Choose a lug (twist-off) cap when the jar is filled hot at 85–95°C or pasteurized and has to hold vacuum on the shelf; choose a continuous-thread (CT) cap when the product is filled cold and opened often. Lug sizes run from 38mm to 82mm and are fixed by the jar finish, so the cap is specified from the jar drawing, together with a plastisol liner and lacquer matched to the product's pH and fat content. Most vacuum failures come from caps and jars that were never fit-tested as a set.
Lug cap or continuous-thread cap
A lug cap carries 3–6 short lugs on its skirt. They catch interrupted threads on the glass, and a quarter turn opens or closes the jar. A CT cap has one unbroken internal thread and needs several turns.
The difference that matters is the seal. Inside a lug cap sits a ring of plastisol that softens under heat and moulds itself to the rim of the jar. As the contents cool, the headspace contracts and draws 0.3–0.6 bar of vacuum, which pulls the button in the centre of the lid down. The short engagement also copes with the expansion of a 90°C fill without loosening, and a twist-off chuck seats and tightens the cap in a single movement.
CT caps suit the opposite case. The thread re-tightens fully each time, so a spice jar opened 30 times still closes properly, and the cap can be PP plastic, which shrugs off drops that would dent tinplate. A standard CT cap does not give the same hot-fill vacuum seal, and most have no button.
| Attribute | Lug / twist-off | CT continuous-thread |
|---|---|---|
| Engagement | 3–6 lugs, quarter turn | Continuous thread, several turns |
| Seal | Flowed-in plastisol gasket, hermetic | Foam or pulp liner, or induction seal |
| Hot fill at 85–95°C | Yes, built for it | Limited, depends on the liner |
| Vacuum button | Available (flip or safety button) | Generally not |
| Reseal after opening | Moderate; the gasket takes a set | Strong |
| Material | Tinplate 0.20–0.24mm | Tinplate or PP |
Two cases fall outside both columns. Retort above 121°C needs special high-temperature liners and its own validation. And a cold-filled product with no vacuum requirement gains nothing from a lug cap. Our guide to choosing a closure covers the other families.
Cap sizes from 38mm to 82mm and the jars they fit
The size of a lug cap is its nominal inner diameter in millimetres, and it must equal the finish of the jar. Let the product set the mouth: pourable sauces sit at 48–58mm, jam and honey at 63–70mm, and whole pickles or peach halves need 70–82mm.
| Cap size | Usual lug count | Cap weight | Typical jar capacity | Typical use | Pieces per carton |
|---|---|---|---|---|---|
| 38mm | 3 | ~7g | 100–200ml | Minis, samples, baby food | ~5,000 |
| 43mm | 3 | ~8g | 150–250ml | Chutney, gourmet sauces | ~4,500 |
| 48mm | 3–4 | 9–11g | 200–350ml | Salsa, single-serve pasta sauce | ~4,000 |
| 53mm | 4 | 11–13g | 250–380ml | Jam, preserves, honey | ~3,200 |
| 58mm | 4 | 13–15g | 300–500ml | Pasta sauce, applesauce | ~2,800 |
| 63mm | 4 | 15–18g | 350–750ml | The most common grocery size | ~2,200 |
| 70mm | 4–6 | 18–21g | 500ml–1L | Honey, pickles, kimchi | ~1,800 |
| 82mm | 6 | 22–26g | 750ml–1.5L | Whole pickles, bulk sauces | ~1,200 |
The weights assume 0.20–0.24mm tinplate with a plastisol liner, and the carton counts are typical for bulk-lined cartons. The finish sizes we work with are listed against each model in the glass jar collection.
The number of lugs rises with diameter: 3 lugs up to about 48mm, 4 lugs at 53–70mm and 6 lugs at 82mm. Extra lugs spread the capping force around a wider rim, which is why a 6-lug 82mm cap survives pasteurization pressure swings that would lift a 4-lug design. Because the glass finish fixes the count, the cap cannot be chosen separately from the jar.
Liner compound and lacquer
The metal shell does not seal the jar. The plastisol does: a PVC-based compound poured into the cap as a liquid and cured into a gasket. Three things have to be specified around it.
- Process temperature. Standard plastisol is validated for hot fill at 85–95°C and pasteurization up to 100°C. Sterilization or retort at 115–121°C calls for a high-temperature compound, and the cap-and-finish pair must go through validation trials.
- Product chemistry. High-acid fills with a pH below 3.5, such as pickles and vinegar-based sauces, can pull plasticizer out of a liner over 18+ months. Oily fills such as pesto or oil-packed vegetables can swell it. We ask what goes in the jar before anything else and set the compound from the pH and fat content.
- Lacquer beneath the liner. The tinplate is coated: gold phenolic lacquer for acid products, white or clear enamel for neutral ones. Leave the acid-resistant lacquer off a pickle programme and rust spots appear at month 6–9, when the batch is already in distribution.
On documents, ask for a food-contact declaration with each lot covering FDA 21 CFR 175.300 for the coatings and 177 for the plastisol. Buyers in the EU should also request the EU 10-2011 migration test summary.
Line settings that decide whether vacuum holds
Application torque
Lug caps usually go on at 0.8–1.6 Nm, roughly 8–14 in-lb, depending on size, and the value is set on the twist-off chuck. Too much torque shears the gasket against the glass and produces leakers. Too little leaves the lugs only partly engaged, and the cap can back off as a 90°C fill expands. Check the chuck against a torque meter at the start of every shift, and run a hot-water inversion test on the first 50 jars of each run.
Headspace
A hot fill needs 6–10% headspace. With less, product boils into the seal area while cooling; with more, the vacuum is weak and the button only half draws. Fill-level references per model are on the pasta sauce jar page and the pickle jar page.
The safety button
The dimple in the middle of the lid is a diaphragm. A sound seal with 0.3–0.6 bar of headspace vacuum holds it concave; if the seal fails or someone opens the jar, it springs up with a click. US grocery treats this as the tamper-evident feature, and store receivers turn away cases with more than 1–2% raised buttons. Write "flip/safety button" on the purchase order, because plain lids without one also exist.
Five failures and how to prevent them
- Caps from one supplier on jars from another. The finish drawings agree on paper, yet the angle of the sealing surface and the pitch of the lug thread differ enough between plants to cause 2–5% vacuum failures. Nothing shows at packing; it shows at the receiving dock. Buy matched sets, or have the cap supplier test samples of the actual jars before the first order.
- Standard liners in a retort. A liner proven at 100°C softens at 121°C and the seal creeps. Put the high-temperature spec on the purchase order and plan for trial runs, typically 2–3 pilot batches.
- Cold product under a hot-fill closure. Plastisol needs heat from the product, or a steam flush across the headspace, to flow and form vacuum. Without either, the cap may grip mechanically while the button never draws, and retailers read that as a failed seal. For an ambient fill, specify a steam-cap head or change closure family.
- No acid-resistant lacquer on acid products. Below pH 3.5 the tinplate needs gold phenolic under the liner. Rust spotting is grounds for a recall.
- Heavy print across the button. Dense ink stiffens the diaphragm and can delay or mute the pop. Leave the button clear, or keep to the button-safe ink zone marked in the lid maker's artwork template.
Shipping and storing caps
Caps are dense freight. A carton of 2,200 × 63mm caps weighs about 35kg, so a pallet of caps alone is expensive to move by sea for what it is worth. Packed into the gaps between jar pallets in the same container, they add almost nothing to the freight bill, which is how we normally ship them.
Plastisol-lined caps keep for 24 months in their original cartons below 30°C and under 65% humidity. In worse conditions the cut edges of the tinplate can start to corrode and the liner can take a compression set, and both appear later as slow vacuum loss. Ordering in 9–12 month tranches is safer than one purchase meant to last several years.
What to send with a cap enquiry
Cap cost follows the tinplate used (diameter and gauge), the liner spec and the print, and steel moves with the coil market, so figures are quoted per project. A complete brief lets us quote caps and jars as a matched set:
- the jar model or drawing, or the product and target capacity so that we can propose jar and cap together;
- what goes in the jar, its pH and fat content if known, the fill temperature, and the pasteurization or retort plan;
- cap size, colour and whether a safety button is required;
- print artwork as AI or PDF files with the number of colours;
- quantity, destination port, and whether the caps travel alone or with the jars.
Questions buyers ask about lug caps
How do I find the cap size for my jars?
Measure across the outside of the top rim, where the cap seals, rather than across the threads or the body. A 63mm rim takes a 63mm cap. A drawing number or a physical jar is more reliable still. For models we source, such as the honey jars and jam jars, the cap size is given on the spec sheet.
Do lug caps fit mason jars?
No. A standard mason jar has a continuous-thread finish and takes a two-piece closure, a flat lid held by a screw band. That system is described on the mason jar page. A twist-off cap needs a lug finish on the glass.
How much force should it take to open the jar?
Once vacuum has formed, a 63mm cap typically reads 0.5–1.2 Nm on removal, plus the break as the button pops. Lids that consumers cannot open usually trace back to over-torque at the capper combined with deep vacuum from too little headspace. Both are line settings, and the cap is not at fault.
Can lug caps go through a pasteurization water bath?
Yes, the plastisol is designed for that cycle. Fill at 85–95°C, cap at once, invert or pass the jars through an 85–95°C bath for the validated time, then cool. The time-temperature table still has to come from your process authority, since lethality is a property of the process.
What is the lead time for printed lids?
It is quoted per project. The print, the quantity and the schedule of the line running the order all change it.
Can you supply caps for jars bought elsewhere?
Yes, after a fit check. We ask for 30–50 jars from your production lot, measure finish diameter, sealing-surface angle and lug engagement across them, then cap and vacuum-test the set. The report also states the tolerances your jar supplier should hold.