridge of plastic into a seal. The sealed gap is what keeps the outside drier and the drink at its temperature for longer than a single wall.
Everything that matters about the product depends on that weld being continuous all the way round.
What goes wrong
| What the customer sees | Usual cause |
|---|---|
| Water, fog or drops between the walls | A gap or weak spot in the weld lets water in during washing |
| Dark spots between the walls | Water that got in and stayed |
| A crack at the rim | A weak weld, or a weld that was stressed by a drop |
| The straw slides out | No stopper ring, or a straw that does not fit the lid |
| Drink on the table | A straw lid sold as leakproof when it is only spill-resistant |
Water between the walls cannot be cleaned or dried by the customer. That is why weld quality decides whether this product comes back.
Checking the weld
Agree these checks and the sample size in the inspection standard:
- Visual: the weld line is continuous all the way round, with no gaps, no burn marks and no flash that a lip will touch.
- Immersion: hold the tumbler under water and watch the weld for a stream of bubbles, as in the immersion test in our leak-testing article. Any bubble stream marks a leak path.
- Warm-water check: fill with warm water — within the material's limit — and look for fog forming between the walls.
- After washing: repeat the immersion check after the agreed dishwasher cycles if the product is sold as dishwasher safe.
Run the weld checks on production, not only on the approved sample. The welding settings can drift, and a weld that was perfect on the sample is no guarantee for the batch.
Material and hot drinks
A double wall does not change the heat limit of the plastic. A cold-drink material in a double-wall tumbler is still a cold-drink material. Choose the material for the drinks you will sell it for — our temperature chart gives the limits — and state them on the pack.
Straw and lid
- A stopper ring near the end of the straw stops it sliding out of the lid.
- Check the straw and the lid seal for food contact separately from the cup.
- A straw lid keeps drinks from splashing; it is not leakproof unless it passes a leak test. Our general leak method is 30 seconds upside down, then a shake, with no drops. Do not print "leakproof" on a lid that has not passed it.
Writing it into the specification
- Material, and the drink temperature the tumbler is sold for.
- Weld checks: visual, immersion and warm-water, with sample size and stage.
- Dishwasher claim, and the checks after the agreed cycles.
- Straw with stopper ring, and lid behaviour — spill-resistant or leakproof.
- Print or decoration position, and its adhesion test.
The weld is invisible on a good tumbler. The specification is what keeps it that way across the batch.