Two ways to make a lid
An injection molded lid is made by forcing molten resin into a closed steel mold, where it cools into a part with a wall thickness set by the tool on both sides. A thermoformed lid starts as a sheet that is heated and drawn or pressed over a single-sided mold; the wall thickness follows the sheet and thins where it stretches most. Both processes make PP lids, and thermoforming also makes PET lids for cold use. On a catalog page the two can look alike. In use they differ in rigidity, fit, decoration and tooling, and those differences decide which one belongs on a given container.
Injection molded lids: strengths and limits
Because the tool controls both faces, an injection lid can carry thicker sections exactly where they are needed: a deep sealing skirt, a rib that stiffens a wide panel, clips, a hinge, a vent or a valve seat. That precision is why reusable storage lids, four-clip lids, vacuum lids and cups with snap rims are almost always injection molded. One partner plant describes its injection molded cup body as food grade PP rated -10 to 120 C and suitable for boiling water; that is the supplier's own statement and is confirmed in testing. Injection parts also accept in-mold labeling, which puts a printed label into the part as it is molded; see in-mold labeling explained.
The limits are tooling and weight. An injection mold is machined steel, takes longer to build and costs more than a thermoforming tool, and the household plastics cluster around Taizhou, with Huangyan known as a mold-making centre, is where most such tools are made. Injection lids are usually heavier than thermoformed lids for the same footprint, which matters in single-use packaging where every gram counts.
Thermoformed lids: strengths and limits
Thermoformed lids dominate single-use takeout and fruit packaging. They are light, fast to produce and come off multi-cavity forming tools at high speed. A partner plant in Foshan, Guangdong, thermoforms PP takeout boxes at 650, 750 and 1,000 ml, including plain, ribbed and two-compartment versions, packed 300 sets per carton; export soup cups from another thermoforming plant run from 350 to 950 ml with the PP code and a microwave marking in the base, per supplier. PET thermoformed lids give high clarity for cold display.
The limits come from the sheet. Corners and deep features thin out as the sheet stretches, so a thermoformed lid cannot hold a thick sealing skirt or a strong clip. Fit depends on a snap profile in the sheet, which can loosen if the sheet is too thin or the parts shrink differently from the base. Thin lids can also crack when frozen. See container lid cracking.
Side by side
| Factor | Injection molded | Thermoformed |
|---|---|---|
| Wall control | Set by the tool on both faces | Follows the sheet, thins at corners |
| Features | Clips, ribs, hinges, valve seats, vents | Snap profile, shallow ribs, vents |
| Materials | PP, also PS or copolyester for storage | PP; PET for cold clarity |
| Weight | Heavier for the same footprint | Light |
| Decoration | In-mold label, print | Print, label, sleeve |
| Tooling | Steel mold, longer build | Forming tool, quicker and simpler |
| Typical use | Reusable storage, vacuum lids, cups | Takeout boxes, soup cups, fruit boxes |
Fit between lid and base
A lid only works on its base, and mixing processes is common: a thermoformed lid on an injection molded tub, or an injection lid on a thermoformed tray. Each process shrinks differently as the part cools, and PP keeps shrinking for some time after molding. If lids and bases come from different lines or different plants, the fit has to be checked with parts from the actual production lots, not from a single sample pair. Storage and shipping temperatures matter as well; lids that fit at the plant can feel tight or loose after weeks in a hot container. See lids not fitting after shipping.
Tooling, sampling and lead time
For a new lid, the route is drawing, sample mold or production tool, T1 trial, corrections and golden sample. For injection tooling the mold cycle is typically 25-45 days to T1, inferred from partner plant practice, and a new mold program usually starts from 50,000-100,000 pieces per item. Thermoforming tools are simpler and quicker, but the sheet gauge and the forming conditions need the same trial discipline. Stock lids that already fit an existing base avoid tooling entirely and start from a few mixed cartons, with printed lids typically from 10,000-50,000 pieces per item, all confirmed in writing for each program. See sample molds and T1 trials.
Cost drivers, without prices
The desk does not publish prices, but the drivers differ clearly. An injection lid carries a higher tool cost, spread over the life of the program, and a unit cost driven by resin, gram weight and cycle time; cavity count on the mold decides output per hour. A thermoformed lid carries a lower tool cost and a unit cost driven by sheet gauge, the number of cavities on the forming tool and trim waste, since the skeleton left after cutting has to be reground or sold. Over a long program at high volume, a light thermoformed lid usually stays the lower-cost route for single use; for reusable products, the extra cost of an injection lid buys the fit and durability the customer is paying for. See mold cost and ownership.
What to test on each
For both: lid fit and opening force, a tilt and inversion test with liquid, a drop test, opening after freezing at -18 C and a microwave test where marked. For injection lids add clip or hinge cycling; for thermoformed lids add a check on corner thickness and a stack test, since thin lids carry the load of the boxes above. Pass criteria are a stated working default, confirmed per order, and seal results are described as leak-resistant in that test.
How to choose
- Reusable storage, vacuum or clip lids, cups with hot drinks: injection molded.
- Single-use takeout, soup cups and fruit boxes at high volume: thermoformed PP, or PET for cold clarity.
- Premium single-use with branding: injection with in-mold label, if the volume justifies the tool.
Send the base, the food and the target volume through the quote form, or check typical quantities on MOQ and lead time.